Regulation of cerebral blood flow by arterial PCO2 independent of metabolic acidosis at 5050 m.

KEY POINTS: We investigated the influence of arterial PCO2 (PaCO2 ) with and without experimentally altered pH on cerebral blood flow (CBF) regulation at sea level and with acclimatization to 5050 m. At sea level and high altitude, we assessed stepwise alterations in PaCO2 following metabolic acidosis (via 2 days of oral acetazolamide; ACZ) with and without acute restoration of pH (via intravenous sodium bicarbonate; ACZ+HCO3- ). Total resting CBF was unchanged between trials at each altitude even though arterial pH and [HCO3- ] (i.e. buffering capacity) were effectively altered. The cerebrovascular responses to changes in arterial [H+ ]/pH were consistent with the altered relationship between PaCO2 and [H+ ]/pH following ACZ at high altitude (i.e. leftward x-intercept shifts). Absolute cerebral blood velocity (CBV) and the sensitivity of CBV to PaCO2 was unchanged between trials at high altitude, indicating that CBF is acutely regulated by PaCO2 rather than arterial pH. ABSTRACT: Alterations in acid-base balance with progressive acclimatization to high altitude have been well-established. However, how respiratory alkalosis and the resultant metabolic compensation interact to regulate cerebral blood flow (CBF) is uncertain. We addressed this via three separate experimental trials at sea level and following partial acclimatization (14 to 20 days) at 5050 m; involving: (1) resting acid-base balance (control); (2) following metabolic acidosis via 2 days of oral acetazolamide at 250 mg every 8 h (ACZ; pH: Δ -0.07 ± 0.04 and base excess: Δ -5.7 ± 1.9 mEq⋅l-1 , trial effects: P < 0.001 and P < 0.001, respectively); and (3) after acute normalization of arterial acidosis via intravenous sodium bicarbonate (ACZ + HCO3- ; pH: Δ -0.01 ± 0.04 and base excess: Δ -1.5 ± 2.1 mEq⋅l-1 , trial effects: P = 1.000 and P = 0.052, respectively). Within each trial, we utilized transcranial Doppler ultrasound to assess the cerebral blood velocity (CBV) response to stepwise alterations in arterial PCO2 (PaCO2 ), i.e. cerebrovascular CO2 reactivity. Resting CBF (via Duplex ultrasound) was unaltered between trials within each altitude, indicating that respiratory compensation (i.e. Δ -3.4 ± 2.3 mmHg PaCO2 , trial effect: P < 0.001) was sufficient to offset any elevations in CBF induced via the ACZ-mediated metabolic acidosis. Between trials at high altitude, we observed consistent leftward shifts in both the PaCO2 -pH and CBV-pH responses across the CO2 reactivity tests with experimentally reduced arterial pH via ACZ. When indexed against PaCO2 - rather than pH - the absolute CBV and sensitivity of CBV-PaCO2 was unchanged between trials at high altitude. Taken together, following acclimatization, CO2 -mediated changes in cerebrovascular tone rather than arterial [H+ ]/pH is integral to CBF regulation at high altitude.

UBC-Nepal expedition: The use of oral antioxidants does not alter cerebrovascular function at sea level or high altitude.

NEW FINDINGS: What is the central question of the study? Does the use of antioxidants alter cerebrovascular function and blood flow at sea level (344 m) and/or high altitude (5050 m)? What is the main finding and its importance? This is the first study to investigate whether antioxidant administration alters cerebrovascular regulation and blood flow in response to hypercapnia, acute hypoxia and chronic hypoxia in healthy humans. We demonstrate that an acute dose of antioxidants does not alter cerebrovascular function and blood flow at sea level (344 m) or after 12 days at high altitude (5050 m). ABSTRACT: Hypoxia is associated with an increase in systemic and cerebral formation of free radicals and associated reactants that may be linked to impaired cerebral vascular function and neurological sequelae. To what extent oral antioxidant prophylaxis impacts cerebrovascular function in humans throughout the course of acclimatization to the hypoxia of terrestrial high altitude has not been examined. Thus, the purpose of the present study was to examine the influence of orally ingested antioxidants at clinically relevant doses (vitamins C and E and α-lipoic acid) on cerebrovascular regulation at sea level (344 m; n = 12; female n = 2 participants) and at high altitude (5050 m; n = 9; female n = 2) in a randomized, placebo-controlled and double-blinded crossover design. Hypercapnic and hypoxic cerebrovascular reactivity tests of the internal carotid artery (ICA) were conducted at sea level, and global and regional cerebral blood flow (CBF; i.e. ICA and vertebral artery) were assessed 10-12 days after arrival at 5050 m. At sea level, acute administration of antioxidants did not alter cerebral hypoxic cerebrovascular reactivity (pre versus post: 1.5 ± 0.7 versus 1.2 ± 0.8%∆CBF/-%∆SpO2; P = 0.96) or cerebral hypercapnic cerebrovascular reactivity (pre versus post: 5.7 ± 2.0 versus 5.8 ± 1.9%∆CBF/∆mmHg; P = 0.33). Furthermore, global CBF (P = 0.43) and cerebral vascular conductance (ICA P = 0.08; vertebral artery P = 0.32) were unaltered at 5050 m after antioxidant administration. In conclusion, these data show that an oral antioxidant cocktail known to attenuate systemic oxidative stress failed to alter cerebrovascular function at sea level and CBF during acclimatization to high altitude.

