Hyperthermia-induced vasoconstriction of the carotid artery, a possible causative factor of heatstroke

Transient Changes in Cerebral Tissue Oxygen, Glucose, and Temperature by Microstrokes: A Computational Study

OBJECTIVE

This study focuses on evaluating the disruptions in key physiological parameters during microstroke events to assess their severity.

METHODS

A mathematical model was developed to simulate the changes in cerebral tissue pO2, glucose concentration, and temperature due to blood flow interruptions. The model considers variations in baseline cerebral blood flow (CBF), capillary density, and blood oxygen/glucose levels, as well as ambient temperature changes.

RESULTS

Simulations indicate that complete blood flow obstruction still allows for limited glucose availability, supporting nonoxidative metabolism and potentially exacerbating lactate buildup and acidosis. Partial obstructions decrease tissue pO2, with minimal impact on glucose level, which can remain almost unchanged or even slightly increase. Reduced CBF, capillary density, or blood oxygen due to aging or disease enhances hypoxia risk at lower obstruction levels, with capillary density having a significant effect on stroke severity by influencing both pO2 and glucose levels. Conditions could lead to co-occurrence of hypoxia/hypoglycemia or hypoxia/hyperglycemia, each worsening outcomes. Temperature effects were minimal in deep brain regions but varied near the skull by 0.2-0.8°C depending on ambient temperature.

CONCLUSIONS

The model provides insights into the conditions driving severe stroke outcomes based on estimated levels of hypoxia, hypoglycemia, hyperglycemia, and temperature changes.

Profile of precipitating factors and its implication in 160 Indian patients with Moyamoya angiopathy

INTRODUCTION

Moyamoya angiopathy (MMA) has been known to manifest with myriad of neurological manifestations, often in association with various precipitating factors. This is the first study to systematically analyze the precipitating triggers to neurological symptoms done on the largest cohort of MMA in India.

METHODS

A single-centered, cross-sectional observational study, recruiting 160 patients with consecutive angiographically proven MMA over a period of 5 years (2016-2021), was undertaken to evaluate the profile of immediate precipitating factors in temporal association to the neurological symptoms, along with their clinical and radiological characteristics. SPSS 25 was used for statistical analysis.

RESULTS

Among the 160 patients (Adult-85, children-75), precipitating factors were seen in 41.3%, significantly higher in children (52%) than adults (31.8%) (p value: 0.011). The commonest triggers included fever (18.8%), emotional stress (8.1%), heavy exercise and diarrhea (6.3% each). Cold bath triggered MMA symptoms in 1.3%. Fever (p value: 0.008) and persistent crying (p value: 0.010) triggered neurological symptoms more commonly in children than in adults. Amongst MMA patients with precipitating factors, the commonest MMA presentation included cerebral infarction type (37.9%) and TIA (31.8%). The majority of precipitating factors that preceded an ischemic event were BP-lowing ones (54.7%).

CONCLUSION

Neurological symptoms of MMA are commonly associated with several precipitating factors, including the lesser known triggers like cold bath. The frequency and profile precipitating factors varies with the age of presentation and type of MMA. It can serve as an early clue to the diagnosis of MMA and its careful avoidance can be largely beneficial in limiting the distressing transient neurological symptoms.

68Ga-PSMA PET/CT: effect of external cooling on salivary gland uptake

Objective

To evaluate the effect that external cooling of the salivary glands (ECSG) has on the uptake of gallium-68-labeled prostate-specific membrane antigen (⁶⁸Ga-PSMA), as an indirect assessment of the capacity of ECSG to reduce the local dose in lutetium-177-PSMA-617 radioligand therapy.

Materials and Methods

Ten patients with prostate cancer were submitted to ⁶⁸Ga-PSMA positron emission tomography/computed tomography with unilateral ECSG. The ECSG was started at 30 min before the injection of the radiotracer and maintained until the end of image acquisition (1 h after injection). Each salivary gland was assessed by determining the maximum, mean, and peak standardized uptake values (SUVmax, SUVmean, and SUVpeak, respectively). The volume of each gland was determined in a volume of interest delineated by a threshold SUVmax of 10%. Paired Student's t-tests were used in order to compare the results.

