Cerebral blood flow response to isocapnic hypoxia during slow-wave sleep and wakefulness

Impact of intermittent hypoxia on human vascular responses during sleep

Exposure to intermittent hypoxia (IH) ≥15 times per hour is believed to be the primary mechanism for the increased risk of cerebrovascular and cardiovascular disease in patients with moderate to severe sleep apnea. Human experimental models of IH used to investigate this link have been predominantly employed during wakefulness, which limits extrapolation of findings to sleep apnea where IH occurs during sleep. Moreover, how IH impacts vascular regulation during sleep has not been measured quantitatively. Therefore, the objective of this study was to assess the impact sleep accompanied by IH on vascular responses to hypoxia and hypercapnia during sleep. Ten males performed two randomly scheduled 6-h overnight sleep studies. One sleep study was performed in room air (normoxia) and the other sleep study was performed during isocapnic IH (60 s hypoxia-60 s normoxia). On each night, cerebrovascular (peak blood velocity through the middle cerebral artery (V¯_P); transcranial Doppler ultrasound) and cardiovascular (blood pressure, heart rate) responses to hypoxia and hypercapnia were measured before sleep onset (PM-Awake), within the first 2 h of sleep (PM-Asleep), in the 5th (out of 6) hours of sleep (AM-Asleep) and after being awoken in the morning (AM-Awake). Sleep accompanied by IH had no impact on the V¯_P and blood pressure responses to hypoxia and hypercapnic at any timepoint (p ≥ 0.103 for all responses). However, the AM-Awake heart rate response to hypoxia was greater following sleep in IH compared to sleep in normoxia. Independent of the sleep environment, the V¯_P response to hypoxia and hypercapnia were reduced during sleep. In conclusion, cerebral blood flow responses are reduced during sleep compared to wakefulness, but 6 h of sleep accompanied by IH does not alter cerebrovascular and cardiovascular response to hypoxia and hypercapnia during wakefulness or sleep in healthy young humans. However, it is likely that longer exposure to IH during sleep (i.e., days-to-weeks) is required to better elucidate IH's impact on vascular regulation in humans.

Revisiting the neurovascular unit

The brain is supplied by an elaborate vascular network that originates extracranially and reaches deep into the brain. The concept of the neurovascular unit provides a useful framework to investigate how neuronal signals regulate nearby microvessels to support the metabolic needs of the brain, but it does not consider the role of larger cerebral arteries and systemic vasoactive signals. Furthermore, the recently emerged molecular heterogeneity of cerebrovascular cells indicates that there is no prototypical neurovascular unit replicated at all levels of the vascular network. Here, we examine the cellular and molecular diversity of the cerebrovascular tree and the relative contribution of systemic and brain-intrinsic factors to neurovascular function. Evidence supports the concept of a 'neurovascular complex' composed of segmentally diverse functional modules that implement coordinated vascular responses to central and peripheral signals to maintain homeostasis of the brain. This concept has major implications for neurovascular regulation in health and disease and for brain imaging.

Nocturnal cerebral tissue oxygenation in lowlanders with chronic obstructive pulmonary disease travelling to an altitude of 2,590 m: Data from a randomised trial

Altitude exposure induces hypoxaemia in patients with chronic obstructive pulmonary disease (COPD), particularly during sleep. The present study tested the hypothesis in patients with COPD staying overnight at high altitude that nocturnal arterial hypoxaemia is associated with impaired cerebral tissue oxygenation (CTO). A total of 35 patients with moderate-to-severe COPD, living at <800 m (mean [SD] age 62.4 [12.3] years, forced expiratory volume in 1 s [FEV₁ ] 61 [16]% predicted, awake pulse oximetry ≥92%) underwent continuous overnight monitoring of pulse oximetry (oxygen saturation [SpO₂ ]) and near-infrared spectroscopy of prefrontal CTO, respectively, at 490 m and 2,590 m. Regression analysis was used to evaluate whether nocturnal arterial desaturation (COPD_Desat , SpO₂ <90% for >30% of night-time) at 490 m predicted CTO at 2,590 m when controlling for baseline variables. At 2,590 m, mean nocturnal SpO₂ and CTO were decreased versus 490 m, mean change -8.8% (95% confidence interval [CI] -10.0 to -7.6) and -3.6% (95% CI -5.7 to -1.6), difference in change ΔCTO-ΔSpO₂ 5.2% (95% CI 3.0 to 7.3; p < .001). Moreover, frequent cyclic desaturations (≥4% dips/hr) occurred in SpO₂ and CTO, mean change from 490 m 35.3/hr (95% CI 24.9 to 45.7) and 3.4/hr (95% CI 1.4 to 5.3), difference in change ΔCTO-ΔSpO₂ -32.8/hr (95% CI -43.8 to -21.8; p < .001). Regression analysis confirmed an association of COPD_Desat with lower CTO at 2,590 m (coefficient -7.6%, 95% CI -13.2 to -2.0; p = .007) when controlling for several confounders. We conclude that lowlanders with COPD staying overnight at 2,590 m experience altitude-induced hypoxaemia and periodic breathing in association with sustained and intermittent cerebral deoxygenation. Although less pronounced than the arterial deoxygenation, the altitude-induced cerebral tissue deoxygenation may represent a risk of brain dysfunction, especially in patients with COPD with nocturnal hypoxaemia at low altitude.

Routine low-dose continuous or nocturnal oxygen for people with acute stroke: three-arm Stroke Oxygen Supplementation RCT

BACKGROUND

Stroke is a major cause of death and disability worldwide. Hypoxia is common after stroke and is associated with worse outcomes. Oxygen supplementation could prevent hypoxia and secondary brain damage.

OBJECTIVES

(1) To assess whether or not routine low-dose oxygen supplementation in patients with acute stroke improves outcome compared with no oxygen; and (2) to assess whether or not oxygen given at night only, when oxygen saturation is most likely to be low, is more effective than continuous supplementation.

DESIGN

Multicentre, prospective, randomised, open, blinded-end point trial.

SETTING

Secondary care hospitals with acute stroke wards.

PARTICIPANTS

Adult stroke patients within 24 hours of hospital admission and 48 hours of stroke onset, without definite indications for or contraindications to oxygen or a life-threatening condition other than stroke.

INTERVENTIONS

Allocated by web-based minimised randomisation to: (1) continuous oxygen: oxygen via nasal cannula continuously (day and night) for 72 hours after randomisation at a flow rate of 3 l/minute if baseline oxygen saturation was ≤ 93% or 2 l/minute if > 93%; (2) nocturnal oxygen: oxygen via nasal cannula overnight (21:00-07:00) for three consecutive nights. The flow rate was the same as the continuous oxygen group; and (3) control: no routine oxygen supplementation unless required for reasons other than stroke.