Aerobic exercise training does not alter vascular structure and function in chronic obstructive pulmonary disease.

NEW FINDINGS: What is the central question of this study? Chronic obstructive pulmonary disease (COPD) is associated with endothelial dysfunction, arterial stiffness and systemic inflammation, which are linked to increased cardiovascular disease risk. We asked whether periodized aerobic exercise training could improve vascular structure and function in patients with COPD. What is the main finding and its importance? Eight weeks of periodized aerobic training did not improve endothelial function, arterial stiffness or systemic inflammation in COPD, despite improvements in aerobic capacity, blood pressure and dyspnoea. Short-term training programmes may not be long enough to improve vascular-related cardiovascular risk in COPD. Chronic obstructive pulmonary disease (COPD) has been associated with endothelial dysfunction and arterial stiffening, which are predictive of future cardiovascular events. Although aerobic exercise improves vascular function in healthy individuals and those with chronic disease, it is unknown whether aerobic exercise can positively modify the vasculature in COPD. We examined the effects of 8 weeks of periodized aerobic training on vascular structure and function and inflammation in 24 patients with COPD (age, 69 ± 7 years; forced expiratory volume in 1 second as a percentage of predicted (FEV1 %pred), 68 ± 19%) and 20 matched control subjects (age, 64 ± 5 years; FEV1 %pred, 113 ± 16%) for comparison. Endothelial function was measured using brachial artery flow-mediated dilatation, whereas central and peripheral pulse wave velocity, carotid artery intima-media thickness, carotid compliance, distensibility and ß-stiffness index were measured using applanation tonometry and ultrasound. Peak aerobic power (V̇O2 peak ) was measured using an incremental cycling test. Upper and lower body cycling training was performed three times per week for 8 weeks, and designed to optimize vascular adaptation by increasing and sustaining vascular shear stress. Flow-mediated dilatation was not increased in COPD patients (+0.15 ± 2.27%, P = 0.82) or control subjects (+0.34 ± 3.20%, P = 0.64) and was not different between groups (P = 0.68). No significant improvements in central pulse wave velocity (COPD, +0.30 ± 1.79 m s-1 versus control subjects, -0.34 ± 1.47 m s-1 ) or other markers of vascular structure or function were found within or between groups. The V̇O2 peak increased significantly in COPD and control subjects, and was greater in control subjects (1.6 ± 1.4 versus 4.1 ± 3.7 ml kg min-1 , P = 0.003), while blood pressure and dyspnoea were reduced in COPD patients (P < 0.05). These findings demonstrate that 8 weeks of aerobic training improved cardiorespiratory fitness and blood pressure in COPD but had little effect on other established markers of cardiovascular disease risk.

Shear-mediated dilation of the internal carotid artery occurs independent of hypercapnia.

Evidence for shear stress as a regulator of carotid artery dilation in response to increased arterial CO2 was recently demonstrated in humans during sustained elevations in CO2 (hypercapnia); however, the relative contributions of CO2 and shear stress to this response remains unclear. We examined the hypothesis that, after a 30-s transient increase in arterial CO2 tension and consequent increase in internal carotid artery shear stress, internal carotid artery diameter would increase, indicating shear-mediated dilation, in the absence of concurrent hypercapnia. In 27 healthy participants, partial pressures of end-tidal O2 and CO2, ventilation (pneumotachography), blood pressure (finger photoplethysmography), heart rate (electrocardiogram), internal carotid artery flow, diameter, and shear stress (high-resolution duplex ultrasound), and middle cerebral artery blood velocity (transcranial Doppler) were measured during 4-min steady-state and transient 30-s hypercapnic tests (both +9 mmHg CO2). Internal carotid artery dilation was lower in the transient compared with steady-state hypercapnia (3.3 ± 1.9 vs. 5.3 ± 2.9%, respectively, P < 0.03). Increases in internal carotid artery shear stress preceded increases in diameter in both transient (time: 16.8 ± 13.2 vs. 59.4 ± 60.3 s, P < 0.01) and steady-state (time: 18.2 ± 14.2 vs. 110.3 ± 79.6 s, P < 0.01) tests. Internal carotid artery dilation was positively correlated with shear rate area under the curve in the transient (r2 = 0.44, P < 0.01) but not steady-state (r2 = 0.02, P = 0.53) trial. Collectively, these results suggest that hypercapnia induces shear-mediated dilation of the internal carotid artery in humans. This study further promotes the application and development of hypercapnia as a clinical strategy for the assessment of cerebrovascular vasodilatory function and health in humans.NEW & NOTEWORTHY Shear stress dilates the internal carotid artery in humans. This vasodilatory response occurs independent of other physiological factors, as demonstrated by our transient CO2 test, and is strongly correlated to shear area under the curve. Assessing carotid shear-mediated dilation may provide a future avenue for assessing cerebrovascular health and the risk of cerebrovascular events.

Ventricular-Arterial Coupling in Breast Cancer Patients After Treatment With Anthracycline-Containing Adjuvant Chemotherapy.

