Cerebrovascular CO2 reactivity in normotensive and hypertensive man

The effect of hypertension on cerebrovascular carbon dioxide reactivity in atrial fibrillation patients

Atrial fibrillation (AF) and hypertension (HTN) are both associated with impaired cerebrovascular carbon dioxide reactivity (CVRCO2), an indicator of cerebral vasodilatory reserve. We hypothesised that CVRCO2 would be lower in patients with both AF and HTN (AF + HTN) compared to normotensive AF patients, due to an additive effect of AF and HTN on CVRCO2. Forty AF (68 ± 9 years) and fifty-seven AF + HTN (68 ± 8 years) patients underwent transcranial Doppler ultrasound measurement of middle cerebral artery blood velocity (MCA Vm) during stepped increases and decreases in end-tidal carbon dioxide (PETCO2). A cerebrovascular conductance index (CVCi) was calculated as the ratio of MCA Vm and mean arterial pressure (MAP). CVRCO2 was determined from the linear slope for MCA Vm and MCA CVCi vs PETCO2. Baseline MAP was higher in AF + HTN than AF (107 ± 9 vs. 98 ± 9 mmHg, respectively; p < 0.001), while MCA Vm was not different (AF + HTN:49.6 [44.1-69.0]; AF:51.7 [45.2-63.3] cm.s-1; p = 0.075), and CVCi was lower in AF + HTN (0.46 [0.42-0.57] vs. 0.54 [0.44-0.63] cm.s-1.mmHg-1; p < 0.001). MCA Vm CVRCO2 was not different (AF + HTN: 1.70 [1.47-2.19]; AF 1.74 [1.54-2.52] cm/s/mmHg-2; p = 0.221), while CVCi CVRCO2 was 13% lower in AF + HTN (0.013 ± 0.004 vs 0.015 ± 0.005 cm.s-1.mmHg-1; p = 0.047). Our results demonstrate blunted cerebral vasodilatory reserve (determined as MCA CVCi CVRCO2) in AF + HTN compared to AF alone. This may implicate HTN as a driver of further cerebrovascular dysfunction in AF that may be important for the development of AF-related cerebrovascular events and downstream cognitive decline. We demonstrated reduced cerebrovascular CO2 responsiveness in atrial fibrillation with hypertension (AF+HTN) vs. atrial fibrillation (AF). Furthermore, AF per se (as opposed to normal sinus rhythm) predicts reduced cerebrovascular CO2 responsiveness. Our findings suggest additional cerebrovascular dysfunction in AF+HTN vs. AF.

Determining the effects of elevated partial pressure of oxygen on hypercapnia-induced cerebrovascular reactivity

Evaluation of cerebrovascular reactivity (CVR) to hypo- and hypercapnia is a valuable test for the assessment of vasodilatory reserve. While hypercapnia-induced CVR testing is usually performed at normoxia, mild hyperoxia may increase tolerability of hypercapnia by reducing the ventilatory distress. However, the effects of mild hyperoxia on CVR was unknown. We therefore recruited 21 patients with a range of steno-occlusive diseases and 12 healthy participants who underwent a standardized 13-minute step plus ramp CVR test with a carbon dioxide gas challenge at the subject's resting end-tidal partial pressure of oxygen or at mild hyperoxia (PetO2 = 150 mmHg) depending on to which group they were assigned. In 11 patients, the second CVR test was at normoxia to examine test-retest differences. CVR was defined as % Δ Signal/ΔPetCO2. We found that there was no significant difference between CVR test results conducted at normoxia and at mild hyperoxia for participants in Groups 1 and 2 for the step and ramp portion. We also found no difference between test and retest CVR at normoxia for patients with cerebrovascular pathology (Group 3) for step and ramp portion. We concluded normoxic CVR is repeatable, and that mild hyperoxia does not affect CVR.

Can Alterations in Cerebrovascular CO2 Reactivity Be Identified Using Transfer Function Analysis without the Requirement for Carbon Dioxide Inhalation?

The present study aimed to examine the validity of a novel method to assess cerebrovascular carbon dioxide (CO₂) reactivity (CVR) that does not require a CO₂ inhalation challenge, e.g., for use in patients with respiratory disease or the elderly, etc. In twenty-one healthy participants, CVR responses to orthostatic stress (50° head-up tilt, HUT) were assessed using two methods: (1) the traditional CO₂ inhalation method, and (2) transfer function analysis (TFA) between middle cerebral artery blood velocity (MCA V) and predicted arterial partial pressure of CO₂ (PaCO₂) during spontaneous respiration. During HUT, MCA V steady-state (i.e., magnitude) and MCA V onset (i.e., time constant) responses to CO₂ inhalation were decreased (p < 0.001) and increased (p = 0.001), respectively, indicative of attenuated CVR. In contrast, TFA gain in the very low-frequency range (VLF, 0.005-0.024 Hz) was unchanged, while the TFA phase in the VLF approached zero during HUT (-0.38 ± 0.59 vs. 0.31 ± 0.78 radians, supine vs. HUT; p = 0.003), indicative of a shorter time (i.e., improved) response of CVR. These findings indicate that CVR metrics determined by TFA without a CO₂ inhalation do not track HUT-evoked reductions in CVR identified using CO₂ inhalation, suggesting that enhanced cerebral blood flow response to a change in CO₂ using CO₂ inhalation is necessary to assess CVR adequately.

Association Between Risk of Obstructive Sleep Apnea and Cerebrovascular Reactivity in Stroke Patients

Background Obstructive sleep apnea (OSA) is present in 60% to 70% of stroke patients. Cerebral vasoreactivity in patients with stroke and OSA has not been well studied and could identify a new pathophysiologic mechanism with potential therapeutic intervention. We aimed to determine whether risk categories for OSA are associated with cerebral vasoreactivity in stroke patients. Methods and Results In this cross-sectional study of a cohort of patients with stroke, we used clinical questionnaires (Sleep Obstructive Apnea Score Optimized for Stroke [SOS] and snoring, tiredness, observed, pressure, bmi, age, neck, gender [STOP-BANG] scores) to assess the risk of OSA and transcranial Doppler to assess cerebral vasoreactivity (breath-holding index and visual evoked flow velocity response). Of the 99 patients included, 77 (78%) had medium or high risk of OSA and 80 performed transcranial Doppler. Mean breath-holding index was 0.52±0.37, and median visual evoked flow velocity response was 10.8% (interquartile range: 8.8-14.5); 54 of 78 (69%) showed impaired anterior circulation vasoreactivity (breath-holding index <0.69) and 53 of 71 (75%) showed impaired posterior circulation vasoreactivity (visual evoked flow velocity response ≤14.0%). There was a significant negative correlation between the risk of OSA calculated by STOP-BANG and the breath-holding index (r_S=-0.284, P=0.012). The following variables were associated with low anterior circulation vasoreactivity: dyslipidemia (odds ratio: 4.7; 95% CI, 1.5-14.2) and STOP-BANG score (odds ratio: 1.7 per 1-point increase; 95% CI, 1.1-1.5). Conclusions A high risk of OSA and impaired vasoreactivity exists in the population that has had stroke. Dyslipidemia and STOP-BANG sleep apnea risk categories were independently associated with impaired anterior circulation vasoreactivity.

End-Tidal Hypocapnia Under Anesthesia Predicts Postoperative Delirium

Background: Postoperative delirium (POD) might be associated with anesthetic management, but research has focused on choice or dosage of anesthetic drugs. We examined potential contributions of intraoperative ventilatory and hemodynamic management to POD.

Methods: This was a sub-study of the ENGAGES-Canada trial (NCT02692300) involving non-cardiac surgery patients enrolled in Winnipeg, Canada. Patients received preoperative psychiatric and cognitive assessments, and intraoperatively underwent high-fidelity data collection of blood pressure, end-tidal respiratory gases and anesthetic agent concentration. POD was assessed by peak and mean POD scores using the Confusion Assessment Method-Severity (CAM-S) tool. Bivariate and multiple linear regression models were constructed controlling for age, psychiatric illness, and cognitive dysfunction in the examination of deviations in intraoperative end-tidal carbon dioxide (areas over (AOC) and under the curve (AUC)) on POD severity scores.

Results: A total of 101 subjects [69 (6) years of age] were studied; 89 had comprehensive intraoperative hemodynamic and end-tidal gas measurements (data recorded at 1 Hz). The incidence of POD was 11.9% (12/101). Age, cognitive dysfunction, anxiety, depression, and intraoperative end-tidal CO₂ (AUC) were significant correlates of POD severity. In the multiple regression model, cognitive dysfunction and AUC end-tidal CO₂ (0.67 kPa below median intra-operative value) were the only independent significant predictors across both POD severity (mean and peak) scores. There was no association between cumulative anesthetic agent exposure and POD.

