Influence of the mode of heating on cerebral blood flow, non-invasive intracranial pressure and thermal tolerance in humans

Heat-related changes in the velocity and kinetic energy of flowing blood influence the human heart's output during hyperthermia

Passive whole-body hyperthermia increases limb blood flow and cardiac output (Q ̇ $\dot Q$ ), but the interplay between peripheral and central thermo-haemodynamic mechanisms remains unclear. Here we tested the hypothesis that local hyperthermia-induced alterations in peripheral blood flow and blood kinetic energy modulate flow to the heart andQ ̇ $\dot Q$ . Body temperatures, regional (leg, arm, head) and systemic haemodynamics, and left ventricular (LV) volumes and functions were assessed in eight healthy males during: (1) 3 h control (normothermic condition); (2) 3 h of single-leg heating; (3) 3 h of two-leg heating; and (4) 2.5 h of whole-body heating. Leg, forearm, and extracranial blood flow increased in close association with local rises in temperature while brain perfusion remained unchanged. Increases in blood velocity with small to no changes in the conduit artery diameter underpinned the augmented limb and extracranial perfusion. In all heating conditions,Q ̇ $\dot Q$ increased in association with proportional elevations in systemic vascular conductance, related to enhanced blood flow, blood velocity, vascular conductance and kinetic energy in the limbs and head (all R2 ≥ 0.803; P < 0.001), but not in the brain. LV systolic (end-systolic elastance and twist) and diastolic functional profiles (untwisting rate), pulmonary ventilation and systemic aerobic metabolism were only altered in whole-body heating. These findings substantiate the idea that local hyperthermia-induced selective alterations in peripheral blood flow modulate the magnitude of flow to the heart andQ ̇ $\dot Q$ through changes in blood velocity and kinetic energy. Localised heat-activated events in the peripheral circulation therefore affect the human heart's output. KEY POINTS: Local and whole-body hyperthermia increases limb and systemic perfusion, but the underlying peripheral and central heat-sensitive mechanisms are not fully established. Here we investigated the regional (leg, arm and head) and systemic haemodynamics (cardiac output:Q ̇ $\dot Q$ ) during passive single-leg, two-leg and whole-body hyperthermia to determine the contribution of peripheral and central thermosensitive factors in the control of human circulation. Single-leg, two-leg, and whole-body hyperthermia induced graded increases in leg blood flow andQ ̇ $\dot Q$ . Brain blood flow, however, remained unchanged in all conditions. Ventilation, extracranial blood flow and cardiac systolic and diastolic functions only increased during whole-body hyperthermia. The augmentedQ ̇ $\dot Q$ with hyperthermia was tightly related to increased limb and head blood velocity, flow and kinetic energy. The findings indicate that local thermosensitive mechanisms modulate regional blood velocity, flow and kinetic energy, thereby controlling the magnitude of flow to the heart and thus the coupling of peripheral and central circulation during hyperthermia.

No large effects on cognitive performance in high versus low solar green-flag WBGT conditions

Excessive solar radiation negatively affects cognitive performance. Occupational guidelines typically combine environmental components into one value, such as wet-bulb globe temperature (WBGT). Here, we evaluated cognitive performance in two similar 28.6 °C WBGT-effective (WBGTeff) that were designed differently; using high or low levels of solar radiation. Eight soldiers were exposed to a virtual-reality environment in a climate chamber set to high (900 Wm-2) or low solar radiation conditions (300 Wm-2). Soldiers walked 3 x 30 min at 5 kmh-1. Cognitive performance was evaluated using a virtual-reality scenario and a computerised test battery. There was no statistically significant effect of condition on the cognitive tasks (p > 0.05). Associations were found between mean body temperature (Tb) and visual detection (P ≤ 0.01). Differences in solar radiation with similar WBGTeff (28.6 °C) do not cause large systematic differences in cognitive performance. Certain aspects of cognitive performance (i.e. response inhibition) seem to be partly associated with Tb rather than solar radiation.Practitioner summary: Cognitive performance was evaluated in two similar WBGT conditions that were designed differently; using high or low levels of solar radiation. Differences in solar radiation with similar WBGT do not cause systematic differences in cognitive performance. Certain aspects of cognition were partly associated with mean body temperature rather than solar radiation.

