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

Hypercapnia elicits differential vascular and blood flow responses in the cerebral circulation and active skeletal muscles in exercising humans

The purpose of this study was to investigate the effects of a rise in arterial carbon dioxide pressure (PaCO₂) on vascular and blood flow responses in the cerebral circulation and active skeletal muscles during dynamic exercise in humans. Thirteen healthy young adults (three women) participated in hypercapnia and normocapnia trials. In both trials, participants performed a two‐legged dynamic knee extension exercise at a constant workload that increased heart rate to roughly 100 beats min⁻¹. In the hypercapnia trial, participants performed the exercise with spontaneous breathing while end‐tidal carbon dioxide pressure (P_ETCO₂), an index of PaCO₂, was held at 60 mmHg by inhaling hypercapnic gas (O₂: 20.3 ± 0.1%; CO₂: 6.0 ± 0.5%). In the normocapnia trial, minute ventilation during exercise was matched to the value in the hypercapnia trial by performing voluntary hyperventilation with P_ETCO₂ clamped at baseline level (i.e., 40–45 mmHg) through inhalation of mildly hypercapnic gas (O₂: 20.6 ± 0.1%; CO₂: 2.7 ± 1.0%). Middle cerebral artery mean blood velocity and the cerebral vascular conductance index were higher in the hypercapnia trial than in the normocapnia trial. By contrast, vascular conductance in the exercising leg was lower in the hypercapnia trial than in the normocapnia trial. Blood flow to the exercising leg did not differ between the two trials. These results demonstrate that hypercapnia‐induced vasomotion in active skeletal muscles is opposite to that in the cerebral circulation. These differential vascular responses may cause a preferential rise in cerebral blood flow.

Relationship Between Cerebral Oxygenation and Skin Blood Flow at the Frontal Lobe during Progressive Hypoxia: Impact of Acute Hypotension

We investigated the relationship between cerebral oxygenation (COX) and skin blood flow (SkBF) at the left frontal lobes of 10 healthy young men during progressive hypoxia (∼ -1 h at each of 21%, 18%, 15%, and 12% of inspired oxygen [FiO₂]). Acute hypotension was manipulated by a thigh-cuff-release technique, where a pressure of 220 mmHg was applied at both thigh muscles for 3 min and the cuff was immediately released to induce acute hypotension. While the resting baseline for COX before the thigh-cuff release manipulation decreased gradually with the reduction of FiO₂ (P < 0.05), the resting baseline for SkBF, mean arterial blood pressure (MAP), and cutaneous vascular conductance (CVC) were unaffected by FiO₂ (P > 0.05). The acute hypotension that was induced by the thigh-cuff release decreased COX, SkBF, MAP, and CVC; thereafter, these values recovered toward their baseline values. During the hypotension phase, while the time to the nadir values for COX slowed progressively with reductions in FiO₂ (P < 0.05), those for SkBF, MAP, and CVC were unaffected by FiO₂ (P > 0.05). These results suggest that COX may not be associated with SkBF for the protocol or with the subjects in the present study.

Lower Body Negative Pressure: Physiological Effects, Applications, and Implementation

This review presents lower body negative pressure (LBNP) as a unique tool to investigate the physiology of integrated systemic compensatory responses to altered hemodynamic patterns during conditions of central hypovolemia in humans. An early review published in Physiological Reviews over 40 yr ago (Wolthuis et al. Physiol Rev 54: 566-595, 1974) focused on the use of LBNP as a tool to study effects of central hypovolemia, while more than a decade ago a review appeared that focused on LBNP as a model of hemorrhagic shock (Cooke et al. J Appl Physiol (1985) 96: 1249-1261, 2004). Since then there has been a great deal of new research that has applied LBNP to investigate complex physiological responses to a variety of challenges including orthostasis, hemorrhage, and other important stressors seen in humans such as microgravity encountered during spaceflight. The LBNP stimulus has provided novel insights into the physiology underlying areas such as intolerance to reduced central blood volume, sex differences concerning blood pressure regulation, autonomic dysfunctions, adaptations to exercise training, and effects of space flight. Furthermore, approaching cardiovascular assessment using prediction models for orthostatic capacity in healthy populations, derived from LBNP tolerance protocols, has provided important insights into the mechanisms of orthostatic hypotension and central hypovolemia, especially in some patient populations as well as in healthy subjects. This review also presents a concise discussion of mathematical modeling regarding compensatory responses induced by LBNP. Given the diverse applications of LBNP, it is to be expected that new and innovative applications of LBNP will be developed to explore the complex physiological mechanisms that underline health and disease.

Independent and interactive effects of incremental heat strain, orthostatic stress, and mild hypohydration on cerebral perfusion

The purpose of this study was to identify the dose-dependent effects of heat strain and orthostasis [via lower body negative pressure (LBNP)], with and without mild hypohydration, on systemic function and cerebral perfusion. Eleven men (means ± SD: 27 ± 7 y; body mass 77 ± 6 kg), resting supine in a water-perfused suit, underwent progressive passive heating [0.5°C increments in core temperature (T_c; esophageal to +2.0°C)] while euhydrated (EUH) or hypohydrated (HYPO; 1.5-2% body mass deficit). At each thermal state, mean cerebral artery blood velocity (MCAv_mean; transcranial Doppler), partial pressure of end-tidal carbon dioxide ([Formula: see text]), heart rate (HR) and mean arterial blood pressure (MAP; photoplethysmography) were measured continuously during LBNP (0, -15, -30, and -45 mmHg). Four subjects became intolerant before +2.0°C T_c, unrelated to hydration status. Without LBNP, decreases in [Formula: see text] accounted fully for reductions in MCAv_mean across all T_c. With LBNP at heat tolerance (+1.5 or +2.0°C), [Formula: see text] accounted for 69 ± 25% of the change in MCAv_mean. The HYPO condition did not affect MCAv_mean or any cardiovascular variables during combined LBNP and passive heat stress (all P > 0.13). These findings indicate that hypocapnia accounted fully for the reduction in MCAv_mean when passively heat stressed in the absence of LBNP and for two- thirds of the reduction when at heat tolerance combined with LBNP. Furthermore, when elevations in T_c are matched, mild hypohydration does not influence cerebrovascular or cardiovascular responses to LBNP, even when stressed by a combination of hyperthermia and LBNP.

Cerebral blood flow regulation in women across menstrual phase: differential contribution of cyclooxygenase to basal, hypoxic, and hypercapnic vascular tone

In healthy young women, basal cerebral blood flow (CBF) and cerebrovascular reactivity may change across the menstrual cycle, but mechanisms remain untested. When compared with the early follicular phase of the menstrual cycle, we hypothesized women in late follicular phase would exhibit: 1) greater basal CBF, 2) greater hypercapnic increases in CBF, 3) greater hypoxic increases in CBF, and 4) increased cyclooxygenase (COX) signaling. We measured middle cerebral artery velocity (MCAv, transcranial Doppler ultrasound) in 11 healthy women (23 ± 1 yr) during rest, hypoxia, and hypercapnia. Subjects completed four visits: two during the early follicular (∼day 3) and two during the late follicular (∼day 14) phases of the menstrual cycle, with and without COX inhibition (oral indomethacin). Isocapnic hypoxia elicited an SPO2 = 90% and SPO2 = 80% for 5 min each. Separately, hypercapnia increased end-tidal CO2 10 mmHg above baseline. Cerebral vascular conductance index (CVCi = MCAv/MABP·100, where MABP is mean arterial blood pressure) was calculated and a positive change reflected vasodilation (ΔCVCi). Basal CVCi was greater in the late follicular phase (P < 0.001). Indomethacin decreased basal CVCi (∼37%) and abolished the phase difference (P < 0.001). Hypoxic ΔCVCi was similar between phases and unaffected by indomethacin. Hypercapnic ΔCVCi was similar between phases, and indomethacin decreased hypercapnic ΔCVCi (∼68%; P < 0.001) similarly between phases. In summary, while neither hypercapnic nor hypoxic vasodilation is altered by menstrual phase, increased basal CBF in the late follicular phase is fully explained by a greater contribution of COX. These data provide new mechanistic insight into anterior CBF regulation across menstrual phases and contribute to our understanding of CBF regulation in women.

