Severe hypoxaemic hypercapnia compounds cerebral oxidative-nitrosative stress during extreme apnoea: Implications for cerebral bioenergetic function.

We examined the extent to which apnoea-induced extremes of oxygen demand/carbon dioxide production impact redox regulation of cerebral bioenergetic function. Ten ultra-elite apnoeists (six men and four women) performed two maximal dry apnoeas preceded by normoxic normoventilation, resulting in severe end-apnoea hypoxaemic hypercapnia, and hyperoxic hyperventilation designed to ablate hypoxaemia, resulting in hyperoxaemic hypercapnia. Transcerebral exchange of ascorbate radicals (by electron paramagnetic resonance spectroscopy) and nitric oxide metabolites (by tri-iodide chemiluminescence) were calculated as the product of global cerebral blood flow (by duplex ultrasound) and radial arterial (a) to internal jugular venous (v) concentration gradients. Apnoea duration increased from 306 ± 62 s during hypoxaemic hypercapnia to 959 ± 201 s in hyperoxaemic hypercapnia (P ≤ 0.001). Apnoea generally increased global cerebral blood flow (all P ≤ 0.001) but was insufficient to prevent a reduction in the cerebral metabolic rates of oxygen and glucose (P = 0.015-0.044). This was associated with a general net cerebral output (v > a) of ascorbate radicals that was greater in hypoxaemic hypercapnia (P = 0.046 vs. hyperoxaemic hypercapnia) and coincided with a selective suppression in plasma nitrite uptake (a > v) and global cerebral blood flow (P = 0.034 to <0.001 vs. hyperoxaemic hypercapnia), implying reduced consumption and delivery of nitric oxide consistent with elevated cerebral oxidative-nitrosative stress. In contrast, we failed to observe equidirectional gradients consistent with S-nitrosohaemoglobin consumption and plasma S-nitrosothiol delivery during apnoea (all P ≥ 0.05). Collectively, these findings highlight a key catalytic role for hypoxaemic hypercapnia in cerebral oxidative-nitrosative stress. KEY POINTS: Local sampling of blood across the cerebral circulation in ultra-elite apnoeists determined the extent to which severe end-apnoea hypoxaemic hypercapnia (prior normoxic normoventilation) and hyperoxaemic hypercapnia (prior hyperoxic hyperventilation) impact free radical-mediated nitric oxide bioavailability and global cerebral bioenergetic function. Apnoea generally increased the net cerebral output of free radicals and suppressed plasma nitrite consumption, thereby reducing delivery of nitric oxide consistent with elevated oxidative-nitrosative stress. The apnoea-induced elevation in global cerebral blood flow was insufficient to prevent a reduction in the cerebral metabolic rates of oxygen and glucose. Cerebral oxidative-nitrosative stress was greater during hypoxaemic hypercapnia compared with hyperoxaemic hypercapnia and coincided with a lower apnoea-induced elevation in global cerebral blood flow, highlighting a key catalytic role for hypoxaemia. This applied model of voluntary human asphyxia might have broader implications for the management and treatment of neurological diseases characterized by extremes of oxygen demand and carbon dioxide production.

Cerebral uptake of microvesicles occurs in normocapnic but not hypocapnic passive hyperthermia in young healthy male adults.