MAIN OUTCOME MEASURES

Primary outcome: disability assessed by the modified Rankin Scale (mRS) at 3 months by postal questionnaire (participant aware, assessor blinded). Secondary outcomes at 7 days: neurological improvement, National Institutes of Health Stroke Scale (NIHSS), mortality, and the highest and lowest oxygen saturations within the first 72 hours. Secondary outcomes at 3, 6, and 12 months: mortality, independence, current living arrangements, Barthel Index, quality of life (European Quality of Life-5 Dimensions, three levels) and Nottingham Extended Activities of Daily Living scale by postal questionnaire.

RESULTS

In total, 8003 patients were recruited between 24 April 2008 and 17 June 2013 from 136 hospitals in the UK [continuous, n = 2668; nocturnal, n = 2667; control, n = 2668; mean age 72 years (standard deviation 13 years); 4398 (55%) males]. All prognostic factors and baseline characteristics were well matched across the groups. Eighty-two per cent had ischaemic strokes. At baseline the median Glasgow Coma Scale score was 15 (interquartile range 15-15) and the mean and median NIHSS scores were 7 and 5 (range 0-34), respectively. The mean oxygen saturation at randomisation was 96.6% in the continuous and nocturnal oxygen groups and 96.7% in the control group. Primary outcome: oxygen supplementation did not reduce disability in either the continuous or the nocturnal oxygen groups. The unadjusted odds ratio for a better outcome (lower mRS) was 0.97 [95% confidence interval (CI) 0.89 to 1.05; p = 0.5] for the combined oxygen groups (both continuous and nocturnal together) (n = 5152) versus the control (n = 2567) and 1.03 (95% CI 0.93 to 1.13; p = 0.6) for continuous versus nocturnal oxygen. Secondary outcomes: oxygen supplementation significantly increased oxygen saturation, but did not affect any of the other secondary outcomes.

LIMITATIONS

Severely hypoxic patients were not included.

CONCLUSIONS

Routine low-dose oxygen supplementation in stroke patients who are not severely hypoxic is safe, but does not improve outcome after stroke.

FUTURE WORK

To investigate the causes of hypoxia and develop methods of prevention.

TRIAL REGISTRATION

Current Controlled Trials ISRCTN52416964 and European Union Drug Regulating Authorities Clinical Trials (EudraCT) number 2006-003479-11.

FUNDING DETAILS

This project was funded by the National Institute for Health Research (NIHR) Research for Patient Benefit and Health Technology Assessment programmes and will be published in full in Health Technology Assessment; Vol. 22, No. 14. See the NIHR Journals Library website for further project information.

Human Genetic Adaptation to High Altitude: Evidence from the Andes

Whether Andean populations are genetically adapted to high altitudes has long been of interest. Initial studies focused on physiological changes in the O₂ transport system that occur with acclimatization in newcomers and their comparison with those of long-resident Andeans. These as well as more recent studies indicate that Andeans have somewhat larger lung volumes, narrower alveolar to arterial O₂ gradients, slightly less hypoxic pulmonary vasoconstrictor response, greater uterine artery blood flow during pregnancy, and increased cardiac O₂ utilization, which overall suggests greater efficiency of O₂ transfer and utilization. More recent single nucleotide polymorphism and whole-genome sequencing studies indicate that multiple gene regions have undergone recent positive selection in Andeans. These include genes involved in the regulation of vascular control, metabolic hemostasis, and erythropoiesis. However, fundamental questions remain regarding the functional links between these adaptive genomic signals and the unique physiological attributes of highland Andeans. Well-designed physiological and genome association studies are needed to address such questions. It will be especially important to incorporate the role of epigenetic processes (i.e.; non-sequence-based features of the genome) that are vital for transcriptional responses to hypoxia and are potentially heritable across generations. In short, further exploration of the interaction among genetic, epigenetic, and environmental factors in shaping patterns of adaptation to high altitude promises to improve the understanding of the mechanisms underlying human adaptive potential and clarify its implications for human health.

Human cerebral blood flow control during hypoxia: focus on chronic pulmonary obstructive disease and obstructive sleep apnea

The brain is a vital organ that relies on a constant and adequate blood flow to match oxygen and glucose delivery with the local metabolic demands of active neurons. Thus exquisite regulation of cerebral blood flow (CBF) is particularly important under hypoxic conditions to prevent a detrimental decrease in the partial pressure of oxygen within the brain tissues. Cerebrovascular sensitivity to hypoxia, assessed as the change in CBF during a hypoxic challenge, represents the capacity of cerebral vessels to respond to, and compensate for, a reduced oxygen supply, and has been shown to be impaired or blunted in a number of conditions. For instance, this is observed with aging, and in clinical conditions such as untreated obstructive sleep apnea (OSA) and in healthy humans exposed to intermittent hypoxia. This review will 1) provide a brief overview of cerebral blood flow regulation and results of pharmacological intervention studies which we have performed to better elucidate the basic mechanisms of cerebrovascular regulation in humans; and 2) present data from studies in clinical and healthy populations, using a translational physiology approach, to investigate human CBF control during hypoxia. Results from studies in patients with chronic obstructive pulmonary disease and OSA will be presented to identify the effects of the disease processes on cerebrovascular sensitivity to hypoxia. Data emerging from experimental human models of intermittent hypoxia during wakefulness will also be reviewed to highlight the effects of intermittent hypoxia on the brain.

Measuring high-altitude adaptation

High altitudes (>8,000 ft or 2,500 m) provide an experiment of nature for measuring adaptation and the physiological processes involved. Studies conducted over the past ~25 years in Andeans, Tibetans, and, less often, Ethiopians show varied but distinct O₂ transport traits from those of acclimatized newcomers, providing indirect evidence for genetic adaptation to high altitude. Short-term (acclimatization, developmental) and long-term (genetic) responses to high altitude exhibit a temporal gradient such that, although all influence O₂ content, the latter also improve O₂ delivery and metabolism. Much has been learned concerning the underlying physiological processes, but additional studies are needed on the regulation of blood flow and O₂ utilization. Direct evidence of genetic adaptation comes from single-nucleotide polymorphism (SNP)-based genome scans and whole genome sequencing studies that have identified gene regions acted upon by natural selection. Efforts have begun to understand the connections between the two with Andean studies on the genetic factors raising uterine blood flow, fetal growth, and susceptibility to Chronic Mountain Sickness and Tibetan studies on genes serving to lower hemoglobin and pulmonary arterial pressure. Critical for future studies will be the selection of phenotypes with demonstrable effects on reproductive success, the calculation of actual fitness costs, and greater inclusion of women among the subjects being studied. The well-characterized nature of the O₂ transport system, the presence of multiple long-resident populations, and relevance for understanding hypoxic disorders in all persons underscore the importance of understanding how evolutionary adaptation to high altitude has occurred.NEW & NOTEWORTHY Variation in O₂ transport characteristics among Andean, Tibetan, and, when available, Ethiopian high-altitude residents supports the existence of genetic adaptations that improve the distribution of blood flow to vital organs and the efficiency of O₂ utilization. Genome scans and whole genome sequencing studies implicate a broad range of gene regions. Future studies are needed using phenotypes of clear relevance for reproductive success for determining the mechanisms by which naturally selected genes are acting.