Additive effect of simultaneously varying respiratory frequency and tidal volume on respiratory sinus arrhythmia

Our aims were to assess, in healthy young females and males, the effects of the linear joint variation of respiratory frequency (RF) and tidal volume (VT) on the logarithmic transformation of high-frequency power of RR intervals (lnHF). ECG and VT were recorded from 18 females and 20 males during three visually guided 30-s breathing maneuvers: linearly increasing RF (RFLI) at constant VT; linearly increasing VT (VTLI) followed by decreasing VT (VTLD) at fixed RF, and RFLI and VTLI-VTLD combined. VT of females was 20% smaller. Instantaneous RF and lnHF were computed from the time-frequency distributions of respiratory series and RR intervals. LnHF-RF and lnHF-VT relations were similar between genders. LnHF and RR intervals control-maneuver differences during combined maneuver were approximately equal to the sum of those of the independent maneuvers. LnHF-RFLI relation showed strong negative correlations in separated and combined conditions, with steeper slope in the latter (p < 0.001). LnHF-VTLI and lnHF-VTLD relations presented, in the independent maneuvers, three combinations of slopes of different sign, all with hysteresis, and in the combined maneuver, strong correlations with negative slope for VTLI and positive slope for VTLD, steeper (p < 0.001) and with greater hysteresis (p < 0.001) than the independent ones. LnHF responses to our fast, non-fatiguing and non-steady-state breathing maneuvers are: similar between genders; consistent attenuation due to RFLI, whether applied alone or combined; ambiguous and with hysteresis to independent VTLI-VTLD variations; systematic greater attenuation during RFLI combined with VTLI-VTLD, equal to the sum of the independent effects, indicating that there is no interference between them.

The effect of hypercapnia on static cerebral autoregulation

Hypercapnia impairs cerebrovascular control during rapid changes in blood pressure (BP); however, data concerning the effect of hypercapnia on steady state, nonpharmacological increases in BP is scarce. We recruited fifteen healthy volunteers (mean ± SD: age, 28 ± 6 years; body mass, 77 ± 12 kg) to assess the effect of hypercapnia on cerebrovascular control during steady-state elevations in mean arterial BP (MAP), induced via lower body positive pressure (LBPP). Following 20 min of supine rest, participants completed 5 min of eucapnic 20 and 40 mm Hg LBPP (order randomized) followed by 5 min of hypercapnia (5% CO2 in air) with and without LBPP (order randomized), and each stage was separated by ≥5 min to allow for recovery. Middle cerebral artery blood velocity (MCAv), BP, partial pressure of end-tidal carbon dioxide (PETCO2) and heart rate were recorded and presented as the change from the preceding baseline. No difference in MCAv was apparent between eupcapnic baseline and LBPPs (grouped mean 65 ± 11 cm·s(-1), all P > 0.05), despite the increased MAP with LBPP (Δ6 ± 5 and Δ8 ± 3 mm Hg for 20 and 40 mm Hg, respectively, both P < 0.001 vs. baseline). Conversely, MCAv during the hypercapnic +40 mm Hg stage (Δ31 ± 13 cm·s(-1)) was greater than hypercapnia alone (Δ25 ± 11 cm·s(-1), P = 0.026), due to an increased MAP (Δ14 ± 7 mm Hg, P < 0.001 vs. hypercapnia alone and P = 0.026 vs. hypercapnia +20 mm Hg). As cardiac output and PETCO2 were similar across all hypercapnic stages (all P > 0.05), our findings indicate that hypercapnia impairs static autoregulation, such that higher blood pressures are translated into the cerebral circulation.

Autonomic dysfunction affects dynamic cerebral autoregulation during Valsalva maneuver: comparison between healthy and autonomic dysfunction subjects

The role of autonomic nervous system (ANS) in adapting cerebral blood flow (CBF) to arterial blood pressure (ABP) fluctuations [cerebral autoregulation (CA)] is still controversial. We aimed to study the repercussion of autonomic failure (AF) on dynamic CA during the Valsalva maneuver (VM). Eight AF subjects with familial amyloidotic polineuropahty (FAP) were compared with eight healthy controls. ABP and CBF velocity (CBFV) were measured continuously with Finapres and transcranial Doppler, respectively. Cerebrovascular response was evaluated by cerebrovascular resistance index (CVRi), critical closing pressure (CrCP), and resistance-area product (RAP) changes. Dynamic CA was derived from continuous estimates of autoregulatory index (ARI) [ARI(t)]. During phase II of VM, FAP subjects showed a more pronounced decrease in normalized CBFV (78 ± 19 and 111 ± 16%; P = 0.002), ABP (78 ± 19 and 124 ± 12%; P = 0.0003), and RAP (67 ± 17 and 89 ± 17%; P = 0.019) compared with controls. CrCP and CVRi increased similarly in both groups during strain. ARI(t) showed a biphasic variation in controls with initial increase followed by a decrease during phase II but in FAP this response was blunted (5.4 ± 3.0 and 2.0 ± 2.9; P = 0.033). Our data suggest that dynamic cerebral autoregulatory response is a time-varying phenomena during VM and that it is disturbed by autonomic dysfunction. This study also emphasizes the fact that RAP + CrCP model allowed additional insights into understanding of cerebral hemodynamics, showing a higher vasodilatory response expressed by RAP in AF and an equal CrCP response in both groups during the increased intracranial and intrathoracic pressure, while classical CVRi paradoxically suggests a cerebral vasoconstriction.

Regional redistribution of blood flow in the external and internal carotid arteries during acute hypotension

The present study examined to what extent an acute bout of hypotension influences blood flow in the external carotid artery (ECA) and the corresponding implications for blood flow regulation in the internal carotid artery (ICA). Nine healthy male participants were subjected to an abrupt decrease in arterial pressure via the thigh-cuff inflation-deflation technique. Duplex ultrasound was employed to measure beat-to-beat ECA and ICA blood flow. Compared with the baseline normotensive control, acute hypotension resulted in a heterogeneous blood flow response. ICA blood flow initially decreased following cuff release and then returned quickly to baseline levels. In contrast, the reduction in ECA blood flow persisted for 30 s following cuff release. Thus, the contribution of common carotid artery blood flow to the ECA circulation decreased during acute hypotension (−10 ± 4%, P < 0.001). This finding suggests that a preserved reduction in ECA blood flow, as well as dynamic cerebral autoregulation likely prevent a further decrease in intracranial blood flow during acute hypotension. The peripheral vasculature of the ECA may, thus, be considered an important vascular bed for intracranial cerebral blood flow regulation.

Cerebral vasoreactivity: impact of heat stress and lower body negative pressure

OBJECTIVE

Cerebrovascular reactivity represents the capacity of the cerebral circulation to raise blood flow in the face of increased demand, and may be reduced in some clinical and physiological conditions. We tested the hypothesis that the hypercapnia-induced increase in cerebral perfusion is attenuated during heat stress (HS) compared to normothermia (NT), and this response is further reduced during the combined challenges of HS and lower body negative pressure (LBNP).

METHODS

Ten healthy individuals (9 men) undertook rebreathing-induced hypercapnia during NT, HS, and HS + 20 mmHg LBNP (HSLBNP), while cerebral perfusion was indexed from middle cerebral artery blood velocity (MCA V mean). Cerebrovascular responses were calculated from the slope of the change in MCA V mean and cerebral vascular conductance (CVCi) relative to the increase in end tidal carbon dioxide ([Formula: see text]) during rebreathing.

RESULTS

MCA V mean was similar in HS (55 ± 19 cm s(-1)) and HSLBNP (52 ± 16 cm s(-1)), and both values were reduced relative to NT (66 ± 20 cm s(-1)), yet the rise in MCA V mean per Torr increase in [Formula: see text] during rebreathing was similar in each condition (NT: 2.5 ± 0.6 cm s(-1) Torr(-1); HS: 2.4 ± 0.8 cm s(-1) Torr(-1); HSLBNP: 2.1 ± 1.1 cm s(-1) Torr(-1)). Likewise, the rate of increase in CVCi was not different between conditions (NT: 2.1 ± 0.65 cm s(-1 )mmHg(-1)100 Torr(-1); HS: 2.4 ± 0.8 cm s(-1) mmHg(-1) 100 Torr(-1); HSLBNP: 2.0 ± 1.0 cm s(-1) mmHg(-1) 100 Torr(-1)).

INTERPRETATIONS

These data indicate that cerebrovascular reactivity is not compromised during whole-body heat stress alone or when combined with mild orthostatic stress relative to normothermic conditions.

Influence of CO2 on neurovascular coupling: interaction with dynamic cerebral autoregulation and cerebrovascular reactivity

PaCO₂ affects cerebral blood flow (CBF) and its regulatory mechanisms, but the interaction between neurovascular coupling (NVC), cerebral autoregulation (CA), and cerebrovascular reactivity to CO₂ (CVR), in response to hypercapnia, is not known. Recordings of cerebral blood flow velocity (CBFv), blood pressure (BP), heart rate, and end‐tidal CO₂ (EtCO₂) were performed in 18 subjects during normocapnia and 5% CO₂ inhalation while performing a passive motor paradigm. Together with BP and EtCO₂, a gate signal to represent the effect of stimulation was used as input to a multivariate autoregressive‐moving average model to calculate their separate effects on CBFv. Hypercapnia led to a depression of dynamic CA at rest and during stimulation in both hemispheres (P <0.02) as well as impairment of the NVC response, particularly in the ipsilateral hemisphere (P <0.01). Neither hypercapnia nor the passive motor stimulation influenced CVR. Dynamic CA was not influenced by the motor paradigm during normocapnia. The CBFv step responses to each individual input (BP, EtCO₂, stimulation) allowed identification of the influences of hypercapnia and neuromotor stimulation on CA, CVR, and NVC, which have not been previously described, and also confirmed the depressing effects of hypercapnia on CA and NVC. The stability of CVR during these maneuvers and the lack of influence of stimulation on dynamic CA are novel findings which deserve further investigation. Dynamic multivariate modeling can identify the complex interplay between different CBF regulatory mechanisms and should be recommended for studies involving similar interactions, such as the effects of exercise or posture on cerebral hemodynamics.