Conclusions: POD was associated with intraoperative ventilatory management, reflected by low end-tidal CO₂ concentrations, but not with cumulative anesthetic drug exposure. These findings suggest that maintenance of intraoperative normocapnia might benefit patients at risk of POD.

Aging modifies the effect of cardiac output on middle cerebral artery blood flow velocity

An association between cerebral blood flow (CBF) and cardiac output (CO) has been established in young healthy subjects. As of yet it is unclear how this association evolves over the life span. To that purpose, we continuously recorded mean arterial pressure (MAP; finger plethysmography), CO (pulse contour; CO‐trek), mean blood flow velocity in the middle cerebral artery (MCAV; transcranial Doppler ultrasonography), and end‐tidal CO₂ partial pressure (PetCO₂) in healthy young (19–27 years), middle‐aged (51–61 years), and elderly subjects (70–79 years). Decreases and increases in CO were accomplished using lower body negative pressure and dynamic handgrip exercise, respectively. Aging in itself did not alter dynamic cerebral autoregulation or cerebrovascular CO₂ reactivity. A linear relation between changes in CO and MCAV_mean was observed in middle‐aged (P < 0.01) and elderly (P = 0.04) subjects but not in young (P = 0.45) subjects, taking concurrent changes in MAP and PetCO₂ into account. These data imply that with aging, brain perfusion becomes increasingly dependent on CO.

Cerebrovascular Reactivity and Vascular Activation in Postmenopausal Women With Histories of Preeclampsia

Cerebrovascular reactivity (CVR) is reduced in patients with cognitive decline. Women with a history of preeclampsia are at increased risk for cognitive decline. This study examined an association between pregnancy history and CVR using a subgroup of 40 age- and parity-matched pairs of women having histories of preeclampsia (n=27) or normotensive pregnancy (n=29) and the association of activated blood elements with CVR. Middle cerebral artery velocity was measured by Doppler ultrasound before and during hypercapnia to assess CVR. Thirty-eight parameters of blood cellular elements, microvesicles, and cell-cell interactions measured in venous blood were assessed for association with CVR using principal component analysis. Middle cerebral artery velocity was lower in the preeclampsia compared with the normotensive group at baseline (63±4 versus 73±3 cm/s; P=0.047) and during hypercapnia (P=0.013-0.056). CVR was significantly lower in the preeclampsia compared with the normotensive group (2.1±1.3 versus 2.9±1.1 cm·s·mm Hg; P=0.009). Globally, the association of the 7 identified principal components with preeclampsia (P=0.107) and with baseline middle cerebral artery velocity (P=0.067) did not reach statistical significance. The interaction between pregnancy history and principal components with respect to CVR (P=0.084) was driven by a nominally significant interaction between preeclampsia and the individual principal component defined by blood elements, platelet aggregation, and interactions of platelets with monocytes and granulocytes (P=0.008). These results suggest that having a history of preeclampsia negatively affects the cerebral circulation years beyond the pregnancy and that this effect was associated with activated blood elements.

Evidence from high-altitude acclimatization for an integrated cerebrovascular and ventilatory hypercapnic response but different responses to hypoxia

Ventilation and cerebral blood flow (CBF) are both sensitive to hypoxia and hypercapnia. To compare chemosensitivity in these two systems, we made simultaneous measurements of ventilatory and cerebrovascular responses to hypoxia and hypercapnia in 35 normal human subjects before and after acclimatization to hypoxia. Ventilation and CBF were measured during stepwise changes in isocapnic hypoxia and iso-oxic hypercapnia. We used MRI to quantify actual cerebral perfusion. Measurements were repeated after 2 days of acclimatization to hypoxia at 3,800 m altitude (partial pressure of inspired O₂ = 90 Torr) to compare plasticity in the chemosensitivity of these two systems. Potential effects of hypoxic and hypercapnic responses on acute mountain sickness (AMS) were assessed also. The pattern of CBF and ventilatory responses to hypercapnia were almost identical. CO₂ responses were augmented to a similar degree in both systems by concomitant acute hypoxia or acclimatization to sustained hypoxia. Conversely, the pattern of CBF and ventilatory responses to hypoxia were markedly different. Ventilation showed the well-known increase with acute hypoxia and a progressive decline in absolute value over 25 min of sustained hypoxia. With acclimatization to hypoxia for 2 days, the absolute values of ventilation and O₂ sensitivity increased. By contrast, O₂ sensitivity of CBF or its absolute value did not change during sustained hypoxia for up to 2 days. The results suggest a common or integrated control mechanism for CBF and ventilation by CO₂ but different mechanisms of O₂ sensitivity and plasticity between the systems. Ventilatory and cerebrovascular responses were the same for all subjects irrespective of AMS symptoms. NEW & NOTEWORTHY Ventilatory and cerebrovascular hypercapnic response patterns show similar plasticity in CO₂ sensitivity following hypoxic acclimatization, suggesting an integrated control mechanism. Conversely, ventilatory and cerebrovascular hypoxic responses differ. Ventilation initially increases but adapts with prolonged hypoxia (hypoxic ventilatory decline), and ventilatory sensitivity increases following acclimatization. In contrast, cerebral blood flow hypoxic sensitivity remains constant over a range of hypoxic stimuli, with no cerebrovascular acclimatization to sustained hypoxia, suggesting different mechanisms for O₂ sensitivity in the two systems.

Bridging macroscopic and microscopic methods for the measurements of cerebral blood flow: Toward finding the determinants in maintaining the CBF homeostasis

Methods exist to evaluate the cerebral blood flow (CBF) at both the macroscopic and microscopic spatial scales. These methods provide complementary information for understanding the mechanism in maintaining an adequate blood supply in response to neural demand. The macroscopic CBF assesses perfusion flow, which is usually measured using radioactive tracers, such as diffusible, nondiffusible, or microsphere. Each of them determines CBF based on indicator dilution principle or particle fraction principle under the assumption that CBF is steady state during the measurement. Macroscopic CBF therefore represents averaged CBF over a certain space and time domains. On the other hand, the microscopic CBF assesses bulk flow, usually measures using real-time microscopy. The method assesses hemodynamics of microvessels, ie, vascular dimensions and flow velocities of fluorescently labeled or nonlabeled RBC and plasma markers. The microscopic CBF continuously fluctuates in time and space. Smoothing out this heterogeneity may lead to underestimation in the macroscopic CBF. To link the two measurements, it is needed to introduce a common parameter which is measurable for the both methods, such as mean transit time. Additionally, applying the defined physiological and/or pharmacological perturbation may provide a good exercise to determine how the specific perturbations interfere the quantitative relationships between the macroscopic and microscopic CBF. Finally, bridging these two-scale methods potentially gives a further indication how the absolute CBF is regulated with respect to a specific type of the cerebrovascular tones or capillary flow velocities in the brain.

Modeling the role of osmotic forces in the cerebrovascular response to CO2

Increases in blood osmolarity have been shown to exert a vasodilatory effect on cerebral and other vasculature, with accompanying increases in blood flow. It has also been shown that, through an influence on blood concentration of the bicarbonate ion and pH, changes in blood levels of CO2 can alter blood osmolarity sufficiently to have an impact on vessel diameter. We propose here that this phenomenon plays a previously unappreciated role in CO2-mediated vasodilation, and present a biophysical model of osmotically driven vasodilation. Our model, which is based on literature data describing CO2-dependent changes in blood osmolarity and hydraulic conductivity (Lp) of the blood-brain barrier, is used to predict the change in cerebral blood flow (CBF) associated with osmotic forces arising from a specific hypercapnic challenge. Modeled changes were then compared with actual CBF changes determined using arterial spin-labeling (ASL) MRI. For changes in the arterial partial pressure of CO2 (PaCO2) of 20 mmHg, our model predicted increases of 80% from baseline CBF with a temporal evolution that was comparable to the measured hemodynamic responses. Our modeling results suggest that osmotic forces could play a significant role in the cerebrovascular response to CO2.

Regional cerebral blood flow and arterial blood volume and their reactivity to hypercapnia in hypertensive and normotensive rats

Chronic hypertension induces cerebrovascular remodeling, changing the inner diameter and elasticity of arterial vessels. To examine cerebrovascular morphologic changes and vasodilatory impairment in early-stage hypertension, we measured baseline (normocapnic) cerebral arterial blood volume (CBV(a)) and cerebral blood flow (CBF) as well as hypercapnia-induced dynamic vascular responses in animal models. All experiments were performed with young (3 to 4 month old) spontaneously hypertensive rats (SHR) and control Wistar-Kyoto rats (WKY) under ∼1% isoflurane anesthesia at 9.4 Tesla. Baseline regional CBF values were similar in both animal groups, whereas SHR had significantly lower CBV(a) values, especially in the hippocampus area. As CBF is maintained by adjusting arterial diameters within the autoregulatory blood pressure range, CBV(a) is likely more sensitive than CBF for detecting hypertensive-mediated alterations. Unexpectedly, hypercapnia-induced CBF and blood-oxygenation-level-dependent (BOLD) response were significantly higher in SHR as compared with WKY, and the CBF reactivity was highly correlated with the BOLD reactivity in both groups. The higher reactivity in early-stage hypertensive animals indicates no significant vascular remodeling occurred. At later stages of hypertension, the reduced vascular reactivity is expected. Thus, CBF and CBV(a) mapping may provide novel insights into regional cerebrovascular impairment in hypertension and its progression as hypertension advances.