Impact of obstructive sleep apnoea and intermittent hypoxia on cardiovascular and cerebrovascular regulation

NEW FINDINGS

What is the topic of this review? This review examines the notion that obstructive sleep apnoea (OSA) and intermittent hypoxia (IH) have hormetic effects on vascular health. What advances does it highlight? Clinical (OSA patient) and experimental animal and human models report that IH is detrimental to vascular regulation. However, mild IH and, by extension, mild OSA also have physiological and clinical benefits. This review highlights clinical and experimental animal and human data linking OSA and IH to vascular disease and discusses how hormetic effects of OSA and IH relate to OSA severity, IH intensity and duration, and patient/subject age. Obstructive sleep apnoea (OSA) is associated with increased risk of cardiovascular and cerebrovascular disease, a consequence attributed in part to chronic intermittent hypoxia (IH) resulting from repetitive apnoeas during sleep. Although findings from experimental animal, and human, models have shown that IH is detrimental to vascular regulation, the severity of IH used in many of these animal studies [e.g. inspired fraction of oxygen (FI,O2) = 2-3%; oxygen desaturation index = 120 events h^-1 ] is considerably greater than that observed in the majority of patients with OSA. This may also explain disparities between animal and recently developed human models of IH, where IH severity is, by necessity, less severe (e.g. FI,O2 = 10-12%; oxygen desaturation index = 15-30 events h^-1 ). In this review, we highlight the current knowledge regarding the impact of OSA and IH on cardiovascular and cerebrovascular regulation. In addition, we critically discuss the recent notion that OSA and IH may have hormetic effects on vascular health depending on conditions such as OSA severity, IH intensity and duration, and age. In general, data support an independent causal link between OSA and vascular disease, particularly for patients with severe OSA. However, the data are equivocal for older OSA patients and patients with mild OSA, because advanced age and short-duration, low-intensity IH have been reported to provide a degree of protection against IH and ischaemic events such as myocardial infarction and stroke, respectively. Overall, additional studies are needed to investigate the beneficial/detrimental effects of mild OSA on the various vascular beds.

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

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

Brain Damage and Motor Cortex Impairment in Chronic Obstructive Pulmonary Disease: Implication of Nonrapid Eye Movement Sleep Desaturation

STUDY OBJECTIVES

Nonrapid eye movement (NREM) sleep desaturation may cause neuronal damage due to the withdrawal of cerebrovascular reactivity. The current study (1) assessed the prevalence of NREM sleep desaturation in nonhypoxemic patients with chronic obstructive pulmonary disease (COPD) and (2) compared a biological marker of cerebral lesion and neuromuscular function in patients with and without NREM sleep desaturation.

METHODS

One hundred fifteen patients with COPD (Global Initiative for Chronic Obstructive Lung Disease [GOLD] grades 2 and 3), resting PaO2 of 60-80 mmHg, aged between 40 and 80 y, and without sleep apnea (apnea-hypopnea index < 15) had polysomnographic sleep recordings. In addition, twenty-nine patients (substudy) were assessed i) for brain impairment by serum S100B (biological marker of cerebral lesion), and ii) for neuromuscular function via motor cortex activation and excitability and maximal voluntary quadriceps strength measurement.

RESULTS

A total of 51.3% patients (n = 59) had NREM sleep desaturation (NREMDes). Serum S100B was higher in the NREMDes patients of the substudy (n = 14): 45.1 [Q1: 37.7, Q3: 62.8] versus 32.9 [Q1: 25.7, Q3: 39.5] pg.ml(-1) (P = 0.028). Motor cortex activation and excitability were lower in NREMDes patients (both P = 0.03), but muscle strength was comparable between groups (P = 0.58).

CONCLUSIONS

Over half the nonhypoxemic COPD patients exhibited NREM sleep desaturation associated with higher values of the cerebral lesion biomarker and lower neural drive reaching the quadriceps during maximal voluntary contraction. The lack of muscle strength differences between groups suggests a compensatory mechanism(s). Altogether, the results are consistent with an involvement of NREM sleep desaturation in COPD brain impairment.

CLINICAL TRIAL REGISTRATION

The study was registered at www.clinicaltrials.gov as NCT01679782.

Influence of Hypoxia on Cerebral Blood Flow Regulation in Humans

The brain is a vital organ that relies on a constant and adequate supply of blood to match oxygen and glucose delivery with the local metabolic demands of active neurones. It is well established that cerebral blood flow is altered in response to both neural activity and humoral stimuli. Thus, augmented neural activation (e.g. visual stimulation) leads to locally increased cerebral blood flow via functional hyperaemia, whereas humoral stimuli (i.e. alterations in arterial PO2 and PCO2) produce global increases in cerebral blood flow. Perhaps not surprisingly, cerebrovascular responses to neural activity and humoral stimuli may not be highly correlated because they reflect different physiological mechanisms for vasodilation. Exquisite regulation of cerebral blood flow is particularly important under hypoxic conditions when cerebral PO2 can be reduced substantially. Indeed, cerebrovascular reactivity to hypoxia determines the capacity of cerebral vessels to respond and compensate for a reduced oxygen supply. This reactivity is dynamic, changing with prolonged exposure to hypoxic environments, and in patients and healthy individuals exposed to chronic intermittent periods of hypoxia. More recently, a number of animal studies have provided evidence that glial cells (i.e. astrocytes) play an important role in regulating cerebral blood flow under normoxic and hypoxic conditions. This review aims to summarize our current understanding of cerebral blood flow control during hypoxia in humans and put into context the underlying neurovascular mechanisms that may contribute to this regulation.