The influence of hypercapnia on dynamic cerebral autoregulation (CA), CO₂ vasoreactivity (CVR), and neurovascular coupling (NVC) was described based on a single recording during motor stimulation coupled to a new multivariate modeling approach. Hypercapnia led to a depression of CA at rest and during stimulation in both hemispheres as well as impairment of the NVC response. Neither hypercapnia nor the passive motor stimulation influenced CVR. Dynamic CA was not influenced by the motor paradigm during normocapnia. The stability of CVR during these maneuvers and the lack of influence of stimulation on dynamic CA are novel findings which deserve further investigation.

Integrative regulation of human brain blood flow

Herein, we review mechanisms regulating cerebral blood flow (CBF), with specific focus on humans. We revisit important concepts from the older literature and describe the interaction of various mechanisms of cerebrovascular control. We amalgamate this broad scope of information into a brief review, rather than detailing any one mechanism or area of research. The relationship between regulatory mechanisms is emphasized, but the following three broad categories of control are explicated: (1) the effect of blood gases and neuronal metabolism on CBF; (2) buffering of CBF with changes in blood pressure, termed cerebral autoregulation; and (3) the role of the autonomic nervous system in CBF regulation. With respect to these control mechanisms, we provide evidence against several canonized paradigms of CBF control. Specifically, we corroborate the following four key theses: (1) that cerebral autoregulation does not maintain constant perfusion through a mean arterial pressure range of 60-150 mmHg; (2) that there is important stimulatory synergism and regulatory interdependence of arterial blood gases and blood pressure on CBF regulation; (3) that cerebral autoregulation and cerebrovascular sensitivity to changes in arterial blood gases are not modulated solely at the pial arterioles; and (4) that neurogenic control of the cerebral vasculature is an important player in autoregulatory function and, crucially, acts to buffer surges in perfusion pressure. Finally, we summarize the state of our knowledge with respect to these areas, outline important gaps in the literature and suggest avenues for future research.

Parasympathetic reflex vasodilation in the cerebral hemodynamics of rats

We investigated the role of parasympathetic reflex vasodilation in the regulation of the cerebral hemodynamics, and whether GABAA receptors modulate the response. We examined the effects of activation of the parasympathetic fibers through trigeminal afferent inputs on blood flow in the internal carotid artery (ICABF) and the cerebral blood vessels (rCBF) in parietal cortex in urethane-anesthetized rats. Electrical stimulation of the central cut end of the lingual nerve (LN) elicited intensity- and frequency-dependent increases in ICABF that were independent of changes in external carotid artery blood flow. Increases in ICABF were elicited by LN stimulation regardless of the presence or absence of sympathetic innervation. The ICABF increases evoked by LN stimulation were almost abolished by the intravenous administration of hexamethonium (10 mg kg(-1)) and were reduced significantly by atropine administration (0.1 mg kg(-1)). Although the LN stimulation alone had no significant effect on rCBF, LN stimulation in combination with a blocker of the GABAA receptor pentylenetetrazole increased the rCBF markedly. This increase in rCBF was reduced significantly by the administration of hexamethonium and atropine. These observations indicate that the increases in both ICABF and rCBF are evoked by parasympathetic activation via the trigeminal-mediated reflex. The rCBF increase evoked by LN stimulation is thought to be limited by the GABAA receptors in the central nervous system. These results suggest that the parasympathetic reflex vasodilation and its modulation mediated by GABA receptors within synaptic transmission in the brainstem are involved in the regulation of the cerebral hemodynamics during trigeminal afferent inputs.

Cerebral hemodynamics and systemic endothelial function are already impaired in well-controlled type 2 diabetic patients, with short-term disease

Objective

Impaired cerebral vasomotor reactivity (VMR) and flow-mediated dilation (FMD) were found in selected subgroups of type 2 diabetes mellitus (T2DM) patients with long-term disease. Our study aimed to evaluate cerebral hemodynamics, systemic endothelial function and sympatho-vagal balance in a selected population of well-controlled T2DM patients with short-term disease and without cardiac autonomic neuropathy (CAN).

Research Design and Methods

Twenty-six T2DM patients with short-term (4.40±4.80 years) and well-controlled (HbA1C = 6.71±1.29%) disease, without any complications, treated with diet and/or metformin, were consecutively recruited. Eighteen controls, comparable by sex and age, were enrolled also.

Results

FMD and shear rate FMD were found to be reduced in T2DM subjects with short-term disease (8.5% SD 3.5 and 2.5 SD 1.3, respectively) compared to controls (15.4% SD 4.1 and 3.5 SD 1.4; p<.001 and p<.05). T2DM patients also displayed reduced VMR values than controls (39.4% SD 12.4 vs 51.7%, SD 15.5; p<.05). Sympatho-vagal balance was not different in T2DM patients compared to healthy subjects. FMD and shear rate FMD did not correlate with VMR in T2DM patients or in controls (p>.05).

Conclusions

In well-controlled T2DM patients with short-term disease cerebral hemodynamics and systemic endothelial function are altered while autonomic balance appeared to be preserved.

Hyperventilation, cerebral perfusion, and syncope

This review summarizes evidence in humans for an association between hyperventilation (HV)-induced hypocapnia and a reduction in cerebral perfusion leading to syncope defined as transient loss of consciousness (TLOC). The cerebral vasculature is sensitive to changes in both the arterial carbon dioxide (PaCO2) and oxygen (PaO2) partial pressures so that hypercapnia/hypoxia increases and hypocapnia/hyperoxia reduces global cerebral blood flow. Cerebral hypoperfusion and TLOC have been associated with hypocapnia related to HV. Notwithstanding pronounced cerebrovascular effects of PaCO2 the contribution of a low PaCO2 to the early postural reduction in middle cerebral artery blood velocity is transient. HV together with postural stress does not reduce cerebral perfusion to such an extent that TLOC develops. However when HV is combined with cardiovascular stressors like cold immersion or reduced cardiac output brain perfusion becomes jeopardized. Whether, in patients with cardiovascular disease and/or defect, cerebral blood flow cerebral control HV-induced hypocapnia elicits cerebral hypoperfusion, leading to TLOC, remains to be established.

Hypercapnia-induced increases in cerebral blood flow do not improve lower body negative pressure tolerance during hyperthermia

Heat-related decreases in cerebral perfusion are partly the result of ventilatory-related reductions in arterial CO2 tension. Cerebral perfusion likely contributes to an individual's tolerance to a challenge like lower body negative pressure (LBNP). Thus increasing cerebral perfusion may prolong LBNP tolerance. This study tested the hypothesis that a hypercapnia-induced increase in cerebral perfusion improves LBNP tolerance in hyperthermic individuals. Eleven individuals (31 ± 7 yr; 75 ± 12 kg) underwent passive heat stress (increased intestinal temperature ∼1.3°C) followed by a progressive LBNP challenge to tolerance on two separate days (randomized). From 30 mmHg LBNP, subjects inhaled either (blinded) a hypercapnic gas mixture (5% CO2, 21% oxygen, balanced nitrogen) or room air (SHAM). LBNP tolerance was quantified via the cumulative stress index (CSI). Mean middle cerebral artery blood velocity (MCAvmean,) and end-tidal CO2 (PetCO2) were also measured. CO2 inhalation of 5% increased PetCO2 at ∼40 mmHg LBNP (by 16 ± 4 mmHg) and at LBNP tolerance (by 18 ± 5 mmHg) compared with SHAM (P < 0.01). Subsequently, MCAvmean was higher in the 5% CO2 trial during ∼40 mmHg LBNP (by 21 ± 12 cm/s, ∼31%) and at LBNP tolerance (by 18 ± 10 cm/s, ∼25%) relative to the SHAM (P < 0.01). However, hypercapnia-induced increases in MCAvmean did not alter LBNP tolerance (5% CO2 CSI: 339 ± 155 mmHg × min; SHAM CSI: 273 ± 158 mmHg × min; P = 0.26). These data indicate that inhaling a hypercapnic gas mixture increases cerebral perfusion during LBNP but does not improve LBNP tolerance when hyperthermic.