Effects of aging on the association between cerebrovascular responses to visual stimulation, hypercapnia and arterial stiffness

Aging is associated with decreased vascular compliance and diminished neurovascular- and hypercapnia-evoked cerebral blood flow (CBF) responses. However, the interplay between arterial stiffness and reduced CBF responses is poorly understood. It was hypothesized that increased cerebral arterial stiffness is associated with reduced evoked responses to both, a flashing checkerboard visual stimulation (i.e., neurovascular coupling), and hypercapnia. To test this hypothesis, 20 older (64 ± 8 year; mean ± SD) and 10 young (30 ± 5 year) subjects underwent a visual stimulation (VS) and a hypercapnic test. Blood velocity through the posterior (PCA) and middle cerebral (MCA) arteries was measured concurrently using transcranial Doppler ultrasound (TCD). Cerebral and systemic vascular stiffness were calculated from the cerebral blood velocity and systemic blood pressure waveforms, respectively. Cerebrovascular (MCA: young = 76 ± 15%, older = 98 ± 19%, p = 0.004; PCA: young = 80 ± 16%, older = 106 ± 17%, p < 0.001) and systemic (young = 59 ± 9% and older = 80 ± 9%, p < 0.001) augmentation indices (AI) were higher in the older group. CBF responses to VS (PCA: p < 0.026) and hypercapnia (PCA: p = 0.018; MCA: p = 0.042) were lower in the older group. A curvilinear model fitted to cerebral AI and age showed AI increases until ~60 years of age, after which the increase levels off (PCA: R (2) = 0.45, p < 0.001; MCA: R (2) = 0.31, p < 0.001). Finally, MCA, but not PCA, hypercapnic reactivity was inversely related to cerebral AI (MCA: R (2) = 0.28, p = 0.002; PCA: R (2) = 0.10, p = 0.104). A similar inverse relationship was not observed with the PCA blood flow response to VS (R (2) = 0.06, p = 0.174). In conclusion, older subjects had reduced neurovascular- and hypercapnia-mediated CBF responses. Furthermore, lower hypercapnia-mediated blood flow responses through the MCA were associated with increased vascular stiffness. These findings suggest the reduced hypercapnia-evoked CBF responses through the MCA, in older individuals may be secondary to vascular stiffening.

Cerebral vasomotor reactivity during hypo- and hypercapnia in sedentary elderly and Masters athletes

Physical activity may influence cerebrovascular function. The objective of this study was to determine the impact of life-long aerobic exercise training on cerebral vasomotor reactivity (CVMR) to changes in end-tidal CO2 (EtCO2) in older adults. Eleven sedentary young (SY, 27±5 years), 10 sedentary elderly (SE, 72±4 years), and 11 Masters athletes (MA, 72±6 years) underwent the measurements of cerebral blood flow velocity (CBFV), arterial blood pressure, and EtCO2 during hypocapnic hyperventilation and hypercapnic rebreathing. Baseline CBFV was lower in SE and MA than in SY while no difference was observed between SE and MA. During hypocapnia, CVMR was lower in SE and MA compared with SY (1.87±0.42 and 1.47±0.21 vs. 2.18±0.28 CBFV%/mm Hg, P<0.05) while being lowest in MA among all groups (P<0.05). In response to hypercapnia, SE and MA exhibited greater CVMR than SY (6.00±0.94 and 6.67±1.09 vs. 3.70±1.08 CBFV1%/mm Hg, P<0.05) while no difference was observed between SE and MA. A negative linear correlation between hypo- and hypercapnic CVMR (R(2)=0.37, P<0.001) was observed across all groups. Advanced age was associated with lower resting CBFV and lower hypocapnic but greater hypercapnic CVMR. However, life-long aerobic exercise training appears to have minimal effects on these age-related differences in cerebral hemodynamics.

The cerebral vasomotor response in varying CO(2) concentrations, as evaluated using cine phase contrast MRI: Flow, volume, and cerebrovascular resistance indices

PURPOSE

Previous studies have identified that impaired cerebral vasomotor reactivity (VMR) is associated with a higher risk of stroke and transient ischemic attack. This study aims to evaluate VMR by measuring the blood flow waveforms of the supplying arteries and dural sinuses using cine phase contrast MRI (PC MRI) and hypercapnic challenge.

METHODS

PC MRI flow quantification was performed on an oblique slice approximately perpendicular to the target vessels to include the left (LICA) and right internal carotid artery (RICA), basilar artery (BA), sinus rectus (SR), and superior sagittal sinus (SSS). A total of four PC MRI scans were performed at different CO(2) concentrations (room air and 3%, 5%, and 7% CO(2)).

RESULTS

The analyses obtained the flow parameters and cerebrovascular resistance parameters for all five vessels. Results indicated that the vascular resistance indices decreased with increasing CO(2) concentration in four vessels (LICA, RICA, BA, and SR). The obtained VMR parameters demonstrated exponential increases with increasing CO(2) concentration.

CONCLUSIONS

Using entire blood flow waveforms, this study applied separate flow dynamics during systolic and diastolic periods to obtain cerebrovascular resistance parameters and extensive flow-related information. It is the first to investigate the cerebrovascular resistance parameters under hypercapnic challenge using cine MRI. This technique could provide a useful tool for clinical application in cerebrovascular disease.

Differential blood flow responses to CO₂ in human internal and external carotid and vertebral arteries

Arterial CO2 serves as a mediator of cerebral blood flow(CBF), and its relative influence on the regulation of CBF is defined as cerebral CO2 reactivity. Our previous studies have demonstrated that there are differences in CBF responses to physiological stimuli (i.e. dynamic exercise and orthostatic stress) between arteries in humans. These findings suggest that dynamic CBF regulation and cerebral CO2 reactivity may be different in the anterior and posterior cerebral circulation. The aim of this study was to identify cerebral CO2 reactivity by measuring blood flow and examine potential differences in CO2 reactivity between the internal carotid artery (ICA), external carotid artery (ECA) and vertebral artery (VA). In 10 healthy young subjects, we evaluated the ICA, ECA, and VA blood flow responses by duplex ultrasonography (Vivid-e, GE Healthcare), and mean blood flow velocity in middle cerebral artery (MCA) and basilar artery (BA) by transcranial Doppler (Vivid-7, GE healthcare) during two levels of hypercapnia (3% and 6% CO2), normocapnia and hypocapnia to estimate CO2 reactivity. To characterize cerebrovascular reactivity to CO2,we used both exponential and linear regression analysis between CBF and estimated partial pressure of arterial CO2, calculated by end-tidal partial pressure of CO2. CO2 reactivity in VA was significantly lower than in ICA (coefficient of exponential regression 0.021 ± 0.008 vs. 0.030 ± 0.008; slope of linear regression 2.11 ± 0.84 vs. 3.18 ± 1.09% mmHg−1: VA vs. ICA, P <0.01). Lower CO2 reactivity in the posterior cerebral circulation was persistent in distal intracranial arteries (exponent 0.023 ± 0.006 vs. 0.037 ± 0.009; linear 2.29 ± 0.56 vs. 3.31 ± 0.87% mmHg−1: BA vs. MCA). In contrast, CO2 reactivity in ECA was markedly lower than in the intra-cerebral circulation (exponent 0.006 ± 0.007; linear 0.63 ± 0.64% mmHg−1, P <0.01). These findings indicate that vertebro-basilar circulation has lower CO2 reactivity than internal carotid circulation, and that CO2 reactivity of the external carotid circulation is markedly diminished compared to that of the cerebral circulation, which may explain different CBF responses to physiological stress.

The effect of continuous positive airway pressure on total cerebral blood flow in healthy awake volunteers

PURPOSE

Continuous positive airway pressure (CPAP) is the gold standard treatment for obstructive sleep apnea. However, the physiologic impact of CPAP on cerebral blood flow (CBF) is not well established. Ultrasound can be used to estimate CBF, but there is no widespread accepted protocol. We studied the physiologic influence of CPAP on CBF using a method integrating arterial diameter and flow velocity (FV) measurements obtained for each vessel supplying blood to the brain.