Single-photon emission computed tomography (SPECT) predicted neurological prognosis in heat stroke: A case report

Heat stroke may cause multi-organ dysfunction and death. Some patients with neurological abnormalities in the acute phase have neurological sequelae, particularly cerebellar ataxia, in the recovery phase. However, there is no method to predict the neurological prognosis, and the usefulness of imaging has not yet been established. We report the case of an 86-year-old woman with dementia brought to our emergency department in a coma and hyperthermia. The patient was diagnosed with heat stroke and promptly treated in the ICU but remained unconscious. The patient gained consciousness on day 19, but difficulty with stillness associated with cerebellar ataxia in her right upper extremity became apparent. On day 1, head magnetic resonance imaging (MRI) showed no obvious abnormality. However, on day 6, high-signal areas, suggestive of edema, were seen in the bilateral cerebellar hemispheres. Single-photon emission computed tomography (SPECT) on day 9 revealed significant hypoperfusion in the right cerebellum. These changes improved at the time of hospital discharge. This was a case of persistent cerebellar ataxia due to heat stroke, in which imaging findings improved over time. In most cases, MRI findings do not match clinical symptoms. However, the low cerebral blood flow in the early SPECT images was consistent with the clinical symptoms. MRI may not be a prognostic indicator; however, SPECT images may be useful for predicting sequelae.

Co-administration of Nanowired Oxiracetam and Neprilysin with Monoclonal Antibodies to Amyloid Beta Peptide and p-Tau Thwarted Exacerbation of Brain Pathology in Concussive Head Injury at Hot Environment

Environmental temperature adversely affects the outcome of concussive head injury (CHI)-induced brain pathology. Studies from our laboratory showed that animals reared at either cold environment or at hot environment exacerbate brain pathology following CHI. Our previous experiments showed that nanowired delivery of oxiracetam significantly attenuated CHI-induced brain pathology and associated neurovascular changes. Military personnel are the most susceptible to CHI caused by explosion, blasts, missile or blunt head trauma leading to lifetime functional and cognitive impairments affecting the quality of life. Severe CHI leads to instant death and/or lifetime paralysis. Military personnel engaged in combat operations are often subjected to extreme high or low environmental temperature zones across the globe. Thus, further exploration of novel therapeutic agents at cold or hot ambient temperatures following CHI are the need of the hour. CHI is also a major risk factor for developing Alzheimer's disease by enhancing amyloid beta peptide deposits in the brain. In this review, effect of hot environment on CHI-induced brain pathology is discussed. In addition, whether nanodelivery of oxiracetam together with neprilysin and monoclonal antibodies (mAb) to amyloid beta peptide and p-tau could lead to superior neuroprotection in CHI is explored. Our results show that co-administration of oxiracetam with neprilysin and mAb to AβP and p-tau significantly induced superior neuroprotection following CHI in hot environment, not reported earlier.

Post-exercise, passive heat acclimation with sauna or hot-water immersion provide comparable adaptations to performance in the heat in a military context

To mitigate the effects of heat during operations in hot environments, military personnel will likely benefit from heat acclimation (HA) conducted prior to deployment. Using post-exercise, passive heating, 25 participants completed a 5 d HA regime in sauna (70 °C, 18% RH) or hot-water immersion (HWI) (40 °C) for ≤40 min, preceded and followed by a heat stress test (1-h walking at 5 km.h^-1 in 33 °C, 77% RH in military uniform (20 kg) before an incremental ramp to exhaustion). Fifteen completed both regimes in a randomised, cross-over manner. While performance did not significantly improve (+14%, [-1, 29], p = .079), beneficial adaptations were observed for mean exercising core temperature (-0.2 °C, [-0.2, -0.2], p <.001), skin temperature (-0.2 °C, [-0.2, -0.2], p = 035) and heart rate (-8 bpm, [-6, -10], p<.001) in both conditions. Post-exercise, passive HA of either modality may benefit military units operating in the heat.Practitioner summary: Strategies are required to prevent health and performance impairments during military operations upon arrival in hot environments. Using a randomised, cross-over design, participants completed five-day passive, post-exercise heat acclimation using sauna or hot-water immersion. Both regimes elicited beneficial albeit modest heat adaptations.Abbreviations: HA: heat acclimation; HST: heat stress test; HWI: hot-water immersion; RH: relative humidity.