Endothelium-dependent control of cerebrovascular functions through age: exercise for healthy cerebrovascular aging

Cognitive performances are tightly associated with the maximal aerobic exercise capacity, both of which decline with age. The benefits on mental health of regular exercise, which slows the age-dependent decline in maximal aerobic exercise capacity, have been established for centuries. In addition, the maintenance of an optimal cerebrovascular endothelial function through regular exercise, part of a healthy lifestyle, emerges as one of the key and primary elements of successful brain aging. Physical exercise requires the activation of specific brain areas that trigger a local increase in cerebral blood flow to match neuronal metabolic needs. In this review, we propose three ways by which exercise could maintain the cerebrovascular endothelial function, a premise to a healthy cerebrovascular function and an optimal regulation of cerebral blood flow. First, exercise increases blood flow locally and increases shear stress temporarily, a known stimulus for endothelial cell maintenance of Akt-dependent expression of endothelial nitric oxide synthase, nitric oxide generation, and the expression of antioxidant defenses. Second, the rise in circulating catecholamines during exercise not only facilitates adequate blood and nutrient delivery by stimulating heart function and mobilizing energy supplies but also enhances endothelial repair mechanisms and angiogenesis. Third, in the long term, regular exercise sustains a low resting heart rate that reduces the mechanical stress imposed to the endothelium of cerebral arteries by the cardiac cycle. Any chronic variation from a healthy environment will perturb metabolism and thus hasten endothelial damage, favoring hypoperfusion and neuronal stress.

Cerebral blood flow, sympathetic nerve activity and stroke risk in obstructive sleep apnoea. Is there a direct link?

Obstructive sleep apnoea (OSA) is significantly associated with the risk of stroke, and this association is independent of other risk factors, including hypertension, atrial fibrillation and diabetes mellitus. Therefore, additional pathogenic mechanisms may exist, which contribute to the increased risk of stroke. OSA is characterized by prolonged sympathetic overactivity; however the role of the sympathetic nervous system in regulating cerebral circulation remains a matter of controversy. Converging data indicate that brain perfusion is significantly distorted in OSA, with reported decreases in cerebral blood flow as well as intermittent surges in blood pressure and cerebral blood flow velocity. Based on recent research, there is accumulating evidence that sympathetic nerve activity is an important element in brain protection against excessive increases in perfusion pressure during blood pressure surges and flow during rapid eye movement sleep. The aim of this article was to review: (i) the current physiological knowledge related to the role of the sympathetic system in the regulation of cerebral blood flow, (ii) how the influence of the sympathetic system on cerebral vessels is affected by apnoea (increased PaCO(2)) and (iii) the potential significance of the pathological sympathetic system/PaCO(2) interplay in OSA. Sympathetic system seems to be at least partially involved in pathogenesis of distorted haemodynamics and stroke in OSA patients. However, there are still several open questions that need to be addressed before the effective therapeutic strategies can be implemented.

Sympathetic regulation of the human cerebrovascular response to carbon dioxide

Although the cerebrovasculature is known to be exquisitely sensitive to CO2, there is no consensus on whether the sympathetic nervous system plays a role in regulating cerebrovascular responses to changes in arterial CO2. To address this question, we investigated human cerebrovascular CO2 reactivity in healthy participants randomly assigned to the α1-adrenoreceptor blockade group (9 participants; oral prazosin, 0.05 mg/kg) or the placebo control (9 participants) group. We recorded mean arterial blood pressure (MAP), heart rate (HR), mean middle cerebral artery flow velocity (MCAV mean), and partial pressure of end-tidal CO2 (PetCO2) during 5% CO2 inhalation and voluntary hyperventilation. CO2 reactivity was quantified as the slope of the linear relationship between breath-to-breath PetCO2 and the average MCAvmean within successive breathes after accounting for MAP as a covariate. Prazosin did not alter resting HR, PetCO2, MAP, or MCAV mean. The reduction in hypocapnic CO2 reactivity following prazosin (−0.48 ± 0.093 cm·s−1·mmHg−1) was greater compared with placebo (−0.19 ± 0.087 cm·s−1·mmHg−1; P < 0.05 for interaction). In contrast, the change in hypercapnic CO2 reactivity following prazosin (−0.23 cm·s−1·mmHg−1) was similar to placebo (−0.31 cm·s−1·mmHg−1; P = 0.50 for interaction). These data indicate that the sympathetic nervous system contributes to CO2 reactivity via α1-adrenoreceptors; blocking this pathway with prazosin reduces CO2 reactivity to hypocapnia but not hypercapnia.

Influence of sympathoexcitation at high altitude on cerebrovascular function and ventilatory control in humans

We sought to determine the influence of sympathoexcitation on dynamic cerebral autoregulation (CA), cerebrovascular reactivity, and ventilatory control in humans at high altitude (HA). At sea level (SL) and following 3-10 days at HA (5,050 m), we measured arterial blood gases, ventilation, arterial pressure, and middle cerebral blood velocity (MCAv) before and after combined α- and β-adrenergic blockade. Dynamic CA was quantified using transfer function analysis. Cerebrovascular reactivity was assessed using hypocapnia and hyperoxic hypercapnia. Ventilatory control was assessed from the hypercapnia and during isocapnic hypoxia. Arterial Pco(2) and ventilation and its control were unaltered following blockade at both SL and HA. At HA, mean arterial pressure (MAP) was elevated (P < 0.01 vs. SL), but MCAv remained unchanged. Blockade reduced MAP more at HA than at SL (26 vs. 15%, P = 0.048). At HA, gain and coherence in the very-low-frequency (VLF) range (0.02-0.07 Hz) increased, and phase lead was reduced (all P < 0.05 vs. SL). Following blockade at SL, coherence was unchanged, whereas VLF phase lead was reduced (-40 ± 23%; P < 0.01). In contrast, blockade at HA reduced low-frequency coherence (-26 ± 20%; P = 0.01 vs. baseline) and elevated VLF phase lead (by 177 ± 238%; P < 0.01 vs. baseline), fully restoring these parameters back to SL values. Irrespective of this elevation in VLF gain at HA (P < 0.01), blockade increased it comparably at SL and HA (∼43-68%; P < 0.01). Despite elevations in MCAv reactivity to hypercapnia at HA, blockade reduced (P < 0.05) it comparably at SL and HA, effects we attributed to the hypotension and/or abolition of the hypercapnic-induced increase in MAP. With the exception of dynamic CA, we provide evidence of a redundant role of sympathetic nerve activity as a direct mechanism underlying changes in cerebrovascular reactivity and ventilatory control following partial acclimatization to HA. These findings have implications for our understanding of CBF function in the context of pathologies associated with sympathoexcitation and hypoxemia.

Brain hypoperfusion in a girl with systemic lupus erythematosus

We describe an adolescent girl with systemic lupus erythematosus, presenting with severe cardiovascular autonomic dysfunction and incapacitating orthostatic hypotension to a degree not previously reported. Further evaluation of cerebral blood flow velocity, using transcranial Doppler testing, demonstrated an abnormal hypercapnic cerebrovascular response. Both the orthostatic hypotension and the abnormal cerebrovascular hypercapnic response improved with intensive medical treatment of her systemic lupus erythematosus. Additional studies are necessary to elucidate the pathogenesis of these cerebrovascular and autonomic abnormalities, especially considering the potential consequences they may exert on cerebral perfusion, which may be subtle, underrecognized, and capable of affecting cognition.

Cerebral vasoreactivity during hypercapnia is reset by augmented sympathetic influence

Sympathetic nerve activity influences cerebral blood flow, but it is unknown whether augmented sympathetic nerve activity resets cerebral vasoreactivity to hypercapnia. This study tested the hypothesis that cerebral vasodilation during hypercapnia is restrained by lower-body negative pressure (LBNP)-stimulated sympathoexcitation. Cerebral hemodynamic responses were assessed in nine healthy volunteers [age 25 yr (SD 3)] during rebreathing-induced increases in partial pressure of end-tidal CO(2) (Pet(CO(2))) at rest and during LBNP. Cerebral hemodynamic responses were determined by changes in flow velocity of middle cerebral artery (MCAV) using transcranial Doppler sonography and in regional cerebral tissue oxygenation (ScO(2)) using near-infrared spectroscopy. Pet(CO(2)) values during rebreathing were similarly increased from 41.9 to 56.5 mmHg at rest and from 40.7 to 56.0 mmHg during LBNP of -15 Torr. However, the rates of increases in MCAV and in ScO(2) per unit increase in Pet(CO(2)) (i.e., the slopes of MCAV/Pet(CO(2)) and ScO(2)/Pet(CO(2))) were significantly (P ≤0.05) decreased from 2.62 ± 0.16 cm·s(-1)·mmHg(-1) and 0.89 ± 0.10%/mmHg at rest to 1.68 ± 0.18 cm·s(-1)·mmHg(-1) and 0.63 ± 0.07%/mmHg during LBNP. In conclusion, the sensitivity of cerebral vasoreactivity to hypercapnia, in terms of the rate of increases in MCAV and in ScO(2), is diminished by LBNP-stimulated sympathoexcitation.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

Sympathetic withdrawal augments cerebral blood flow during acute hypercapnia in sleeping lambs

STUDY OBJECTIVES

Cerebral sympathetic activity constricts cerebral vessels and limits increases in cerebral blood flow (CBF), particularly in conditions such as hypercapnia which powerfully dilate cerebral vessels. As hypercapnia is common in sleep, especially in sleep disordered breathing, we tested the hypothesis that sympathetic innervation to the cerebral circulation attenuates the CBF increase that accompanies increases in PaCO2 in sleep, particularly in REM sleep when CBF is high.