METHODS

FV and lumen diameter of the left and right internal carotid, vertebral, and middle cerebral arteries were measured using duplex Doppler ultrasound with and without CPAP at 15 cm H(2)O, applied in a random order. Transcutaneous carbon dioxide (PtcCO(2)), heart rate (HR), blood pressure (BP), and oxygen saturation were monitored. Results were compared with a theoretical prediction of CBF change based on the effect of partial pressure of carbon dioxide on CBF.

RESULTS

Data were obtained from 23 healthy volunteers (mean ± SD; 12 male, age 25.1 ± 2.6 years, body mass index 21.8 ± 2.0 kg/m(2)). The mean experimental and theoretical CBF decrease under CPAP was 12.5 % (p < 0.001) and 11.9 % (p < 0.001), respectively. The difference between experimental and theoretical CBF reduction was not statistically significant (3.84 ± 79 ml/min, p = 0.40). There was a significant reduction in PtcCO(2) with CPAP (p = <0.001) and a significant increase in mean BP (p = 0.0017). No significant change was observed in SaO(2) (p = 0.21) and HR (p = 0.62).

CONCLUSION

Duplex Doppler ultrasound measurements of arterial diameter and FV allow for a noninvasive bedside estimation of CBF. CPAP at 15 cm H(2)O significantly decreased CBF in healthy awake volunteers. This effect appeared to be mediated predominately through the hypocapnic vasoconstriction coinciding with PCO(2) level reduction. The results suggest that CPAP should be used cautiously in patients with unstable cerebral hemodynamics.

A case of posterior reversible encephalopathy syndrome after emergence from anesthesia

Posterior reversible encephalopathy syndrome (PRES) is a relatively new clinical entity characterized by reversible neurological symptoms with findings indicating leukoencephalopathy on imaging studies. Reports of PRES in the field of anesthesiology have been quite limited. A patient with therapeutic anticoagulant developed PRES immediately after emergence from anesthesia, in which her status was initially recognized as delayed recovery from anesthesia with transient hypertension because an emergent head computed tomography (CT) scan was almost normal. Subsequently, magnetic resonance imaging (MRI) was also performed according to a radiologist's recommendation because the CT results showed areas of slightly low attenuation in the frontoparieto-occipital lobes bilaterally, suggesting PRES; otherwise, ischemic events. MRI showed subcortical increased T(2) and fluid-attenuated inversion recovery (FLAIR) intensity in the occipitoparietal regions bilaterally with slight increase in the apparent diffusion coefficient signal on diffusion-weighted imaging, which confirmed a diagnosis of PRES. Gradually, the patient regained consciousness and became responsive with antihypertensive therapy. A prompt and accurate diagnosis of PRES is important to avoid irreversible brain damage, for example, intracranial hemorrhage, especially in a patient receiving anticoagulation therapy.

Blood pressure and stroke in heart failure in the REasons for Geographic And Racial Differences in Stroke (REGARDS) study

BACKGROUND AND PURPOSE

The prevalence of stroke is increased in individuals with heart failure (HF). The stroke mechanism in HF may be cardiogenic embolism or cerebral hypoperfusion. Stroke risk increases with decreasing ejection fraction and low cardiac output is associated with hypotension and poor survival. We examine the relationship among blood pressure level, history of stroke/transient ischemic attack (TIA), and HF.

METHODS

We compared the prevalence of self-reported history of stroke or TIA in the REasons for Geographic And Racial Differences in Stroke (REGARDS) participants with HF (as defined by current digoxin use) and without HF. We excluded participants with atrial fibrillation or missing data. We examined the relationship between HF and history of stroke/TIA within tertiles of systolic blood pressure (SBP) adjusting for patient demographic and health characteristics.

RESULTS

Prevalent stroke/TIA were reported by 66 (26.3%) of 251 participants with and 1805 (8.5%) of 21 202 participants without HF (P<0.0001). Within each tertile of SBP, the unadjusted OR (95% CI) for prior stroke/TIA among those with HF compared with those without HF (the reference group) was, 4.0 (2.8 to 5.8) for SBP <119.5 mm Hg, 2.7 (1.8 to 3.9) for SBP >or=119.5 but <131.5 mm Hg, and 2.3 (1.6 to 3.2) for SBP >or=131.5 mm Hg. After adjustment, the relationship between prior stroke/TIA and HF remained significant only within the lowest tertile of SBP (<119.5 mm Hg; 3.0; 1.5 to 6.1).

CONCLUSIONS

The odds of prevalent self-reported stroke/TIA are increased in participants with HF and most markedly increased in participants with low SBP. Longitudinal data are needed to determine whether this reflects stroke/TIA secondary to thromboembolism from poor cardiac function or secondary to cerebral hypoperfusion.

Relationship between middle cerebral artery blood velocity and end-tidal PCO2 in the hypocapnic-hypercapnic range in humans

This study examined the relationship between cerebral blood flow (CBF) and end-tidal PCO2 (PETCO2) in humans. We used transcranial Doppler ultrasound to determine middle cerebral artery peak blood velocity responses to 14 levels of PETCO2 in a range of 22 to 50 Torr with a constant end-tidal PO2 (100 Torr) in eight subjects. PETCO2 and end-tidal PO2 were controlled by using the technique of dynamic end-tidal forcing combined with controlled hyperventilation. Two protocols were conducted in which PETCO2 was changed by 2 Torr every 2 min from hypocapnia to hypercapnia (protocol I) and vice-versa (protocol D). Over the range of PETCO2 studied, the sensitivity of peak blood velocity to changes in PETCO2 (CBF-PETCO2 sensitivity) was nonlinear with a greater sensitivity in hypercapnia (4.7 and 4.0%/Torr, protocols I and D, respectively) compared with hypocapnia (2.5 and 2.2%/Torr). Furthermore, there was evidence of hysteresis in the CBF-PETCO2 sensitivity; for a given PETCO2, there was greater sensitivity during protocol I compared with protocol D. In conclusion, CBF-PETCO2 sensitivity varies depending on the level of PETCO2 and the protocol that is used. The mechanisms underlying these responses require further investigation.

Cerebral vasomotor reactivity before and after blood pressure reduction in hypertensive patients

BACKGROUND

Hypertension is associated with cerebrovascular remodeling and endothelial dysfunction, which may reduce cerebral vasomotor reactivity to CO2. Treatment combining blood pressure (BP) reduction with inhibition of vascular effects of angiotensin II may reverse these changes. However, the reduction in BP at the onset of treatment can compromise cerebral perfusion and exhaust vasomotor reserve, leading to impaired CO2 reactivity.

METHODS

Eleven patients (nine men, two women) with newly diagnosed, untreated mild-to-moderate hypertension aged (mean (s.d.)) 52 (9) years, and eight controls (seven men, one woman) aged 46 (10) years were studied. Patients received losartan/hydrochlorothiazide (50/12.5 or 100/25 mg) to reduce BP to <140/<90 mm Hg within 1-2 weeks. BP (Finapres), heart rate (HR), CBFV (cerebral blood flow velocity, transcranial Doppler), cerebrovascular resistance, and CO2 reactivity were measured at baseline, after the rapid BP reduction, and after long-term treatment (3-4 months).

RESULTS

At baseline, hypertension was not associated with reduced CO2 reactivity. Treatment effectively lowered BP from 148 (12)/89 (7) to 130 (15)/80 (9) after 1-2 weeks and 125 (10)/77 (7) mm Hg after 3-4 months (P = 0.003). CO2 reactivity was not affected by the reduction in BP within 2 weeks, and long-term treatment did not augment reactivity.

CONCLUSIONS

In hypertension without diabetes or advanced cerebrovascular disease, CO2 reactivity is not reduced, and rapid normalization (within 2 weeks) of BP does not exhaust vasomotor reserve. CO2 reactivity did not change between 2 and 12 weeks of treatment, which argues against a direct vascular effect of angiotensin II inhibition within this period.