BACKGROUND: Anthracycline-containing chemotherapy (Anth-C) is associated with long-term cardiovascular mortality. Although cardiovascular risk assessment has traditionally focused on the heart, evidence has demonstrated that vascular dysfunction also occurs during and up to 1 year following Anth-C. Whether vascular dysfunction persists long-term or negatively influences cardiac function remains unknown. Hence, the present study evaluated ventricular-arterial coupling, in concert with measures of vascular structure and function, in the years following Anth-C. METHODS: Arterial elastance (Ea), end-systolic elastance (Ees), and ventricular-arterial coupling (Ea/Ees) were measured during rest and exercise using echocardiography. Resting vascular function (flow-mediated dilation) and structure (carotid intima-media thickness, arterial stiffness) were also measured. RESULTS: Thirty breast cancer survivors (6.5 ± 3.6 years after Anth-C) with normal left ventricular ejection fraction (LVEF) (60% ± 6%) and 30 matched controls were studied. At rest, no differences were found in Ea, Ees, Ea/Ees, or LVEF between groups. The normal exercise-induced increase in Ees was attenuated in survivors at 50% and 75% of maximal workload (p < .01). Ea/Ees was also higher at all workloads in the survivors compared with the controls (p < .01). No differences in vascular structure and function were observed between the two groups (p > .05). CONCLUSION: In the years after Anth-C, ventricular-arterial coupling was significantly attenuated during exercise, primarily owing to decreased LV contractility (indicated by a reduced Ees). This subclinical dysfunction appears to be isolated to the heart, as no differences in Ea were observed. The previously reported adverse effects of Anth-C on the vasculature appear to not persist in the years after treatment, as vascular structure and function were comparable to controls. IMPLICATIONS FOR PRACTICE: Anthracycline-induced cardiotoxicity results in significantly impaired ventricular-arterial coupling in the years following chemotherapy, owing specifically to decreased left ventricular contractility. This subclinical dysfunction was identified only under exercise stress. A comprehensive evaluation of vascular structure and function yielded no differences between those treated with anthracyclines and controls. Combined with a stress stimulus, ventricular-arterial coupling might hold significant value beyond characterization of integrative cardiovascular function, in particular as a part of a risk-stratification strategy after anthracycline-containing chemotherapy. Although vascular function and structure were not different in this cohort, this does not undermine the importance of identifying vascular (dys)function in this population, because increases in net arterial load during exercise might amplify the effect of reductions in contractility on cardiovascular function after anthracycline-containing chemotherapy.

Impact of sympathetic nervous system activity on post-exercise flow-mediated dilatation in humans.

Transient reduction in vascular function following systemic large muscle group exercise has previously been reported in humans. The mechanisms responsible are currently unknown. We hypothesised that sympathetic nervous system activation, induced by cycle ergometer exercise, would contribute to post-exercise reductions in flow-mediated dilatation (FMD). Ten healthy male subjects (28 ± 5 years) undertook two 30 min sessions of cycle exercise at 75% HR(max). Prior to exercise, individuals ingested either a placebo or an α1-adrenoreceptor blocker (prazosin; 0.05 mg kg(-1)). Central haemodynamics, brachial artery shear rate (SR) and blood flow profiles were assessed throughout each exercise bout and in response to brachial artery FMD, measured prior to, immediately after and 60 min after exercise. Cycle exercise increased both mean and antegrade SR (P < 0.001) with retrograde SR also elevated under both conditions (P < 0.001). Pre-exercise FMD was similar on both occasions, and was significantly reduced (27%) immediately following exercise in the placebo condition (t-test, P = 0.03). In contrast, FMD increased (37%) immediately following exercise in the prazosin condition (t-test, P = 0.004, interaction effect P = 0.01). Post-exercise FMD remained different between conditions after correction for baseline diameters preceding cuff deflation and also post-deflation SR. No differences in FMD or other variables were evident 60 min following recovery. Our results indicate that sympathetic vasoconstriction competes with endothelium-dependent dilator activity to determine post-exercise arterial function. These findings have implications for understanding the chronic impacts of interventions, such as exercise training, which affect both sympathetic activity and arterial shear stress.

Technical recommendations for the use of carotid duplex ultrasound for the assessment of extracranial blood flow.

Duplex ultrasound is an evolving technology that allows the assessment of volumetric blood flow in the carotid and vertebral arteries during a range of interventions along the spectrum of health and chronic disease. Duplex ultrasound can provide high-resolution diameter and velocity information in real-time and is noninvasive with minimal risks or contraindications. However, this ultrasound approach is a specialized technique requiring intensive training and stringent control of multiple complex settings; results are highly operator-dependent, and analysis approaches are inconsistent. Importantly, therefore, methodological differences can invalidate comparisons between different imaging modalities and studies; such methodological errors have potential to discredit study findings completely. The task of this review is to provide the first comprehensive, user-friendly technical guideline for the application of duplex ultrasound in measuring extracranial blood flow in human research. An update on recent developments in the use of edge-detection software for offline analysis is highlighted, and suggestions for future directions in this field are provided. These recommendations are presented in an attempt to standardize measurements across research groups and, hence, ultimately to improve the accuracy and reproducibility of measuring extracranial blood flow both within subjects and between groups.