Results

In terms of the SUV parameters, there were no statistically significant differences between the cooled and contralateral salivary glands. However, the mean volume was 27% lower in the cooled parotid glands than in the contralateral parotid glands (p = 0.004).

Conclusion

The use of ECSG does not appear to reduce ⁶⁸Ga-PSMA uptake by the salivary glands. In addition, there is yet no evidence that ECSG is effective in preventing salivary gland toxicity.

Ethanol potentiates heat response in the carotid artery via TRPV1

AIM

Ethanol is one of the most widely used recreational drugs in the world. At high concentrations, it can induce carotid artery vasoconstriction. Hyperthermia potentiates its effects resulting in carotid artery vasoconstriction at any concentration. The aim of this study is to investigate the interaction between ethanol and heating and to understand the underlying mechanisms leading to their synergistic effect.

MATERIALS AND METHODS

Isometric tension of rabbit carotid artery ring segments suspended in organ baths filled with Krebs solution was recorded. Different concentrations of ethanol were examined at 37°C and during temperature elevation to39-43°C. Capsaicin and capsazepine were used to examine the mechanism of action of ethanol.

KEY FINDINGS

Ethanol induced contraction at 37°C when the concentration reached 100mM. Contraction was observed at any concentration at higher temperatures. Ethanol potentiated heat-induced contraction. Capsaicin, the vanilloid receptor subtype1 (TRPV1) agonist, potentiated the vasoconstriction due to heating. While capsazepine, TRPV1 antagonist, abolished the effect of ethanol and its potentiation of heating-induced contraction, but it did not abolish the heating effect.

SIGNIFICANCE

Ethanol's mechanism of action and its effect on heating induced-vasoconstriction of the carotid artery is being mediated by TRPV1. The combination of ethanol and hyperthermia can lead to a synergistic effect on carotid vasoconstriction. This effect may induce brain damage and heat stroke. Development of new drugs act as TRPV1 antagonist can be used to prevent these fatal effects.

Cerebral Vascular Control and Metabolism in Heat Stress

This review provides an in-depth update on the impact of heat stress on cerebrovascular functioning. The regulation of cerebral temperature, blood flow, and metabolism are discussed. We further provide an overview of vascular permeability, the neurocognitive changes, and the key clinical implications and pathologies known to confound cerebral functioning during hyperthermia. A reduction in cerebral blood flow (CBF), derived primarily from a respiratory-induced alkalosis, underscores the cerebrovascular changes to hyperthermia. Arterial pressures may also become compromised because of reduced peripheral resistance secondary to skin vasodilatation. Therefore, when hyperthermia is combined with conditions that increase cardiovascular strain, for example, orthostasis or dehydration, the inability to preserve cerebral perfusion pressure further reduces CBF. A reduced cerebral perfusion pressure is in turn the primary mechanism for impaired tolerance to orthostatic challenges. Any reduction in CBF attenuates the brain's convective heat loss, while the hyperthermic-induced increase in metabolic rate increases the cerebral heat gain. This paradoxical uncoupling of CBF to metabolism increases brain temperature, and potentiates a condition whereby cerebral oxygenation may be compromised. With levels of experimentally viable passive hyperthermia (up to 39.5-40.0 °C core temperature), the associated reduction in CBF (∼ 30%) and increase in cerebral metabolic demand (∼ 10%) is likely compensated by increases in cerebral oxygen extraction. However, severe increases in whole-body and brain temperature may increase blood-brain barrier permeability, potentially leading to cerebral vasogenic edema. The cerebrovascular challenges associated with hyperthermia are of paramount importance for populations with compromised thermoregulatory control--for example, spinal cord injury, elderly, and those with preexisting cardiovascular diseases.