Carotid body hyperexcitability underlies heat-induced hyperventilation in exercising humans

Physical activity is the most common source of heat strain for humans. The thermal strain of physical activity causes overbreathing (hyperventilation) and this has adverse physiological repercussions. The mechanisms underlying heat-induced hyperventilation during exercise are unknown, but recent evidence supports a primary role of carotid body hyperexcitability (increased tonic activity and sensitivity) underpinning hyperventilation in passively heated humans. In a repeated-measures crossover design, 12 healthy participants (6 female) completed two low-intensity cycling exercise conditions (25% maximal aerobic power) in randomized order, one with core temperature (T_C) kept relatively stable near thermoneutrality, and the other with progressive heat strain to +2°C T_C. To provide a complete examination of carotid body function under graded heat strain, carotid body tonic activity was assessed indirectly by transient hyperoxia, and its sensitivity estimated by responses to both isocapnic and poikilocapnic hypoxia. Carotid body tonic activity was increased by 220 ± 110% during cycling alone, and by 400 ± 290% with supplemental thermal strain to +1°C T_C, and 600 ± 290% at +2°C T_C (interaction, P = 0.0031). During exercise with heat stress at both +1°C and +2°C T_C, carotid body suppression by hyperoxia decreased ventilation below the rates observed during exercise without heat stress (P < 0.0147). Carotid body sensitivity was increased by up to 230 ± 190% with exercise alone, and by 290 ± 250% with supplemental heating to +1°C T_C and 510 ± 470% at +2°C T_C (interaction, P = 0.0012). These data indicate that the carotid body is further activated and sensitized by heat strain during exercise and this largely explains the added drive to breathe.NEW & NOTEWORTHY Physical activity is the most common way humans increase their core temperature, and excess breathing in the heat can limit heat tolerance and performance, and may increase the risk of heat-related injury. Dose-dependent increases in carotid body tonic activity and sensitivity with core heating provide compelling evidence that carotid body hyperexcitability is the primary cause of heat-induced hyperventilation during exercise.

Moving in a hotter world: Maintaining adequate childhood fitness as a climate change countermeasure

Children cope with high temperatures differently than adults do, largely because of slight alterations in their body proportions and heat loss mechanisms compared to fully mature humans. Paradoxically, all current tools of assessing thermal strain have been developed on adults. As the Earth's warming continues to accelerate, children are set to bear the health risk brunt of rising global temperatures. Physical fitness has a direct impact on heat tolerance, yet children are less fit and more obese than ever before. Longitudinal research reveals that children have 30% lower aerobic fitness than their parents did at the same age; this deficit is greater than can be recovered by training alone. So, as the planet's climate and weather patterns become more extreme, children may become less capable of tolerating it. This comprehensive review provides an outline of child thermoregulation and assessment of thermal strain, before moving to summarize how aerobic fitness can modulate hyperthermia, heat tolerance, and behavioral thermoregulation in this under-researched population. The nature of child physical activity, physical fitness, and one's physical literacy journey as an interconnected paradigm for promoting climate change resilience is explored. Finally, future research foci are suggested to encourage continued exploration of this dynamic field, notable since more extreme, multifactorial environmental stressors are expected to continue challenging the physiological strain of the human population for the foreseeable future.

Contribution of the carotid body to thermally mediated hyperventilation in humans

Humans hyperventilate under heat and cold strain. This hyperventilatory response has detrimental consequences including acid-base dysregulation, dyspnoea, decreased cerebral blood flow and accelerated brain heating. The ventilatory response to hypoxia is exaggerated under whole-body heating and cooling, indicating that altered carotid body function might contribute to thermally mediated hyperventilation. To address whether the carotid body might contribute to heat- and cold-induced hyperventilation, we indirectly measured carotid body tonic activity via hyperoxia, and carotid body sensitivity via hypoxia, under graded heat and cold strain in 13 healthy participants in a repeated-measures design. We hypothesised that carotid body tonic activity and sensitivity would be elevated in a dose-dependent manner under graded heat and cold strain, thereby supporting its role in driving thermally mediated hyperventilation. Carotid body tonic activity was increased in a dose-dependent manner with heating, reaching 175% above baseline (P < 0.0005), and carotid body suppression with hyperoxia removed all of the heat-induced increase in ventilation (P = 0.9297). Core cooling increased carotid body activity by up to 250% (P < 0.0001), but maximal values were reached with mild cooling and thereafter plateaued. Carotid body sensitivity to hypoxia was profoundly increased by up to 180% with heat stress (P = 0.0097), whereas cooling had no detectable effect on hypoxic sensitivity. In summary, cold stress increased carotid body tonic activity and this effect was saturated with mild cooling, whereas heating had clear dose-dependent effects on carotid body tonic activity and sensitivity. These dose-dependent effects with heat strain indicate that the carotid body probably plays a primary role in driving heat-induced hyperventilation. KEY POINTS: Humans over-breathe (hyperventilate) when under heat and cold stress, and though this has detrimental physiological repercussions, the mechanisms underlying this response are unknown. The carotid body, a small organ that is responsible for driving hyperventilation in hypoxia, was assessed under incremental heat and cold strain. The carotid body drive to breathe, as indirectly assessed by transient hyperoxia, increased in a dose-dependent manner with heating, reaching 175% above baseline; cold stress similarly increased the carotid body drive to breathe, but did not show dose-dependency. Carotid body sensitivity, as indirectly assessed by hypoxic ventilatory responses, was profoundly increased by 70-180% with mild and severe heat strain, whereas cooling had no detectable effect. Carotid body hyperactivity and hypersensitivity are two interrelated mechanisms that probably underlie the increased drive to breathe with heat strain, whereas carotid body hyperactivity during mild cooling may play a subsidiary role in cold-induced hyperventilation.