Impact of transient hypotension on regional cerebral blood flow in humans.

We examined the impact of progressive hypotension with and without hypocapnia on regional extracranial cerebral blood flow (CBF) and intracranial velocities. Participants underwent progressive lower-body negative pressure (LBNP) until pre-syncope to inflict hypotension. End-tidal carbon dioxide was clamped at baseline levels (isocapnic trial) or uncontrolled (poikilocapnic trial). Middle cerebral artery (MCA) and posterior cerebral artery (PCA) blood velocities (transcranial Doppler; TCD), heart rate, blood pressure and end-tidal carbon dioxide were obtained continuously. Measurements of internal carotid artery (ICA) and vertebral artery (VA) blood flow (ICABF and VABF respectively) were also obtained. Overall, blood pressure was reduced by ~20% from baseline in both trials (P<0.001). In the isocapnic trial, end-tidal carbon dioxide was successfully clamped at baseline with hypotension, whereas in the poikilocapnic trial it was reduced by 11.1 mmHg (P<0.001) with hypotension. The decline in the ICABF with hypotension was comparable between trials (-139 ± 82 ml; ~30%; P<0.0001); however, the decline in the VABF was -28 ± 22 ml/min (~21%) greater in the poikilocapnic trial compared with the isocapnic trial (P=0.002). Regardless of trial, the blood flow reductions in ICA (-26 ± 14%) and VA (-27 ± 14%) were greater than the decline in MCA (-21 ± 15%) and PCA (-19 ± 10%) velocities respectively (P ≤ 0.01). Significant reductions in the diameter of both the ICA (~5%) and the VA (~7%) contributed to the decline in cerebral perfusion with systemic hypotension, independent of hypocapnia. In summary, our findings indicate that blood flow in the VA, unlike the ICA, is sensitive to changes hypotension and hypocapnia. We show for the first time that the decline in global CBF with hypotension is influenced by arterial constriction in the ICA and VA. Additionally, our findings suggest TCD measures of blood flow velocity may modestly underestimate changes in CBF during hypotension with and without hypocapnia, particularly in the posterior circulation.

The contribution of arterial blood gases in cerebral blood flow regulation and fuel utilization in man at high altitude.

The effects of partial acclimatization to high altitude (HA; 5,050 m) on cerebral metabolism and cerebrovascular function have not been characterized. We hypothesized (1) increased cerebrovascular reactivity (CVR) at HA; and (2) that CO2 would affect cerebral metabolism more than hypoxia. PaO2 and PaCO2 were manipulated at sea level (SL) to simulate HA exposure, and at HA, SL blood gases were simulated; CVR was assessed at both altitudes. Arterial-jugular venous differences were measured to calculate cerebral metabolic rates and cerebral blood flow (CBF). We observed that (1) partial acclimatization yields a steeper CO2-H(+) relation in both arterial and jugular venous blood; yet (2) CVR did not change, despite (3) mean arterial pressure (MAP)-CO2 reactivity being doubled at HA, thus indicating effective cerebral autoregulation. (4) At SL hypoxia increased CBF, and restoration of oxygen at HA reduced CBF, but neither had any effect on cerebral metabolism. Acclimatization resets the cerebrovasculature to chronic hypocapnia.

Hypoxia, not pulmonary vascular pressure, induces blood flow through intrapulmonary arteriovenous anastomoses.

KEY POINTS: Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) is increased by acute hypoxia during rest by unknown mechanisms. Oral administration of acetazolamide blunts the pulmonary vascular pressure response to acute hypoxia, thus permitting the observation of IPAVA blood flow with minimal pulmonary pressure change. Hypoxic pulmonary vasoconstriction was attenuated in humans following acetazolamide administration and partially restored with bicarbonate infusion, indicating that the effects of acetazolamide on hypoxic pulmonary vasoconstriction may involve an interaction between arterial pH and PCO2. We observed that IPAVA blood flow during hypoxia was similar before and after acetazolamide administration, even after acid-base status correction, indicating that pulmonary pressure, pH and PCO2 are unlikely regulators of IPAVA blood flow. ABSTRACT: Blood flow through intrapulmonary arteriovenous anastomoses (IPAVA) is increased with exposure to acute hypoxia and has been associated with pulmonary artery systolic pressure (PASP). We aimed to determine the direct relationship between blood flow through IPAVA and PASP in 10 participants with no detectable intracardiac shunt by comparing: (1) isocapnic hypoxia (control); (2) isocapnic hypoxia with oral administration of acetazolamide (AZ; 250 mg, three times a day for 48 h) to prevent increases in PASP; and (3) isocapnic hypoxia with AZ and 8.4% NaHCO3 infusion (AZ + HCO3 (-) ) to control for AZ-induced acidosis. Isocapnic hypoxia (20 min) was maintained by end-tidal forcing, blood flow through IPAVA was determined by agitated saline contrast echocardiography and PASP was estimated by Doppler ultrasound. Arterial blood samples were collected at rest before each isocapnic-hypoxia condition to determine pH, [HCO3(-)] and Pa,CO2. AZ decreased pH (-0.08 ± 0.01), [HCO3(-)] (-7.1 ± 0.7 mmol l(-1)) and Pa,CO2 (-4.5 ± 1.4 mmHg; P < 0.01), while intravenous NaHCO3 restored arterial blood gas parameters to control levels. Although PASP increased from baseline in all three hypoxic conditions (P < 0.05), a main effect of condition expressed an 11 ± 2% reduction in PASP from control (P < 0.001) following AZ administration while intravenous NaHCO3 partially restored the PASP response to isocapnic hypoxia. Blood flow through IPAVA increased during exposure to isocapnic hypoxia (P < 0.01) and was unrelated to PASP, cardiac output and pulmonary vascular resistance for all conditions. In conclusion, isocapnic hypoxia induces blood flow through IPAVA independent of changes in PASP and the influence of AZ on the PASP response to isocapnic hypoxia is dependent upon the H(+) concentration or Pa,CO2.