Neck-cooling improves repeated sprint performance in the heat

The present study evaluated the effect of neck-cooling during exercise on repeated sprint ability in a hot environment. Seven team-sport playing males completed two experimental trials involving repeated sprint exercise (5 × 6 s) before and after two 45 min bouts of a football specific intermittent treadmill protocol in the heat (33.0 ± 0.2°C; 53 ± 2% relative humidity). Participants wore a neck-cooling collar in one of the trials (CC). Mean power output and peak power output declined over time in both trials but were higher in CC (540 ± 99 v 507 ± 122 W, d = 0.32; 719 ± 158 v 680 ± 182 W, d = 0.24 respectively). The improved power output was particularly pronounced (d = 0.51-0.88) after the 2nd 45 min bout but the CC had no effect on % fatigue. The collar lowered neck temperature and the thermal sensation of the neck (P < 0.001) but had no effect on heart rate, fluid loss, fluid consumption, lactate, glucose, plasma volume change, cortisol, or thermal sensation (P > 0.05). There were no trial differences but interaction effects were demonstrated for prolactin concentration and rating of perceived exertion (RPE). Prolactin concentration was initially higher in the collar cold trial and then was lower from 45 min onwards (interaction trial × time P = 0.04). RPE was lower during the football intermittent treadmill protocol in the collar cold trial (interaction trial × time P = 0.01). Neck-cooling during exercise improves repeated sprint performance in a hot environment without altering physiological or neuroendocrinological responses. RPE is reduced and may partially explain the performance improvement.

Human cardiovascular responses to passive heat stress

Heat stress increases human morbidity and mortality compared to normothermic conditions. Many occupations, disease states, as well as stages of life are especially vulnerable to the stress imposed on the cardiovascular system during exposure to hot ambient conditions. This review focuses on the cardiovascular responses to heat stress that are necessary for heat dissipation. To accomplish this regulatory feat requires complex autonomic nervous system control of the heart and various vascular beds. For example, during heat stress cardiac output increases up to twofold, by increases in heart rate and an active maintenance of stroke volume via increases in inotropy in the presence of decreases in cardiac preload. Baroreflexes retain the ability to regulate blood pressure in many, but not all, heat stress conditions. Central hypovolemia is another cardiovascular challenge brought about by heat stress, which if added to a subsequent central volumetric stress, such as hemorrhage, can be problematic and potentially dangerous, as syncope and cardiovascular collapse may ensue. These combined stresses can compromise blood flow and oxygenation to important tissues such as the brain. It is notable that this compromised condition can occur at cardiac outputs that are adequate during normothermic conditions but are inadequate in heat because of the increased systemic vascular conductance associated with cutaneous vasodilation. Understanding the mechanisms within this complex regulatory system will allow for the development of treatment recommendations and countermeasures to reduce risks during the ever-increasing frequency of severe heat events that are predicted to occur.

3D perfused brain phantom for interstitial ultrasound thermal therapy and imaging: design, construction and characterization

Thermal therapy has emerged as an independent modality of treating some tumors. In many clinics the hyperthermia, one of the thermal therapy modalities, has been used adjuvant to radio- or chemotherapy to substantially improve the clinical treatment outcomes. In this work, a methodology for building a realistic brain phantom for interstitial ultrasound low dose-rate thermal therapy of the brain is proposed. A 3D brain phantom made of the tissue mimicking material (TMM) had the acoustic and thermal properties in the 20-32 °C range, which is similar to that of a brain at 37 °C. The phantom had 10-11% by mass of bovine gelatin powder dissolved in ethylene glycol. The TMM sonicated at 1 MHz, 1.6 MHz and 2.5 MHz yielded the amplitude attenuation coefficients of 62  ±  1 dB m(-1), 115  ±  4 dB m(-1) and 175  ±  9 dB m(-1), respectively. The density and acoustic speed determination at room temperature (~24 °C) gave 1040  ±  40 kg m(-3) and 1545  ±  44 m s(-1), respectively. The average thermal conductivity was 0.532 W m(-1) K(-1). The T1 and T2 values of the TMM were 207  ±  4 and 36.2  ±  0.4 ms, respectively. We envisage the use of our phantom for treatment planning and for quality assurance in MRI based temperature determination. Our phantom preparation methodology may be readily extended to other thermal therapy technologies.

