Coronary blood flow responses to physiological stress in humans

Right ventricular performance during acute hypoxic exercise

Acute hypoxia increases pulmonary arterial (PA) pressures, though its effect on right ventricular (RV) function is controversial. The objective of this study was to characterize exertional RV performance during acute hypoxia. Ten healthy participants (34 ± 10 years, 7 males) completed three visits: visits 1 and 2 included non-invasive normoxic (fraction of inspired oxygen (F i O 2 ${F_{{\mathrm{i}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) = 0.21) and isobaric hypoxic (F i O 2 ${F_{{\mathrm{i}}{{\mathrm{O}}_{\mathrm{2}}}}}$  = 0.12) cardiopulmonary exercise testing (CPET) to determine normoxic/hypoxic maximal oxygen uptake (V ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ ). Visit 3 involved invasive haemodynamic assessments where participants were randomized 1:1 to either Swan-Ganz or conductance catheterization to quantify RV performance via pressure-volume analysis. Arterial oxygen saturation was determined by blood gas analysis from radial arterial catheterization. During visit 3, participants completed invasive submaximal CPET testing at 50% normoxicV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ and again at 50% hypoxicV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ (F i O 2 ${F_{{\mathrm{i}}{{\mathrm{O}}_{\mathrm{2}}}}}$  = 0.12). Median (interquartile range) values for non-invasiveV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ values during normoxic and hypoxic testing were 2.98 (2.43, 3.66) l/min and 1.84 (1.62, 2.25) l/min, respectively (P < 0.0001). Mean PA pressure increased significantly when transitioning from rest to submaximal exercise during normoxic and hypoxic conditions (P = 0.0014). Metrics of RV contractility including preload recruitable stroke work, dP/dtmax, and end-systolic pressure increased significantly during the transition from rest to exercise under normoxic and hypoxic conditions. Ventricular-arterial coupling was maintained during normoxic exercise at 50%V ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ . During submaximal exercise at 50% of hypoxicV ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ , ventricular-arterial coupling declined but remained within normal limits. In conclusion, resting and exertional RV functions are preserved in response to acute exposure to hypoxia at anF i O 2 ${F_{{\mathrm{i}}{{\mathrm{O}}_{\mathrm{2}}}}}$  = 0.12 and the associated increase in PA pressures. KEY POINTS: The healthy right ventricle augments contractility, lusitropy and energetics during periods of increased metabolic demand (e.g. exercise) in acute hypoxic conditions. During submaximal exercise, ventricular-arterial coupling decreases but remains within normal limits, ensuring that cardiac output and systemic perfusion are maintained. These data describe right ventricular physiological responses during submaximal exercise under conditions of acute hypoxia, such as occurs during exposure to high altitude and/or acute hypoxic respiratory failure.

Effects of short-term dietary nitrate supplementation on exercise and coronary blood flow responses in patients with peripheral artery disease

Background

Peripheral arterial disease (PAD) is a prevalent vascular disorder characterized by atherosclerotic occlusion of peripheral arteries, resulting in reduced blood flow to the lower extremities and poor walking ability. Older patients with PAD are also at a markedly increased risk of cardiovascular events, including myocardial infarction. Recent evidence indicates that inorganic nitrate supplementation, which is abundant in certain vegetables, augments nitric oxide (NO) bioavailability and may have beneficial effects on walking, blood pressure, and vascular function in patients with PAD.

Objective

We sought to determine if short-term nitrate supplementation (via beetroot juice) improves peak treadmill time and coronary hyperemic responses to plantar flexion exercise relative to placebo (nitrate-depleted juice) in older patients with PAD. The primary endpoints were peak treadmill time and the peak coronary hyperemic response to plantar flexion exercise.

Methods

Eleven PAD patients (52-80 yr.; 9 men/2 women; Fontaine stage II) were randomized (double-blind) to either nitrate-rich (Beet-IT, 0.3 g inorganic nitrate twice/day; BRnitrate) or nitrate-depleted (Beet-IT, 0.04 g inorganic nitrate twice/day, BRplacebo) beetroot juice for 4 to 6 days, followed by a washout of 7 to 14 days before crossing over to the other treatment. Patients completed graded plantar flexion exercise with their most symptomatic leg to fatigue, followed by isometric handgrip until volitional fatigue at 40% of maximum on day 4 of supplementation, and a treadmill test to peak exertion 1-2 days later while continuing supplementation. Hemodynamics and exercise tolerance, and coronary blood flow velocity (CBV) responses were measured.

Results

Although peak walking time and claudication onset time during treadmill exercise did not differ significantly between BRplacebo and BRnitrate, the diastolic blood pressure response at the peak treadmill walking stage was significantly lower in the BRnitrate condition. Increases in CBV from baseline to peak plantar flexion exercise after BRplacebo and BRnitrate showed a trend for a greater increase in CBV at the peak workload of plantar flexion with BRnitrate (p = 0.06; Cohen's d = 0.56).

Conclusion

Overall, these preliminary findings suggest that inorganic nitrate supplementation in PAD patients is safe, well-tolerated, and may improve the coronary hyperemic and blood pressure responses when their calf muscles are most predisposed to ischemia.Clinical trial registration:https://clinicaltrials.gov/, identifier NCT02553733.

Common carotid artery responses to the cold-pressor test are impaired in individuals with cervical spinal cord injury

Cervical spinal cord injury (SCI) leads to autonomic cardiovascular dysfunction that underlies the three- to fourfold elevated risk of cardiovascular disease in this population. Reduced common carotid artery (CCA) dilatory responsiveness during the cold-pressor test (CPT) is associated with greater cardiovascular disease risk and progression. The cardiovascular and CCA responses to the CPT may provide insight into cardiovascular autonomic dysfunction and cardiovascular disease risk in individuals with cervical SCI. Here, we used CPT to perturb the autonomic nervous system in 14 individuals with cervical SCI and 12 uninjured controls, while measuring cardiovascular responses and CCA diameter. The CCA diameter responses were 55% impaired in those with SCI compared with uninjured controls (P = 0.019). The CCA flow, velocity, and shear response to CPT were reduced in SCI by 100% (P < 0.001), 113% (P = 0.001), and 125% (P = 0.002), respectively. The association between mean arterial pressure and CCA dilation observed in uninjured individuals (r = 0.54, P = 0.004) was absent in the SCI group (r = 0.22, P = 0.217). Steady-state systolic blood pressure (P = 0.020), heart rate (P = 0.003), and cardiac contractility (P < 0.001) were reduced in those with cervical SCI, whereas total peripheral resistance was increased compared with uninjured controls (P = 0.042). Relative cerebral blood velocity responses to CPT were increased in the SCI group and reduced in controls (middle cerebral artery, P = 0.010; posterior cerebral artery, P = 0.026). The CCA and cardiovascular responsiveness to CPT are impaired in those with cervical SCI.NEW & NOTEWORTHY This is the first study demonstrating that CCA responses during CPT are suppressed in SCI. Specifically, CCA diameter, flow, velocity, and shear rate were reduced. The relationship between changes in MAP and CCA dilatation in response to CPT was absent in individuals with SCI, despite similar cardiovascular activation between SCI and uninjured controls. These findings support the notion of elevated cardiovascular disease risk in SCI and that the cardiovascular responses to environmental stimuli are impaired.

The coronary vascular response to the metaboreflex at low-altitude and during acute and prolonged high-altitude in males

Myocardial oxygen delivery is primarily regulated through changes in vascular tone to match increased metabolic demands. In males, activation of the muscle metaboreflex during acute isocapnic hypoxia results in a paradoxical coronary vasoconstriction. Whether coronary blood velocity is reduced by metaboreflex activation following travel and/or adaptation to high-altitude is unknown. This study determined if the response of the coronary vasculature to muscle metaboreflex activation at low-altitude differs from acute (1/2 days) and prolonged (8/9 days) high-altitude. Healthy males (n=16) were recruited and performed isometric handgrip exercise (30 % max) followed by post-exercise circulatory occlusion (PECO) to isolate the muscle metaboreflex at low-altitude and following acute and prolonged high-altitude (3,800 m). Mean left anterior descending coronary artery blood velocity (LADv_mean, transthoracic Doppler echocardiography), heart rate, mean arterial pressure (MAP), ventilation, and respired gases were assessed during baseline and PECO at all time-points. Coronary vascular conductance index (CVC_i) was calculated as LAD_Vmean/MAP. The change in LADv_mean (acute altitude: -1.7 ± 3.9 cm/s, low-altitude: 2.6 ± 3.4 cm/s, P = 0.01) and CVC_i (acute altitude: -0.05 ± 0.04 cm/s/mmHg, low-altitude: -0.01 ± 0.03 cm/s/mmHg, P = 0.005) induced by PECO differed significantly between acute high-altitude and low-altitude. The change in LAD_Vmean and CVC_i induced by PECO following prolonged high-altitude was not different from low-altitude. Our results suggest that coronary vasoconstriction with metaboreflex activation in males is greatest following acute ascent to high-altitude and restored to low-altitude levels following 8-9 days of acclimatization.

Sympathetic regulation of coronary circulation during handgrip exercise and isolated muscle metaboreflex activation in men

NEW FINDINGS

What is the central question of this study? What is the role of β- and α-adrenergic receptors in the control of the coronary circulation during handgrip exercise and isolated muscle metaboreflex activation in humans? What is the main finding and its importance? β-Adrenergic receptor, but not α-adrenergic receptor, blockade significantly blunted the increases in coronary blood velocity observed during handgrip. Coronary blood velocity was unchanged from baseline during isolated muscle metaboreflex activation. This highlights the important role of β-adrenergic receptors in the coronary circulation during handgrip in humans, and the more limited involvement of the α-adrenergic receptors.