Chemoreceptor Responsiveness at Sea Level Does Not Predict the Pulmonary Pressure Response to High Altitude.

BACKGROUND: The hypoxic ventilatory response (HVR) at sea level (SL) is moderately predictive of the change in pulmonary artery systolic pressure (PASP) to acute normobaric hypoxia. However, because of progressive changes in the chemoreflex control of breathing and acid-base balance at high altitude (HA), HVR at SL may not predict PASP at HA. We hypothesized that resting oxygen saturation as measured by pulse oximetry (Spo2) at HA would correlate better than HVR at SL with PASP at HA. METHODS: In 20 participants at SL, we measured normobaric, isocapnic HVR (L/min · -%Spo2⁻¹) and resting PASP using echocardiography. Both resting Spo2 and PASP measures were repeated on day 2 (n = 10), days 4 to 8 (n = 12), and 2 to 3 weeks (n = 8) after arrival at 5,050 m. These data were also collected at 5,050 m in life-long HA residents (ie, Sherpa [n = 21]). RESULTS: Compared with SL, Spo2 decreased from 98.6% to 80.5% (P < .001), whereas PASP increased from 21.7 to 34.0 mm Hg (P < .001) after 2 to 3 weeks at 5,050 m. Isocapnic HVR at SL was not related to Spo2 or PASP at any time point at 5,050 m (all P > .05). Sherpa had lower PASP (P < .01) than lowlanders on days 4 to 8 despite similar Spo2. Upon correction for hematocrit, Sherpa PASP was not different from lowlanders at SL but was lower than lowlanders at all HA time points. At 5,050 m, although Spo2 was not related to PASP in lowlanders at any point (all R² ≤ 0.05, P > .50), there was a weak relationship in the Sherpa (R² = 0.16, P = .07). CONCLUSIONS: We conclude that neither HVR at SL nor resting Spo2 at HA correlates with elevations in PASP at HA.

Impact of hypocapnia and cerebral perfusion on orthostatic tolerance.

We examined two novel hypotheses: (1) that orthostatic tolerance (OT) would be prolonged when hyperventilatory-induced hypocapnia (and hence cerebral hypoperfusion) was prevented; and (2) that pharmacological reductions in cerebral blood flow (CBF) at baseline would lower the 'CBF reserve', and ultimately reduce OT. In study 1 (n = 24; aged 25 ± 4 years) participants underwent progressive lower-body negative pressure (LBNP) until pre-syncope; end-tidal carbon dioxide (P ET , CO 2) was clamped at baseline levels (isocapnic trial) or uncontrolled. In study 2 (n = 10; aged 25 ± 4 years), CBF was pharmacologically reduced by administration of indomethacin (INDO; 1.2 mg kg(-1)) or unaltered (placebo) followed by LBNP to pre-syncope. Beat-by-beat measurements of middle cerebral artery blood flow velocity (MCAv; transcranial Doppler), heart rate (ECG), blood pressure (BP; Finometer) and end-tidal gases were obtained continuously. In a subset of subjects' arterial-to-jugular venous differences were obtained to examine the independent impact of hypocapnia or cerebral hypoperfusion (following INDO) on cerebral oxygen delivery and extraction. In study 1, during the isocapnic trial, P ET , CO 2 was successfully clamped at baseline levels at pre-syncope (38.3 ± 2.7 vs. 38.5 ± 2.5 mmHg respectively; P = 0.50). In the uncontrolled trial, P ET , CO 2 at pre-syncope was reduced by 10.9 ± 3.9 mmHg (P ≤ 0.001). Compared to the isocapnic trial, the decline in mean MCAv was 15 ± 4 cm s(-1) (35%; P ≤ 0.001) greater in the uncontrolled trial, yet the time to pre-syncope was comparable between trials (544 ± 130 vs. 572 ± 180 s; P = 0.30). In study 2, compared to placebo, INDO reduced resting MCAv by 19 ± 4 cm s(-1) (31%; P ≤ 0.001), but time to pre-syncope remained similar between trials (placebo: 1123 ± 138 s vs. INDO: 1175 ± 212 s; P = 0.53). The brain extracted more oxygen in face of hypocapnia (34% to 53%) or cerebral hypoperfusion (34% to 57%) to compensate for reductions in delivery. In summary, cerebral hypoperfusion either at rest or induced by hypocapnia at pre-syncope does not impact OT, probably due to a compensatory increase in oxygen extraction.