Heat and α1-adrenergic responsiveness in human skeletal muscle feed arteries: the role of nitric oxide

Increased local temperature exerts a sympatholytic effect on human skeletal muscle feed arteries. We hypothesized that this attenuated α(1)-adrenergic receptor responsiveness may be due to a temperature-induced increase in nitric oxide (NO) bioavailability, thereby reducing the impact of the α(1)-adrenergic receptor agonist phenylephrine (PE). Thirteen human skeletal muscle feed arteries were harvested, and wire myography was used to generate PE concentration-response curves at 37 °C and 39 °C, with and without the NO synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (L-NMMA). A subset of arteries (n = 4) were exposed to 37 °C or 39 °C, and the protein content of endothelial NOS (eNOS) and α(1)-adrenergic receptors was determined by Western blot analysis. Additionally, cultured bovine endothelial cells were exposed to static or shear stress conditions at 37 °C and 39 °C and assayed for eNOS activation (phosphorylation at Ser(1177)), eNOS expression, and NO metabolites [nitrate + nitrite (NOx)]. Maximal PE-induced vasocontraction (PE(max)) was lower at 39 °C than at 37 °C [39 ± 10 vs. 84 ± 30% maximal response to 100 mM KCl (KCl(max))]. NO blockade restored vasocontraction at 39 °C to that achieved at 37 °C (80 ± 26% KCl(max)). Western blot analysis of the feed arteries revealed that heating increased eNOS protein, but not α(1)-adrenergic receptors. Heating of bovine endothelial cells resulted in greater shear stress-induced eNOS activation and NOx production. Together, these data reveal for the first time that, in human skeletal muscle feed arteries, NO blockade can restore the heat-attenuated α(1)-adrenergic receptor-mediated vasocontraction and implicate endothelium-derived NO bioavailability as a major contributor to heat-induced sympatholysis. Consequently, these findings highlight the important role of vasodilators in modulating the vascular response to vasoconstrictors.

Thermally-mediated ultrasound-induced contraction of equine muscular arteries in vitro and an investigation of the associated cellular mechanisms

We have previously shown that MHz frequency ultrasound causes contraction of the carotid artery in vitro. We now extend this investigation to equine mesenteric arteries and investigate the cellular mechanisms. In vitro exposure of the large lateral cecal mesenteric artery to 4-min periods of 3.2 MHz continuous wave ultrasound at acoustic powers up to 145 mW induced reversible repeatable contraction. The magnitude of the response was linearly dependent on acoustic power and, at 145 mW, the mean increase in wall stress was 0.020 ± 0.017 mN/mm(2) (n = 34). These results are consistent with our previous study and the effect was hypothesised to be thermally mediated. A 2°C temperature rise produced an increase in intracellular calcium, measured by Fluo-4 fluorescence. Inhibition of the inward-rectifier potassium ion channel with BaCl(2) (4 μM) increased the response to ultrasound by 55% ± 49%, indicating a similar electrophysiologic basis to the response to mild hyperthermia. In small mesenteric arteries (0.5-1.0 mm diameter) mounted in a perfusion myograph, neither ultrasound exposure nor heating produced measureable vasoconstriction or a rise in intracellular calcium and we conclude that temperature-sensitive channels are absent or inactive in these small vessels. It, therefore, appears that response of blood vessels to ultrasound depends not only on the thermal properties of the vessels and surrounding tissues but also on the electrophysiology of the smooth muscle cells.