Human cardiorespiratory and cerebrovascular function during severe passive hyperthermia: effects of mild hypohydration

The influence of severe passive heat stress and hypohydration (Hypo) on cardiorespiratory and cerebrovascular function is not known. We hypothesized that 1) heating-induced hypocapnia and peripheral redistribution of cardiac output (Q̇) would compromise blood flow velocity in the middle cerebral artery (MCAv) and cerebral oxygenation; 2) Hypo would exacerbate the hyperthermic-induced hypocapnia, further decreasing MCAv; and 3) heating would reduce MCAv-CO2 reactivity, thereby altering ventilation. Ten men, resting supine in a water-perfused suit, underwent progressive hyperthermia [0.5°C increments in core (esophageal) temperature (TC) to +2°C] while euhydrated (Euh) or Hypo by 1.5% body mass (attained previous evening). Time-control (i.e., non-heat stressed) data were obtained on six of these subjects. Cerebral oxygenation (near-infrared spectroscopy), MCAv, end-tidal carbon dioxide (PetCO2) and arterial blood pressure, Q̇ (flow model), and brachial and carotid blood flows (CCA) were measured continuously each 0.5°C change in TC. At each level, hypercapnia was achieved through 3-min administrations of 5% CO2, and hypocapnia was achieved with controlled hyperventilation. At baseline in Hypo, heart rate, MCAv and CCA were elevated ( P < 0.05 vs. Euh). MCAv-CO2 reactivity was unchanged in both groups at all TC levels. Independent of hydration, hyperthermic-induced hyperventilation caused a severe drop in PetCO2 (−8 ± 1 mmHg/°C), which was related to lower MCAv (−15 ± 3%/°C; R2 = 0.98; P < 0.001). Elevations in Q̇ were related to increases in brachial blood flow ( R2 = 0.65; P < 0.01) and reductions in MCAv ( R2 = 0.70; P < 0.01), reflecting peripheral distribution of Q̇. Cerebral oxygenation was maintained, presumably via enhanced O2-extraction or regional differences in cerebral perfusion.

Anxiety, respiration, and cerebral blood flow: implications for functional brain imaging

Brain functional imaging methods, such as fMRI, are sensitive to changes in cerebral blood flow (CBF) that are normally associated with changes in regional neural activation. However, other endogenous and exogenous factors can alter CBF independently of brain neural activity, thus complicating the interpretation of functional imaging data. The presence of an anxiety disorder, as well as change in state anxiety, is often accompanied by respiratory alterations that affect arterial CO(2) tensions and produce significant changes in CBF that are independent of task-related neural activation. Therefore, the effects of trait and state anxiety need to be given close consideration in interpreting functional imaging findings. In this paper, we review the dependence of most brain functional imaging methods on localized changes in CBF and the potentially confounding effects of anxiety-related alterations of respiration on interpreting patterns of functional activation. Approaches for addressing these effects are discussed.

The relationship between cerebral blood flow and volume in humans

The purpose of this study was to establish the relationship between regional CBF and CBV at normal, resting cerebral metabolic rates. Eleven healthy volunteers were investigated with PET during baseline conditions, and during hyper- and hypocapnia. Values for rCBF and rCBV were obtained using 15O-labelled water and carbon monoxide, respectively. The mean value of rCBF using PET was 62 +/- 18 ml 100 g(-1) min(-1) during baseline conditions, with an average increase of 46% during hypercapnia, and a decrease of 29% during hypocapnia; baseline rCBV was 7.7 ml/100 g, with 27% increase during hypercapnia and no significant decrease during hypocapnia. A regionally uniform exponential relationship was confirmed between PaCO2 and rCBF as well as rCBV. It is shown that the theoretical implication of this is that the rCBV vs. rCBF relationship should be modelled by a power function; however, due to pronounced intersubject variability, the goodness of fit for linear and nonlinear models were not significantly different. The results of the study are applied to a numerical estimation of regional brain deoxy-haemoglobin content. Independently of the choice of model for the rCBV vs. rCBF relationship, a nonlinear deoxy-haemoglobin vs. rCBF relationship was predicted, and the implications for the BOLD response are discussed.

Influence of moderate and profound hyperventilation on cerebral blood flow, oxygenation and metabolism

OBJECTIVE

The aim of the present study was to examine the impact of moderate and profound hyperventilation on regional cerebral blood flow (rCBF), oxygenation and metabolism.

MATERIALS AND METHODS

Twelve anesthetized pigs were subjected to moderate (mHV) and profound (pHV) hyperventilation (target arterial pO(2): 30 and 20 mmHg, respectively) for 30 min each, after baseline normoventilation (BL) for 1 h. Local cerebral extracellular fluid (ECF) concentrations of glucose, lactate, pyruvate and glutamate as well as brain tissue oxygenation (p(ti)O(2)) were monitored using microdialysis and a Licox oxygen sensor, respectively. In nine pigs, regional cerebral blood flow (rCBF) was also continuously measured via a thermal diffusion system.

RESULTS

Both moderate and profound hyperventilation resulted in a significant decrease in rCBF (BL: 37.9+/-4.3 ml/100 g/min; mHV: 29.4+/-3.6 ml/100 g/min; pHV: 23.6+/-4.7 ml/100 g/min; p<0.05) and p(ti)O(2) (BL: 22.7+/-4.1 mmHg; mHV: 18.9+/-4.9 mmHg; pHV: 13.0+/-2.2 mmHg; p<0.05). A p(ti)O(2) decrease below the critical threshold of 10 mmHg was induced in three animals by moderate hyperventilation and in five animals by profound hyperventilation. Furthermore, significant increases in lactate (BL: 1.06+/-0.18 mmol/l; mHV: 1.36+/-0.20 mmol/l; pHV: 1.67+/-0.17 mmol/l; p<0.005), pyruvate (BL: 46.4+/-7.8 micromol/l; mHV: 58.0+/-10.3 micromol/l; pHV: 66.1+/-12.7 micromol/l; p<0.05), and lactate/glucose ratio were observed during hyperventilation. (Data are presented as mean+/-S.E.M.)

CONCLUSIONS

Both moderate and profound hyperventilation may result in insufficient regional oxygen supply and anaerobic metabolism, even in the uninjured brain. Therefore, the use of hyperventilation cannot be considered as a safe procedure and should either be avoided or used with extreme caution.

Ventilatory, cerebrovascular, and cardiovascular interactions in acute hypoxia: regulation by carbon dioxide

This study examined the effect of high, normal, and uncontrolled end-tidal Pco2 (PetCO2) on the ventilatory, peak cerebral blood flow velocity ( V̄p), and mean arterial blood pressure (MAP) responses to acute hypoxia. Nine healthy subjects undertook, in random order, three hypoxic protocols (end-tidal Po2 was held at eight steps between 300 and 45 Torr) in conditions of hypercapnia, isocapnia, or poikilocapnia (PetCO2 +7.5 Torr, +1.0 Torr, or uncontrolled, respectively). Transcranial Doppler ultrasound was used to measure V̄p in the middle cerebral artery. The slopes of the linear regressions of ventilation, V̄p, and MAP with arterial O2 saturation were significantly greater in hypercapnia than in both isocapnia and poikilocapnia ( P < 0.05). Strong, significant correlations were observed between ventilation, V̄p, and MAP with each PetCO2 condition. These data suggest that 1) a high acute hypoxic ventilatory response (AHVR) decreases the acute hypoxic cerebral blood flow responses during poikilocapnia hypoxia, due to hypocapnic-induced cerebral vasoconstriction; and 2) in hypercapnic hypoxia, a high AHVR is associated with a high acute hypoxic cerebral blood flow response, demonstrating a linkage of individual sensitivities of ventilation and cerebral blood flow to the interaction of PetCO2 and hypoxia. In summary, the between-individual variability in AHVR is shown to be firmly linked to the variability in V̄p and MAP responses to hypoxia. Individuals with a high AHVR are found also to have high V̄p and MAP responses to hypoxia.

Baroreflex-induced sympathetic activation does not alter cerebrovascular CO2 responsiveness in humans

We investigated the effect of baroreflex-induced sympathetic activation, produced by lower body negative pressure (LBNP) at -40 mmHg, on cerebrovascular responsiveness to hyper- and hypocapnia in healthy humans. Transcranial Doppler ultrasound was used to measure blood flow velocity (CFV) in the middle cerebral artery during variations in end-tidal carbon dioxide pressure (PET,CO2) of +10, +5, 0, -5, and -10 mmHg relative to eupnoea. The slopes of the linear relationships between PET,CO2 and CFV were computed separately for hyper- and hypocapnia during the LBNP and no-LBNP conditions. LBNP decreased pulse pressure, but did not change mean arterial pressure. LBNP evoked an increase in ventilation that resulted in a 9 +/- 2 mmHg decrease in PET,CO2, which was corrected by CO2 supplementation of the inspired air. LBNP did not affect cerebrovascular CO2 response slopes during steady-state hypercapnia (3.14 +/- 0.24 vs. 2.96 +/- 0.26 cm s-1 mmHg-1) or hypocapnia (1.31 +/- 0.18 vs. 1.32 +/- 0.19 cm s-1 mmHg-1), or the CFV responses to voluntary apnoea (+51 +/- 19 vs. +50 +/- 18 %). Thus, cerebrovascular CO2 responsiveness was not altered by baroreflex-induced sympathetic activation. Our data challenge the concept that sympathetic activation restrains cerebrovascular responses to alterations in CO2 pressure.