ABSTRACT

We sought to investigate the role of β- and α-adrenergic receptors in coronary circulation during static handgrip exercise and isolated muscle metaboreflex activation in humans. Seventeen healthy young men underwent two experimental sessions, consisting of 3 min of static handgrip exercise at a target force of 40% maximum voluntary force (not achieved for the full 3 min), and 3 min of metaboreflex activation (post-exercise ischaemia) in two conditions: (1) control and β-blockade (oral propranolol), and (2) control and α-blockade (oral prazosin). In both sessions, coronary blood velocity (CBV, echocardiography) was increased during handgrip (Δ8.0 ± 7.4 cm s^-1 ) but unchanged with metaboreflex activation (Δ2.5 ± 3.2 cm s^-1 ) under control conditions. β-Blockade abolished the increase in CBV during handgrip, while CBV was unchanged from control with α-blockade. Cardiac work, estimated from rate pressure product (RPP; systolic blood pressure multiplied by heart rate), increased during handgrip and metaboreflex in control conditions in both sessions. β-Blockade reduced RPP responses to handgrip and metaboreflex, whereas α-blockade increased RPP, but the responses to handgrip and metaboreflex were unchanged. CBV and RPP were only significantly correlated during handgrip under control (r = 0.71, P < 0.01) and β-blockade (r = 0.54, P = 0.03) conditions, and the slope of this relationship was unaltered with β-blockade. Collectively, these findings indicate that β-adrenergic receptors play the primary role to the increase of coronary circulation during handgrip exercise, but CBV is unchanged with metaboreflex activation, while α-adrenergic receptor stimulation seems to exert no effect in the control of the coronary circulation during handgrip exercise and isolated muscle metaboreflex activation in humans.

Oxygen Delivery Approaches to Augment Cell Survival After Myocardial Infarction: Progress and Challenges

Myocardial infarction (MI), triggered by blockage of a coronary artery, remains the most common cause of death worldwide. After MI, the capability of providing sufficient blood and oxygen significantly decreases in the heart. This event leads to depletion of oxygen from cardiac tissue and consequently leads to massive cardiac cell death due to hypoxemia. Over the past few decades, many studies have been carried out to discover acceptable approaches to treat MI. However, very few have addressed the crucial role of efficient oxygen delivery to the injured heart. Thus, various strategies were developed to increase the delivery of oxygen to cardiac tissue and improve its function. Here, we have given an overall discussion of the oxygen delivery mechanisms and how the current technologies are employed to treat patients suffering from MI, including a comprehensive view on three major technical approaches such as oxygen therapy, hemoglobin-based oxygen carriers (HBOCs), and oxygen-releasing biomaterials (ORBs). Although oxygen therapy and HBOCs have shown promising results in several animal and clinical studies, they still have a few drawbacks which limit their effectiveness. More recent studies have investigated the efficacy of ORBs which may play a key role in the future of oxygenation of cardiac tissue. In addition, a summary of conducted studies under each approach and the remaining challenges of these methods are discussed.

Modeling Left Ventricle Perfusion in Healthy and Stenotic Conditions

A theoretical fluid mechanical model is proposed for the investigation of myocardial perfusion in healthy and stenotic conditions. The model hinges on Terzaghi’s consolidation theory and reformulates the related unsteady flow equation for the simulation of the swelling–drainage alternation characterizing the diastolic–systolic phases. When compared with the outcome of experimental in vivo observations in terms of left ventricle transmural perfusion ratio (T.P.R.), the analytical solution provided by the present study for the time-dependent blood pressure and flow rate across the ventricle wall proves to consistently reproduce the basic mechanisms of both healthy and ischemic perfusion. Therefore, it could constitute a useful interpretative support to improve the comprehension of the basic hemodynamic mechanisms leading to the most common cardiac diseases. Additionally, it could represent the mathematical basis for the application of inverse methods aimed at estimating the characteristic parameters of ischemic perfusion (i.e., location and severity of coronary stenoses) via downstream ventricular measurements, possibly inspiring their assessment via non-invasive myocardial imaging techniques.

Oxygen therapy in ST-elevation myocardial infarction

Aims

To determine whether supplemental oxygen in patients with ST-elevation myocardial infarction (STEMI) impacts on procedure-related and clinical outcomes.

Methods and results

The DETermination of the role of Oxygen in suspected Acute Myocardial Infarction (DETO2X-AMI) trial randomized patients with suspected myocardial infarction (MI) to receive oxygen at 6 L/min for 6-12 h or ambient air. In this pre-specified analysis, we included only STEMI patients who underwent percutaneous coronary intervention (PCI). In total, 2807 patients were included, 1361 assigned to receive oxygen, and 1446 assigned to ambient air. The pre-specified primary composite endpoint of all-cause death, rehospitalization with MI, cardiogenic shock, or stent thrombosis at 1 year occurred in 6.3% (86 of 1361) of patients allocated to oxygen compared to 7.5% (108 of 1446) allocated to ambient air [hazard ratio (HR) 0.85, 95% confidence interval (95% CI) 0.64-1.13; P = 0.27]. There was no difference in the rate of death from any cause (HR 0.86, 95% CI 0.61-1.22; P = 0.41), rate of rehospitalization for MI (HR 0.92, 95% CI 0.57-1.48; P = 0.73), rehospitalization for cardiogenic shock (HR 1.05, 95% CI 0.21-5.22; P = 0.95), or stent thrombosis (HR 1.27, 95% CI 0.46-3.51; P = 0.64). The primary composite endpoint was consistent across all subgroups, as well as at different time points, such as during hospital stay, at 30 days and the total duration of follow-up up to 1356 days.

Conclusions

Routine use of supplemental oxygen in normoxemic patients with STEMI undergoing primary PCI did not significantly affect 1-year all-cause death, rehospitalization with MI, cardiogenic shock, or stent thrombosis.

Role of autonomic dysfunction in the regulation of myocardial blood flow in systemic sclerosis evaluated by cardiac magnetic resonance

AIM

Autonomic dysfunction (AD) is an early feature of systemic sclerosis (SSc). A regular endothelial function is a prerequisite for normal response of the myocardial blood flow (MBF) to cold pressure test (CPT). The aim of the study was to evaluate the relation between MBF and AD at rest and after CPT in asymptomatic SSc patients.

METHODS

Twenty SSc patients and 10 age-, sex- and body mass index-matched healthy controls underwent cardiac magnetic resonance at rest and after CPT. All subjects underwent 24 hours ambulatory 3-channel electrocardiogram Holter to evaluate AD by heart rate variability.

RESULTS

We did not observe any significant difference in MBF (mL/g/min) at rest and after CPT between SSc patients and healthy controls. Delta of MBF (difference between MBF after CPT and rest MBF) was lower (P = 0.039) in SSc patients than healthy controls (0.28 [0.04-0.40] vs 0.33 [0.24-0.54]). The low frequency/high frequency (LF/HF) was higher (P = 0.002) in SSc patients than healthy controls (3 [1.7-6] vs 1.8 [1.1-2.8]). The high frequencies (HF), modulated mainly by paraympathetic system, was lower (P = 0.003) in SSc patients than healthy controls (30 [16-42] vs 36.5 [24-44]). Sympathetic hyperactivity, due to reduction of parasympathetic activity (HF), is present in SSc patients. A negative correlation was observed between Delta of MBF and LF/HF (r = -0.572, P = 0.0031).

CONCLUSION

AD, characterized by sympathovagal imbalance due to a reduced parasympathetic tone with high LF/HF ratio, could be responsible for the reduced myocardial vasodilatory response after CPT.

Influence of myocardial oxygen demand on the coronary vascular response to arterial blood gas changes in humans

It remains unclear if the human coronary vasculature is inherently sensitive to changes in arterial Po₂ and Pco₂ or if coronary vascular responses are the result of concomitant increases in myocardial O₂ consumption/demand ([Formula: see text]). We hypothesized that the coronary vascular response to Po₂ and Pco₂ would be attenuated in healthy men when [Formula: see text] was attenuated with β₁-adrenergic receptor blockade. Healthy men (age: 25 ± 1 yr, n = 11) received intravenous esmolol (β₁-adrenergic receptor antagonist) or volume-matched saline in a double-blind, randomized crossover study and were exposed to poikilocapnic hypoxia, isocapnic hypoxia, and hypercapnic hypoxia. Measurements made at baseline and after 5 min of steady state at each gas manipulation included left anterior descending coronary blood velocity (LAD_V; Doppler echocardiography), heart rate, and arterial blood pressure. LAD_V values at the end of each hypoxic condition were compared between esmolol and placebo. The rate-pressure product (RPP) and left ventricular mechanical energy (ME_LV) were calculated as indexes of [Formula: see text]. All gas manipulations augmented RPP, ME_LV, and LAD_V, but only RPP and ME_LV were attenuated (4-18%) after β₁-adrenergic receptor blockade ( P < 0.05). Despite attenuated RPP and ME_LV responses, β₁-adrenergic receptor blockade did not attenuate the mean LAD_V vasodilatory response compared with placebo during poikilocapnic hypoxia (29.4 ± 2.2 vs. 27.3 ± 1.6 cm/s) and isocapnic hypoxia (29.5 ± 1.5 vs. 30.3 ± 2.2 cm/s). Hypercapnic hypoxia elicited a feedforward coronary dilation that was blocked by β₁-adrenergic receptor blockade. These results indicate a direct influence of arterial Po₂ on coronary vascular regulation that is independent of [Formula: see text]. NEW & NOTEWORTHY In humans, arterial hypoxemia led to an increase in epicardial coronary artery blood velocity. β₁-Adrenergic receptor blockade did not diminish the hypoxemic coronary response despite reduced myocardial O₂ demand. These data indicate hypoxemia can regulate coronary blood flow independent of myocardial O₂ consumption. A plateau in the mean left anterior descending coronary artery blood velocity-rate-pressure product relationship suggested β₁-adrenergic receptor-mediated, feedforward epicardial coronary artery dilation. In addition, we observed a synergistic effect of Po₂ and Pco₂ during hypercapnic hypoxia.