Human skeletal muscle feed arteries studied in vitro: the effect of temperature on α(1)-adrenergic responsiveness

Heat and cold exposure can decrease and increase total peripheral resistance, respectively, in humans. With unique access to human skeletal muscle feed arteries, we sought both to characterize these vessels and to determine the interaction between temperature and α(1)-adrenergic receptor responsiveness. We hypothesized that α(1)-mediated vasocontraction of human feed arteries would be attenuated in response to 39 or 35°C. Skeletal muscle feed arteries were harvested from thirty-two human volunteers and studied using isometric techniques. Vessel function was assessed using KCl, sodium nitroprusside (SNP), phenylephrine (PE) and ACh dose-response curves to characterize non-receptor- and receptor-mediated vasocontraction and vasorelaxation. Single doses of PE (1 mm) and KCl (100 mm) were administered at 37°C and then, in a balanced design, repeated at both 35 and 39°C. The KCl and PE dose-response curves elicited significant vasocontraction (2009 ± 407 and 1974 ± 508 mg developed tension, respectively), whereas SNP and ACh induced the expected vasorelaxation (102 ± 6 and 73 ± 10% relaxation, respectively). Altering the temperature had no effect on inherent smooth muscle function (KCl response), but both a reduction (35°C) and an increase in temperature (39°C) decreased the vasocontractile response to 1 mm PE (37°C, 1478 ± 338 mg; 35°C, 546 ± 104 mg; and 39°C, 896 ± 202 mg; P < 0.05) or across PE dose (P < 0.05, 35 and 39 versus 37°C). Despite clear heterogeneity between both the human volunteers and the feed arteries themselves, this novel approach to the procurement of human vessels revealed a robust 'inverted U' response to altered temperature, such that α(1)-mediated vasocontraction was attenuated with either warming or cooling.

Local heating, but not indirect whole body heating, increases human skeletal muscle blood flow

For decades it was believed that direct and indirect heating (the latter of which elevates blood and core temperatures without directly heating the area being evaluated) increases skin but not skeletal muscle blood flow. Recent results, however, suggest that passive heating of the leg may increase muscle blood flow. Using the technique of positron-emission tomography, the present study tested the hypothesis that both direct and indirect heating increases muscle blood flow. Calf muscle and skin blood flows were evaluated from eight subjects during normothermic baseline, during local heating of the right calf [only the right calf was exposed to the heating source (water-perfused suit)], and during indirect whole body heat stress in which the left calf was not exposed to the heating source. Local heating increased intramuscular temperature of the right calf from 33.4 ± 1.0°C to 37.4 ± 0.8°C, without changing intestinal temperature. This stimulus increased muscle blood flow from 1.4 ± 0.5 to 2.3 ± 1.2 ml·100 g⁻¹·min⁻¹ (P < 0.05), whereas skin blood flow under the heating source increased from 0.7 ± 0.3 to 5.5 ± 1.5 ml·100 g⁻¹·min⁻¹ (P < 0.01). While whole body heat stress increased intestinal temperature by ∼1°C, muscle blood flow in the calf that was not directly exposed to the water-perfused suit (i.e., indirect heating) did not increase during the whole body heat stress (normothermia: 1.6 ± 0.5 ml·100 g⁻¹·min⁻¹; heat stress: 1.7 ± 0.3 ml·100 g⁻¹·min⁻¹; P = 0.87). Whole body heating, however, reflexively increased calf skin blood flow (to 4.0 ± 1.5 ml·100 g⁻¹·min⁻¹) in the area not exposed to the water-perfused suit. These data show that local, but not indirect, heating increases calf skeletal muscle blood flow in humans. These results have important implications toward the reconsideration of previously accepted blood flow distribution during whole body heat stress.