Effects of alterations in arterial CO2 tension on cerebral blood flow during acute intracranial hypertension in rats

Cerebrovascular reactivity to CO2 in clinical and experimental studies has been found to be impaired during increased intracranial pressure (ICP). However, from previous study results it has not been possible to estimate whether the impairment was caused by elevated ICP, or caused by decreased cerebral perfusion pressure (CPP). The current study was carried out in a group of unmanipulated control rats and in six investigation groups of six rats each: two groups with elevated ICP (30 and 50 mm Hg) and spontaneous arterial blood pressure (MABP), two groups with spontaneous ICP and arterial hypotension (77 and 64 mm Hg), and two groups with elevated ICP (30 and 50 mm Hg) and arterial hypertension (124 mm Hg). Intracranial hypertension was induced by continuous infusion of lactated Ringer's solution into the cisterna magna, arterial hypotension by controlled bleeding, and arterial hypertension by continuous administration of norepinephrine intravenously. Cerebral blood flow (CBF) was measured repetitively by the intraarterial 133Xe method at different levels of arterial PCO2. In each individual animal, CO2 reactivity was calculated from an exponential regression line obtained from the corresponding CBF/PaCO2 values. By plotting each individual value of CO2 reactivity against the corresponding CPP value from the seven investigation groups, CPP was significantly and directly related to CO2 reactivity of CBF (P < .001). No correlation was found by plotting CO2 reactivity values against the corresponding MABP values or the corresponding ICP values. Thus, the results show that CO2 reactivity is at least partially determined by CPP and that the impaired CO2 reactivity observed at intracranial hypertension and arterial hypotension may be caused by reduced CPP.

Transcranial doppler sonography and ambulatory blood pressure monitoring in patients with hypertension

To appraise the value of transcranial Doppler sonography (TCD) for assessment of hypertensive cerebrovascular damage, the relationship between ambulatory blood pressure (BP) and indices of cerebral circulation determined by TCD was investigated. Subjects were 55 inpatients with or without hypertension, including 13 patients with histories of cerebrovascular attacks. Mean flow velocity (MFV) in the middle cerebral artery was measured by TCD, then the cerebrovascular resistance index (CVRI; mean BP/MFV) and the Fourier PI1 (pulsatility index of the first Fourier harmonic of the flow-velocity waveform) were determined as indices of cerebrovascular resistance. CO2 reactivity of MFV was estimated as an index of cerebrovascular flow reserve. CVRI positively correlated with both daytime and nighttime BP as well as with age (p<0.01). Fourier PI1 positively correlated with nighttime BP and age (p<0.01). CO2 reactivity did not correlate with any of the ambulatory BP parameters, but negatively correlated with age (p<0.01). LV mass index significantly correlated with ambulatory BP parameters, CVRI, and Fourier PI1 but did not correlate with CO2 reactivity. Multiple regression analyses showed that nighttime systolic BP was a significant correlate for CVRI and Fourier PI1, but not for CO2 reactivity, and that history of cerebrovascular attack was significant for CVRI and CO2 reactivity. We conclude that cerebrovascular resistance determined by TCD accords with the results of ambulatory BP and LVMI, and thus could be successfully used to detect the early stage of hypertensive cerebrovascular change. Cerebrovascular flow reserve would be relatively preserved in hypertensive patients without cerebrovascular diseases.

Changes in human regional cerebral blood flow and cerebral blood volume during visual stimulation measured by positron emission tomography

The hemodynamic mechanism of increase in cerebral blood flow (CBF) during neural activation has not been elucidated in humans. In the current study, changes in both regional CBF and cerebral blood volume (CBV) during visual stimulation in humans were investigated. Cerebral blood flow and CBV were measured by positron emission tomography using H(2)(15)O and (11)CO, respectively, at rest and during 2-Hz and 8-Hz photic flicker stimulation in each of 10 subjects. Changes in CBF in the primary visual cortex were 16% +/- 16% and 68% +/- 20% for the visual stimulation of 2 Hz and 8 Hz, respectively. The changes in CBV were 10% +/- 13% and 21% +/- 5% for 2-Hz and 8-Hz stimulation, respectively. Significant differences between changes in CBF and CBV were observed for visual stimulation of 8 Hz. The relation between CBF and CBV values during rest and visual stimulation was CBV = 0.88CBF(0.30). This indicates that when the increase in CBF during neural activation is great, that increase is caused primarily by the increase in vascular blood velocity rather than by the increase in CBV. This observation is consistent with reported findings obtained during hypercapnia.

Assessment of cerebrovascular reactivity with functional magnetic resonance imaging: comparison of CO(2) and breath holding

Cerebral blood flow (CBF) and oxygenation changes following both a simple breath holding test (BHT) and a CO(2) challenge can be detected with functional magnetic resonance imaging techniques. The BHT has the advantage of not requiring a source of CO(2) and acetazolamide and therefore it can easily be performed during a routine MR examination. In this study we compared global hemodynamic changes induced by breath holding and CO(2) inhalation with blood oxygenation level dependent (BOLD) and CBF sensitized fMRI techniques. During each vascular challenge BOLD and CBF signals were determined simultaneously with a combined BOLD and flow-sensitive alternating inversion recovery (FAIR) pulse sequence. There was a good correlation between the global BOLD signal intensity changes during breath holding and CO(2) inhalation supporting the notion that the BHT is equivalent to CO(2) inhalation in evaluating the hemodynamic reserve capacity with BOLD fMRI. In contrast, there was no correlation between relative CBF changes during both vascular challenges, which was probably due to the reduced temporal resolution of the combined BOLD and FAIR pulse sequence.

Cerebral vasomotor reactivity at high altitude in humans

The purpose of this study was twofold: 1) to determine whether at high altitude cerebral blood flow (CBF) as assessed during CO2 inhalation and during hyperventilation in subjects with acute mountain sickness (AMS) was different from that in subjects without AMS and 2) to compare the CBF as assessed under similar conditions in Sherpas at high altitude and in subjects at sea level. Resting control values of blood flow velocity in the middle cerebral artery (VMCA), pulse oxygen saturation (SaO2), and transcutaneous PCO2 were measured at 4,243 m in 43 subjects without AMS, 17 subjects with AMS, 20 Sherpas, and 13 subjects at sea level. Responses of CO2 inhalation and hyperventilation on VMCA, SaO2, and transcutaneous PCO2 were measured, and the cerebral vasomotor reactivity (VMR = DeltaVMCA/PCO2) was calculated as the fractional change of VMCA per Torr change of PCO2, yielding a hypercapnic VMR and a hypocapnic VMR. AMS subjects showed a significantly higher resting control VMCA than did no-AMS subjects (74 +/- 22 and 56 +/- 14 cm/s, respectively; P < 0.001), and SaO2 was significantly lower (80 +/- 8 and 88 +/- 3%, respectively; P < 0.001). Resting control VMCA values in the sea-level group (60 +/- 15 cm/s), in the no-AMS group, and in Sherpas (59 +/- 13 cm/s) were not different. Hypercapnic VMR values in AMS subjects were 4.0 +/- 4.4, in no-AMS subjects were 5.5 +/- 4. 3, in Sherpas were 5.6 +/- 4.1, and in sea-level subjects were 5.6 +/- 2.5 (not significant). Hypocapnic VMR values were significantly higher in AMS subjects (5.9 +/- 1.5) compared with no-AMS subjects (4.8 +/- 1.4; P < 0.005) but were not significantly different between Sherpas (3.8 +/- 1.1) and the sea-level group (2.8 +/- 0.7). We conclude that AMS subjects have greater cerebral hemodynamic responses to hyperventilation, higher VMCA resting control values, and lower SaO2 compared with no-AMS subjects. Sherpas showed a cerebral hemodynamic pattern similar to that of normal subjects at sea level.

Hypercapnic cerebral blood flow in spontaneously hypertensive rats

OBJECTIVE

Hypercapnic cerebral vasodilation appears to be endothelium-dependent, as it involves nitric oxide and prostaglandins. Since chronic hypertension has been associated with impaired endothelial function, we designed a study to find out whether hypercapnic cerebral blood flow and its nitric oxide- and prostaglandin-sensitive component is reduced in spontaneously hypertensive rats (SHR) compared with normotensive controls.

METHODS

Cerebral blood flow was measured in enflurane-anesthetized SHR (n=53), Wistar-Kyoto (WKY, n=20) and Sprague-Dawley (n=50) rats using the hydrogen clearance method. Cerebral blood flow was measured during eucapnia and hypercapnia; it was also assessed after administering either nonisoform-selective or isoform-selective neuronal nitric oxide synthase inhibitors and during inhibition of prostaglandin production.