DESIGN

Newborn lambs (n = 5) were instrumented to record CBF, arterial pressure (AP) intracranial pressure (ICP), and sleep-wake state (quiet wakefulness (QW), NREM, and REM sleep). Cerebral vascular resistance was calculated as CVR = [AP-ICP]/CBF. Lambs were subjected to 60-sec tests of hypercapnia (FICO2 = 0.08) during spontaneous sleep-wake states before (intact) and after sympathectomy (bilateral superior cervical ganglionectomy).

RESULTS

During hypercapnia in intact animals, CBF increased and CVR decreased in all sleep-wake states, with the greatest changes occurring in REM (CBF 39.3% +/- 6.1%, CVR -26.9% +/- 3.6%, P < 0.05). After sympathectomy, CBF increases (26.5% +/- 3.6%) and CVR decreases (-21.8% +/- 2.1%) during REM were less (P < 0.05). However the maximal CBF (27.8 +/- 4.2 mL/min) and minimum CVR (1.8 +/- 0.3 mm Hg/ min/mL) reached during hypercapnia were similar to intact values.

CONCLUSION

Hypercapnia increases CBF in sleep and wakefulness, with the increase being greatest in REM. Sympathectomy increases baseline CBF, but decreases the response to hypercapnia. These findings suggest that cerebral sympathetic nerve activity is normally withdrawn during hypercapnia in REM sleep, augmenting the CBF response.

Contrasting effects of isocapnic and hypocapnic hyperventilation on orthostatic circulatory control

The effects of hyperventilation (HV) on mean arterial pressure (MAP) are variable. To identify factors affecting the MAP response to HV, we dissected the effects of hypocapnic HV (HHV) and isocapnic HV (IHV) and evaluated the effects of acute vs. prolonged HHV. In 11 healthy subjects the cardio- and cerebrovascular effects of HHV and IHV vs. normal ventilation were examined for 15 min in the supine position and also for 15 min during 60° head-up tilt. The end-tidal CO2 of the HHV condition was set at 15–20 mmHg. With HHV in the supine position, mean cerebral blood flow velocity (mCBFV) declined [95% confidence interval (CI) −43 to −34%], heart rate (HR) increased (95% CI 7 to 16 beats/min), but MAP did not change (95% CI −1 to 6 mmHg). However, an augmentation of the supine MAP was observed in the last 10 min of HHV compared with the first 5 min of HHV (95% CI 2 to 12 mmHg). During HHV in the tilted position mCBFV declined (95% CI −28 to −12%) and MAP increased (95% CI 3 to 11 mmHg) without changes in HR. With supine IHV, mCBFV decreased (95% CI −14 to −4%) and MAP increased (95% CI 1 to 13 mmHg) without changes in HR. During IHV in the tilted position MAP was further augmented (95% CI 11 to 20 mmHg) without changes in CBFV or HR. Preventing hypocapnia during HV resulted in a higher MAP, suggesting two contrasting effects of HV on MAP: hypocapnia causing vasodepression and hyperpnea without hypocapnia acting as a vasopressor.

Interaction between the ventilatory and cerebrovascular responses to hypo- and hypercapnia at rest and during exercise

Cerebrovascular reactivity to changes in the partial pressure of arterial carbon dioxide (P(a,CO(2))) via limiting changes in brain [H(+)] modulates ventilatory control. It remains unclear, however, how exercise-induced alterations in respiratory chemoreflex might influence cerebral blood flow (CBF), in particular the cerebrovascular reactivity to CO(2). The respiratory chemoreflex system controlling ventilation consists of two subsystems: the central controller (controlling element), and peripheral plant (controlled element). In order to examine the effect of exercise-induced alterations in ventilatory chemoreflex on cerebrovascular CO(2) reactivity, these two subsystems of the respiratory chemoreflex system and cerebral CO(2) reactivity were evaluated (n = 7) by the administration of CO(2) as well as by voluntary hypo- and hyperventilation at rest and during steady-state exercise. During exercise, in the central controller, the regression line for the P(a,CO(2))-minute ventilation (VE) relation shifted to higher VE and P(a,CO(2)) with no change in gain (P = 0.84). The functional curve of the peripheral plant also reset rightward and upward during exercise. However, from rest to exercise, gain of the peripheral plant decreased, especially during the hypercapnic condition (-4.1 +/- 0.8 to -2.0 +/- 0.2 mmHg l(-1) min(-1), P = 0.01). Therefore, under hypercapnia, total respiratory loop gain was markedly reduced during exercise (-8.0 +/- 2.3 to -3.5 +/- 1.0 U, P = 0.02). In contrast, cerebrovascular CO(2) reactivity at each condition, especially to hypercapnia, was increased during exercise (2.4 +/- 0.2 to 2.8 +/- 0.2% mmHg(-1), P = 0.03). These findings indicate that, despite an attenuated chemoreflex system controlling ventilation, elevations in cerebrovascular reactivity might help maintain CO(2) homeostasis in the brain during exercise.

Autonomic neural control of the cerebral vasculature: acute hypotension

BACKGROUND AND PURPOSE

The effect of antihypertensive drugs on autonomic neural control of the cerebral circulation remains unclear. This study was designed to compare middle cerebral artery mean blood velocity responses to acute hypotension with and without alpha(1)-adrenoreceptor blockade (Prazosin) in young, healthy humans.

METHODS

Acute hypotension was induced nonpharmacologically in 6 healthy subjects (mean+/-SE; 28+/-2 years) by releasing bilateral thigh cuffs after 9 minutes of suprasystolic resting ischemia before and after an oral dose of Prazosin (1 mg/20 kg body weight).

RESULTS

Prazosin had no effect on thigh cuff release-induced reductions in mean arterial pressure and middle cerebral artery mean blood velocity. However, Prazosin attenuated the amount of peripheral vasoconstriction through the arterial baroreflex as evidenced by a slower return of mean arterial pressure to baseline (P=0.03). Immediately after cuff release, cerebral vascular conductance index increased through cerebral autoregulation and returned to resting values as a result of an increased perfusion pressure mediated through arterial baroreflex mechanisms. The rate of regulation, an index of cerebral autoregulation, was attenuated with Prazosin (control versus Prazosin; rate of regulation=0.204+/-0.020 versus 0.006+/-0.053/s, P=0.037). In addition, as mean arterial pressure was returning to resting values, the rate of change in cerebral vascular conductance index was decreased with Prazosin (0.005+/-0.006/s) compared with control (0.025+/-0.005/s; P=0.010).

CONCLUSIONS

These data suggest that during recovery from acute hypotension, decreases in cerebral vascular conductance index were mediated by increases in arterial blood pressure and sympathetically mediated cerebral vasoconstriction.

Cerebrovascular response to arousal from NREM and REM sleep

STUDY OBJECTIVE

To determine the effect of arousal from sleep on cerebral blood flow velocity (CBFV) in relation to associated ventilatory and systemic hemodynamic changes.

PARTICIPANTS

Eleven healthy individuals (6 men, 5 women).

MEASUREMENTS

Pulsed Doppler ultrasonography was used to measure CBFV in the middle cerebral artery with simultaneous measurements of sleep state (EEG, EOG, and EMG), ventilation (inductance plethysmography), heart rate (ECG), and arterial pressure (finger plethysmography). Arousals were induced by auditory tones (range: 40-80 dB; duration: 0.5 sec). Cardiovascular responses were examined beat-by-beat for 30 sec before and 30 sec after auditory tones.

RESULTS

During NREM sleep, CBFV declined following arousals (-15% +/- 2%; group mean +/- SEM) with a nadir at 9 sec after the auditory tone, followed by a gradual return to baseline. Mean arterial pressure (MAP; +20% +/- 1%) and heart rate (HR; +17% +/- 2%) increased with peaks at 5 and 3 sec after the auditory tone, respectively. Minute ventilation (VE) was increased (+35% +/- 10%) for 2 breaths after the auditory tone. In contrast, during REM sleep, CBFV increased following arousals (+15% +/- 3%) with a peak at 3 sec. MAP (+17% +/- 2%) and HR (+15% +/- 2%) increased during arousals from REM sleep with peaks at 5 and 3 sec post tone. VE increased (+16% +/- 7%) in a smaller, more sustained manner during arousals from REM sleep.