Static autoregulation in humans: a review and reanalysis.

INTRODUCTION: Cerebral autoregulation (CA) is a theoretical construct characterized by the relationship between mean arterial pressure (MAP) and cerebral blood flow (CBF). We performed a comprehensive literature search to provide an up-to-date review on the static relationship between MAP and CBF. METHODS: The results are based on 40 studies (49 individual experimental protocols) in healthy subjects between 18 and 65 years. Exclusion criteria were: a ΔMAP <5%, hypoxia/hyperoxia or hypo/hypercapnia, and unstable levels (<2 min stages). The partial pressure of arterial CO2 (PaCO2) was measured in a subset of the included studies (n=28); therefore, CBF was also adjusted to account for small changes in PaCO2. RESULTS: The linear regression coefficient between MAP and CBF (or velocity) of 0.82±0.77%ΔCBF/%ΔMAP during decreases in MAP (n=23 experiments) was significantly different than the relationship of 0.21±0.47%ΔCBF/%ΔMAP during increases (n=26 experiments; p<0.001). After correction for increases/decreases in PaCO2, the slopes were not significantly different: 0.64±1.16%ΔCBF/%ΔMAP (n=16) and 0.39±0.30%ΔCBF/%ΔMAP (n=12) for increased vs. decreased MAP changes, respectively (p=0.60). CONCLUSION: The autoregulatory ability of the cerebral circulation appears to be more active in buffering increases in MAP as compared to reductions in MAP. However, the statistical finding of hysteresis is lost following an attempt to correct for PaCO2.

Effect of acute hypoxia on regional cerebral blood flow: effect of sympathetic nerve activity.

We examined 1) whether global cerebral blood flow (CBF) would increase across a 6-h bout of normobaric poikilocapnic hypoxia and be mediated by a larger increase in blood flow in the vertebral artery (VA) than in the internal carotid artery (ICA); and 2) whether additional increases in global CBF would be evident following an α1-adrenergic blockade via further dilation of the ICA and VA. In 11 young normotensive individuals, ultrasound measures of ICA and VA flow were obtained in normoxia (baseline) and following 60, 210, and 330 min of hypoxia (FiO2 = 0.11). Ninety minutes prior to final assessment, participants received an α1-adrenoreceptor blocker (prazosin, 1 mg/20 kg body mass) or placebo. Compared with baseline, following 60, 220, and 330 min of hypoxia, global CBF [(ICAFlow + VAFlow) ∗ 2] increased by 160 ± 52 ml/min (+28%; P = 0.05), 134 ± 23 ml/min (+23%; P = 0.02), and 113 ± 51 (+19%; P = 0.27), respectively. Compared with baseline, ICAFlow increased by 23% following 60 min of hypoxia (P = 0.06), after which it progressively declined. The percentage increase in VA flow was consistently larger than ICA flow during hypoxia by ∼20% (P = 0.002). Compared with baseline, ICA and VA diameters increased during hypoxia by ∼9% and ∼12%, respectively (P ≤ 0.05), and were correlated with reductions in SaO2. Flow and diameters were unaltered following α1 blockade (P ≥ 0.10). In conclusion, elevations in global CBF during acute hypoxia are partly mediated via greater increases in VA flow compared with ICA flow; this regional difference was unaltered following α1 blockade, indicating that a heightened sympathetic nerve activity with hypoxia does not constrain further dilation of larger extracranial blood vessels.

Resting pulmonary haemodynamics and shunting: a comparison of sea-level inhabitants to high altitude Sherpas.

The incidence of blood flow through intracardiac shunt and intrapulmonary arteriovenous anastomoses (IPAVA) may differ between Sherpas permanently residing at high altitude (HA) and sea-level (SL) inhabitants as a result of evolutionary pressure to improve gas exchange and/or resting pulmonary haemodynamics. To test this hypothesis we compared sea-level inhabitants at SL (SL-SL; n = 17), during acute isocapnic hypoxia (SL-HX; n = 7) and following 3 weeks at 5050 m (SL-HA; n = 8 non-PFO subjects) to Sherpas at 5050 m (n = 14). SpO2, heart rate, pulmonary artery systolic pressure (PASP) and cardiac index (Qi) were measured during 5 min of room air breathing at SL and HA, during 20 min of isocapnic hypoxia (SL-HX; PETO2 = 47 mmHg) and during 5 min of hyperoxia (FIO2 = 1.0; Sherpas only). Intracardiac shunt and IPAVA blood flow was evaluated by agitated saline contrast echocardiography. Although PASP was similar between groups at HA (Sherpas: 30.0 ± 6.0 mmHg; SL-HA: 32.7 ± 4.2 mmHg; P = 0.27), it was greater than SL-SL (19.4 ± 2.1 mmHg; P < 0.001). The proportion of subjects with intracardiac shunt was similar between groups (SL-SL: 41%; Sherpas: 50%). In the remaining subjects, IPAVA blood flow was found in 100% of subjects during acute isocapnic hypoxia at SL, but in only 4 of 7 Sherpas and 1 of 8 SL-HA subjects at rest. In conclusion, differences in resting pulmonary vascular regulation, intracardiac shunt and IPAVA blood flow do not appear to account for any adaptation to HA in Sherpas. Despite elevated pulmonary pressures and profound hypoxaemia, IPAVA blood flow in all subjects at HA was lower than expected compared to acute normobaric hypoxia.