RESULTS

Hypercapnic cerebral blood flow did not differ among the strains. Nitric oxide synthase inhibition with intracortical N(G)-monomethyl-L-arginine reduced hypercapnic cerebral blood flow in SHR by 23+/-4% and in Sprague-Dawley rats by 23+/-7% without affecting eucapnic flow. Intraperitoneal administration of the inhibitor of neuronal nitric oxide synthase, 7-nitroindazole, reduced eucapnic flow by 18+/-5% in SHR and 27+/-5% in WKY rats, and hypercapnic flow by 48+/-3 and by 51+/-6%, respectively. Indomethacin produced a similar decrease in hypercapnic flow in Sprague-Dawley rats and SHR (49+/-5 and 62+/-4%, respectively).

CONCLUSION

Hypercapnic cerebral blood flow was not impaired in SHR. The contribution of nitric oxide- and prostaglandin-dependent vasodilation appeared to be intact Our results are consistent with the hypothesis that neuronal rather than endothelial production of nitric oxide may be responsible for maintaining hypercapnic cerebral vasodilation in SHR.

Assessment of cerebral pressure autoregulation in humans--a review of measurement methods

Assessment of cerebral autoregulation is an important adjunct to measurement of cerebral blood flow for diagnosis, monitoring or prognosis of cerebrovascular disease. The most common approach tests the effects of changes in mean arterial blood pressure on cerebral blood flow, known as pressure autoregulation. A 'gold standard' for this purpose is not available and the literature shows considerable disparity of methods and criteria. This is understandable because cerebral autoregulation is more a concept rather than a physically measurable entity. Static methods utilize steady-state values to test for changes in cerebral blood flow (or velocity) when mean arterial pressure is changed significantly. This is usually achieved with the use of drugs, shifts in blood volume or by observing spontaneous changes. The long time interval between measurements is a particular concern in many of the studies reviewed. Parallel changes in other critical variables, such as pCO2, haematocrit, brain activation and sympathetic tone, are rarely controlled for. Proposed indices of static autoregulation are based on changes in cerebrovascular resistance, on parameters of the linear regression of flow/velocity versus pressure changes, or only on the absolute changes in flow. The limitations of studies which assess patient groups rather than individual cases are highlighted. Newer methods of dynamic assessment are based on transient changes in cerebral blood flow (or velocity) induced by the deflation of thigh cuffs, Valsalva manoeuvres, tilting and induced or spontaneous oscillations in mean arterial blood pressure. Dynamic testing overcomes several limitations of static methods but it is not clear whether the two approaches are interchangeable. Classification of autoregulation performance using dynamic methods has been based on mathematical modelling, coherent averaging, transfer function analysis, crosscorrelation function or impulse response analysis. More research on reproducibility and inter-method comparisons is urgently needed, particularly involving the assessment of pressure autoregulation in individuals rather than patient groups.

Cerebral hemodynamics in young hypertensive subjects and effects of atenolol treatment

The aim of this study was to evaluate changes in cerebral hemodynamics in young patients with uncomplicated hypertension before and after effective antihypertensive treatment with a beta-blocker drug. Changes in mean flow velocity in the middle cerebral artery from normal condition to hypercapnia were evaluated by means of a transcranial Doppler in 42 hypertensive patients and 21 healthy subjects comparable for age and sex distribution. We obtained hypercapnia with breath-holding and evaluated cerebrovascular reactivity with the breath-holding index (BHI). After a baseline evaluation (time 0), patients were randomly assigned to a placebo (group 1) or atenolol (group 2) therapy. The evaluation was repeated after 30 (time 1) and 60 (time 2) days of treatment. Before treatment, hypertensive patients had significantly lower BHI values (0.96 +/- 0.1 group 1 and 0.85 +/- 0.3 group 2) than controls (1.69 +/- 0.4) (P < 0.0001). During treatment, mean blood pressure significantly decreased in group 2 patients. In the same group, BHI values significantly increased with respect to the pre-treatment evaluation: 1.39 +/- 0.2 at time 1 and 1.44 +/- 0.2 at time 2 (P < 0.0001). On the contrary, mean blood pressure and BHI values remained unchanged in the placebo group. Furthermore, BHI values were significantly higher in group 2 than in group 1 patients at times 1 (P < 0.001) and 2 (P < 0.0001). These findings suggest that hypertension causes reduced capability of cerebral vessels to adapt to functional changes. This condition, which is reversible after treatment, could be implicated in the increased susceptibility to ischemic stroke in hypertension.

Effects of sevoflurane on intracranial pressure, cerebral blood flow and cerebral metabolism. A dose-response study in patients subjected to craniotomy for cerebral tumours

BACKGROUND

Studies concerning the cerebrovascular effects of sevoflurane in patients with space-occupying lesions are few. This study was carried out as a dose-response study comparing the effects of increasing sevoflurane concentration (1.5% (0.7 MAC) to 2.5% (1.3 MAC)) on cerebral blood flow (CBF), intracranial pressure (ICP), cerebrovascular resistance (CVR), metabolic rate of oxygen (CMRO2) and CO2-reactivity in patients subjected to craniotomy for supratentorial brain tumours.

METHODS

Anaesthesia was induced with propofol/fentanyl/atracurium and maintained with 1.5% sevoflurane in air/oxygen at normocapnia. Blood pressure was maintained constant by ephedrine. In group 1 (n = 10), the patients received continuously 1.5% sevoflurane. Subdural ICP, CBF and CMRO2 were measured twice at 30-min intervals. In group 2 (n = 10), sevoflurane concentration was increased from 1.5% to 2.5% after CBF1. CBF2 was measured after 20 min during 2.5% sevoflurane. Finally, CO2-reactivity was studied in both groups.

RESULTS

In group 1, no time-dependent alterations in CBF, CVR, ICP and CMRO2 were found. In group 2, an increase in sevoflurane from 1.5% to 2.5% resulted in an increase in CBF from 29 +/- 10 to 34 +/- 12 ml 100 g-1 min-1 and a decrease in CVR from 2.7 +/- 0.9 to 2.3 +/- 1.2 mmHg ml-1 min 100 g (P < 0.05), while ICP and CMRO2 were unchanged. CO2-reactivity was maintained at 1.5% and 2.5% sevoflurane.

CONCLUSION

Sevoflurane is a cerebral vasodilator in patients with cerebral tumours. Sevoflurane increases CBF and decreases CVR in a dose-dependent manner. CO2-reactivity is preserved during 1.5% and 2.5% sevoflurane.

Cerebrovascular reactivity in hypertensive patients: a transcranial Doppler study

PURPOSE

We studied the usefulness of transcranial Doppler sonography for assessing changes in vasoreactivity in patients with hypertension and the hemodynamic consequences of hypertension.

METHODS

The study group comprised 25 patients with chronic severe hypertension and 25 age- and sex-matched healthy subjects. Cerebrovascular reserve capacity was assessed by transcranial Doppler recording of the blood flow velocity in both middle cerebral arteries before and 5, 10, 15, and 20 minutes after intravenous injection of 1 g of acetazolamide (Diamox). Blood pressure, blood gases, and other blood parameters were also measured before and after acetazolamide injection. The sizes of the left atrium, left ventricle, and aortic root were measured by echocardiography and correlated with the vasoreactivity after acetazolamide injection.

RESULTS

After acetazolamide injection, no significant changes in blood pressure were observed in either group. The mean blood flow velocity in the middle cerebral arteries of hypertensive patients (60.8 +/- 2.6 cm/sec) was not significantly different from that of controls (58.8 +/- 1.9 cm/sec) before acetazolamide injection. Ten minutes after acetazolamide injection, the percentage change in blood flow velocity was significantly lower in the hypertensive group (36.2 +/- 4.5%) than in the controls (52.6 +/- 3.7%). A significant negative correlation (p < 0.05) between decreased vasoreactivity and increased size of the left atrium and aortic root was observed.

CONCLUSIONS

Vasoreactivity decreases in hypertensive patients without neurologic deficits or computed tomography abnormalities. Enlargement of the left atrium correlates well with the severity of the impairment in vasoreactivity. Transcranial Doppler sonography can be a sensitive tool in the investigation of vascular impairment caused by hypertension and in the follow-up of hypertensive patients.