Regional Cerebrovascular Responses to Hypercapnia and Hypoxia

A limited number of studies using differing imaging approaches suggest that there are regional variation in the cerebrovascular response to hypercapnia and hypoxia. However there are limitations to these studies. In particular, it is not clear if existing studies of hypoxia have fully accounted for the confounding effects of the changes in arterial PCO2 on cerebral perfusion that, if uncontrolled, will accompany the hypoxic stimulus. We determined quantitative maps of grey matter cerebral blood flow using a multi-slice pulsed arterial spin labelling MRI method at 3 T at rest, during conditions of isocapnic euoxia, hypercapnia, and mild isocapnic hypoxia. From these data, we determined grey matter cerebrovascular reactivity maps which show the spatial distribution of the responses to these interventions. Whilst, overall, cerebral perfusion increased with hypercapnia and hypoxia, hypoxia cerebrovascular reactivity maps showed very high variation both within and between individuals: most grey matter regions exhibiting a positive cerebrovascular reactivity, but some exhibiting a negative reactivity. The physiological explanation for this variation remains unclear and it is not known if these local differences will vary with state or with regional brain activity. The potential interaction between hypoxic or hypercapnic cerebrovascular changes and neurally related changes in brain perfusion is of particular interest for functional imaging studies of brain activation in which arterial blood gases are altered. We have determined the interaction between global hypoxia and hypercapnia-induced blood oxygen level-dependent (BOLD) MRI signal and local neurally related BOLD signal. Although statistically significant interactions were present, physiologically the effects were weak and, in practice, they did not change the statistical outcome related to the analysis of the neurally related signals. These data suggest that such respiratory-related confounds can be successfully accounted for in functional imaging studies.

A Biphasic Change of Regional Blood Volume in the Frontal Cortex during Non-Rapid Eye Movement Sleep: A Near-Infrared Spectroscopy Study

STUDY OBJECTIVES

Current knowledge on hemodynamics in sleep is limited because available techniques do not allow continuous recordings and mainly focus on cerebral blood flow while neglecting other important parameters, such as blood volume (BV) and vasomotor activity.

DESIGN

Observational study.

PARTICIPANTS AND SETTINGS

Continuous measures of hemodynamics over the left forehead and biceps were performed using near-infrared spectroscopy (NIRS) during nocturnal polysomnography in 16 healthy participants in sleep laboratory.

MEASUREMENTS AND RESULTS

Temporal dynamics and mean values of cerebral and muscular oxygenated hemoglobin (HbO2), deoxygenated hemoglobin (HHb), and BV during different sleep stages were compared. A biphasic change of cerebral BV was observed which contrasted a monotonic increase of muscular BV during non-rapid eye movement sleep. A significant decrement in cerebral HbO2 and BV accompanied by an increase of HHb was recorded at sleep onset (Phase I). Prior to slow wave sleep (SWS) HbO2 and BV turned to increase whereas HHb began to decrease in subsequent Phase II suggested increased brain perfusion during SWS. The cerebral HbO2 slope correlated to BV slope in Phase I and II, but it only correlated to HHb slope in Phase II. The occurrence time of inflection points correlated to SWS latencies.

CONCLUSION

Initial decrease of brain perfusion with decreased blood volume (BV) and oxygenated hemoglobin (HbO2) together with increasing muscular BV fit thermoregulation process at sleep onset. The uncorrelated and correlated slopes of HbO2 and deoxygenated hemoglobin indicate different mechanisms underlying the biphasic hemodynamic process in light sleep and slow wave sleep (SWS). In SWS, changes in vasomotor activity (i.e., increased vasodilatation) may mediate increasing cerebral and muscular BV.

Statistical analysis plan for the Stroke Oxygen Study (SO₂S): a multi-center randomized controlled trial to assess whether routine oxygen supplementation in the first 72 hours after a stroke improves long-term outcome

BACKGROUND

The Stroke Oxygen Study (SO₂S) is a multi-center randomized controlled trial of oxygen supplementation in patients with acute stroke. The main hypothesis for the trial is that fixed-dose oxygen treatment during the first 3 days after an acute stroke improves outcome. The secondary hypothesis is that restricting oxygen supplementation to night time only is more effective than continuous supplementation. This paper describes the statistical analysis plan for the study.

METHODS AND DESIGN

Patients (n = 8000) are randomized to three groups: (1) continuous oxygen supplementation for 72 hours; (2) nocturnal oxygen supplementation for three nights; and (3) no routine oxygen supplementation. Outcomes are recorded at 7 days, 90 days, 6 months, and 12 months. The primary outcome measure is the modified Rankin scale at 90 days. Data will be analyzed according to the intention-to-treat principle. Methods of statistical analysis are described, including the handling of missing data, the covariates used in adjusted analyses, planned subgroups analyses, and planned sensitivity analyses.

TRIAL REGISTRATION

This trial is registered with the ISRCTN register, number ISRCTN52416964 (30 September 2005).

The Stroke Oxygen Study (SO2S) - a multi-center, study to assess whether routine oxygen treatment in the first 72 hours after a stroke improves long-term outcome: study protocol for a randomized controlled trial

BACKGROUND

Mild hypoxia is common in stroke patients and may have significant adverse effects on the ischemic brain after stroke. The use of oxygen treatment is rapidly increasing in European stroke units but is not without side effects. It impedes early mobilization, could pose an infection risk, and may encourage the formation of toxic free radicals, leading to further damage to the ischemic brain. In the Stroke Oxygen Pilot Study (2 or 3 L/min for 72 hours) neurological recovery at one week was better in the oxygen group than in controls, and after correction for difference in baseline stroke severity and prognostic factors, there was a trend to better outcome with oxygen at six months. Oxygen was as effective in mild as in severe strokes.Oxygen saturation is lower at night than during the day, and episodes of oxygen desaturation are common during sleep. Nocturnal oxygen supplementation is likely to reduce the burden of hypoxia without interfering with daytime mobilization and rehabilitation.Before wider use of oxygen supplementation becomes established it is important to obtain better evidence on which patients benefit from such treatment.

METHODS

Participants will be randomized to one of three groups: the first will receive continuous oxygen for 72 hours (at a rate of 2 or 3 L/min depending on baseline oxygen saturation), the second group will receive nocturnal oxygen only (at a rate of 2 or 3 L/min depending on baseline oxygen saturation) and the third group will not receive any oxygen (control). A baseline assessment is performed at randomization and a one-week follow-up completed. Outcome data at three, six and twelve months will be obtained via a questionnaire sent to the patient by the trial center.

DISCUSSION

This study will provide evidence on the effectiveness of oxygen supplementation for the treatment of stroke and whether nocturnal oxygen is a potentially beneficial therapy regimen.

TRIAL REGISTRATION

This trial is registered with the ISRCTN register ID number ISRCTN52416964.

The Young Everest Study: preliminary report of changes in sleep and cerebral blood flow velocity during slow ascent to altitude in unacclimatised children

BACKGROUND

Cerebral blood flow velocity (CBFV) and sleep physiology in healthy children exposed to hypoxia and hypocarbia are under-researched.