Impact of Different Perfusion Modalities on Coronary and Carotid Blood Flow Velocities in an Adult ECLS Swine Model

The objective of this study was to compare the effects of nonpulsatile and ECG-synchronized pulsatile extracorporeal life support on coronary and carotid blood flow velocities using transthoracic echocardiography and vascular ultrasound, respectively. Nine adult swine were randomly separated into nonpulsatile (NP, n = 5) and pulsatile (P, N = 4) groups and placed on ECLS for 24 h using an i-cor ECLS system. Noninvasive transthoracic images of the left and right coronary artery and the left carotid artery were acquired at the pre-ECLS (baseline), 30 min, 3, 6, 9, 12, and 24 h on-ECLS stages. The mean diastolic velocity of the left and right coronary arteries in the NP group significantly decreased after 24 h on ECLS compared to the baseline and 30 min ECLS stages (P < 0.05). There was no statistical difference in the mean diastolic velocity of the coronary arteries in the P group at 30 min, 3-, 6-, 9-, 12-, and 24-h ECLS compared to baseline. The P group showed a smaller decrease in the mean diastolic velocity of coronary arteries between the 30-min ECLS and 3-, 6-, 9-, 13-, 24-h ECLS stages compared to the NP group. The diastolic velocity of the left carotid artery in the NP group significantly decreased during 24-h ECLS compared to the P group (P < 0.05). An ECG-synchronized pulsatile ECLS system appeared to maintain coronary and carotid artery diastolic velocities better than conventional nonpulsatile ECLS. Further investigation of the perfusion modes during ECLS is warranted.

Peripheral revascularization attenuates the exercise pressor reflex and increases coronary exercise hyperemia in peripheral arterial disease

Peripheral arterial disease (PAD) is associated with augmented blood pressure (BP) and impaired coronary blood flow responses to exercise, which may increase cardiovascular risk. We investigated the effects of leg revascularization on the BP and coronary blood flow responses to exercise in PAD. Seventeen PAD patients (11 men, 66 ± 2 yr) performed single-leg plantar flexion exercise 24 h before and 1 mo following leg revascularization. BP and heart rate (HR) were measured continuously, and rate pressure product (systolic BP × HR) was calculated as an index of myocardial oxygen demand. Coronary blood velocity was obtained by transthoracic Doppler echocardiography in 8/17 subjects. The mean BP response to plantar flexion exercise was attenuated by leg revascularization (pre-revascularization: 15 ± 4 vs. post-revascularization: 7 ± 3 mmHg, P = 0.025). The HR response to plantar flexion was also attenuated following leg revascularization (pre-revascularization: 9 ± 1 vs. post-revascularization: 6 ± 1 beats/min, P = 0.006). The change in coronary blood velocity with exercise was greater at the post-revascularization visit: 4 ± 1 vs. pre-revascularization: -1 ± 2 cm/s ( P = 0.038), even though the change in rate pressure product was not greater following revascularization in these subjects (pre-revascularization: 2,796 ± 871 vs. post-revascularization: 1,766 ± 378 mmHg·beats/min, P = 0.082). These data suggest that leg revascularization alters reflex control of BP, HR, and coronary blood flow in response to exercise in patients with PAD. NEW & NOTEWORTHY We found that peripheral revascularization procedures lowered exercise blood pressure and improved coronary blood flow in patients with peripheral arterial disease.

Similarity between carotid and coronary artery responses to sympathetic stimulation and the role of α1-receptors in humans

Carotid artery (CCA) dilation occurs in healthy subjects during cold pressor test (CPT), while the magnitude of dilation relates to cardiovascular risk. To further explore this phenomenon and mechanism, we examined carotid artery responses to different sympathetic tests, with and without α₁-receptor blockade and assessed similarity to these responses between carotid and coronary arteries. In randomized order, 10 healthy participants (25 ± 3 yr) underwent sympathetic stimulation using the CPT (3-min left-hand immersion in ice-slush) and lower-body negative pressure (LBNP). Before and during sympathetic tests, CCA diameter and velocity (Doppler ultrasound) and left anterior descending (LAD) coronary artery velocity (echocardiography) were recorded across 3 min. Measures were repeated 90 min following selective α₁-receptor blockade via oral prazosin (0.05 mg/kg body wt). CPT significantly increased CCA diameter, LAD maximal velocity, and velocity-time integral area-under-the-curve (all P < 0.05). In contrast, LBNP resulted in a decrease in CCA diameter, LAD maximal velocity, and velocity time integral (VTI; all P < 0.05). Following α₁-receptor blockade, CCA and LAD velocity responses to CPT were diminished. In contrast, during LBNP (-30 mmHg), α₁-receptor blockade did not alter CCA or LAD responses. Finally, changes in CCA diameter and LAD VTI responses to sympathetic stimulation were positively correlated ( r = 0.66, P < 0.01). We found distinct carotid artery responses to different tests of sympathetic stimulation, where α₁ receptors partly contribute to CPT-induced responses. Finally, we found agreement between carotid and coronary artery responses. These data indicate similarity between carotid and coronary responses to sympathetic tests and the role of α₁ receptors that is dependent on the nature of the sympathetic challenge. NEW & NOTEWORTHY We showed distinct carotid artery responses to cold pressor test (CPT; i.e., dilation) and lower-body negative pressure (LBNP; i.e., constriction). Blockade of α₁-receptors significantly attenuated dilator responses in carotid and coronary arteries during CPT, while no changes were found during LBNP. Our findings indicate strong similarity between carotid and coronary artery responses to distinct sympathetic stimuli, and for the role of α-receptors.

An Injectable Oxygen Release System to Augment Cell Survival and Promote Cardiac Repair Following Myocardial Infarction

Oxygen deficiency after myocardial infarction (MI) leads to massive cardiac cell death. Protection of cardiac cells and promotion of cardiac repair are key therapeutic goals. These goals may be achieved by re-introducing oxygen into the infarcted area. Yet current systemic oxygen delivery approaches cannot efficiently diffuse oxygen into the infarcted area that has extremely low blood flow. In this work, we developed a new oxygen delivery system that can be delivered specifically to the infarcted tissue, and continuously release oxygen to protect the cardiac cells. The system was based on a thermosensitive, injectable and fast gelation hydrogel, and oxygen releasing microspheres. The fast gelation hydrogel was used to increase microsphere retention in the heart tissue. The system was able to continuously release oxygen for 4 weeks. The released oxygen significantly increased survival of cardiac cells under the hypoxic condition (1% O₂) mimicking that of the infarcted hearts. It also reduced myofibroblast formation under hypoxic condition (1% O₂). After implanting into infarcted hearts for 4 weeks, the released oxygen significantly augmented cell survival, decreased macrophage density, reduced collagen deposition and myofibroblast density, and stimulated tissue angiogenesis, leading to a significant increase in cardiac function.

[Comparison of safe duration of apnea and intubation time in face mask ventilation with air versus 100% oxygen during induction of general anesthesia]

OBJECTIVE

To compare the safe duration of apnea and intubation time between face mask ventilation with air and 100% oxygen during induction of general anesthesia.

METHODS

Eighty adult patients with ASA class I or II without predicted difficult airways were scheduled for elective surgery under general anesthesia. The patients were randomized to receive anesthesia induction with preoxygenation [Group 1, n=40, fraction of inspired oxygen (FiO₂)=1] or without preoxygenation (Group2, n=40, FiO₂=0.21). Two experienced anesthesiologists performed the mask ventilation and tracheal intubation during induction, and the assistants adjusted the oxygen concentration and recorded the pulse oxygen saturation (SpO₂) and other variables. The cases where SpO₂ decreased to below 90% before accomplishment of intubation were considered unsuccessful, and mask ventilation with 100% oxygen was given. After tracheal intubation, mechanical ventilation was not initiated until the SpO₂ decreased to 90%. The number of unsuccessful cases, the safe duration of apnea and intubation time were recorded in the two groups.

RESULTS

There was no unsuccessful case in either groups. The safe duration of apnea was 469.5∓143.0 s in Group 1 and 63.6∓20.0 s in Group 2, and the intubation time was 34.4∓12.6 s and 32.8∓9.6 s, respectively. The safe duration of apnea was significantly longer than the intubation time in both groups (P<0.01). The intubation time and the number of cases with SpO₂≥90% before completion of tracheal intubation were similar between the two groups. The safe duration of apnea was significantly shorter in Group 2 than in Group 1 (P<0.01) and was correlated with the body mass index of the patients (P<0.05).

CONCLUSION

Anesthesia induction without preoxygenation can provide sufficient time for experienced anesthesiologists to complete tracheal intubation.

Metaboreflex-mediated hemodynamic abnormalities in individuals with coronary artery disease without overt signs or symptoms of heart failure

This study was devised to investigate the effect of coronary artery disease (CAD) without overt signs of heart failure on the cardiovascular responses to muscle metaboreflex activation. We hypothesized that any CAD-induced preclinical systolic and/or diastolic dysfunction could impair hemodynamic response to the metaboreflex test. Twelve men diagnosed with CAD without any sign or symptoms of heart failure and 11 age-matched healthy control (CTL) subjects participated in the study. Subjects performed a postexercise muscle ischemia (PEMI) test to activate the metaboreflex. They also performed a control exercise recovery test to compare data from the PEMI test. The main results were that the CAD group reached a similar mean arterial blood pressure response as the CTL group during PEMI. However, the mechanism by which this response was achieved was different between groups. In particular, CAD achieved the target mean arterial blood pressure by increasing systemic vascular resistance (+383.8 ± 256.6 vs. +91.2 ± 293.5 dyn·s^-1·cm^-5 for the CAD and CTL groups, respectively), the CTL group by increasing cardiac preload (-0.92 ± 8.53 vs. 5.34 ± 4.29 ml in end-diastolic volume for the CAD and CTL groups, respectively), which led to an enhanced stroke volume and cardiac output. Furthermore, the ventricular filling rate response was higher in the CTL group than in the CAD group during PEMI ( P < 0.05 for all comparisons). This study confirms that diastolic function is pivotal for normal hemodynamics during the metaboreflex. Moreover, it provides evidence that early signs of diastolic impairment attributable to CAD can be detected by the metaboreflex test. NEW & NOTEWORTHY Individuals suffering from coronary artery disease without overt signs of heart failure may show early signs of diastolic dysfunction, which can be detected by the metaboreflex test. During the metaboreflex, these subjects show impaired preload and stroke volume responses and exaggerated vasoconstriction compared with controls.