Estimation of cerebral blood flow at bedside by continuous jugular thermodilution

The Kety-Schmidt technique can be regarded as the reference method for the measurement of cerebral blood flow (CBF). However, the method is somewhat cumbersome for routine use in the intensive care unit (ICU) at the beside. The continuous thermodilution technique developed many years ago for the measurement of coronary sinus blood flow can be applied for the measurement of jugular blood flow (JBF). However, the measurement of JBF by thermodilution has never been validated using the Kety-Schmidt reference method. We first validate the continuous thermodilution in vitro by comparison with a volumetric flow. The thermodilution method is accurate for flows between 50 and 900 ml min-1 with a mean difference volumetric-thermodilution flow of -1 +/- 18 ml min-1 (mean +/- SD), and precise with a coefficient of variability ranging between 1.21% and 2.50%. In vivo accuracy was assessed by comparing in 15 comatose patients CBF measured using the Kety-Schmidt (CBFKS) method and estimated from JBF measured by thermodilution (CBFTH) at four levels of arterial PaCO2 (25, 30, 35, and 40 mm Hg). The mean difference CBFKS-CBFTH is -0.9 +/- 3.6 ml min-1 100 g-1. In vivo precision of the method was good, with a coefficient of variability of 4.1% in mean. We conclude that jugular continuous thermodilution technique is a reliable method for estimating CBF at the bedside. This technique allows repeated measurements jugular bulb blood sampling for brain metabolic studies.

Cerebral blood flow and vascular response to hypercapnia in hypertensive patients with leukoaraiosis

Both arteriosclerosis and leukoaraiosis have a close relationship with hypertension, but the relationship between cerebral hemodynamics and leukoaraiosis in hypertensive patients has not been fully examined. To clarify this issue, we measured the regional cerebral blood flow (rCBF) and cerebrovascular response to hypercapnia in hypertensive patients with various degrees of leukoaraiosis. The subjects consisted of 7 normotensive normal controls and 17 hypertensive patients. The hypertensive patients were divided into three groups according to the severity of white matter lesions (leukoaraiosis) on MRI and the presence of dementia, namely, (1) negative or mild leukoaraiosis without dementia, (2) moderate to severe leukoaraiosis without dementia and (3) severe leukoaraiosis with dementia. Both the rCBF and the cerebrovascular response to hypercapnia were measured by the O-15 H2O bolus-injection method and positron emission tomography. The rCBF in hypertensive patients without dementia did not decrease when compared with the normotensive controls, but the rCBF in hypertensive patients with dementia markedly decreased in the cerebral cortices and white matter. On the other hand, the cerebrovascular response to hypercapnia declined with the severity of leukoaraiosis, and it decreased most severely in patients with severe leukoaraiosis and dementia. Our results indicate that the reduction in the cerebral hemodynamic reserve capacity has a close relationship with the severity of leukoaraiosis in hypertensive patients, although the rCBF is maintained in hypertensive patients without dementia, and suggest that arteriosclerotic change reduces cerebrovascular CO2 response and causes a leukoaraiosis in hypertensive patients.

CO2 response for the brain stem artery blood flow velocity in man

We examined changes in the blood flow velocity of brain stem artery (BSA) and middle cerebral artery (MCA) in response to hypercapnic, normocapnic and hypocapnic hyperventilation in seven awake subjects with a transcranial Doppler to determine if there are differences in blood flow control in regional brain perfused by these respective arteries, and to separate the effects of CO2 and ventilation itself on blood flow velocity during CO2 loading. During hypercapnic hyperventilation, BSA flow velocity increased linearly with an increase in end-tidal partial pressure of CO2 (PETCO2). During hypocapnic hyperventilation, BSA flow velocity decreased linearly with decrease in PETCO2, but did not change during normocapnic hyperventilation. The mean CO2 reactivity of BSA was 2.8%/mmHg. The responses of MCA to these hyperventilations and CO2 reactivity were similar to those of BSA. These findings suggest that CO2 rather than ventilation per se is the important stimulus to changes in brain blood flow velocity and that the CO2 responses of brain arteries are not affected by differences in vascular beds.

Evidence for adaptive autoregulatory displacement in hypotensive cortical territories adjacent to arteriovenous malformations. Columbia University AVM Study Project

We hypothesized that chronic hypotension in normal vascular territories fed by arteriovenous malformation pedicles may reset the lower limit of autoregulation and allow flow to remain constant over a lower pressure range. We studied the effect of increasing systemic mean arterial pressure (SMAP) with phenylephrine on cerebral blood flow using a novel technique. Fourteen patients undergoing 15 procedures were studied before endovascular embolization of arteriovenous malformations under neuroleptic conscious sedation. Mean pressures were transduced via a 1.5-F intracranial microcatheter, which was passed under fluoroscopic guidance into the target feeding artery. The microcatheter was positioned (unwedged) at a point that was relatively hypotensive to systemic pressure but that irrigated normal cortex on angiography; feeding mean arterial pressure (FMAP) and SMAP were recorded. A bolus of 133Xe in saline was injected into the microcatheter, and washout was recorded for 3 minutes by a scintillation detector placed over the vascular territory of the injected pedicle. SMAP was then increased approximately 25 mm Hg by phenylephrine infusion, a second bolus was given, and washout was recorded. After exclusion of the shunt spike, initial slope was calculated. The SMAP (mean +/- standard error) increased from 65 +/- 3 to 89 +/- 2 mm Hg (P < 0.0001), and FMAP increased from 46 +/- 3 to 63 +/- 3 mm Hg (P < 0.0001); cerebral blood flow did not change (40 +/- 2 to 40 +/- 2 ml/100 g per min, P = 0.9199). Dividing the cases on the basis of the baseline FMAP into a "severe" hypotensive group (FMAP = 38 +/- 2; n = 7) and a "moderate" hypotensive group (FMAP = 54 +/- 3; n = 8), cerebral blood flow did not change in either group during phenylephrine challenge. Chronic hypotension does not necessarily result in "vasomotor paralysis" with loss of the ability to vasoconstrict to acute increases in perfusion pressure. Instead, it appears to displace adaptively the lower limit of autoregulation in affected vascular territories by a shift of the autoregulatory curve to the left, conceptually analogous to the adaptive displacement seen with chronic hypertension and its treatment.

Is the mechanism of prefrontal hypofunction in depression the same as in schizophrenia? Regional cerebral blood flow during cognitive activation

To test the hypothesis that depressed and schizophrenic patients have a common pathophysiological mechanism for hypofunction of the prefrontal cortex ('hypofrontality'), we measured regional cortical blood flow (rCBF) in ten depressed patients, ten patients with schizophrenia, and 20 age- and sex-matched normal controls. Blood flow was measured during three different cognitive conditions: a resting state, a simple number-matching sensorimotor control task, and the Wisconsin Card Sorting test (WCS). The schizophrenic patients had lower prefrontal rCBF during the WCS. There were no differences in global or regional flow between the depressed patients and the normal subjects during any testing condition. Analysis of rCBF lateralisation showed that during the WCS normal subjects had relatively more left parietal blood flow than depressed patients, who had more right parietal blood flow. Since the testing condition that has most consistently revealed hypofrontality in schizophrenia (i.e. the WCS) was not associated with abnormal rCBF in the depressed patients, these data suggest that the pathophysiological mechanisms underlying prefrontal hypofunction in depression and schizophrenia are different.

Temporal lobe CO2 vasoreactivity in patients with complex partial seizures

The topography of CO2 vasoreactivity during hyperventilation in 8 patients with complex partial seizure (CPS) was visualized using the regional cerebral blood flow (rCBF) as measured by H(2)15O-PET (positron emission tomography) and compared with that of 10 normal volunteers. In the normal volunteers, the vascular response to CO2 (VrCO2 = delta CBF%/delta PaCO2) in the temporal lobe was 2.46 +/- 0.56 (%/mmHg). In the patients with CPS, VrCO2 in the temporal lobe of the affected side was 2.08 +/- 0.40 (%/mmHg), while VrCO2 on the contralateral side was 2.30 +/- 0.46 (%/mmHg). There was a significant difference in VrCO2 between the affected side of the temporal lobes and the temporal lobes of the normal volunteers. Furthermore, there was a tendency for VrCO2 to be lower in the affected than in the contralateral side of the temporal lobe in patients with CPS. As CO2 is the main regulator of CBF, this impaired vasoreactivity may reflect the brain dysfunction in the seizure focus and adjacent areas.

Carbon dioxide reactivity in the evaluation of cerebral ischemia

Carbon dioxide reactivity, as measured by transcranial Doppler, has been determined in a group of patients with carotid artery disease and compared to a control group. CO2 reactivity was readily evaluated using transcranial Doppler by having the patients breathe 5% CO2 via a rebreathing circuit. There were significant differences (P less than 0.01) between the symptomatic patients and asymptomatic controls, as well as between symptomatic and asymptomatic hemispheres (P less than 0.05) in the patients with carotid artery disease. Five patients had revascularization procedures with all showing improved CO2 reactivity in the symptomatic and asymptomatic hemispheres. The improvement occurred early in patients after carotid endarterectomy. Two patients demonstrated improved reactivity at 6 to 13 months after extracranial-to-intracranial bypass. Transcranial Doppler has proven to be an easily performed and repeatable method of evaluating CO2 reactivity and its response to treatment.