Cerebral pressure-flow relationship in lowlanders and natives at high altitude.

We investigated if dynamic cerebral pressure-flow relationships in lowlanders are altered at high altitude (HA), differ in HA natives and after return to sea level (SL). Lowlanders were tested at SL (n=16), arrival to 5,050 m, after 2-week acclimatization (with and without end-tidal PO2 normalization), and upon SL return. High-altitude natives (n=16) were tested at 5,050 m. Testing sessions involved resting spontaneous and driven (squat-stand maneuvers at very low (VLF, 0.05 Hz) and low (LF, 0.10 Hz) frequencies) measures to maximize blood pressure (BP) variability and improve assessment of the pressure-flow relationship using transfer function analysis (TFA). Blood flow velocity was assessed in the middle (MCAv) and posterior (PCAv) cerebral arteries. Spontaneous VLF and LF phases were reduced and coherence was elevated with acclimatization to HA (P<0.05), indicating impaired pressure-flow coupling. However, when BP was driven, both the frequency- and time-domain metrics were unaltered and comparable with HA natives. Acute mountain sickness was unrelated to TFA metrics. In conclusion, the driven cerebral pressure-flow relationship (in both frequency and time domains) is unaltered at 5,050 m in lowlanders and HA natives. Our findings indicate that spontaneous changes in TFA metrics do not necessarily reflect physiologically important alterations in the capacity of the brain to regulate BP.

Conduit artery structure and function in lowlanders and native highlanders: relationships with oxidative stress and role of sympathoexcitation.

Research detailing the normal vascular adaptions to high altitude is minimal and often confounded by pathology (e.g., chronic mountain sickness) and methodological issues. We examined vascular function and structure in: (1) healthy lowlanders during acute hypoxia and prolonged (∼2 weeks) exposure to high altitude, and (2) high-altitude natives at 5050 m (highlanders). In 12 healthy lowlanders (aged 32 ± 7 years) and 12 highlanders (Sherpa; 33 ± 14 years) we assessed brachial endothelium-dependent flow-mediated dilatation (FMD), endothelium-independent dilatation (via glyceryl trinitrate; GTN), common carotid intima-media thickness (CIMT) and diameter (ultrasound), and arterial stiffness via pulse wave velocity (PWV; applanation tonometry). Cephalic venous biomarkers of free radical-mediated lipid peroxidation (lipid hydroperoxides, LOOH), nitrite (NO2-) and lipid soluble antioxidants were also obtained at rest. In lowlanders, measurements were performed at sea level (334 m) and between days 3-4 (acute high altitude) and 12-14 (chronic high altitude) following arrival to 5050 m. Highlanders were assessed once at 5050 m. Compared with sea level, acute high altitude reduced lowlanders' FMD (7.9 ± 0.4 vs. 6.8 ± 0.4%; P = 0.004) and GTN-induced dilatation (16.6 ± 0.9 vs. 14.5 ± 0.8%; P = 0.006), and raised central PWV (6.0 ± 0.2 vs. 6.6 ± 0.3 m s(-1); P = 0.001). These changes persisted at days 12-14, and after allometrically scaling FMD to adjust for altered baseline diameter. Compared to lowlanders at sea level and high altitude, highlanders had a lower carotid wall:lumen ratio (∼19%, P ≤ 0.04), attributable to a narrower CIMT and wider lumen. Although both LOOH and NO2- increased with high altitude in lowlanders, only LOOH correlated with the reduction in GTN-induced dilatation evident during acute (n = 11, r = -0.53) and chronic (n = 7, r = -0.69; P ≤ 0.01) exposure to 5050 m. In a follow-up, placebo-controlled experiment (n = 11 healthy lowlanders) conducted in a normobaric hypoxic chamber (inspired O2 fraction (F IO 2) = 0.11; 6 h), a sustained reduction in FMD was evident within 1 h of hypoxic exposure when compared to normoxic baseline (5.7 ± 1.6 vs. 8.0 ±1.3%; P < 0.01); this decline in FMD was largely reversed following α1-adrenoreceptor blockade. In conclusion, high-altitude exposure in lowlanders caused persistent impairment in vascular function, which was mediated partially via oxidative stress and sympathoexcitation. Although a lifetime of high-altitude exposure neither intensifies nor attenuates the impairments seen with short-term exposure, chronic high-altitude exposure appears to be associated with arterial remodelling.

Regional changes in brain blood flow during severe passive hyperthermia: effects of PaCO2 and extracranial blood flow.