Beta-1 vs. beta-2 adrenergic control of coronary blood flow during isometric handgrip exercise in humans

During exercise, β-adrenergic receptors are activated throughout the body. In healthy humans, the net effect of β-adrenergic stimulation is an increase in coronary blood flow. However, the role of vascular β1 vs. β2 receptors in coronary exercise hyperemia is not clear. In this study, we simultaneously measured noninvasive indexes of myocardial oxygen supply (i.e., blood velocity in the left anterior descending coronary artery; Doppler echocardiography) and demand [i.e., rate pressure product (RPP) = heart rate × systolic blood pressure) and tested the hypothesis that β1 blockade with esmolol improves coronary exercise hyperemia compared with nonselective β-blockade with propranolol. Eight healthy young men received intravenous infusions of esmolol, propranolol, and saline on three separate days in a single-blind, randomized, crossover design. During each infusion, subjects performed isometric handgrip exercise until fatigue. Blood pressure, heart rate, and coronary blood velocity (CBV) were measured continuously, and RPP was calculated. Changes in parameters from baseline were compared with paired t-tests. Esmolol (Δ = 3296 ± 1204) and propranolol (Δ = 2997 ± 699) caused similar reductions in peak RPP compared with saline (Δ = 5384 ± 1865). In support of our hypothesis, ΔCBV with esmolol was significantly greater than with propranolol (7.3 ± 2.4 vs. 4.5 ± 1.6 cm/s; P = 0.002). This effect was also evident when normalizing ΔCBV to ΔRPP. In summary, not only does selective β1 blockade reduce myocardial oxygen demand during exercise, but it also unveils β2-receptor-mediated coronary exercise hyperemia.NEW & NOTEWORTHY In this study, we evaluated the role of vascular β1 vs. β2 receptors in coronary exercise hyperemia in a single-blind, randomized, crossover study in healthy men. In response to isometric handgrip exercise, blood flow velocity in the left anterior descending coronary artery was significantly greater with esmolol compared with propranolol. These findings increase our understanding of the individual and combined roles of coronary β1 and β2 adrenergic receptors in humans.

Arterial CO2 as a Potent Coronary Vasodilator: A Preclinical PET/MR Validation Study with Implications for Cardiac Stress Testing

Myocardial blood flow (MBF) is the critical determinant of cardiac function. However, its response to increases in partial pressure of arterial CO₂ (PaCO₂), particularly with respect to adenosine, is not well characterized because of challenges in blood gas control and limited availability of validated approaches to ascertain MBF in vivo. Methods: By prospectively and independently controlling PaCO₂ and combining it with ¹³N-ammonia PET measurements, we investigated whether a physiologically tolerable hypercapnic stimulus (∼25 mm Hg increase in PaCO₂) can increase MBF to that observed with adenosine in 3 groups of canines: without coronary stenosis, subjected to non-flow-limiting coronary stenosis, and after preadministration of caffeine. The extent of effect on MBF due to hypercapnia was compared with adenosine. Results: In the absence of stenosis, mean MBF under hypercapnia was 2.1 ± 0.9 mL/min/g and adenosine was 2.2 ± 1.1 mL/min/g; these were significantly higher than at rest (0.9 ± 0.5 mL/min/g, P < 0.05) and were not different from each other (P = 0.30). Under left-anterior descending coronary stenosis, MBF increased in response to hypercapnia and adenosine (P < 0.05, all territories), but the effect was significantly lower than in the left-anterior descending coronary territory (with hypercapnia and adenosine; both P < 0.05). Mean perfusion defect volumes measured with adenosine and hypercapnia were significantly correlated (R = 0.85) and were not different (P = 0.12). After preadministration of caffeine, a known inhibitor of adenosine, resting MBF decreased; and hypercapnia increased MBF but not adenosine (P < 0.05). Conclusion: Arterial blood CO₂ tension when increased by 25 mm Hg can induce MBF to the same level as a standard dose of adenosine. Prospectively targeted arterial CO₂ has the capability to evolve as an alternative to current pharmacologic vasodilators used for cardiac stress testing.

Baseline values of cardiovascular and respiratory parameters predict response to acute hypoxia in young healthy men

The majority of the available works have studied distinct hypoxic responses of respiratory and cardiovascular systems. This study examines how these systems interact while responding to hypoxia and whether baseline metrics moderate reactions to a hypoxic challenge. Central hemodynamic, aortic wave reflection, and gas exchange parameters were measured in 27 trained young men before and after 10-min normobaric isocapnic hypoxia (10 % O2). Associations were assessed by correlation and multiple regression analyses. Hypoxic changes in the parameters of pulse wave analysis such as augmentation index (-114 %, p=0.007), pulse pressure amplification (+6 %, p=0.020), time to aortic reflection wave (+21 %, p<0.001) report on the increase in arterial distensibility. Specifically, initially compliant arteries blunt the positive cardiac chronotropic response to hypoxia and facilitate the myocardial workload. The degree of blood oxygen desaturation is directly correlated with both baseline values and hypoxic responses of aortic and peripheral blood pressures. The hypoxia-induced gain in ventilation (VE), while controlling for basal VE and heart rate (HR), is inversely associated with deltaHR and deltasystolic blood pressure. The study suggests that cardiovascular and respiratory systems mutually supplement each other when responding to hypoxic challenge.

Impaired peripheral vasodilation during graded systemic hypoxia in healthy older adults: role of the sympathoadrenal system

Systemic hypoxia is a physiological and pathophysiological stress that activates the sympathoadrenal system and, in young adults, leads to peripheral vasodilation. We tested the hypothesis that peripheral vasodilation to graded systemic hypoxia is impaired in older healthy adults and that this age-associated impairment is due to attenuated β-adrenergic mediated vasodilation and elevated α-adrenergic vasoconstriction. Forearm blood flow was measured (Doppler ultrasound), and vascular conductance (FVC) was calculated in 12 young (24 ± 1 yr) and 10 older (63 ± 2 yr) adults to determine the local dilatory responses to graded hypoxia (90, 85, and 80% O₂ saturations) in control conditions, following local intra-arterial blockade of β-receptors (propranolol), and combined blockade of α- and β-receptors (phentolamine + propranolol). Under control conditions, older adults exhibited impaired vasodilation to hypoxia compared with young participants at all levels of hypoxia (peak ΔFVC at 80% [Formula: see text] = 4 ± 6 vs. 35 ± 8%; P < 0.01). During β-blockade, older adults actively constricted at 85 and 80% [Formula: see text] (peak ΔFVC at 80% [Formula: see text] = -13 ± 6%; P < 0.05 vs. control), whereas the response in the young was not significantly impacted (peak ΔFVC = 28 ± 8%). Combined α- and β-blockade increased the dilatory response to hypoxia in young adults; however, older adults failed to significantly vasodilate (peak ΔFVC at 80% [Formula: see text]= 12 ± 11% vs. 58 ± 11%; P < 0.05). Our findings indicate that peripheral vasodilation to graded systemic hypoxia is significantly impaired in older adults, which cannot be fully explained by altered sympathoadrenal control of vascular tone. Thus, the impairment in hypoxic vasodilation is likely due to attenuated local vasodilatory and/or augmented vasoconstrictor signaling with age.NEW & NOTEWORTHY We found that the lack of peripheral vasodilation during graded systemic hypoxia with aging is not mediated by the sympathoadrenal system, strongly implicating local vascular control mechanisms in this impairment. Understanding these mechanisms may lead to therapeutic advances for improving tissue blood flow and oxygen delivery in aging and disease.

Effect of adrenergic agonists on coronary blood flow: a laboratory study in healthy volunteers

Myocardial oxygen supply and demand mismatch is fundamental to the pathophysiology of ischemia and infarction. The sympathetic nervous system, through α‐adrenergic receptors and β‐adrenergic receptors, influences both myocardial oxygen supply and demand. In animal models, mechanistic studies have established that adrenergic receptors contribute to coronary vascular tone. The purpose of this laboratory study was to noninvasively quantify coronary responses to adrenergic receptor stimulation in humans. Fourteen healthy volunteers (11 men and 3 women) performed isometric handgrip exercise to fatigue followed by intravenous infusion of isoproterenol. A subset of individuals also received infusions of phenylephrine (n = 6), terbutaline (n = 10), and epinephrine (n = 4); all dosages were based on fat‐free mass and were infused slowly to achieve steady‐state. The left anterior descending coronary artery was visualized using Doppler echocardiography. Beat‐by‐beat heart rate (HR), blood pressure (BP), peak diastolic coronary velocity (CBVpeak), and coronary velocity time integral were calculated. Data are presented as M ± SD. Isometric handgrip elicited significant increases in BP, HR, and CBVpeak (from 23.3 ± 5.3 to 34.5 ± 9.9 cm/sec). Isoproterenol raised HR and CBVpeak (from 22.6 ± 4.8 to 43.9 ± 12.4 cm/sec). Terbutaline and epinephrine evoked coronary hyperemia whereas phenylephrine did not significantly alter CBVpeak. Different indices of coronary hyperemia (changes in CBVpeak and velocity time integral) were significantly correlated (R = 0.803). The current data indicate that coronary hyperemia occurs in healthy humans in response to isometric handgrip exercise and low‐dose, steady‐state infusions of isoproterenol, terbutaline, and epinephrine. The contribution of β1 versus β2 receptors to coronary hyperemia remains to be determined. In this echocardiographic study, we demonstrate that coronary blood flow increases when β‐adrenergic receptors are stimulated (i.e., during exercise and different intravenous infusions). Our infusion paradigms and beat‐by‐beat imaging methodologies can be used in future studies to evaluate age‐, sex‐, and disease‐ differences in adrenergic control of coronary blood flow.