Effect of CO2 on the ventilatory sensitivity to rising body temperature during exercise

We examined the degree to which ventilatory sensitivity to rising body temperature (the slope of the regression line relating ventilation and body temperature) is altered by restoration of arterial Pco2 to the eucapnic level during prolonged exercise in the heat. Thirteen subjects exercised for ∼60 min on a cycle ergometer at 50% of peak O2 uptake with and without inhalation of CO2-enriched air. Subjects began breathing CO2-enriched air at the point that end-tidal Pco2 started to decline. Esophageal temperature (Tes), minute ventilation (V̇e), tidal volume (VT), respiratory frequency ( fR), respiratory gases, middle cerebral artery blood velocity, and arterial blood pressure were recorded continuously. When V̇e, VT, fR, and ventilatory equivalents for O2 uptake (V̇e/V̇o2) and CO2 output (V̇e/V̇co2) were plotted against changes in Tes from the start of the CO2-enriched air inhalation (ΔTes), the slopes of the regression lines relating V̇e, VT, V̇e/V̇o2, and V̇e/V̇co2 to ΔTes (ventilatory sensitivity to rising body temperature) were significantly greater when subjects breathed CO2-enriched air than when they breathed room air (V̇e: 19.8 ± 10.3 vs. 8.9 ± 6.7 l·min−1·°C−1, VT: 18 ± 120 vs. −81 ± 92 ml/°C; V̇e/V̇o2: 7.4 ± 5.5 vs. 2.6 ± 2.3 units/°C, and V̇e/V̇co2: 7.6 ± 6.6 vs. 3.4 ± 2.8 units/°C). The increase in V̇e was accompanied by increases in VT and fR. These results suggest that restoration of arterial Pco2 to nearly eucapnic levels increases ventilatory sensitivity to rising body temperature by around threefold.

Differential effect of hyperthermia on nerves and smooth muscle of the mouse ileum

BACKGROUND AND OBJECTIVES

Cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC) gained wide acceptance as the treatment of choice for selected patients with peritoneal surface malignancies. Patients tend to suffer from prolonged ileus following CRS + HIPEC, complicating their recovery. We studied the effects of hyperthermia on the intestine to gain insight into mechanisms of ileus post-HIPEC.

METHODS

Segments of mouse ileum were incubated at 36°C. Electrical field stimulation (EFS) was applied, stimulating nerves, and the resultant muscle contraction was measured. The response was measured at varying temperatures (38-43°C) at exposure times of up to 120 min. We also stimulated the tissues with 10(-6) M carbachol, a muscarinic receptor agonist, which acts directly on smooth muscle.

RESULTS

Response to EFS decreased at high temperatures, especially above 41°C. This effect was irreversible for 120 min after decreasing temperature. When stimulating with carbachol, both transient and plateau responses decreased at 43°C (plateau more than transient) but the effect reversed on returning to 36°C.

CONCLUSION

The irreversible decline in responses to nerve stimulation when exposed to high temperatures was not seen with direct muscle stimulation. This indicates that smooth muscle is resilient and that the main effect of hyperthermia is on nerves. These results have significance for HIPEC.

Moyamoya disease: a clinical spectrum, literature review and case series from a tertiary care hospital in Pakistan

Background

Moyamoya is a rare cerebrovascular disease of unknown etiology. The data on moyamoya disease from Pakistan is sparse. We report a case series of 13 patients who presented with moyamoya disease to a tertiary care hospital in Pakistan with a national referral base.

Methods

We conducted a retrospective review of thirteen patients who presented to The Aga Khan University and diagnosed with "Moyamoya Disease" during the period 1988 – 2006. These patients were identified from existing hospital database via ICD-9 codes. A predesigned questionnaire containing information about clinical presentation, management and neuroimaging was administered to all identified patients.

Results

There were seven males and six females. Mean age at presentation was 16.5 years and a female predominance was found in the pediatric age group (n = 10, 71.4%). Stroke (n = 11, 84.2%) was the most common presentation with motor deficit being the universal cortical symptom. Fever was a common symptom in the lower age groups (n = 4, 51.7%). Cerebral Angiography and Magnetic Resonance Angiography showed bilateral involvement of the vessels in eleven patients while unilateral in two. Subarachnoid and interventricular haemorrhage appeared in 2(15.4%) adults. Twelve (92.3%) patients were discharged as independent with minor deficits regardless of therapeutic modality. Only three (23.0%) patients underwent surgery whereas the remaining were managed conservatively.