AIM

To investigate associations between sleep variables, daytime end-tidal carbon dioxide (EtCO2) and CBFV in children during high-altitude ascent.

METHODS

Vital signs, overnight cardiorespiratory sleep studies and transcranial Doppler were undertaken in nine children (aged 6-13 years) at low altitude (130 m), and then at moderate (1300 m) and high (3500 m) altitude during a 5-day ascent.

RESULTS

Daytime (130 m: 98%; 3500 m: 90%, p=0.004) and mean (130 m: 97%, 1300 m: 94%, 3500: 87%, p=0.0005) and minimum (130 m: 92%, 1300 m: 84%, 3500 m: 79%, p=0.0005) overnight pulse oximetry oxyhaemoglobin saturation decreased, and the number of central apnoeas increased at altitude (130 m: 0.2/h, 1300 m: 1.2/h, 3500 m: 3.5/h, p=0.2), correlating inversely with EtCO2 (R(2) 130 m: 0.78; 3500 m: 0.45). Periodic breathing occurred for median (IQR) 0.0 (0; 0.3)% (130 m) and 0.2 (0; 1.2)% (3500 m) of total sleep time. At 3500 m compared with 130 m, there were increases in middle (MCA) (mean (SD) left 29.2 (42.3)%, p=0.053; right 9.9 (12)%, p=0.037) and anterior cerebral (ACA) (left 65.2 (69)%, p=0.024; right 109 (179)%; p=0.025) but not posterior or basilar CBFV. The right MCA CBFV increase at 3500 m was predicted by baseline CBFV and change in daytime SpO2 and EtCO2 at 3500 m (R(2) 0.92); these associations were not seen on the left.

CONCLUSIONS

This preliminary report suggests that sleep physiology is disturbed in children even with slow ascent to altitude. The regional variations in CBFV and their association with hypoxia and hypocapnia require further investigation.

Cerebral hemodynamic changes in obstructive sleep apnea syndrome after continuous positive airway pressure treatment

BACKGROUND

Patients with obstructive sleep apnea syndrome (OSAS) are at increased risk for cerebrovascular diseases. The underlying mechanisms remain obscure. It may occur through a reduction in cerebral vascular reactivity. Continuous positive airway pressure (CPAP) is effective in reducing the occurrence of apneas. We hypothesized that treatment with CPAP improves cerebral vascular reactivity.

METHODS

This is a prospective study with OSAS patients. The apnea test (ApT) was calculated as an increase of mean artery velocity during apnea: [Artery velocity in apnea minus Resting artery velocity]/Resting artery velocity expressed as percentage. After 2 years of CPAP treatment, the test was repeated.

RESULTS

Seventy-six patients represented the study pool. After 2 years of treatment with CPAP, we were able to conduct a reassessment in 65 patients. Of the 65 patients who finished the clinical study, 56 were men, and 9 were women, with an average age of 48.1 ± 10.4 years. There was an improvement in the ApT after CPAP treatment (30.8 ± 12.1 vs 39.8 ± 15.1; p:0.000). The values of cerebral blood flow velocities, diastolic blood pressure in apnea, and basal heart rate decreased.

CONCLUSIONS

Cerebral vascular reactivity in OSAS patients measured by ApT improved after 2 years of CPAP.

Pathogenesis of cognitive dysfunction in patients with obstructive sleep apnea: a hypothesis with emphasis on the nucleus tractus solitarius

OSA is characterized by the quintessential triad of intermittent apnea, hypoxia, and hypoxemia due to pharyngeal collapse. This paper highlights the upstream mechanisms that may trigger cognitive decline in OSA. Three interrelated steps underpin cognitive dysfunction in OSA patients. First, several risk factors upregulate peripheral inflammation; these crucial factors promote neuroinflammation, cerebrovascular endothelial dysfunction, and oxidative stress in OSA. Secondly, the neuroinflammation exerts negative impact globally on the CNS, and thirdly, important foci in the neocortex and brainstem are rendered inflamed and dysfunctional. A strong link is known to exist between neuroinflammation and neurodegeneration. A unique perspective delineated here underscores the importance of dysfunctional brainstem nuclei in etiopathogenesis of cognitive decline in OSA patients. Nucleus tractus solitarius (NTS) is the central integration hub for afferents from upper airway (somatosensory/gustatory), respiratory, gastrointestinal, cardiovascular (baroreceptor and chemoreceptor) and other systems. The NTS has an essential role in sympathetic and parasympathetic systems also; it projects to most key brain regions and modulates numerous physiological functions. Inflamed and dysfunctional NTS and other key brainstem nuclei may play a pivotal role in triggering memory and cognitive dysfunction in OSA. Attenuation of upstream factors and amelioration of the NTS dysfunction remain important challenges.

Spontaneous hemodynamic oscillations during human sleep and sleep stage transitions characterized with near-infrared spectroscopy

Understanding the interaction between the nervous system and cerebral vasculature is fundamental to forming a complete picture of the neurophysiology of sleep and its role in maintaining physiological homeostasis. However, the intrinsic hemodynamics of slow-wave sleep (SWS) are still poorly known. We carried out 30 all-night sleep measurements with combined near-infrared spectroscopy (NIRS) and polysomnography to investigate spontaneous hemodynamic behavior in SWS compared to light (LS) and rapid-eye-movement sleep (REM). In particular, we concentrated on slow oscillations (3-150 mHz) in oxy- and deoxyhemoglobin concentrations, heart rate, arterial oxygen saturation, and the pulsation amplitude of the photoplethysmographic signal. We also analyzed the behavior of these variables during sleep stage transitions. The results indicate that slow spontaneous cortical and systemic hemodynamic activity is reduced in SWS compared to LS, REM, and wakefulness. This behavior may be explained by neuronal synchronization observed in electrophysiological studies of SWS and a reduction in autonomic nervous system activity. Also, sleep stage transitions are asymmetric, so that the SWS-to-LS and LS-to-REM transitions, which are associated with an increase in the complexity of cortical electrophysiological activity, are characterized by more dramatic hemodynamic changes than the opposite transitions. Thus, it appears that while the onset of SWS and termination of REM occur only as gradual processes over time, the termination of SWS and onset of REM may be triggered more abruptly by a particular physiological event or condition. The results suggest that scalp hemodynamic changes should be considered alongside cortical hemodynamic changes in NIRS sleep studies to assess the interaction between the autonomic and central nervous systems.