Coronary Exercise Hyperemia Is Impaired in Patients with Peripheral Arterial Disease

BACKGROUND

Peripheral arterial disease (PAD) is an atherosclerotic vascular disease that affects over 200 million people worldwide. The hallmark of PAD is ischemic leg pain and this condition is also associated with an augmented blood pressure response to exercise, impaired vascular function, and high risk of myocardial infarction and cardiovascular mortality. In this study, we tested the hypothesis that coronary exercise hyperemia is impaired in PAD.

METHODS

Twelve patients with PAD and no overt coronary disease (65 ± 2 years, 7 men) and 15 healthy control subjects (64 ± 2 years, 9 men) performed supine plantar flexion exercise (30 contractions/min, increasing workload). A subset of subjects (n = 7 PAD, n = 8 healthy) also performed isometric handgrip exercise (40% of maximum voluntary contraction to fatigue). Coronary blood velocity in the left anterior descending artery was measured by transthoracic Doppler echocardiography; blood pressure and heart rate were monitored continuously.

RESULTS

Coronary blood velocity responses to 4 min of plantar flexion exercise (PAD: Δ2.4 ± 1.2, healthy: Δ6.0 ± 1.6 cm/sec, P = 0.039) and isometric handgrip exercise (PAD: Δ8.3 ± 4.2, healthy: Δ16.9 ± 3.6, P = 0.033) were attenuated in PAD patients.

CONCLUSION

These data indicate that coronary exercise hyperemia is impaired in PAD, which may predispose these patients to myocardial ischemia.

The effects of graded changes in oxygen and carbon dioxide tension on coronary blood velocity independent of myocardial energy demand

In humans, coronary blood flow is tightly regulated by microvessels within the myocardium to match myocardial energy demand. However, evidence regarding inherent sensitivity of the microvessels to changes in arterial partial pressure of carbon dioxide and oxygen is conflicting because of the accompanied changes in myocardial energy requirements. This study aimed to investigate the changes in coronary blood velocity while manipulating partial pressures of end-tidal CO2 (Petco2) and O2 (Peto2). It was hypothesized that an increase in Petco2 (hypercapnia) or decrease in Peto2 (hypoxia) would result in a significant increase in mean blood velocity in the left anterior descending artery (LADVmean) due to an increase in both blood gases and energy demand associated with the concomitant cardiovascular response. Cardiac energy demand was assessed through noninvasive measurement of the total left ventricular mechanical energy. Healthy subjects (n = 13) underwent a euoxic CO2 test (Petco2 = -8, -4, 0, +4, and +8 mmHg from baseline) and an isocapnic hypoxia test (Peto2 = 64, 52, and 45 mmHg). LADVmean was assessed using transthoracic Doppler echocardiography. Hypercapnia evoked a 34.6 ± 8.5% (mean ± SE; P < 0.01) increase in mean LADVmean, whereas hypoxia increased LADVmean by 51.4 ± 8.8% (P < 0.05). Multiple stepwise regressions revealed that both mechanical energy and changes in arterial blood gases are important contributors to the observed changes in LADVmean (P < 0.01). In summary, regulation of the coronary vasculature in humans is mediated by metabolic changes within the heart and an inherent sensitivity to arterial blood gases.

Impaired coronary and retinal vasomotor function to hyperoxia in Individuals with Type 2 diabetes

PURPOSE

Adults with diabetes are at a high risk of developing coronary heart disease. The purpose of this study was to assess coronary artery vascular function non-invasively in individuals with and without Type 2 diabetes and to compare these coronary responses to another microvascular bed (i.e. retina). We hypothesized that individuals with diabetes would have impaired coronary reactivity and that these impairments would be associated with impairments in retinal reactivity.

METHODS

Coronary blood velocity (Transthoracic Doppler Echocardiography) and retinal diameters (Dynamic Vessel Analyzer) were measured continuously during five minutes of breathing 100% oxygen (i.e. hyperoxia) in 15 persons with Type 2 diabetes and 15 age-matched control subjects. Using fundus photographs, retinal vascular calibers were also measured (central retinal arteriole and venule equivalents).

RESULTS

Individuals with diabetes compared to controls had impaired coronary (-2.34±16.64% vs. -14.27±10.58%, P=0.03) and retinal (arteriole: -0.04±3.34% vs. -3.65±5.07%, P=0.03; venule: -1.65±3.68% vs. -5.23±5.47%, P=0.05) vasoconstrictor responses to hyperoxia, and smaller central arteriole-venule equivalent ratios (0.83±0.07 vs. 0.90±0.07, P=0.014). Coronary reactivity was associated with central retinal arteriole equivalents (r=-0.516, P=0.005) and retinal venular reactivity (r=0.387, P=0.034).

CONCLUSION

Diabetes impairs coronary and retinal microvascular function to hyperoxia. Impaired vasoconstrictor responses may be part of a systemic diabetic vasculopathy, which may contribute to adverse cardiovascular events in individuals with diabetes.

Coronary heart disease risk factors and outcomes in the twenty-first century: findings from the REasons for Geographic and Racial Differences in Stroke (REGARDS) Study

REasons for Geographic and Racial Differences in Stroke (REGARDS) is a longitudinal study supported by the National Institutes of Health to determine the disparities in stroke-related mortality across USA. REGARDS has published a body of work designed to understand the disparities in prevalence, awareness, treatment, and control of coronary heart disease (CHD) and its risk factors in a biracial national cohort. REGARDS has focused on racial and geographical disparities in the quality and access to health care, the influence of lack of medical insurance, and has attempted to contrast current guidelines in lipid lowering for secondary prevention in a nationwide cohort. It has described CHD risk from nontraditional risk factors such as chronic kidney disease, atrial fibrillation, and inflammation (i.e., high-sensitivity C-reactive protein) and has also assessed the role of depression, psychosocial, environmental, and lifestyle factors in CHD risk with emphasis on risk factor modification and ideal lifestyle factors. REGARDS has examined the utility of various methodologies, e.g., the process of medical record adjudication, proxy-based cause of death, and use of claim-based algorithms to determine CHD risk. Some valuable insight into less well-studied concepts such as the reliability of current troponin assays to identify "microsize infarcts," caregiving stress, and CHD, heart failure, and cognitive decline have also emerged. In this review, we discuss some of the most important findings from REGARDS in the context of the existing literature in an effort to identify gaps and directions for further research.

Noninvasive study of coronary microcirculation response to a cold pressor test

BACKGROUND

The aims of this study were to noninvasively (i) assess the coronary microcirculation changes in response to a cold pressor test (CPT) in control subjects, nondiabetic obese patients and patients with type 2 diabetes and (ii) investigate the response of the coronary microcirculation in patients with diabetes according to the presence or the absence of silent myocardial ischaemia (SMI), asymptomatic coronary stenosis (CS) and left ventricle hypertrophy (LVH).

METHODS

The mean left anterior descending coronary flow velocity (mCFV) was measured using transthoracic Doppler before and after a CPT in 16 control subjects, 11 obese and 66 asymptomatic diabetic patients with a high cardiovascular risk. Patients with diabetes were screened for SMI using stress myocardial scintigraphy and/or echocardiography. A coronary angiography was performed in those with SMI.

RESULTS

At baseline, pressure-rate product (PRP) was correlated with mCFV (r = 0.23; P < 0.05) and left ventricle mass (r = 0.26; P < 0.05) in the whole population. Changes in PRP and mCFV during CPT were correlated with controls (r = 0.58, P < 0.05), obese (r = 0.75, P < 0.01) and diabetic patients without CS (r = 0.56, P < 0.0001) or without LVH (r = 0.63, P < 0.05) but not in diabetic patients with CS or with LVH. In patients with diabetes, SMI was associated with mCFV changes, independent of other parameters (P < 0.05).

CONCLUSION

Transthoracic coronary Doppler allows noninvasive study of changes in the coronary microcirculation during CPT. In asymptomatic patients with type 2 diabetes, this method showed that SMI was associated with mCFV changes during CPT and the presence of CS or LVH was associated with a mismatch between coronary microcirculation and myocardial oxygen demand.

β-Adrenergic receptor blockade impairs coronary exercise hyperemia in young men but not older men

Patients with coronary artery disease have attenuated coronary vasodilator responses to physiological stress, which is partially attributed to a β-adrenergic receptor (β-AR)-mediated mechanisms. Whether β-ARs contribute to impaired coronary vasodilation seen with healthy aging is unknown. The purpose of this study was to investigate the role of β-ARs in coronary exercise hyperemia in healthy humans. Six young men (26 ± 1 yr) and seven older men (67 ± 4 yr) performed isometric handgrip exercise at 30% maximal voluntary contraction for 2 min after receiving intravenous propranolol, a β-AR antagonist, and no treatment. Isoproterenol, a β-AR agonist, was infused to confirm the β-AR blockade. Blood pressure and heart rate were monitored continuously, and coronary blood flow velocity (CBV, left anterior descending artery) was measured by transthoracic Doppler echocardiography. Older men had an attenuated ΔCBV to isometric exercise (3.8 ± 1.3 vs. 9.7 ± 2.1 cm/s, P = 0.02) compared with young men. Propranolol decreased the ΔCBV at peak handgrip exercise in young men (9.7 ± 2.1 vs. 2.7 ± 0.9 cm/s, P = 0.008). However, propranolol had no effect on ΔCBV in older men (3.8 ± 1.3 vs. 4.2 ± 1.9 cm/s, P = 0.9). Older men also had attenuated coronary hyperemia to low-dose isoproterenol. These data indicate that β-AR control of coronary blood flow is impaired in healthy older men.