Passive hyperthermia causes cerebral hypoperfusion primarily from heat-induced respiratory alkalosis. However, despite the cerebral hypoperfusion, it is possible that the mild alkalosis might help to attenuate cerebral inflammation. In this study, the cerebral exchange of extracellular vesicles (microvesicles), which are known to elicit pro-inflammatory responses when released in conditions of stress, were examined in hyperthermia with and without respiratory alkalosis. Ten healthy male adults were heated passively, using a warm water-perfused suit, up to core temperature + 2°C. Blood samples were taken from the radial artery and internal jugular bulb. Microvesicle concentrations were determined in platelet-poor plasma via cells expressing CD62E (activated endothelial cells), CD31+ /CD42b- (apoptotic endothelial cells), CD14 (monocytes) and CD45 (pan-leucocytes). Cerebral blood flow was measured via duplex ultrasound of the internal carotid and vertebral arteries to determine cerebral exchange kinetics. From baseline to poikilocapnic (alkalotic) hyperthermia, there was no change in microvesicle concentration from any cell origin measured (P-values all >0.05). However, when blood CO2 tension was normalized to baseline levels in hyperthermia, there was a marked increase in cerebral uptake of microvesicles expressing CD62E (P = 0.028), CD31+ /CD42b- (P = 0.003) and CD14 (P = 0.031) compared with baseline, corresponding to large increases in arterial but not jugular venous concentrations. In a subset of seven participants who underwent hypercapnia and hypocapnia in the absence of heating, there was no change in microvesicle concentrations or cerebral exchange, suggesting that hyperthermia potentiated the CO2 /pH-mediated cerebral uptake of microvesicles. These data provide insight into a potential beneficial role of respiratory alkalosis in heat stress. KEY POINTS: The hyperthermia-induced hyperventilatory response is observed in most humans, despite causing potentially harmful reductions in cerebral blood flow. We tested the hypothesis that the respiratory-induced alkalosis is associated with lower circulating microvesicle concentrations, specifically in the brain, despite the reductions in blood flow. At core temperature + 2°C with respiratory alkalosis, microvesicles derived from endothelial cells, monocytes and leucocytes were at concentrations similar to baseline in the arterial and cerebral venous circulation, with no changes in cross-brain microvesicle kinetics. However, when core temperature was increased by 2°C with CO2 /pH normalized to resting levels, there was a marked cerebral uptake of microvesicles derived from endothelial cells and monocytes. The CO2 /pH-mediated alteration in cerebral microvesicle uptake occurred only in hyperthermia. These new findings suggest that the heat-induced hyperventilatory response might serve a beneficial role by preventing potentially inflammatory microvesicle uptake in the brain.

Short-term ketone monoester supplementation improves cerebral blood flow and cognition in obesity: A randomized cross-over trial.

Adults with obesity are at increased risk of neurocognitive impairments, partly as a result of reduced cerebral blood flow and brain-derived neurotrophic factor (BDNF). Ketone supplements containing ß-hydroxybutyrate (ß-OHB) are a purported therapeutic strategy for improving brain health in at-risk populations. We tested the hypothesis that short-term ß-OHB supplementation will elevate cerebral blood flow and BDNF, as well as improve cognition in adults with obesity. In a placebo-controlled double-blind, cross-over design, 14 adults with obesity (10 females; aged 56 ± 12 years; body mass index = 33.8 ± 6.9 kg m-2 ) consumed 30 mL (12 g) of ß-OHB or placebo thrice-daily for 14 days. Blood flow (Q) and cerebrovascular conductance (CVC) were measured in the common carotid (CCA), internal carotid (ICA) and vertebral (VA) arteries by duplex ultrasound. BDNF was measured by an enzyme-linked immunosorbent assay. Cognition was assessed by the digit-symbol substitution (DSST), Stroop and task-switching tests. Following 14 days of ketone supplementation, we observed significant improvements in cerebrovascular outcomes including QCCA (+12%), QVA (+11%), VACVC (+12%) and VA shear rate (+10%). DSST performance significantly improved following ketone supplementation (+2.7 correct responses) and improved DSST performance was positively associated improvements in cerebrovascular outcomes including QCCA , CCACVC , QVA and VACVC . By contrast to one hypothesis, ß-OHB did not impact fasting serum and plasma BDNF. ß-OHB supplementation improved cognition in adults with obesity, which may be partly facilitated by improvements in cerebral blood flow. ß-OHB supplementation was well-tolerated and appears to be safe for cerebrovascular health, suggesting potential therapeutic benefits of ß-OHB in a population at risk of neurocognitive impairment. KEY POINTS: People with obesity are at increased risk of neurocognitive dysfunction, partly as a result of -induced reductions in cerebral blood flow (CBF) and brain-derived neurotrophic factor (BDNF). Ketone supplements containing ß-hydroxybutyrate (ß-OHB) reduce postprandial hyperglycaemia, which may increase CBF and BDNF, thereby protecting against obesity-related cognitive dysfunction. We show for the first time that 14 days of thrice-daily ß-OHB supplementation improves aspects of cognition and increases cerebrovascular flow, conductance and shear rate in the extracranial arteries of adults with obesity. Our preliminary data indicate a significant positive relationship between elevated CBF and improved cognition following ß-OHB supplementation. This trial provides a foundation for the potential non-pharmacological therapeutic application of ß-OHB supplementation in patient groups at risk of hyperglycaemic cerebrovascular disease and cognitive dysfunction.