Dynamic changes in brain bioenergetics during obstructive sleep apnea

Repetitive collapse of the upper airway during obstructive sleep apnea/hypopnea (OSA) exposes the brain of sufferers to frequent, transient, hypoxic episodes. The loss of cerebrovascular reactivity in sleep, and particularly in OSA, means that physiologic compensatory mechanisms may not ensure adequate brain oxygen levels. This (31)P magnetic resonance spectroscopy study, of 13 males with severe, untreated OSA undertaken after overnight sleep deprivation, represents the first, seconds time-scale analysis of human brain bioenergetics during transient hypoxia and demonstrates that a moderate degree of oxygen desaturation during sleep has significant effects on brain bioenergetic status. Oxygen desaturation >10% of sleeping baseline resulted in decreases in brain adenosine triphosphate levels (P<0.01), and increases in inorganic phosphate (P<0.0001) with no concomitant changes in phosphocreatine or brain pH. This indicates that the mechanism of adenosine triphosphate depletion in these patients is different to that observed in normoxic, awake working brain. These data show that the buffering capacity of phosphocreatine and the creatine kinase system is not active in mild transient hypoxia and that cerebrovascular compensatory mechanisms are not adequate to prevent decrements in brain high-energy phosphates in OSA. Transient hypoxia experienced during sleep may impair brain function more than previously thought.

Hypoxaemia and stroke

The topic of hypoxaemia after stroke was last reviewed in this journal in 2001. Since then a lot of new information on the subject has been published, and while some questions have been resolved, new problems have emerged. This article discusses new research in the light of what is already known and outlines areas of persisting uncertainty.

Human cerebral arteriovenous vasoactive exchange during alterations in arterial blood gases

Cerebral blood flow (CBF) is highly regulated by changes in arterial Pco2and arterial Po2. Evidence from animal studies indicates that various vasoactive factors, including release of norepinephrine, endothelin, adrenomedullin, C-natriuretic peptide (CNP), and nitric oxide (NO), may play a role in arterial blood gas-induced alterations in CBF. For the first time, we directly quantified exchange of these vasoactive factors across the human brain. Using the Fick principle and transcranial Doppler ultrasonography, we measured CBF in 12 healthy humans at rest and during hypercapnia (4 and 8% CO2), hypocapnia (voluntary hyperventilation), and hypoxia (12 and 10% O2). At each level, blood was sampled simultaneously from the internal jugular vein and radial artery. With the exception of CNP and NO, the simultaneous quantification of norepinephrine, endothelin, or adrenomedullin showed no cerebral uptake or release during changes in arterial blood gases. Hypercapnia, but not hypocapnia, increased CBF and caused a net cerebral release of nitrite (a marker of NO), which was reflected by an increase in the venous-arterial difference for nitrite: 57 ± 18 and 150 ± 36 μmol/l at 4% and 8% CO2, respectively (both P < 0.05). Release of cerebral CNP was also observed during changes in CO2(hypercapnia vs. hypocapnia, P < 0.05). During hypoxia, there was a net cerebral uptake of nitrite, which was reflected by a decreased venous-arterial difference for nitrite: −96 ± 14 μmol/l at 10% O2( P < 0.05). These data indicate that there is a differential exchange of NO across the brain during hypercapnia and hypoxia and that CNP may play a complementary role in CO2-induced CBF changes.

Mapping of the cerebral vascular response to hypoxia and hypercapnia using quantitative perfusion MRI at 3 T

Changes in breathing change the concentration of oxygen and carbon dioxide in arterial blood resulting in changes in cerebral blood flow (CBF). This mechanism can be described by the cerebral vascular response (CVR), which has been shown to be altered in different physiological and pathophysiological states. CBF maps of grey matter (GM) were determined with a pulsed arterial spin labelling technique at 3 T in a group of 19 subjects under baseline conditions, hypoxia, and hypercapnia. Experimental conditions allowed a change in either arterial oxygen (hypoxia) or carbon dioxide (hypercapnia) concentration compared with the baseline, leaving the other variable constant, in order to separate the effects of these two variables. From these results, maps were calculated showing the regional distribution of the CVR to hypoxia and hypercapnia in GM. Maps of CVR to hypoxia showed very high intra-subject variations, with some GM regions exhibiting a positive response and others a negative response. Per 10% decrease in arterial oxygen saturation, there was a statistically significant 7.0 +/- 2.9% (mean +/- SEM) increase in GM-CBF for the group. However, 70% of subjects showed an overall positive CVR (positive responders), and the remaining 30% an overall negative CVR (negative responders). Maps of CVR to hypercapnia showed less intra-subject variation. Per 1 mm Hg increase in partial pressure of end-tidal carbon dioxide, there was a statistically significant 5.8 +/- 0.9% increase in GM-CBF, all subjects showing an overall positive CVR. As the brain is particularly vulnerable to hypoxia, a condition associated with cardiorespiratory diseases, CVR maps may help in the clinic to identify the areas most prone to damage because of a reduced CVR.

The BOLD response and vascular reactivity during visual stimulation in the presence of hypoxic hypoxia

A disproportionate increase in cerebral blood flow (CBF) relative to the cerebral metabolic rate of oxygen (CMRO(2)), in response to neuronal activation, results in a decreased oxygen extraction fraction (OEF) and hence local 'hyperoxygenation'. The mismatch is the key 'physiological substrate' for blood oxygenation level dependent (BOLD) fMRI. The mismatch may reflect inefficient O(2) diffusion in the brain tissue, a factor requiring maintenance of a steep [O(2)] gradient between capillary bed and neural cell mitochondria. The aim of this study was to assess vascular responsiveness to reduced blood oxygen saturation, using both BOLD fMRI and the CBV-weighted vascular space occupancy (VASO)-dependent fMRI technique, during visual activation in hypoxic hypoxia. Our fMRI results show decreased amplitude and absence of initial sharp overshoot in the BOLD response, while VASO signal was not influenced by decreasing oxygen saturation down to 0.85. The results suggest that the OEF during visual activation may be different in hypoxia relative to normoxia, due to a more efficient oxygen extraction under compromised oxygen availability. The data also indicate that vascular reactivity to brain activation is not affected by mild hypoxia.