Coronary responses to cold air inhalation following afferent and efferent blockade

Cardiac ischemia and angina pectoris are commonly experienced during exertion in a cold environment. In the current study we tested the hypotheses that oropharyngeal afferent blockade (i.e., local anesthesia of the upper airway with lidocaine) as well as systemic β-adrenergic receptor blockade (i.e., intravenous propranolol) would improve the balance between myocardial oxygen supply and demand in response to the combined stimulus of cold air inhalation (-15 to -30°C) and isometric handgrip exercise (Cold + Grip). Young healthy subjects underwent Cold + Grip following lidocaine, propranolol, and control (no drug). Heart rate, blood pressure, and coronary blood flow velocity (CBV, from Doppler echocardiography) were continuously measured. Rate-pressure product (RPP) was calculated, and changes from baseline were compared between treatments. The change in RPP at the end of Cold + Grip was not different between lidocaine (2,441 ± 376) and control conditions (3,159 ± 626); CBV responses were also not different between treatments. With propranolol, heart rate (8 ± 1 vs. 14 ± 3 beats/min) and RPP responses to Cold + Grip were significantly attenuated. However, at peak exercise propranolol also resulted in a smaller ΔCBV (1.4 ± 0.8 vs. 5.3 ± 1.4 cm/s, P = 0.035), such that the relationship between coronary flow and cardiac metabolism was impaired under propranolol (0.43 ± 0.37 vs. 2.1 ± 0.63 arbitrary units). These data suggest that cold air breathing and isometric exercise significantly influence efferent control of coronary blood flow. Additionally, β-adrenergic vasodilation may play a significant role in coronary regulation during exercise.

Assessment of myocardial reactivity to controlled hypercapnia with free-breathing T2-prepared cardiac blood oxygen level-dependent MR imaging

PURPOSE

To examine whether controlled and tolerable levels of hypercapnia may be an alternative to adenosine, a routinely used coronary vasodilator, in healthy human subjects and animals.

MATERIALS AND METHODS

Human studies were approved by the institutional review board and were HIPAA compliant. Eighteen subjects had end-tidal partial pressure of carbon dioxide (PetCO2) increased by 10 mm Hg, and myocardial perfusion was monitored with myocardial blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging. Animal studies were approved by the institutional animal care and use committee. Anesthetized canines with (n = 7) and without (n = 7) induced stenosis of the left anterior descending artery (LAD) underwent vasodilator challenges with hypercapnia and adenosine. LAD coronary blood flow velocity and free-breathing myocardial BOLD MR responses were measured at each intervention. Appropriate statistical tests were performed to evaluate measured quantitative changes in all parameters of interest in response to changes in partial pressure of carbon dioxide.

RESULTS

Changes in myocardial BOLD MR signal were equivalent to reported changes with adenosine (11.2% ± 10.6 [hypercapnia, 10 mm Hg] vs 12% ± 12.3 [adenosine]; P = .75). In intact canines, there was a sigmoidal relationship between BOLD MR response and PetCO2 with most of the response occurring over a 10 mm Hg span. BOLD MR (17% ± 14 [hypercapnia] vs 14% ± 24 [adenosine]; P = .80) and coronary blood flow velocity (21% ± 16 [hypercapnia] vs 26% ± 27 [adenosine]; P > .99) responses were similar to that of adenosine infusion. BOLD MR signal changes in canines with LAD stenosis during hypercapnia and adenosine infusion were not different (1% ± 4 [hypercapnia] vs 6% ± 4 [adenosine]; P = .12).

CONCLUSION

Free-breathing T2-prepared myocardial BOLD MR imaging showed that hypercapnia of 10 mm Hg may provide a cardiac hyperemic stimulus similar to adenosine.

β-Adrenergic blockade enhances coronary vasoconstrictor response to forehead cooling

Forehead cooling activates the sympathetic nervous system and can trigger angina pectoris in susceptible individuals. However, the effect of forehead cooling on coronary blood flow velocity (CBV) is not well understood. In this human experiment, we tested the hypotheses that forehead cooling reduces CBV (i.e., coronary vasoconstriction) and that this vasoconstrictor effect would be enhanced under systemic β-adrenergic blockade. A total of 30 healthy subjects (age range, 23-79 years) underwent Doppler echocardiography evaluation of CBV in response to 60 s of forehead cooling (1°C ice bag on forehead). A subset of subjects (n = 10) also underwent the procedures after an intravenous infusion of propranolol. Rate pressure product (RPP) was used as an index of myocardial oxygen demand. Consistent with our first hypothesis, forehead cooling reduced CBV from 19.5 ± 0.7 to 17.5 ± 0.8 cm/s (P < 0.001), whereas mean arterial pressure increased by 11 ± 2 mmHg (P < 0.001). Consistent with our second hypothesis, forehead cooling reduced CBV under propranolol despite a significant rise in RPP. The current studies indicate that forehead cooling elicits a sympathetically mediated pressor response and a reduction in CBV, and this effect is augmented under β-blockade. The results are consistent with sympathetic activation of β-receptor coronary vasodilation in humans, as has been demonstrated in animals.

Intravenous phentolamine abolishes coronary vasoconstriction in response to mild central hypovolemia

Animal studies indicate alpha-adrenergic coronary vasoconstriction helps maintain left ventricular function during physiological stress. Whether this process occurs in humans is unknown. In the current study, we used transthoracic Doppler echocardiography to test the effect of lower body negative pressure (LBNP) on coronary blood flow velocity (CBV, left anterior descending coronary artery) and myocardial function in eight young healthy subjects before and after systemic infusion of phentolamine, a nonselective alpha blocker. Heart rate (HR) and blood pressure (BP) were monitored on a beat-by-beat basis. Peak diastolic CBV and myocardial systolic and diastolic tissue velocities (Sm and Em), were quantified at baseline, and at -5 mmHg, -10 mmHg, and -15 mmHg LBNP. Coronary vascular resistance index (CVRI) was calculated as the quotient of diastolic BP and CBV. Phentolamine reduced baseline diastolic BP and increased HR but did not affect the reflex adjustments to LBNP. The reduction in CBV due to LBNP was blunted by phentolamine at -10 mmHg and -15 mmHg. Importantly, the increase in CVRI (i.e., coronary vasoconstriction) was abolished by phentolamine at -5 mmHg (0.21 ± 0.06 vs. 0.83 ± 0.13), -10 mmHg (0.24 ± 0.03 vs. 1.68 ± 0.31), and -15 mmHg (0.27 ± 0.10 vs. 2.34 ± 0.43). These data indicate that alpha-adrenergic coronary vasoconstriction is present during low levels of LBNP. With alpha blockade, more coronary flow is needed to maintain cardiac function. Our data suggest that alpha-adrenergic tone enhances coronary flow efficiency, presumably by redistributing flow from the epicardium to the endocardium.

Cardiac MR imaging to measure myocardial blood flow response to the cold pressor test in healthy smokers and nonsmokers

PURPOSE

To determine if myocardial perfusion cardiac magnetic resonance (MR) imaging can show changes in myocardial blood flow (MBF) during the cold pressor test (CPT) and can allow identification of the differing endothelial function of smokers and nonsmokers when compared during adenosine stress.

MATERIALS AND METHODS

The study was approved by the institutional ethics review board and all participants gave informed written consent. Twenty-nine healthy volunteers (19 nonsmokers, 10 smokers; mean age ± standard deviation, 22 years ± 4) underwent 1.5-T MR imaging and analysis. Myocardial perfusion was assessed during rest, peak CPT, and adenosine hyperemia with a saturation-recovery gradient-echo pulse sequence (spatial resolution, 2.4 × 2.4 × 10 mm). Global, endocardial, and epicardial MBF were calculated by using Fermi-constrained deconvolution. Paired and independent t test statistical analyses were used to compare the responses between tests and groups. Regression analysis was performed to identify predictors of MBF change.

RESULTS

MBF at rest was similar between the nonsmoking and smoking groups (0.97 mL/g/min ± 0.4 vs 0.96 mL/g/min ± 0.3, respectively; P = .96). Nonsmokers responded to CPT with a 47% increase in MBF (1.43 mL/g/min ± 0.5) and smokers responded with a 27% increase (1.22 mL/g/min ± 0.4; P < .001). An endocardial-to-epicardial gradient existed at rest (nonsmokers, 1.10 [P = .002]; smokers, 1.30 [P = .01]) and CPT (nonsmokers, 1.19 [P < .001] smokers, 1.28 [P = .04]) but reversed during adenosine stress (nonsmokers, 0.89 [P = .03]; smokers, 0.92 [P = .42]).

CONCLUSION

Myocardial perfusion cardiac MR imaging during CPT can allow assessment of changes in MBF globally and in the separate myocardial layers in healthy smokers and nonsmokers. This allows the combined assessment of endothelium-dependent (CPT) and endothelium-independent (adenosine stress test) MBF reserve in a single study.