Negligible influence of moderate to severe hyperthermia on blood-brain barrier permeability and neuronal parenchymal integrity in healthy men.

With growing use for hyperthermia as a cardiovascular therapeutic, there is surprisingly little information regarding the acute effects it may have on the integrity of the neurovascular unit (NVU). Indeed, relying on animal data would suggest hyperthermia comparable to levels attained in thermal therapy will disrupt the blood-brain barrier (BBB) and damage the cerebral parenchymal cells. We sought to address the hypothesis that controlled passive hyperthermia is not sufficient to damage the NVU in healthy humans. Young men (n = 11) underwent acute passive heating until +2°C or absolute esophageal temperature of 39.5°C. The presence of BBB opening was determined by trans-cerebral exchange kinetics (radial-arterial and jugular venous cannulation) of S100B. Neuronal parenchymal damage was determined by the trans-cerebral exchange of tau protein, neuron-specific enolase (NSE), and neurofilament-light protein (NF-L). Cerebral blood flow to calculate exchange kinetics was measured by duplex ultrasound of the right internal carotid and left vertebral artery. Passive heating was performed via a warm-water perfused suit. In hyperthermia, there was no increase in the cerebral exchange of S100B (P = 0.327), tau protein (P = 0.626), NF-L (P = 0.447), or NSE (P = 0.908) suggesting the +2°C core temperature is not sufficient to acutely stress the NVU in healthy men. However, there was a significant condition effect (P = 0.028) of NSE, corresponding to a significant increase in arterial (P = 0.023) but not venous (P = 0.173) concentrations in hyperthermia, potentially indicating extra-cerebral release of NSE. Collectively, results from the present study support the notion that in young men there is little concern for NVU damage with acute hyperthermia of +2 °C.NEW & NOTEWORTHY The acute effects of passive whole-body hyperthermia on the integrity of the neurovascular unit (NVU) in humans have remained unclear. We demonstrate that passive heating for ∼1 h until an increase of +2°C esophageal temperature in healthy men does not increase the cerebral release of neuronal parenchymal stress biomarkers, suggesting the NVU integrity is maintained. This preliminary study indicates passive heating is safe for the brain, at least in young healthy men.

Distinct contributions of skin and core temperatures to flow-mediated dilation of the brachial artery following passive heating.

We measured acute vascular responses to heat stress to examine the hypothesis that macrovascular endothelial-dependent dilation is improved in a shear-dependent manner, which is further modified by skin temperature. Twelve healthy males performed whole body heating (+1.3°C esophageal temperature), bilateral forearm heating (∼38°C skin temperature), and a time-matched (∼60 min) control condition on separate days in a counterbalanced order. Bilateral assessments of blood flow and brachial artery flow-mediated dilation (FMD) were performed before and 10 min after each condition with duplex Doppler ultrasound. To isolate the influence of shear stress, a pneumatic cuff was inflated (∼90 mmHg) around the right forearm during each condition to attenuate heat-induced rises in blood flow and shear stress. After forearm heating, FMD increased [cuffed: 4.7 (2.9)% to 6.8 (1.5)% and noncuffed: 5.1 (2.8)% to 6.4 (2.6)%] in both arms (time P < 0.01). Whole body heating also increased FMD in the noncuffed arm from 3.6 (2.2)% to 9.2 (3.2)% and in the cuffed arm from to 5.6 (3.0)% to 8.6 (4.9)% (time P < 0.01). After the time control, FMD decreased [cuffed: 6.3 (2.4)% to 4.7 (2.2)% and noncuffed: 6.1 (3.0)% to 4.5 (2.6)%] in both arms (time P = 0.03). Multiple linear regression (adjusted R2 = 0.421 P = 0.003) revealed that changes in esophageal temperature, skin temperatures, and heart rate explained the majority of the variance in this model (34%, 31%, and 21%, respectively). Our findings indicate that, in addition to shear stress, skin and core temperatures are likely important contributors to passive heating-induced vascular adaptations.NEW & NOTEWORTHY The primary determinant of vascular adaptations to lifestyle interventions, such as exercise and heat therapy, is repeated elevations in vascular shear stress. Whether skin or core temperatures also modulate the vascular adaptation to acute heat exposure is unknown, likely due to difficulty in dissociating the thermal and hemodynamic responses to heat. We found that skin and core temperatures modify the acute vascular responses to passive heating irrespective of the magnitude of increase in shear stress.