CONCLUSIONS

Arousals from NREM sleep transiently reduce CBFV, whereas arousals from REM sleep transiently increase CBFV, despite qualitatively and quantitatively similar increases in MAP, HR, and VE in the two sleep states.

Differential effects of acute hypoxia and high altitude on cerebral blood flow velocity and dynamic cerebral autoregulation: alterations with hyperoxia

We hypothesized that 1) acute severe hypoxia, but not hyperoxia, at sea level would impair dynamic cerebral autoregulation (CA); 2) impairment in CA at high altitude (HA) would be partly restored with hyperoxia; and 3) hyperoxia at HA and would have more influence on blood pressure (BP) and less influence on middle cerebral artery blood flow velocity (MCAv). In healthy volunteers, BP and MCAv were measured continuously during normoxia and in acute hypoxia (inspired O2 fraction = 0.12 and 0.10, respectively; n = 10) or hyperoxia (inspired O2 fraction, 1.0; n = 12). Dynamic CA was assessed using transfer-function gain, phase, and coherence between mean BP and MCAv. Arterial blood gases were also obtained. In matched volunteers, the same variables were measured during air breathing and hyperoxia at low altitude (LA; 1,400 m) and after 1-2 days after arrival at HA ( approximately 5,400 m, n = 10). In acute hypoxia and hyperoxia, BP was unchanged whereas it was decreased during hyperoxia at HA (-11 +/- 4%; P < 0.05 vs. LA). MCAv was unchanged during acute hypoxia and at HA; however, acute hyperoxia caused MCAv to fall to a greater extent than at HA (-12 +/- 3 vs. -5 +/- 4%, respectively; P < 0.05). Whereas CA was unchanged in hyperoxia, gain in the low-frequency range was reduced during acute hypoxia, indicating improvement in CA. In contrast, HA was associated with elevations in transfer-function gain in the very low- and low-frequency range, indicating CA impairment; hyperoxia lowered these elevations by approximately 50% (P < 0.05). Findings indicate that hyperoxia at HA can partially improve CA and lower BP, with little effect on MCAv.

Capillary-oxygenation-level-dependent near-infrared spectrometry in frontal lobe of humans

Brain function requires oxygen and maintenance of brain capillary oxygenation is important. We evaluated how faithfully frontal lobe near-infrared spectroscopy (NIRS) follows haemoglobin saturation (SCap) and how calculated mitochondrial oxygen tension (PMitoO2) influences motor performance. Twelve healthy subjects (20 to 29 years), supine and seated, inhaled O2 air-mixtures (10% to 100%) with and without added 5% carbon dioxide and during hyperventilation. Two measures of frontal lobe oxygenation by NIRS (NIRO-200 and INVOS) were compared with capillary oxygen saturation (SCap) as calculated from the O2 content of brachial arterial and right internal jugular venous blood. At control SCap (78%+/-4%; mean+/-s.d.) was halfway between the arterial (98%+/-1%) and jugular venous oxygenation (SvO2; 61%+/-6%). Both NIRS devices monitored SCap (P<0.001) within approximately 5% as SvO2 increased from 39%+/-5% to 79%+/-7% with an increase in the transcranial ultrasound Doppler determined middle cerebral artery flow velocity from 29+/-8 to 65+/-15 cm/sec. When SCap fell below approximately 70% with reduced flow and inspired oxygen tension, PMitoO2 decreased (P<0.001) and brain lactate release increased concomitantly (P<0.001). Handgrip strength correlated with the measured (NIRS) and calculated capillary oxygenation values as well as with PMitoO2 (r>0.74; P<0.05). These results show that NIRS is an adequate cerebral capillary-oxygenation-level-dependent (COLD) measure during manipulation of cerebral blood flow or inspired oxygen tension, or both, and suggest that motor performance correlates with the frontal lobe COLD signal.

Autonomic ganglionic blockade does not prevent reduction in cerebral blood flow velocity during orthostasis in humans

BACKGROUND AND PURPOSE

The underlying mechanisms for reductions in cerebral blood flow (CBF) during orthostasis are not completely understood. This study tested the hypothesis that sympathetic activation causes cerebral vasoconstriction leading to reductions in CBF during lower body negative pressure (LBNP).

METHODS

CBF velocity, arterial pressure, and end-tidal CO(2) were measured during LBNP (-30 to -50 mm Hg) in 11 healthy subjects before and after autonomic ganglionic blockade with trimethaphan. Arterial partial pressure of CO(2) also was measured in a subgroup of 5 subjects. Mean arterial pressure during LBNP after blockade was maintained by infusion of phenylephrine.

RESULTS

Before blockade, mean arterial pressure did not change during LBNP. However, CBF velocity was reduced in all subjects by 14% (P<0.05). Systolic and pulsatile (systolic-diastolic) CBF velocity were reduced by 18% and 28%, respectively, associated with significant reductions in pulse arterial pressure and end-tidal CO(2) (all P<0.05). After blockade, mean arterial pressure during LBNP was well-maintained and even increased slightly with infusion of phenylephrine. However, reductions in mean, systolic, and pulsatile CBF velocity, pulse arterial pressure, and ETCO(2) were similar to those before blockade. In contrast to reductions in end-tidal CO(2), arterial partial pressure of CO(2) did not change during LBNP.

CONCLUSIONS

These data, contrary to our hypothesis, demonstrate that sympathetic vasoconstriction is not the primary mechanism underlying reductions in CBF during moderate LBNP. We speculate that diminished pulse arterial pressure or pulsatile blood flow may reduce cerebral vessel wall shear stress and contribute to reductions in CBF during orthostasis through flow mediated regulatory mechanisms.

Alterations in cerebral dynamics at high altitude following partial acclimatization in humans: wakefulness and sleep

We tested the hypothesis that, following exposure to high altitude, cerebrovascular reactivity to CO2 and cerebral autoregulation would be attenuated. Such alterations may predispose to central sleep apnea at high altitude by promoting changes in brain Pco2 and thus breathing stability. We measured middle cerebral artery blood flow velocity (MCAv; transcranial Doppler ultrasound) and arterial blood pressure during wakefulness in conditions of eucapnia (room air), hypocapnia (voluntary hyperventilation), and hypercapnia (isooxic rebeathing), and also during non-rapid eye movement (stage 2) sleep at low altitude (1,400 m) and at high altitude (3,840 m) in five individuals. At each altitude, sleep was studied using full polysomnography, and resting arterial blood gases were obtained. During wakefulness and polysomnographic-monitored sleep, dynamic cerebral autoregulation and steady-state changes in MCAv in relation to changes in blood pressure were evaluated using transfer function analysis. High altitude was associated with an increase in central sleep apnea index (0.2 ± 0.4 to 20.7 ± 23.2 per hour) and an increase in mean blood pressure and cerebrovascular resistance during wakefulness and sleep. MCAv was unchanged during wakefulness, whereas there was a greater decrease during sleep at high altitude compared with low altitude (−9.1 ± 1.7 vs. −4.8 ± 0.7 cm/s; P < 0.05). At high altitude, compared with low altitude, the cerebrovascular reactivity to CO2 in the hypercapnic range was unchanged (5.5 ± 0.7 vs. 5.3 ± 0.7%/mmHg; P = 0.06), while it was lowered in the hypocapnic range (3.1 ± 0.7 vs. 1.9 ± 0.6%/mmHg; P < 0.05). Dynamic cerebral autoregulation was further reduced during sleep ( P < 0.05 vs. low altitude). Lowered cerebrovascular reactivity to CO2 and reduction in both dynamic cerebral autoregulation and MCAv during sleep at high altitude may be factors in the pathogenesis of breathing instability.

Regulation of middle cerebral artery blood velocity during recovery from dynamic exercise in humans

We sought to examine the regulation of cerebral blood flow during 10 min of recovery from mild, moderate, and heavy cycling exercise by measuring middle cerebral artery blood velocity (MCA V). Transfer function analyses between changes in arterial blood pressure and MCA V were used to assess the frequency components of dynamic cerebral autoregulation (CA). After mild and moderate exercise, the decreases in mean arterial pressure (MAP) and mean MCA V (MCA Vm) were small. However, following heavy exercise, MAP was rapidly and markedly reduced, whereas MCA Vm decreased slowly (−23 ± 4 mmHg and −4 ± 1 cm/s after 1 min for MAP and MCA Vm, respectively; means ± SE). Importantly, for each workload, the normalized low-frequency transfer function gain between MAP and MCA Vm remained unchanged from rest to exercise and during recovery, indicating a maintained dynamic CA. Similar results were found for the systolic blood pressure and systolic MCA V relationship. In contrast, the normalized low-frequency transfer function gain between diastolic blood pressure and diastolic MCA V (MCA Vd) increased from rest to exercise and remained elevated in the recovery period ( P < 0.05). However, MCA Vd was quite stable on the cessation of exercise. These findings suggest that MCA V is well maintained following mild to heavy dynamic exercise. However, the increased transfer function gain between diastolic blood pressure and MCA Vd suggests that dynamic CA becomes less effective in response to rapid decreases in blood pressure during the initial 10 min of recovery from dynamic exercise.