High intensity exercise and passive hot water immersion cause similar post intervention changes in peripheral and cerebral shear

Passive hot water immersion (PHWI) provides a peripheral vasculature shear stimulus comparable to low intensity exercise within the active skeletal muscle, whereas moderate and high intensity exercise elicit substantially greater shear rates in the peripheral vasculature, likely conferring greater vascular benefits. Few studies have compared post intervention shear rates in the peripheral and cerebral vasculature following high intensity exercise and PHWI, especially considering that the post intervention recovery period represents a key window in which adaptation occurs. Therefore, we aimed to compare shear rates in the internal carotid artery (ICA), vertebral artery (VA) and common femoral artery (CFA) between high intensity exercise and PHWI for up to 80 minutes post intervention. Fifteen healthy (27 ± 4 years), moderately trained individuals underwent three-time matched interventions in a randomised order which included 30 minutes of whole-body immersion in a 42°C hot bath, 30 minutes of treadmill running and 5x4 minute high intensity intervals (HIIE). There were no differences in ICA (P= 0.4643) and VA (P=0.1940) shear rates between PHWI and exercise (both continuous and HIIE) post intervention. All three interventions elicited comparable increases in CFA shear rate post intervention (P=0.0671), however, CFA shear rate was slightly higher 40 minutes post threshold running (P=0.0464) and, slightly higher, although not statically for HIIE (P=0.0565) compared with PHWI. Our results suggest that time and core temperature matched high intensity exercise and PHWI elicit limited changes in cerebral shear and comparable increases in peripheral vasculature shear rates when measured for up to 80 minutes post intervention.

Acute physiological and psychophysical responses to different modes of heat stress

New Findings

What is the central question of this study?

What are the profiles of acute physiological and psychophysical strain during and in recovery from different modes of heating, and to what extent do these diminish after repeated exposure?

What is the main finding and its importance?

Mode of heating affects the strain profiles during heat stress and recovery. Exercise in the heat incurred the greatest cardiovascular strain during heating and recovery. Humid heat was poorly tolerated despite heat strain being no greater than in other heating modes, and tolerance did not improve with multiple exposures.

Abstract

Heat stress is common and arises endogenously and exogenously. It can be acutely hazardous while also increasingly advocated to drive health and performance‐related adaptations. Yet, the nature of strain (deviation in regulated variables) imposed by different heating modes is not well established, despite the potential for important differences. We, therefore, compared three modes of heat stress for thermal, cardiovascular and perceptual strain profiles during exposure and recovery when experienced as a novel stimulus and an accustomed stimulus. In a crossover design, 13 physically active participants (five females) underwent 5 days of 60‐min exposures to hot water immersion (40°C), sauna (55°C, 54% relative humidity) and exercise in the heat (40°C, 52% relative humidity), and a thermoneutral water immersion control (36.5°C), each separated by ≥4 weeks. Physiological (thermal, cardiovascular, haemodynamic) and psychophysical strain responses were assessed on days 1 and 5. Sauna evoked the warmest skin (40°C; P < 0.001) but exercise in the heat caused the largest increase in core temperature, sweat rate, heart rate (post hoc comparisons all P < 0.001) and systolic blood pressure (P ≤ 0.002), and possibly decrease in diastolic blood pressures (P ≤ 0.130), regardless of day. Thermal sensation and feeling state were more favourable on day 5 than on day 1 (P ≤ 0.021), with all modes of heat being equivalently uncomfortable (P ≥ 0.215). Plasma volume expanded the largest extent during immersions (P < 0.001). The current data highlight that exercising in the heat generates a more complex strain profile, while passive heat stress in humid heat has lower tolerance and more cardiovascular strain than hot water immersion.