A ketone monoester drink reduces the glycemic response to an oral glucose challenge in individuals with obesity: a randomized trial.

BACKGROUND: Exogenous ketones make it possible to reach a state of ketosis that may improve metabolic control in humans. OBJECTIVES: The main objective of this study was to determine whether the ingestion of a ketone monoester (KE) drink before a 2-h oral-glucose-tolerance test (OGTT) would lower blood glucose concentrations. Secondary objectives were to determine the impact of KE on nonesterified fatty acid (NEFA) concentration and glucoregulatory hormones. METHODS: We conducted a randomized controlled crossover experiment in 15 individuals with obesity (mean ± SD age: 47 ± 10 y; BMI: 34 ± 5 kg/m2). After an overnight fast, participants consumed a KE drink [(R)-3-hydroxybutyl (R)-3-hydroxybutyrate; 0.45 mL/kg body weight] or taste-matched control drink 30 min before completing a 75-g OGTT. Participants and study personnel performing laboratory analyses were blinded to each condition. RESULTS: The KE increased d-ß-hydroxybutyrate to a maximum of ∼3.4 mM (P < 0.001) during the OGTT. Compared with the control drink, KE reduced glucose (-11%, P = 0.002), NEFA (-21%, P = 0.009), and glucagon-like peptide 1 (-31%, P = 0.001) areas under the curve (AUCs), whereas glucagon AUC increased (+11%, P = 0.030). No differences in triglyceride, C-peptide, and insulin AUCs were observed after the KE drink. Mean arterial blood pressure decreased and heart rate increased after the KE drink (both P < 0.01). CONCLUSIONS: A KE drink consumed before an OGTT lowered glucose and NEFA AUCs with no increase in circulating insulin. Our results suggest that a single drink of KE may acutely improve metabolic control in individuals with obesity. Future research is warranted to examine whether KE could be used safely to have longer-term effects on metabolic control. This trial was registered at clinicaltrials.gov as NCT03461068.

Cerebrovascular function is preserved during mild hyperthermia in cervical spinal cord injury.

STUDY DESIGN: Experimental study. OBJECTIVES: Compromised cerebrovascular function likely contributes to elevated neurological risk in spinal cord injury (SCI). Passive heating offers many cardiovascular and neurological health benefits; therefore, we aimed to determine the effects of an acute bout of heating on cerebrovascular function in chronic SCI. METHODS: Persons with cervical SCI (n = 15) and uninjured controls (CON; n = 15) completed 60 min of lower limb hot water immersion (40 °C). Assessments of middle cerebral (MCA) and posterior cerebral artery (PCA) velocities, pulsatilities, and neurovascular coupling (NVC) were performed using transcranial Doppler ultrasound. Duplex ultrasonography was used to index cerebral blood flow via the internal carotid artery (ICA), and carotid-femoral pulse-wave velocity (PWV) was measured using tonometry. The NVC response was quantified as the peak hyperemic value during 30-s cycles of visual stimulation. RESULTS: Mean arterial pressure changed differentially with heating [mean (standard deviation); SCI: +6(14) mmHg, CON: -8(12) mmHg; P = 0.01]. There were no differences in any intracranial artery measures (all P > 0.05), except for small (~10%) increases in MCA conductance in CON after heating vs. SCI (interaction P = 0.006). Resting ICA flow was greater in SCI vs. CON (P = 0.03) but did not change with heating in either group (interaction P = 0.34). There were also no between-group differences in the NVC response (ΔPCA conductance) pre- [SCI: 29(19)% vs. CON: 30(9)%] or post-heating [SCI 30(9)% vs. 25(9)%; interaction P = 0.22]. CONCLUSIONS: Mild acute heating does not impair or improve cerebrovascular function in SCI or CON. Thus, further study of the effects of chronic heating interventions are warranted.