Differential sensitivities of cerebral and brachial blood flow to hypercapnia in humans

Although it is known that the vasculatures of the brain and the forearm are sensitive to changes in arterial Pco2, previous investigations have not made direct comparisons of the sensitivities of cerebral blood flow (CBF) (middle cerebral artery blood velocity associated with maximum frequency of Doppler shift; V̄p) and brachial blood flow (BBF) to hypercapnia. We compared the sensitivities of V̄p and BBF to hypercapnia in humans. On the basis of the critical importance of the brain for the survival of the organism, we hypothesized that V̄p would be more sensitive than BBF to hypercapnia. Nine healthy males (30.1 ± 5.2 yr, mean ± SD) participated. Euoxic hypercapnia (end-tidal Po2 = 88 Torr, end-tidal Pco2 = 9 Torr above resting) was achieved by using the technique of dynamic end-tidal forcing. V̄p was measured by transcranial Doppler ultrasound as an index of CBF, whereas BBF was measured in the brachial artery by echo Doppler. V̄p and BBF were measured during two 60-min trials of hypercapnia, each trial separated by 60 min. Since no differences in the responses were found between trials, data from both trials were averaged to make comparisons between V̄p and BBF. During hypercapnia, V̄p and BBF increased by 34 ± 8 and 14 ± 8%, respectively. V̄p remained elevated throughout the hypercapnic period, but BBF returned to baseline levels by 60 min. The V̄p CO2 sensitivity was greater than BBF (4 ± 1 vs. 2 ± 1%/Torr; P < 0.05). Our findings confirm that V̄p has a greater sensitivity than BBF in response to hypercapnia and show an adaptive response of BBF that is not evident in V̄p.

Cerebral hemodynamics during orthostatic stress assessed by nonlinear modeling

The effects of orthostatic stress, induced by lower body negative pressure (LBNP), on cerebral hemodynamics were examined in a nonlinear context. Spontaneous fluctuations of beat-to-beat mean arterial blood pressure (MABP) in the finger, mean cerebral blood flow velocity (MCBFV) in the middle cerebral artery, as well as breath-by-breath end-tidal CO2 concentration (PetCO2) were measured continuously in 10 healthy subjects under resting conditions and during graded LBNP to presyncope. A two-input nonlinear Laguerre-Volterra network model was employed to study the dynamic effects of MABP and PetCO2 changes, as well as their nonlinear interactions, on MCBFV variations in the very low (VLF; below 0.04 Hz), low (LF; 0.04–0.15 Hz), and high frequency (HF; 0.15–0.30 Hz) ranges. Dynamic cerebral autoregulation was described by the model terms corresponding to MABP, whereas cerebral vasomotor reactivity was described by the model PetCO2 terms. The nonlinear model terms reduced the output prediction normalized mean square error substantially (by 15–20%) and had a prominent effect in the VLF range, both under resting conditions and during LBNP. Whereas MABP fluctuations dominated in the HF range and played a significant role in the VLF and LF ranges, changes in PetCO2 accounted for a considerable fraction of the VLF and LF MCBFV variations, especially at high LBNP levels. The magnitude of the linear and nonlinear MABP-MCBFV Volterra kernels increased substantially above −30 mmHg LBNP in the VLF range, implying impaired dynamic autoregulation. In contrast, the magnitude of the PetCO2-MCBFV kernels reduced during LBNP at all frequencies, suggesting attenuated cerebral vasomotor reactivity under dynamic conditions. We speculate that these changes may reflect a progressively reduced cerebrovascular reserve to compensate for the increasingly unstable systemic circulation during orthostatic stress that could ultimately lead to cerebral hypoperfusion and syncope.

Interaction between central-peripheral chemoreflexes and cerebro-cardiovascular control

We investigated the interaction between hypoxia and hypercapnia on ventilation and on cerebro-cardio-vascular control. A group of 12 healthy subjects performed rebreathing tests to determine the ventilatory response to hypoxia, at different levels of carbon dioxide (CO(2)), and to normoxic hypercapnia. Oxygen saturation (SaO(2)), end-tidal CO(2) (et-CO(2)), minute ventilation, blood pressure, R-R interval and mid-cerebral artery flow velocity (MCFV) were continuously recorded. The hypoxic ventilatory response significantly increased under hypercapnia and decreased under hypocapnia (slopes L/min/% Sa O(2): -0.33 +/- 0.05, -0.74 +/- 0.02 and -1.59 +/- 0.3, p < 0.0001, in hypocapnia, normocapnia and hypercapnia, respectively). At similar degrees of ventilation, MCFV increased more markedly during normocapnic hypoxia than normoxic hypercapnia; the slopes linking MCFV to hypoxia remained unchanged at increasing levels of et-CO(2), whereas the regression lines were shifted upward. The R-R interval decreased more markedly during normocapnic hypoxia than normoxic hypercapnia and the arterial baroreflex sensitivity was decreased only by hypoxia. Cardiovascular responses to hypoxia were not affected by different levels of et-CO(2). We conclude that concomitant hypoxia and hypercapnia, while increasing ventilation synergistically, exert an additive effect on cerebral blood flow. Increased sympathetic activity (and reduced baroreflex sensitivity) is one of the mechanisms by which hypoxia stimulates cardiac sympathetic activity.

The postural reduction in middle cerebral artery blood velocity is not explained by PaCO2

In the normocapnic range, middle cerebral artery mean velocity (MCA Vmean) changes approximately 3.5% per mmHg carbon-dioxide tension in arterial blood (PaCO2) and a decrease in PaCO2 will reduce the cerebral blood flow by vasoconstriction (the CO2 reactivity of the brain). When standing up MCA Vmean and the end-tidal carbon-dioxide tension (PETCO2) decrease, suggesting that PaCO2 contributes to the reduction in MCA Vmean. In a fixed body position, PETCO2 tracks changes in the PaCO2 but when assuming the upright position, cardiac output (Q) decreases and its distribution over the lung changes, while ventilation (VE) increases suggesting that PETCO2 decreases more than PaCO2. This study evaluated whether the postural reduction in PaCO2 accounts for the postural decline in MCA Vmean). From the supine to the upright position, VE, Q, PETCO2, PaCO2, MCA Vmean, and the near-infrared spectrophotometry determined cerebral tissue oxygenation (CO2Hb) were followed in seven subjects. When standing up, MCA Vmean (from 65.3+/-3.8 to 54.6+/-3.3 cm s(-1) ; mean +/- SEM; P<0.05) and cO2Hb (-7.2+/-2.2 micromol l(-1) ; P<0.05) decreased. At the same time, the VE/Q ratio increased 49+/-14% (P<0.05) with the postural reduction in PETCO2 overestimating the decline in PaCO2 (-4.8+/-0.9 mmHg vs. -3.0+/-1.1 mmHg; P<0.05). When assuming the upright position, the postural decrease in MCA Vmean seems to be explained by the reduction in PETCO2 but the small decrease in PaCO2 makes it unlikely that the postural decrease in MCA Vmean can be accounted for by the cerebral CO2 reactivity alone.

Control of cerebral blood flow during sleep and the effects of hypoxia

During wakefulness, cerebral blood flow (CBF) is closely coupled to regional cerebral metabolism; however CBF is also strongly modulated by breathing, increasing in response to both hypercapnia and hypoxia. During stage III/IV non-rapid eye (NREM) sleep, cerebral metabolism and CBF decrease whilst the partial pressure of arterial CO2 increases due to a reduction in alveolar ventilation. The reduction in CBF during NREM sleep therefore occurs despite a relative state of hypercapnia. We have used transcranial Doppler ultrasound to determine middle cerebral artery velocity, as an index of CBF, and have determined that NREM sleep is associated with a reduction in the cerebrovascular response to hypercapnia. This reduction in reactivity would, at least in part, allow the observed reductions in CBF in this state. Similarly, we have observed that the CBF response to hypoxia is absent during stage III/IV NREM sleep. Nocturnal hypoxia and hypercapnia are major pathogenic factor associated with cardio-respiratory diseases. These marked changes in cerebrovascular control that occur during sleep suggest that the cerebral circulation may be particularly vulnerable to cardio-respiratory insults during this period.