We investigated 1) the regional distribution of cerebral blood flow (CBF), 2) the influence of end-tidal Pco2 (PetCO2) on CBF, and 3) the potential for an extracranial blood "steal" from the anterior brain region during passive hyperthermia. Nineteen (13 male) volunteers underwent supine passive heating until a steady-state esophageal temperature of 2°C above resting was established. Measurements were obtained 1) during normothermia (Normo), 2) during poikilocapnic hyperthermia (Hyper), and 3) during hyperthermia with PetCO2 and end-tidal Po2 clamped to Normo levels (Hyper-clamp). Blood flow in the internal carotid (Qica), vertebral (QVA), and external carotid (Qeca) arteries (Duplex ultrasound), blood velocity of the middle cerebral (MCAv) and posterior cerebral (PCAv) arteries (transcranial Doppler), and cutaneous vascular conductance on the cheek (cheek CVC; Doppler velocimetry) were measured at each stage. During Hyper, PetCO2 was lowered by 7.0 ± 5.2 mmHg, resulting in a reduction in Qica (-18 ± 17%), Qva (-31 ± 21%), MCAv (-22 ± 13%), and PCAv (-18 ± 10%) compared with Normo (P < 0.05). The reduction in QVA was greater than that in QICA (P = 0.017), MCAv (P = 0.047), and PCAv (P = 0.034). Blood flow/velocity was completely restored in each intracranial vessel (ICA, VA, MCA, and PCA) during Hyper-clamp. Despite a ∼250% increase in QECA and a subsequent increase in cheek CVC during Hyper compared with Normo, reductions in QICA were unrelated to changes in QECA. These data provide three novel findings: 1) hyperthermia attenuates QVA to a greater extent than QICA, 2) reductions in CBF during hyperthermia are governed primarily by reductions in arterial Pco2, and 3) increased QECA is unlikely to compromise QICA during hyperthermia.

Slow breathing as a means to improve orthostatic tolerance: a randomized sham-controlled trial.

Endogenous oscillations in blood pressure (BP) and cerebral blood flow have been associated with improved orthostatic tolerance. Although slow breathing induces such responses, it has not been tested as a therapeutic strategy to improve orthostatic tolerance. With the use of a randomized, crossover sham-controlled design, we tested the hypothesis that breathing at six breaths/min (vs. spontaneous breathing) would improve orthostatic tolerance via inducing oscillations in mean arterial BP (MAP) and cerebral blood flow. Sixteen healthy participants (aged 25 ± 4 yr; mean ± SD) had continuous beat-to-beat measurements of middle cerebral artery blood velocity (MCAv), BP (finometer), heart rate (ECG), and end-tidal carbon dioxide partial pressure during an incremental orthostatic stress test to presyncope by combining head-up tilt with incremental lower-body negative pressure. Tolerance time to presyncope was improved (+15%) with slow breathing compared with spontaneous breathing (29.2 ± 5.4 vs. 33.7 ± 6.0 min; P < 0.01). The improved tolerance was reflected in elevations in low-frequency (LF; 0.07-0.2 Hz) oscillations of MAP and mean MCAv, improved metrics of dynamic cerebrovascular control (increased LF phase and reduced LF gain), and a reduced rate of decline for MCAv (-0.60 ± 0.27 vs. -0.99 ± 0.51 cm·s(-1)·min(-1); P < 0.01) and MAP (-0.50 ± 0.37 vs. -1.03 ± 0.80 mmHg/min; P = 0.01 vs. spontaneous breathing) across time from baseline to presyncope. Our findings show that orthostatic tolerance can be improved within healthy individuals with a simple, nonpharmacological breathing strategy. The mechanisms underlying this improvement are likely mediated via the generation of negative intrathoracic pressure during slow and deep breathing and the related beneficial impact on cerebrovascular and autonomic function.

Initial orthostatic hypotension and cerebral blood flow regulation: effect of α1-adrenoreceptor activity.

We examined the hypothesis that α(1)-adrenergic blockade would lead to an inability to correct initial orthostatic hypotension (IOH) and cerebral hypoperfusion, leading to symptoms of presyncope. Twelve normotensive humans (aged 25 ± 1 yr; means ± SE) attempted to complete a 3-min upright stand, 90 min after the administration of either α(1)-blockade (prazosin, 1 mg/20 kg body wt) or placebo. Continuous beat-to-beat measurements of middle cerebral artery velocity (MCAv; Doppler), blood pressure (finometer), heart rate, and end-tidal Pco(2) were obtained. Compared with placebo, the α(1)-blockade reduced resting mean arterial blood pressure (MAP) (-15%; P < 0.01); MCAv remained unaltered (P ≥ 0.28). Upon standing, although the absolute level of MAP was lower following α(1)-blockade (39 ± 10 mmHg vs. 51 ± 14 mmHg), the relative difference in IOH was negligible in both trials (mean difference in MAP: 2 ± 2 mmHg; P = 0.50). Compared with the placebo trial, the declines in MCAv and Pet(CO(2)) during IOH were greater in the α(1)-blockade trial by 12 ± 4 cm/s and 4.4 ± 1.3 mmHg, respectively (P ≤ 0.01). Standing tolerance was markedly reduced in the α(1)-blockade trial (75 ± 17 s vs. 180 ± 0 s; P < 0.001). In summary, while IOH was little affected by α(1)-blockade, the associated decline in MCAv was greater in the blockade condition. Unlike in the placebo trial, the extent of IOH and cerebral hypoperfusion failed to recover toward baseline in the α(1)-blockade trial leading to presyncope. Although the development of IOH is not influenced by the α(1)-adrenergic receptor pathway, this pathway is critical in the recovery from IOH to prevent cerebral hypoperfusion and ultimately syncope.