Comparison of cardiovascular and biomechanical parameters of supine lower body negative pressure and upright lower body positive pressure to simulate activity in 1/6 G and 3/8 G

For future space exploration missions, it is important to determine the best method of simulating on Earth cardiovascular and biomechanical conditions for lunar and Martian gravities. For this purpose, we compared exercise performed within a lower body negative pressure (LBNP) and a lower body positive pressure (LBPP) chamber. Twelve subjects underwent a protocol of resting and walking (0.25 Froude) within supine LBNP and upright LBPP simulation. Each protocol was performed in simulated 1/6 G and 3/8 G. We assessed heart rate (HR), mean arterial blood pressure, oxygen consumption (Vo2), normalized stride length, normalized vertical peak ground reaction force, duty factor, cadence, perceived exertion (Borg), and comfort of the subject. A mixed linear model was employed to determine effects of the simulation on the respective parameters. Furthermore, parameters were compared with predicted values for lunar and Martian gravities to determine the method that showed the best agreement. During walking, all cardiovascular and biomechanical parameters were unaffected by the simulation used for lunar and Martian gravities. During rest, HR and Vo2 were lower in supine LBNP compared with upright LBPP. HR, Vo2, and normalized vertical peak ground reaction force obtained with supine LBNP and upright LBPP showed good agreement with predicted values. Since supine LBNP and upright LBPP are lacking significant differences, we conclude that both simulations are suited to simulate the cardiovascular and biomechanical conditions during activity in lunar and Martian gravities. Operational characteristics and the intended application should be considered when choosing either supine LBNP or upright LBPP to simulate partial gravities on Earth.

Oxygen therapy in acute coronary syndrome: are the benefits worth the risk?

Oxygen supplementation is a standard treatment for all patients who present with acute coronary syndrome, regardless of oxygen saturation levels. Most of the data regarding the function of oxygen in myocardial infarction is based on a limited number of basic and clinical studies. We performed a systematic literature review that explores the basic and clinical data on the function of oxygen in ischaemic heart disease and myocardial infarction. This review discusses many aspects of oxygen treatment: (i) basic studies on the effects of oxygen in ischaemia and the potential cardiovascular effects of oxygen metabolites; (ii) clinical trials that have assessed the value of inhaled oxygen, supersaturated oxygen, and intracoronary injection of hyperoxaemic solutions in myocardial infarction; and (iii) the haemodynamic effects of oxygen in various clinical scenarios and its direct effects on the coronary vasculature. Our findings suggest that there are conflicting data on the effects of oxygen treatment. Further, the potential harmful effects of oxygen must be considered, particularly in myocardial infarction. These findings question the current guidelines and recommendations and emphasize the need for large clinical trials.

Contribution of sympathetic activation to coronary vasodilatation during the cold pressor test in healthy men: effect of ageing

The sympathetic nervous system is an important regulator of coronary blood flow. The cold pressor test (CPT) is a powerful sympathoexcitatory stressor. We tested the hypotheses that: (1) CPT-induced sympathetic activation elicits coronary vasodilatation in young adults that is impaired with advancing age and (2) combined α- and β-adrenergic blockade diminishes/abolishes these age-related differences. Vascular responses of the left anterior descending artery to the CPT were determined by transthoracic Doppler echocardiography before (pre-blockade) and during (post-blockade) systemic co-administration of α- and β-adrenergic antagonists in young (n = 9; 26 ± 1 years old, mean ± SEM) and older healthy men (n = 9; 66 ± 2 years old). Coronary vascular resistance (CVR; mean arterial pressure/coronary blood velocity) was used as an index of vascular tone. CPT decreased CVR (i.e. coronary vasodilatation occurred) in young ( -33 ± 6%), but not older men ( -3 ± 4%; P < 0.05 vs. young) pre-blockade. Adrenergic blockade abolished CPT-induced coronary vasodilatation in young men ( -33 ± 6% vs. 0 ± 6%, pre-blockade vs. post-blockade, respectively; P < 0.05) such that responses post-blockade mirrored those of older men ( -3 ± 4% vs. 8 ± 9%; both P > 0.05 compared to young pre-blockade). Impaired CPT-induced coronary vasodilatation could not be explained by a reduced stimulus for vasodilatation as group and condition effects persisted when CVR responses were expressed relative to myocardial oxygen demand (rate-pressure product). These data indicate that the normal coronary vascular response to sympathetic activation in young men is pronounced vasodilatation and this effect is lost with age as the result of an adrenergic mechanism. These findings may help explain how acute sympathoexcitation may precipitate angina and coronary ischaemic events, particularly in older adults.

Exercise and diet-induced weight loss attenuates oxidative stress related-coronary vasoconstriction in obese adolescents

Obesity is a disease of oxidative stress (OS). Acute hyperoxia (breathing 100 % O(2)) can evoke coronary vasoconstriction by the oxidative quenching of nitric oxide (NO). To examine if weight loss would alter the hyperoxia-related coronary constriction seen in obese adolescents, we measured the coronary blood flow velocity (CBV) response to hyperoxia using transthoracic Doppler echocardiography before and after a 4-week diet and exercise regimen in 6 obese male adolescents (age 13-17 years, BMI 36.5 ± 2.3 kg/m(2)). Six controls of similar age and BMI were also studied. The intervention group lost 9 ± 1 % body weight, which was associated with a reduced resting heart rate (HR), reduced diastolic blood pressure (BP), and reduced RPP (all P < 0.05). Before weight loss, hyperoxia reduced CBV by 33 ± 3 %. After weight loss, CBV only fell by 15 ± 3 % (P < 0.05). In the control group, CBV responses to hyperoxia were unchanged during the two trials. Thus weight loss: (1) reduces HR, BP, and RPP; and (2) attenuates the OS-related coronary constrictor response seen in obese adolescents. We postulate that: (1) the high RPP before weight loss led to higher myocardial O(2) consumption, higher coronary flow and greater NO production, and in turn a large constrictor response to hyperoxia; and (2) weight loss decreased myocardial oxygen demand and NO levels. Under these circumstances, hyperoxia-induced vasoconstriction was attenuated.

Cardiac mechanics are impaired during fatiguing exercise and cold pressor test in healthy older adults

We sought to determine how the aging left ventricle (LV) responds to sympathetic nervous system (SNS) activation. Three separate echocardiographic experiments were conducted in 11 healthy young (26 ± 1 yr) and 11 healthy older (64 ± 1 yr) adults. Tissue Doppler imaging was used to measure systolic myocardial velocity (S(m)), early diastolic myocardial velocity (E(m)), and late diastolic myocardial velocity (A(m)) during isometric fatiguing handgrip (IFHG), a 2-min cold pressor test (CPT), and 5 min of normobaric hypoxia. Heart rate (HR) and mean arterial pressure (MAP) were also monitored on a beat-by-beat basis; rate pressure product (RPP) was used as an index of myocardial oxygen demand. At peak IFHG, the groups had similar increases in RPP, but the ΔS(m) was significantly greater (i.e., larger impairment) in the older subjects (-0.82 ± 0.13 cm/s) compared with the young subjects (0.37 ± 0.30 cm/s). At peak IFHG, the ΔE(m) was similar between older (-1.59 ± 0.68 cm/s) and young subjects (-1.06 ± 0.76 cm/s). In response to the CPT, both S(m) and E(m) were reduced in the older adults but did not change relative to baseline in the young subjects. Normobaric hypoxia elevated HR and RPP in both groups but did not alter Tissue Doppler parameters. These data indicate that S(m) and E(m) are reduced in healthy older adults during IFHG and CPT. We speculate that suboptimal LV adaptations to SNS stress may partly explain why acute heavy exertion can trigger myocardial ischemia.

Aging attenuates the coronary blood flow response to cold air breathing and isometric handgrip in healthy humans

The purpose of this echocardiography study was to measure peak coronary blood flow velocity (CBV(peak)) and left ventricular function (via tissue Doppler imaging) during separate and combined bouts of cold air inhalation (-14 ± 3°C) and isometric handgrip (30% maximum voluntary contraction). Thirteen young adults and thirteen older adults volunteered to participate in this study and underwent echocardiographic examination in the left lateral position. Cold air inhalation was 5 min in duration, and isometric handgrip (grip protocol) was 2 min in duration; a combined stimulus (cold + grip protocol) and a cold pressor test (hand in 1°C water) were also performed. Heart rate, blood pressure, O(2) saturation, and inspired air temperature were monitored on a beat-by-beat basis. The rate-pressure product (RPP) was used as an index of myocardial O(2) demand, and CBV(peak) was used as an index of myocardial O(2) supply. The RPP response to the grip protocol was significantly blunted in older subjects (Δ1,964 ± 396 beats·min(-1)·mmHg) compared with young subjects (Δ3,898 ± 452 beats·min(-1)·mmHg), and the change in CBV(peak) was also blunted (Δ6.3 ± 1.2 vs. 11.2 ± 2.0 cm/s). Paired t-tests showed that older subjects had a greater change in the RPP during the cold + grip protocol [Δ2,697 ± 391 beats·min(-1)·mmHg compared with the grip protocol alone (Δ2,115 ± 375 beats·min(-1)·mmHg)]. An accentuated RPP response to the cold + grip protocol (compared with the grip protocol alone) without a concomitant increase in CBV(peak) may suggest a dissociation between the O(2) supply and demand in the coronary circulation. In conclusion, older adults have blunted coronary blood flow responses to isometric exercise.

Vitamin C prevents hyperoxia-mediated coronary vasoconstriction and impairment of myocardial function in healthy subjects

Supplementary oxygen is commonly administered in current medical practice. Recently it has been suggested that hyperoxia causes acute oxidative stress and produces prompt and substantial changes in coronary resistance in patients with ischemic heart disease. In this report, we examined whether the effects of hyperoxia on coronary blood velocity (CBV) would be associated with a reduction in myocardial function. We were also interested in determining if the postulated changes in left ventricular (LV) function seen with tissue Doppler imaging (TDI) could be reversed with intravenous vitamin C, a potent, acute anti-oxidant. LV function was determined in eight healthy subjects with transthoracic echocardiography and TDI before and after hyperoxia and with and without infusing vitamin C. Hyperoxia compared with room air promptly reduced CBV by 28 ± 3% (from 23.50 ± 2.31 cm/s down to 17.00 ± 1.79 cm/s) and increased relative coronary resistance by 34 ± 5% (from 5.63 ± 0.88 up to 7.32 ± 0.94). Meanwhile, LV myocardial systolic velocity decreased by 11 ± 6% (TDI). These effects on flow and function were eliminated by the infusion of vitamin C, suggesting that these changes are mediated by vitamin C-quenchable substances acting on the coronary microcirculation.