Enhanced cerebral CO2 reactivity during strenuous exercise in man

Light and moderate exercise elevates the regional cerebral blood flow by approximately 20% as determined by ultrasound Doppler sonography (middle cerebral artery mean flow velocity; MCA V(mean)). However, strenuous exercise, especially in the heat, appears to reduce MCA V(mean) more than can be accounted for by the reduction in the arterial CO(2) tension (P(a)CO(2)). This study evaluated whether the apparently large reduction in MCA V(mean) at the end of exhaustive exercise relates to an enhanced cerebrovascular CO(2) reactivity. The CO(2) reactivity was evaluated in six young healthy male subjects by the administration of CO(2) as well as by voluntary hypo- and hyperventilation at rest and during exercise with and without hyperthermia. At rest, P(a)CO(2) was 5.1 +/- 0.2 kPa (mean +/- SEM) and MCA V(mean) 50.7 +/- 3.8 cm s(-1) and the relationship between MCA V(mean) and P(a)CO(2 )was linear (double-log slope 1.1 +/- 0.1). However, the relationship became curvilinear during exercise (slope 1.8 +/- 0.1; P < 0.01 vs. rest) and during exercise with hyperthermia (slope 2.3 +/- 0.3; P < 0.05 vs. control exercise). Accordingly, the cerebral CO(2) reactivity increased from 30.5 +/- 2.7% kPa(-1) at rest to 61.4 +/- 10.1% kPa(-1) during exercise with hyperthermia (P < 0.05). At exhaustion P(a)CO(2) decreased 1.1+/- 0.2 kPa during exercise with hyperthermia, which, with the determined cerebral CO(2) reactivity, accounted for the 28 +/- 10% decrease in MCA V(mean). The results suggest that during exercise changes in cerebral blood flow are dominated by the arterial carbon dioxide tension.

The effect of changes in cardiac output on middle cerebral artery mean blood velocity at rest and during exercise

We examined the relationship between changes in cardiac output and middle cerebral artery mean blood velocity (MCA V(mean)) in seven healthy volunteer men at rest and during 50% maximal oxygen uptake steady-state submaximal cycling exercise. Reductions in were accomplished using lower body negative pressure (LBNP), while increases in were accomplished using infusions of 25% human serum albumin. Heart rate (HR), arterial blood pressure and MCA V(mean) were continuously recorded. At each stage of LBNP and albumin infusion was measured using an acetylene rebreathing technique. Arterial blood samples were analysed for partial pressure of carbon dioxide tension (P(a,CO2). During exercise HR and were increased above rest (P < 0.001), while neither MCA V(mean) nor P(a,CO2) was altered (P > 0.05). The MCA V(mean) and were linearly related at rest (P < 0.001) and during exercise (P = 0.035). The slope of the regression relationship between MCA V(mean) and at rest was greater (P = 0.035) than during exercise. In addition, the phase and gain between MCA V(mean) and mean arterial pressure in the low frequency range were not altered from rest to exercise indicating that the cerebral autoregulation was maintained. These data suggest that the associated with the changes in central blood volume influence the MCA V(mean) at rest and during exercise and its regulation is independent of cerebral autoregulation. It appears that the exercise induced sympathoexcitation and the change in the distribution of between the cerebral and the systemic circulation modifies the relationship between MCA V(mean) and .

Cerebral blood flow changes associated with fluctuations in alpha and theta rhythm during sleep onset in humans

Cerebral blood flow (CBF) is typically reduced during stable non-rapid eye movement (non-REM) sleep compared with the waking level. It is not known when in the sleep cycle these changes occur. However, spontaneous fluctuations in alpha and theta rhythm during sleep onset are associated with marked changes in cardio-respiratory control. The aim of this study was to test the hypothesis that changes in CBF would occur during sleep onset and would be related to changes in cortical activity. Middle cerebral artery velocity (MCAV) was measured using transcranial Doppler ultrasound, as an index of CBF, in 10 healthy subjects. Sleep state, ventilation, end tidal carbon dioxide (PET,CO2), arterial oxygen saturation (SaO2), mean arterial blood pressure (MABP) and cardiac R-R interval (RR) were monitored simultaneously. Immediately following the transition from alpha to theta rhythm (the transition from wake to sleep), ventilation decreased by 13.4% and tidal volume (VT) by 12.2% (P<0.01); PET,CO2 increased by 1.9% (P<0.01); respiratory frequency (fR) and SaO2 did not change significantly. MCAV increased by 9.7% (P<0.01); MABP decreased by 3.2% (P<0.01) but RR did not change significantly. Immediately following the transition from theta to alpha rhythm (spontaneous awakening), increased by 13.3% (P<0.01); VT increased by 11.4% (P<0.01); PET,CO2 decreased by 1.9% (P<0.01); MCAV decreased by 11.1% (P<0.01) and MABP decreased by 7.5%; fR, SaO2 and RR did not change significantly. These changes in MCAV during sleep onset cannot be attributed to changes in ventilation or MABP. We speculate that the changes in cerebral vascular tone during sleep onset are mediated neurally, by regulatory mechanisms linked to the changes in cortical state, and that these mechanisms are different from those regulating the longer-term reduction in CBF associated with stable non-REM sleep.

Cerebrovascular Response to Carbon Dioxide in Patients with Congestive Heart Failure

RATIONALE

Cerebrovascular reactivity to CO(2) provides an important counterregulatory mechanism that serves to minimize the change in H(+) at the central chemoreceptor, thereby stabilizing the breathing pattern in the face of perturbations in Pa(CO(2)). However, there are no studies relating cerebral circulation abnormality to the presence or absence of central sleep apnea in patients with heart failure.

OBJECTIVES

To determine whether patients with congestive heart failure and central sleep apnea have an attenuated cerebrovascular responsibility to CO(2).

METHODS

Cerebral blood flow velocity in the middle cerebral artery was measured in patients with stable congestive heart failure with (n = 9) and without (n = 8) central sleep apnea using transcranial ultrasound during eucapnia (room air), hypercapnia (inspired CO(2), 3 and 5%), and hypocapnia (voluntary hyperventilation). In addition, eight subjects with apnea and nine without apnea performed a 20-second breath-hold to investigate the dynamic cerebrovascular response to apnea.

MEASUREMENTS AND MAIN RESULTS

The overall cerebrovascular reactivity to CO(2) (hyper- and hypocapnia) was lower in patients with apnea than in the control group (1.8 +/- 0.2 vs. 2.5 +/- 0.2%/mm Hg, p < 0.05), mainly due to the prominent reduction of cerebrovascular reactivity to hypocapnia (1.2 +/- 0.3 vs. 2.2 +/- 0.1%/mm Hg, p < 0.05). Similarly, brain blood flow demonstrated a smaller surge after a 20-second breath-hold (peak velocity, 119 +/- 4 vs. 141 +/- 8% of baseline, p < 0.05).

CONCLUSION

Patients with central sleep apnea have a diminished cerebrovascular response to PET(CO(2)), especially to hypocapnia. The compromised cerebrovascular reactivity to CO(2) might affect stability of the breathing pattern by causing ventilatory overshooting during hypercapnia and undershooting during hypocapnia.

Differential responses to CO2 and sympathetic stimulation in the cerebral and femoral circulations in humans

The relative importance of CO2 and sympathetic stimulation in the regulation of cerebral and peripheral vasculatures has not been previously studied in humans. We investigated the effect of sympathetic activation, produced by isometric handgrip (HG) exercise, on cerebral and femoral vasculatures during periods of isocapnia and hypercapnia. In 14 healthy males (28.1 +/- 3.7 (mean +/- S.D.) years), we measured flow velocity (VP; transcranial Doppler ultrasound) in the middle cerebral artery during euoxic isocapnia (ISO, +1 mmHg above rest) and two levels of euoxic hypercapnia (HC5, end-tidal P(CO(2)), P(ET,CO2), = +5 mmHg above ISO; HC10, P(ET,CO2) = +10 above ISO). Each P(ET,CO2) level was maintained for 10 min using the dynamic end-tidal forcing technique, during which increases in sympathetic activity were elicited by a 2-min HG at 30% of maximal voluntary contraction. Femoral blood flow (FBF; Doppler ultrasound), muscle sympathetic nerve activity (MSNA; microneurography) and mean arterial pressure (MAP; Portapres) were also measured. Hypercapnia increased VP and FBF by 5.0 and 0.6% mmHg-1, respectively, and MSNA by 20-220%. Isometric HG increased MSNA by 50% and MAP by 20%, with no differences between ISO, HC5 and HC10. During the ISO HG there was an increase in cerebral vascular resistance (CVR; 20 +/- 11%), while VP remained unchanged. During HC5 and HC10 HG, VP increased (13% and 14%, respectively), but CVR was unchanged. In contrast, HG-induced sympathetic stimulation increased femoral vascular resistance (FVR) during ISO, HC5 and HC10 (17-41%), while there was a general decrease in FBF below ISO. The HG-induced increases in MSNA were associated with increases in FVR in all conditions (r = 0.76-0.87), whereas increases in MSNA were associated with increases in CVR only during ISO (r = 0.91). In summary, in the absence of hypercapnia, HG exercise caused cerebral vasoconstriction, myogenically and/or neurally, which was reflected by increases in CVR and a maintained VP. In contrast, HG increased FVR during conditions of ISO, HC5 and HC10. Therefore, the cerebral circulation is more responsive to alterations in PCO2, and less responsive to sympathetic stimulation than the femoral circulation.