Conclusion

Physicians when dealing with childhood strokes and characteristic deficits in adult population should consider Moyamoya disease.

Hyperthermia-induced vasoconstriction of the carotid artery and the role of potassium channels

Clinical experience and experimental studies have shown that hyperthermia can cause cerebral ischaemia and brain damage. By in vitro experiments with heating, we previously were able to induce carotid artery constriction. The objective of the present study was to clarify the mechanism of this thermal response. Isometric tension was recorded in rabbit carotid artery specimens using organ baths during stepwise temperature elevation. The heating responses were investigated at basal tone, in precontracted vessels, after blocking of adrenergic responses and administration of potassium (K)-channel activators and inhibitors. Stepwise heating of carotid artery strips from 37 degrees C to 47 degrees C induced reproducible graded contraction. The hyperthermic responses were not due to adrenergic stimulation, which were reduced and resistant to neurogenic blockade by tetrodotoxin. Heating-induced contractions were potentiated by the K-channel inhibitors tetraethylammonium, BaCl2, charybdotoxin, and the Na+/K+ ATPase inhibitor ouabain. Levcromakalim (BRL), a K+-channel activator, reduced heating induced contractions. Heating of carotid artery preparations induced reversible graded vasoconstriction proportional to temperature. The heating-induced contractions were not mediated by an adrenergenic process, but rather were due to inhibition of K+ channels, which increases Ca2+ entry. In vivo, this reaction may lead to a disturbance of autoregulation of cerebral blood flow and ischemia with brain damage.

Effects of chemoreflexes on hyperthermic hyperventilation and cerebral blood velocity in resting heated humans

We tested the hypothesis that hyperthermic hyperventilation in part reflects enhanced chemoreceptor ventilatory O(2) drive, and that the resultant hypocapnia attenuates ventilatory responses and/or middle cerebral artery mean blood velocity (MCAV(mean)) in resting humans. Eleven healthy subjects were passively heated for 50-80 min, causing oesophageal temperature (T(oes)) to increase by 1.6 degrees C. During heating, minute ventilation increased (P < 0.05), while end-tidal CO(2) pressure (P(ET,CO(2))) and MCAV(mean) declined. A hyperoxia test in which three breaths of hyperoxic air were inspired was performed once before heating and three times during the heating. When we observed hypocapnia (P(ET,CO(2)) below 40 mmHg), P(ET,CO(2)) was restored to the eucapnic level by adding 100% CO(2) to the inspired air immediately before the last two tests. Minute ventilation was significantly reduced by hyperoxia, and that reduction gradually increased with increasing T(oes). However, the percentage decrease in from the normoxic level was small (20-29%) and unchanged during heating. When P(ET,CO(2)) was restored to eucapnic levels, was unchanged, but MCAV(mean) was partly restored to the level seen prior to heating (28.1% restoration at T(oes) 37.6 degrees C and 38.1% restoration at T(oes) 38.0 degrees C). These findings suggest that although hyperthermia increases chemoreceptor ventilatory O(2) drive in resting humans, the relative contribution of the chemoreceptor ventilatory O(2) drive to hyperthermic hyperventilation is small ( approximately 20%) and unaffected by increasing core temperature. Moreover, hypocapnia induced by hyperthermic hyperventilation reduces cerebral blood flow but not ventilatory responses.

Influence of heat stress on the reactivity of isolated chicken carotid artery to vasoactive agents