Effect of cold air inhalation and isometric exercise on coronary blood flow and myocardial function in humans

The effects of cold air inhalation and isometric exercise on coronary blood flow are currently unknown, despite the fact that both cold air and acute exertion trigger angina in clinical populations. In this study, we used transthoracic Doppler echocardiography to measure coronary blood flow velocity (CBV; left anterior descending coronary artery) and myocardial function during cold air inhalation and handgrip exercise. Ten young healthy subjects underwent the following protocols: 5 min of inhaling cold air (cold air protocol), 5 min of inhaling thermoneutral air (sham protocol), 2 min of isometric handgrip at 30% of maximal voluntary contraction (grip protocol), and 5 min of isometric handgrip at 30% maximal voluntary contraction while breathing cold air (cold + grip protocol). Heart rate, blood pressure, inspired air temperature, CBV, myocardial function (tissue Doppler imaging), O(2) saturation, and pulmonary function were measured. The rate-pressure product (RPP) was used as an index of myocardial O(2) demand, whereas CBV was used as an index of myocardial O(2) supply. Compared with the sham protocol, the cold air protocol caused a significantly higher RPP, but there was a significant reduction in CBV. The cold + grip protocol caused a significantly greater increase in RPP compared with the grip protocol (P = 0.045), but the increase in CBV was significantly less (P = 0.039). However, myocardial function was not impaired during the cold + grip protocol relative to the grip protocol alone. Collectively, these data indicate that there is a supply-demand mismatch in the coronary vascular bed when cold ambient air is breathed during acute exertion but myocardial function is preserved, suggesting an adequate redistribution of blood flow.

Oxygen therapy for acute myocardial infarction-then and now. A century of uncertainty

For about 100 years, inhaled oxygen has been administered to all patients suspected of having an acute myocardial infarction. The basis for this practice was the belief that oxygen supplementation raised often-deficient arterial oxygen content to improve myocardial oxygenation, thereby reducing infarct size. This assumption is conditional and not evidence-based. While such physiological changes may pertain in some patients who are hypoxemic, considerable data suggest that oxygen therapy may be detrimental in others. Acute oxygen therapy may raise blood pressure and lower cardiac index, heart rate, cardiac oxygen consumption, and blood flow in the cerebral and renal beds. Oxygen also may lower capillary density and redistribute blood in the microcirculation. Several reports now confirm that these changes occur in humans. In patients with both acute coronary syndromes and stable coronary disease, oxygen administration may constrict the coronary vessels, lower myocardial oxygen delivery, and may actually worsen ischemia. There are no large, contemporary, randomized studies that examine clinical outcomes after this intervention. Hence, this long-accepted but potentially harmful tradition urgently needs reevaluation. Clinical guidelines appear to be changing, favoring use of oxygen only in hypoxemic patients, and then cautiously titrating to individual oxygen tensions.

Altered coronary vascular control during cold stress in healthy older adults

Cardiovascular-related mortality increases in the cold winter months, particularly in older adults. Previously, we reported that determinants of myocardial O(2) demand, such as the rate-pressure product, increase more in older adults compared with young adults during cold stress. The aim of the present study was to determine if aging influences the coronary hemodynamic response to cold stress in humans. Transthoracic Doppler echocardiography was used to noninvasively measure peak coronary blood velocity in the left anterior descending artery before and during acute (20 min) whole body cold stress in 10 young adults (25 ± 1 yr) and 11 older healthy adults (65 ± 2 yr). Coronary vascular resistance (diastolic blood pressure/peak coronary blood velocity), coronary perfusion time fraction (coronary perfusion time/R-R interval), and left ventricular wall stress were calculated. We found that cooling (via a water-perfused suit) increased left ventricular wall stress, a primary determinant of myocardial O(2) consumption, in both young and older adults, although the magnitude of this increase was nearly twofold greater in older adults (change of 9.1 ± 3.5% vs. 17.6 ± 3.2%, P < 0.05, change from baseline in young and older adults and young vs. older adults). Despite the increased myocardial O(2) demand during cooling, coronary vasodilation (decreased coronary vascular resistance) occurred only in young adults (3.22 ± 0.23 to 2.85 ± 0.18 mmHg·cm(-1)·s(-1), P < 0.05) and not older adults (3.97 ± 0.24 to 3.79 ± 0.27 mmHg·cm(-1)·s(-1), P > 0.05). Consistent with a blunted coronary vascular response, absolute coronary perfusion time tended to decrease (P = 0.13) and coronary perfusion time fraction decreased (P < 0.05) during cooling in older adults but not young adults. Collectively, these data suggest that older adults demonstrate an altered coronary hemodynamic response to acute cold stress.

Cerebral and myocardial blood flow responses to hypercapnia and hypoxia in humans

In humans, cerebrovascular responses to alterations in arterial Pco2 and Po2 are well documented. However, few studies have investigated human coronary vascular responses to alterations in blood gases. This study investigated the extent to which the cerebral and coronary vasculatures differ in their responses to euoxic hypercapnia and isocapnic hypoxia in healthy volunteers. Participants ( n = 15) were tested at rest on two occasions. On the first visit, middle cerebral artery blood velocity ( V̄P) was assessed using transcranial Doppler ultrasound. On the second visit, coronary sinus blood flow (CSBF) was measured using cardiac MRI. For comparison with V̄P, CSBF was normalized to the rate pressure product [an index of myocardial oxygen consumption; normalized (n)CSBF]. Both testing sessions began with 5 min of euoxic [end-tidal Po2 (PetO2) = 88 Torr] isocapnia [end-tidal Pco2 (PetCO2) = +1 Torr above resting values]. PetO2 was next held at 88 Torr, and PetCO2 was increased to 40 and 45 Torr in 5-min increments. Participants were then returned to euoxic isocapnia for 5 min, after which PetO2 was decreased from 88 to 60, 52 and 45 Torr in 5-min decrements. Changes in V̄P and nCSBF were normalized to isocapnic euoxic conditions and indexed against PetCO2 and arterial oxyhemoglobin saturation. The V̄P gain for euoxic hypercapnia (%/Torr) was significantly higher than nCSBF ( P = 0.030). Conversely, the V̄P gain for isocapnic hypoxia (%/%desaturation) was not different from nCSBF ( P = 0.518). These findings demonstrate, compared with coronary circulation, that the cerebral circulation is more sensitive to hypercapnia but similarly sensitive to hypoxia.

Coronary vasoconstrictor responses are attenuated in young women as compared with age-matched men

Recent work in humans suggests coronary vasoconstriction occurs with static handgrip with a time course that suggests a sympathetic constrictor mechanism. These findings are consistent with animal studies that suggest this effect helps maintain transmural myocardial perfusion. It is known that oestrogen can attenuate sympathetic responsiveness, however it is not known if sympathetic constrictor responses vary in men and women. To examine this issue we studied young men (n = 12; 28 ± 1 years) and women (n = 14; 30 ± 1 years). Coronary blood flow velocity (CBV; Duplex Ultrasound), heart rate (ECG) and blood pressure (BP; Finapres) were measured during static handgrip (20 s) at 10% and 70% of maximum voluntary contraction. Measurements were also obtained during graded lower body negative pressure (LBNP; activates baroreflex-mediated sympathetic system) and the cold pressor test (CPT; a non-specific sympathetic stimulus). A coronary vascular resistance index (CVR) was calculated as diastolic BP/CBV. Increases in CVR with handgrip were greater in men vs. women (1.25 ± 0.49 vs. 0.26 ± 0.38 units; P < 0.04) and CBV tended to fall in men but not in women (−0.9 ± 0.9 vs. 1.7 ± 0.8 cm s−1; P < 0.01). Changes in CBV with handgrip were linked to the myocardial oxygen consumption in women but not in men. CBV reductions were greater in men vs. women during graded LBNP (P < 0.04). Men and women had similar coronary responses to CPT (P = n.s.). We conclude that coronary vasoconstrictor tone is greater in men than women during static handgrip and LBNP.

The association of remotely-sensed outdoor temperature with blood pressure levels in REGARDS: a cross-sectional study of a large, national cohort of African-American and white participants

BACKGROUND

Evidence is mounting regarding the clinically significant effect of temperature on blood pressure.

METHODS

In this cross-sectional study the authors obtained minimum and maximum temperatures and their respective previous week variances at the geographic locations of the self-reported residences of 26,018 participants from a national cohort of blacks and whites, aged 45+. Linear regression of data from 20,623 participants was used in final multivariable models to determine if these temperature measures were associated with levels of systolic or diastolic blood pressure, and whether these relations were modified by stroke-risk region, race, education, income, sex hypertensive medication status, or age.

RESULTS

After adjustment for confounders, same-day maximum temperatures 20 °F lower had significant associations with 1.4 mmHg (95% CI: 1.0, 1.9) higher systolic and 0.5 mmHg (95% CI: 0.3, 0.8) higher diastolic blood pressures. Same-day minimum temperatures 20 °F lower had a significant association with 0.7 mmHg (95% CI: 0.3, 1.0) higher systolic blood pressures but no significant association with diastolic blood pressure differences. Maximum and minimum previous-week temperature variabilities showed significant but weak relationships with blood pressures. Parameter estimates showed effect modification of negligible magnitude.

CONCLUSIONS

This study found significant associations between outdoor temperature and blood pressure levels, which remained after adjustment for various confounders including season. This relationship showed negligible effect modification.