Gray matter blood flow change is unevenly distributed during moderate isocapnic hypoxia in humans

Hypoxia increases cerebral blood flow (CBF), but it is unknown whether this increase is uniform across all brain regions. We used H(2)(15)O positron emission tomography imaging to measure absolute blood flow in 50 regions of interest across the human brain (n = 5) during normoxia and moderate hypoxia. Pco(2) was kept constant ( approximately 44 Torr) throughout the study to avoid decreases in CBF associated with the hypocapnia that normally occurs with hypoxia. Breathing was controlled by mechanical ventilation. During hypoxia (inspired Po(2) = 70 Torr), mean end-tidal Po(2) fell to 45 +/- 6.3 Torr (means +/- SD). Mean global CBF increased from normoxic levels of 0.39 +/- 0.13 to 0.45 +/- 0.13 ml/g during hypoxia. Increases in regional CBF were not uniform and ranged from 9.9 +/- 8.6% in the occipital lobe to 28.9 +/- 10.3% in the nucleus accumbens. Regions of interest that were better perfused during normoxia generally showed a greater regional CBF response. Phylogenetically older regions of the brain tended to show larger vascular responses to hypoxia than evolutionary younger regions, e.g., the putamen, brain stem, thalamus, caudate nucleus, nucleus accumbens, and pallidum received greater than average increases in blood flow, while cortical regions generally received below average increases. The heterogeneous blood flow distribution during hypoxia may serve to protect regions of the brain with essential homeostatic roles. This may be relevant to conditions such as altitude, breath-hold diving, and obstructive sleep apnea, and may have implications for functional brain imaging studies that involve hypoxia.

Cerebrovascular responses to altitude

The regulation of cerebral blood flow (CBF) is a complex process that is altered significantly with altitude exposure. Acute exposure produces a marked increase in CBF, in proportion to the severity of the hypoxia and mitigated by hyperventilation-induced hypocapnia when CO(2) is uncontrolled. A number of mediators contribute to the hypoxia-induced cerebral vasodilation, including adenosine, potassium channels, substance P, prostaglandins, and NO. Upon acclimatization to altitude, CBF returns towards normal sea-level values in subsequent days and weeks, mediated by a progressive increase in PO2, first through hyperventilation followed by erythropoiesis. With long-term altitude exposure, a number of mechanisms play a role in regulating CBF, including acid-base balance, hematological modifications, and angiogenesis. Finally, several cerebrovascular disorders are associated with altitude exposure. Existing gaps in our knowledge of CBF and altitude, and areas of future investigation include effects of longer exposures, intermittent hypoxia, and gender differences in the CBF responses to altitude.

Measuring the ventilatory response to hypoxia

After defining the current approach to measuring the hypoxic ventilatory response this paper explains why this method is not appropriate for comparisons between individuals or conditions, and does not adequately measure the parameters of the peripheral chemoreflex. A measurement regime is therefore proposed that incorporates three procedures. The first procedure measures the peripheral chemoreflex responsiveness to both hypoxia and CO(2) in terms of hypoxia's effects on the sensitivity and ventilatory recruitment threshold of the peripheral chemoreflex response to CO(2). The second and third procedures employ current methods for measuring the isocapnic and poikilocapnic ventilatory responses to hypoxia, respectively, over a period of 20 min. The isocapnic measure is used to determine the time course characteristics of hypoxic ventilatory decline and the poikilocapnic measure shows the ventilatory response to a hypoxic environment. A measurement regime incorporating these three procedures will permit a detailed assessment of the peripheral chemoreflex response to hypoxia that allows comparisons to be made between individuals and different physiological and environmental conditions.

The effects of adaptive servo ventilation on cerebral vascular reactivity in patients with congestive heart failure and sleep-disordered breathing

STUDY OBJECTIVE

Hypercapnic cerebral vascular reactivity (HCVR) is reduced in patients with congestive heart failure (CHF) and sleep-disordered breathing (SDB); this may be associated with an increased risk of stroke. We tested the hypothesis that reversal of SDB in CHF patients using adaptive servo ventilation (ASV) would increase morning HCVR.

DESIGN

Interventional, cross-over clinical study.

SETTING

Research sleep laboratory.

PATIENTS

Ten CHF patients with SDB, predominantly obstructive sleep apnea.

INTERVENTIONS

The HCVR was measured from the change in middle cerebral artery velocity, using pulsed Doppler ultrasound. HCVR was determined during the evening (before) and morning (after) 1 night of sleep on ASV and 1 night of spontaneous sleep (control).

MEASUREMENTS AND RESULTS

Compared with the control situation, ASV decreased the apnea-hypopnea index (group mean +/- SEM, control: 48 +/- 12, ASV: 4 +/- 1 events per hour). HCVR was 23% lower in the morning, compared with the evening, on the control night (evening: 1.3 +/- 0.2, morning: 1.0 +/- 0.2 cm/sec per mm Hg, P < 0.05) and 27% lower following the ASV night (evening: 1.5 +/- 0.2, morning: 1.1 +/- 0.2 cm/sec per mm Hg, P < 0.05). The effect of ASV on the evening-to-morning reduction in HCVR was not significant, compared with the control night (0.02 cm/sec per mm Hg, 95% confidence interval: -0.28, 0.32 P = 0.89).

CONCLUSIONS

In CHF patients with SDB, HCVR was reduced in the morning compared with the evening. However, removal of SDB for 1 night did not reverse the reduced HCVR. The relatively low morning HCVR could be linked with an increased risk of stroke.

Obstructive sleep apnea and insulin resistance: a role for microcirculation?

Obstructive sleep apnea is an increasingly recognized medical problem. The recent attention to its frequency in the general population and its important role in metabolic, vascular, and behavioral aspects have sharply increased the number and nature of investigations, thereby revealing new aspects that open new approaches in research. Whereas obstructive sleep apnea is a well-known phenomenon accompanying obesity and diabetes, new findings strongly suggest that this close relationship may also operate in the opposite direction. Indeed obstructive sleep apnea may be a primary feature inducing or aggravating a series of vascular and metabolic disturbances closely resembling the metabolic syndrome. This review will discuss established and potential mechanisms responsible for these changes. Obstructive sleep apnea indeed appears to gather all the elements necessary to induce insulin resistance, hypertension, and possibly heart failure. After careful analysis of these modifications and considering how they are intertwined, we propose that microcirculation could represent the common denominator mediating the progression of this pathology, as it is eventually the case in the metabolic syndrome and diabetes domain. This plausible hypothesis is discussed in detail and should be verified by appropriate preclinical and clinical protocols, which are now achievable by using noninvasive techniques in humans.