Acute heat stress reduces biomarkers of endothelial activation but not macro- or microvascular dysfunction in cervical spinal cord injury.

Cardiovascular diseases (CVD) are highly prevalent in spinal cord injury (SCI), and peripheral vascular dysfunction might be a contributing factor. Recent evidence demonstrates that exposure to heat stress can improve vascular function and reduce the risk of CVD in uninjured populations. We therefore aimed to examine the extent of vascular dysfunction in SCI and the acute effects of passive heating. Fifteen participants with cervical SCI and 15 uninjured control (CON) participants underwent ultrasound assessments of vascular function and venous blood sampling for biomarkers of endothelial activation (i.e., CD62e+) and apoptosis (i.e., CD31+/42b-) before and after a 60-min exposure to lower limb hot water immersion (40°C). In SCI, macrovascular endothelial function was reduced in the brachial artery [SCI: 4.8 (3.2)% vs. CON: 7.6 (3.4)%, P = 0.04] but not the femoral artery [SCI: 3.7 (2.6)% vs. CON: 4.0 (2.1)%, P = 0.70]. Microvascular function, via reactive hyperemia, was ~40% lower in SCI versus CON in both the femoral and brachial arteries ( P < 0.01). Circulating concentrations of CD62e+ were elevated in SCI versus CON [SCI: 152 (106) microparticles/µl vs. CON: 58 (24) microparticles/µl, P < 0.05]. In response to heating, macrovascular and microvascular function remained unchanged, whereas increases (+83%) and decreases (-93%) in antegrade and retrograde shear rates, respectively, were associated with heat-induced reductions of CD62e+ concentrations in SCI to levels similar to CON ( P = 0.05). These data highlight the potential of acute heating to provide a safe and practical strategy to improve vascular function in SCI. The chronic effects of controlled heating warrant long-term testing. NEW & NOTEWORTHY Individuals with cervical level spinal cord injury exhibit selectively lower flow-mediated dilation in the brachial but not femoral artery, whereas peak reactive hyperemia was lower in both arteries compared with uninjured controls. After 60 min of lower limb hot water immersion, femoral artery blood flow and shear patterns were acutely improved in both groups. Elevated biomarkers of endothelial activation in the spinal cord injury group decreased with heating, but these biomarkers remained unchanged in controls.

Differential influence of vitamin C on the peripheral and cerebral circulation after diving and exposure to hyperoxia.

We examined if the diving-induced vascular changes in the peripheral and cerebral circulation could be prevented by oral antioxidant supplementation. Fourteen divers performed a single scuba dive to eighteen meter sea water for 47 min. Twelve of the divers participated in a follow-up study involving breathing 60% of oxygen at ambient pressure for 47 min. Before both studies, participants ingested vitamin C (2 g/day) or a placebo capsule for 6 days. After a 2-wk washout, the study was repeated with the different condition. Endothelium-dependent vasodilator function of the brachial artery was assessed pre- and postintervention using the flow-mediated dilation (FMD) technique. Transcranial Doppler ultrasound was used to measure intracranial blood velocities pre- and 90 min postintervention. FMD was reduced by ∼32.8% and ∼21.2% postdive in the placebo and vitamin C trial and posthyperoxic condition in the placebo trial by ∼28.2% ( P < 0.05). This reduction in FMD was attenuated by ∼10% following vitamin C supplementation in the hyperoxic study ( P > 0.05). Elevations in intracranial blood velocities 30 min after surfacing from diving were reduced in the vitamin C study compared with the placebo trial ( P < 0.05). O2 breathing had no postintervention effects on intracranial velocities ( P > 0.05). Prophylactic ingestion of vitamin C effectively abrogated peripheral vascular dysfunction following exposure to 60% O2 but did not abolish the postdive decrease in FMD. Transient elevations of intracranial velocities postdive were reduced by vitamin C. These findings highlight the differential influence of vitamin C on peripheral and cerebral circulations following scuba diving, which are only partly mediated via hyperoxia.