Physiological Function during Exercise and Environmental Stress in Humans-An Integrative View of Body Systems and Homeostasis

Claude Bernard’s milieu intérieur (internal environment) and the associated concept of homeostasis are fundamental to the understanding of the physiological responses to exercise and environmental stress. Maintenance of cellular homeostasis is thought to happen during exercise through the precise matching of cellular energetic demand and supply, and the production and clearance of metabolic by-products. The mind-boggling number of molecular and cellular pathways and the host of tissues and organ systems involved in the processes sustaining locomotion, however, necessitate an integrative examination of the body’s physiological systems. This integrative approach can be used to identify whether function and cellular homeostasis are maintained or compromised during exercise. In this review, we discuss the responses of the human brain, the lungs, the heart, and the skeletal muscles to the varying physiological demands of exercise and environmental stress. Multiple alterations in physiological function and differential homeostatic adjustments occur when people undertake strenuous exercise with and without thermal stress. These adjustments can include: hyperthermia; hyperventilation; cardiovascular strain with restrictions in brain, muscle, skin and visceral organs blood flow; greater reliance on muscle glycogen and cellular metabolism; alterations in neural activity; and, in some conditions, compromised muscle metabolism and aerobic capacity. Oxygen supply to the human brain is also blunted during intense exercise, but global cerebral metabolism and central neural drive are preserved or enhanced. In contrast to the strain seen during severe exercise and environmental stress, a steady state is maintained when humans exercise at intensities and in environmental conditions that require a small fraction of the functional capacity. The impact of exercise and environmental stress upon whole-body functions and homeostasis therefore depends on the functional needs and differs across organ systems.

Sodium bicarbonate ingestion mitigates the heat-induced hyperventilation and reduction in cerebral blood velocity during exercise in the heat

Hyperthermia during exercise in the heat causes minute ventilation ([Formula: see text]) to increase, which leads to reductions in arterial CO₂ partial pressure ([Formula: see text]) and cerebral blood flow. On the other hand, sodium bicarbonate ingestion reportedly results in metabolic alkalosis, leading to decreased [Formula: see text] and increased [Formula: see text] during prolonged exercise in a thermoneutral environment. Here, we investigated whether sodium bicarbonate ingestion suppresses heat-induced hyperventilation and the resultant hypocapnia and cerebral hypoperfusion during prolonged exercise in the heat. Eleven healthy men ingested a solution of sodium bicarbonate (0.3 g/kg body wt) (NaHCO₃ trial) or sodium chloride (0.208 g/kg) (NaCl trial). Ninety minutes after the ingestion, the subjects performed a cycle exercise for 60 min at 50% of peak oxygen uptake in the heat (35°C and 40% relative humidity). Esophageal temperature did not differ between the trials throughout (P = 0.56, main effect of trial). [Formula: see text] gradually increased with exercise duration in the NaCl trial, but the increases in [Formula: see text] were attenuated in the NaHCO₃ trial (P = 0.01, main effect of trial). Correspondingly, estimated [Formula: see text] and middle cerebral artery blood velocity (an index of anterior cerebral blood flow) were higher in the NaHCO₃ than the NaCl trial (P = 0.002 and 0.04, main effects of trial). Ratings of perceived exertion were lower in the NaHCO₃ than the NaCl trial (P = 0.02, main effect of trial). These results indicate that sodium bicarbonate ingestion mitigates heat-induced hyperventilation and reductions in [Formula: see text] and cerebral blood velocity during prolonged exercise in the heat.NEW & NOTEWORTHY Hyperthermia causes hyperventilation and concomitant hypocapnia and cerebral hypoperfusion. The cerebral hypoperfusion may underlie central fatigue. We demonstrate that sodium bicarbonate ingestion reduces heat-induced hyperventilation and attenuates hypocapnia-related cerebral hypoperfusion during prolonged exercise in the heat. In addition, we show that sodium bicarbonate ingestion reduces ratings of perceived exertion during the exercise. This study provides new insight into the development of effective strategies for preventing central fatigue during exercise in the heat.