Effects of oxygen saturation on BOLD and arterial spin labelling perfusion fMRI signals studied in a motor activation task

Effects of oxygen availability on blood oxygenation level dependent (BOLD) and arterial spin labelling (ASL) perfusion functional magnetic resonance imaging (fMRI) signal changes upon motor activation were studied. Mild hypoxic hypoxia was induced by reducing the inspired oxygen content (FIO(2)) to 12%, decreasing blood oxygen saturation (Y) from 0.99 +/- 0.01 to 0.85 +/- 0.03. The fMRI signal characteristics were determined during finger tapping. BOLD activation volume decreased as a function of declining Y in the brain structures involved in execution of the motor task, however, the BOLD signal increase in activated parenchyma was not influenced by Y. ASL fMRI showed that the baseline CBF of 61.8 +/- 3.6 ml/100 g/min was not affected by hypoxic hypoxia. Similar to the BOLD fMRI, the volume of motor cortex areas displaying increase in perfusion by ASL fMRI decreased, but the signal change due to perfusion increase was not influenced in hypoxia. The present fMRI results show distinct patterns of haemodynamic and metabolic responses in the brain to motor task between normoxia and hypoxia. On one hand, neither BOLD nor ASL fMRI signal changes are influenced by hypoxia during motor activation. On the other hand, hypoxia attenuates increase in both BOLD and perfusion fMRI signals upon finger tapping from the levels determined in normoxia. These observations indicate that haemodynamic and metabolic responses may be heterogeneous in brain during execution of motor functions in mild hypoxia.

Heterogeneous oxygen extraction in the visual cortex during activation in mild hypoxic hypoxia revealed by quantitative functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) techniques were used to study haemodynamic and metabolic responses in human visual cortex during varying arterial blood oxygen saturation levels (Y(sat), determined by pulse-oximeter) and stimulation with contrast-reversing checkerboards. The visual-evoked potential amplitude remained constant at lowered Y(sat) of 0.82+/-0.03. Similarly, fMRI cerebral blood flow (CBF) responses were unchanged during reduced Y(sat). In contrast, visual cortex volume displaying blood oxygen level-dependent (BOLD) fMRI response decreased as a function of Y(sat), but the BOLD signal change of 3.6%+/-1.4% was constant. Oxygen extraction ratio (OER) during visual activation showed values of 0.26+/-0.03 for normal Y(sat). At lowered Y(sat), two OER patterns were observed. Firstly, a reduced OER of 0.14+/-0.03 in the visual cortex structures showing BOLD in hypoxia was observed. Secondly, signs of much higher OER in other parts of visual cortex were obtained. T2*-weighted magnetic resonance imaging revealed signal increases by 0.8%+/-0.4% with visual activation during lowered Y(sat) in the visual cortex structures, which showed BOLD of 3.6% in magnitude under normoxia. Because the CBF response in the visual cortex was quantitatively similar during stimulation in normoxia and hypoxia, attenuated T2*-weighted signal increase in parts of visual cortex indicated high OER during visual activation in hypoxia, which was close to that encountered in the resting brain. These spatially localised regions of tissue oxygen extraction and metabolism argue for dissociation between CBF and BOLD fMRI signals in mild hypoxia. The findings point to heterogeneity with regard to oxygen requirement and its coupling to the haemodynamic response in the brain.

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.

Overnight Changes in the Cerebral Vascular Response to Isocapnic Hypoxia and Hypercapnia in Healthy Humans

Background and Purpose— The reduction in hypercapnic cerebral vascular reactivity that occurs in the morning after sleep is associated with an increased risk of cerebral ischemia and stroke. It is not known if the cerebral vascular response to hypoxia is similarly reduced in the morning, but such a reduction could be considered a further risk factor for cerebral vascular disease.

Methods— To test if the cerebral vascular response to hypoxia is reduced in the morning, the overnight changes in the left middle cerebral artery velocity (MCAV) in response to isocapnic hypoxia (IH) and hypercapnia before and after a normal night sleep were determined in 18 individuals.

Results— From evening to morning, hypercapnic cerebral vascular reactivity decreased significantly (evening 2.0±0.4, morning 1.3±0.2 cm/sec/mm Hg; P <0.05); in contrast, the increase in MCAV in response to IH (−10% SaO 2 ) was unchanged (evening 9.0±1.4, morning 8.7±2.2%; P >0.05).

Conclusions— Our findings indicate that substantial differences exist in the regulation of the cerebral circulation in response to hypoxia and hypercapnia on waking from sleep. An intact cerebral vascular response to IH, during this time period, could be interpreted as a protective mechanism against cerebral ischemia and stroke; this is of particular relevance to patients with obstructive sleep apnea who arouse from sleep during hypoxia.

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 responses to hypoxia and hypocapnia in high-altitude dwellers

Cerebral blood flow is known to increase in response to hypoxia and to decrease with hypocapnia. It is not known, however, whether these responses are altered in high-altitude dwellers who are not only chronically hypoxic and hypocapnic, but also polycythaemic. Here we examined cerebral blood flow responses to hypoxia and hypocapnia, separately and together, in Andean high-altitude dwellers, including some with chronic mountain sickness (CMS), which is characterized by excessive polycythaemia. Studies were carried out at high altitude (Cerro de Pasco (CP), Peru; barometric pressure (P(B)) 450 mmHg) and repeated, following relief of the hypoxia, on the day following arrival at sea level (Lima, Peru; P(B) 755 mmHg). We compared these results with those from eight sea-level residents studied at sea level. In nine high-altitude normal subjects (HA) and nine CMS patients, we recorded middle cerebral artery mean blood flow velocity (MCAVm) using transcranial Doppler ultrasonography, and expressed responses as changes from baseline. MCAVm responses to hypoxia were determined by changing end-tidal partial pressure of oxygen (P(ET,O2)) from 100 to 50 mmHg, with end-tidal partial pressure of carbon dioxide clamped. MCAVm responses to hypocapnia were studied by voluntary hyperventilation with (P(ET,O2)) clamped at 100 and 50 mmHg. There were no significant differences between the cerebrovascular responses of the two groups to any of the interventions at either location. In both groups, the MCAVm responses to hypoxia were significantly greater at Lima than at CP (HA, 12.1 +/- 1.3 and 6.1 +/- 1.0%; CMS, 12.5 +/- 0.8 and 5.6 +/- 1.2%; P < 0.01 both groups). The responses at Lima were similar to those in the sea-level subjects (13.6 +/- 2.3%). The responses to normoxic hypocapnia in the altitude subjects were also similar at both locations and greater than those in sea-level residents. During hypoxia, both high-altitude groups showed responses to hypocapnia that were significantly smaller at Lima than at CP (HA, 2.17 +/- 0.23 and 3.29 +/- 0.34% mmHg(-1), P < 0.05; CMS, 1.87 +/- 0.16 and 3.23 +/- 0.24% mmHg(-1); P < 0.01). The similarity of the results from the two groups of altitude dwellers suggests that haematocrit is unlikely to greatly affect cerebrovascular reactivity to hypoxia and hypocapnia. The smaller vasodilatation to hypoxia and larger vasoconstriction to hypoxic hypocapnia at high altitude suggest that cerebrovascular responses may be impaired at the high altitude, i.e. a maladaptation. The changes in the responses within less than 24 h at sea level indicate that this impairment is rapidly reversible.