Clinical pathophysiology of hypoxic ischemic brain injury after cardiac arrest: a "two-hit" model.

Hypoxic ischemic brain injury (HIBI) after cardiac arrest (CA) is a leading cause of mortality and long-term neurologic disability in survivors. The pathophysiology of HIBI encompasses a heterogeneous cascade that culminates in secondary brain injury and neuronal cell death. This begins with primary injury to the brain caused by the immediate cessation of cerebral blood flow following CA. Thereafter, the secondary injury of HIBI takes place in the hours and days following the initial CA and reperfusion. Among factors that may be implicated in this secondary injury include reperfusion injury, microcirculatory dysfunction, impaired cerebral autoregulation, hypoxemia, hyperoxia, hyperthermia, fluctuations in arterial carbon dioxide, and concomitant anemia.Clarifying the underlying pathophysiology of HIBI is imperative and has been the focus of considerable research to identify therapeutic targets. Most notably, targeted temperature management has been studied rigorously in preventing secondary injury after HIBI and is associated with improved outcome compared with hyperthermia. Recent advances point to important roles of anemia, carbon dioxide perturbations, hypoxemia, hyperoxia, and cerebral edema as contributing to secondary injury after HIBI and adverse outcomes. Furthermore, breakthroughs in the individualization of perfusion targets for patients with HIBI using cerebral autoregulation monitoring represent an attractive area of future work with therapeutic implications.We provide an in-depth review of the pathophysiology of HIBI to critically evaluate current approaches for the early treatment of HIBI secondary to CA. Potential therapeutic targets and future research directions are summarized.

Evidence against redox regulation of energy homoeostasis in humans at high altitude.

The present study examined if free radicals and associated inflammatory sequelae influenced metabolic biomarkers involved in the neuro-endocrinological regulation of energy homoeostasis at high altitude. Sixteen mountaineers (11 males/five females) were matched for physical fitness and caloric intake and assigned in a double-blind manner to either antioxidant (n=8) or placebo (n=8) supplementation, which was enforced for 7 days at sea level and during an 11-day ascent to 4780 m. Enteral prophylaxis incorporated a daily bolus dose of 1 g of L-ascorbate, 400 international units of D,L-alpha-tocopherol acetate and 600 mg of alpha-lipoic acid. EPR (electron paramagnetic resonance) spectroscopic detection of PBN (alpha-phenyl-tert-butylnitrone) adducts confirmed an increase in the venous concentration of carbon-centred radicals at high altitude in the placebo group, whereas a decrease was observed in the antioxidant group (P<0.05 compared with that at sea level). EPR detection of DMSO/A*- (DMSO-supplemented ascorbate free radical) demonstrated that the increase in carbon-centred radicals at high altitude was associated with a decrease in ascorbate (r2=0.63; P<0.05). Ascent to high altitude (pooled placebo+antioxidant groups) also increased the expression of pro-inflammatory cytokines (P<0.05 compared with that at sea level) and biomarkers of skeletal tissue damage (P<0.05). Despite a general decrease in leptin, insulin and glucose at high altitude (pooled placebo+antioxidant groups; P<0.05 compared with that at sea level), persistent anorexia resulted in a selective loss of body fat (P<0.05). In conclusion, antioxidant prophylaxis decreased the concentration of carbon-centred radicals at high altitude (P<0.05 compared with the placebo group), but did not influence markers of inflammation, appetite-related peptides, ad libitum nutrient intake or body composition. Thus free radicals do not appear to be involved in the inflammatory response and subsequent control of eating behaviour at high altitude.