Cerebral ischaemia is considered to be an important cause of central nervous system dysfunction in heat stress. We hypothesized that heat stress would alter the reactivity of isolated carotid artery to vasoactive agents. Carotid arteries were isolated from broiler chickens maintained either at 23-24 degrees C with 55-65% humidity (control conditions) or exposed to 40 +/- 1 degrees C with 35% humidity for 4 h (heat stress). Contractions were elicited with vasoconstrictors such as 5-HT, phenylephrine, guanfacine and CaCl(2) (K(+)-depolarized) in endothelium-denuded arterial rings. Heat stress significantly increased the potency of 5-HT, but had no effect on the sensitivity of the vessel to phenylephrine or guanfacine. In contrast, it markedly decreased the potency and efficacy of CaCl(2). Vasodilator responses to ACh (endothelium-intact) and sodium nitroprusside (endothelium-denuded), however, were unaffected. Although cyclopiazonic acid (10 microm) significantly decreased 5-HT responses in both the conditions, the agonist was still more potent in heat stress. Extracellular Ca(2)(+) removal had no effect on contractions caused by 5-HT in control conditions, but it significantly decreased the agonist potency in heat stress. Interestingly, nifedipine (1 microm) markedly inhibited 5-HT-induced contractions both in control conditions and in heat stress, implying an inhibitory effect on both Ca(2)(+) influx and release. Thus, nifedipine had a markedly greater inhibitory effect on 5-HT-induced contractions in heat stress compared with control conditions. The results suggest that heat stress increased the vasoconstrictor responses to 5-HT by a mechanism that involved extracellular Ca(2)(+) influx through nifedipine-sensitive L-type calcium channels.

Influence of hyperthermia on carotid blood flow using 99mTc-HMPAO

Hyperthermia can be the result of many causes such as environmental conditions, brain tumors and infectious diseases. Since hyperthermia is very common, its role in causing stroke through a decrease in cerebral blood flow needed further emphasis. The aim of this study was to record cerebral blood flow in vitro by using isolated rabbit carotid artery strips and in-vivo using radioactive isotope scanning during temperature elevation. The recording of isometric tension in rabbit carotid artery strips in organ baths, and the scintigraphic cerebral imaging of technetium-99m-hexamethyl-propyleneamineoxime (99mTc-HMPAO) using Gamma camera, were acquired at control and higher body temperature by 4 degrees C. Blood pressure was measured through femoral artery and cerebral blood flow was measured through carotid artery. Elevating temperature by 4 degrees C induced reproducible contraction. During hyperthermia, the carotid artery contraction leads to a decrease in cerebral blood flow although the blood pressure did not decrease. The uptake of 99mTc-HMPAO in the brain was significantly reduced. This decrease in cerebral perfusion is regionally dependent, which is more in the frontal area, the cerebral hemispheres than the cerebellum. The decrease was 36+/-3, 37+/-2, 22+/-2%, respectively. Hyperthermia causes carotid artery contraction leading to decrease in cerebral blood flow, which was confirmed by 99mTc-HMPAO images. The decrease is regionally dependent. Since the blood pressure did not decrease by heating, the reduction in cerebral perfusion is mainly due to carotid contraction. The applied neck cooling may be considered as a promising therapeutic strategy for the hyperthermic patient to avoid brain damage. This can be achieved by external application of an ice-water-perfused neck collar.

Synergistic effects of ethanol and hyperthermia on carotid artery vasoconstriction

BACKGROUND

Heatstroke is a serious condition and clinical studies indicate that vascular stroke increases with excessive consumption of alcohol (ethanol). It was our objective to test the influence of ethanol on cerebral perfusion at normal and higher temperatures.

METHODS

Recording of isometric tension in rabbit carotid artery strips in organ baths with different concentrations of ethanol at 37 degrees C and during hyperthermia (39-43 degrees C) and scintigraphic cerebral imaging of a radioactive isotope in the control situation and during hyperthermia.

RESULTS

Stepwise heating induced reproducible reversible graded contraction, proportional to temperature. At high concentrations (toxic levels), ethanol induced an increase in tension and heating potentiated these responses. Extracellular Mg(2+) potentiated both heat-induced contraction and ethanol-induced contraction while extracellular Ca(2+) had no effect on these responses. During hyperthermia and ethanol scintigraphic isotope uptake was reduced in cortical and cerebellar regions.

CONCLUSIONS

Carotid artery vasomotor tone is temperature dependent and heating induces vasoconstriction. Alcohol (ethanol) at 37 degrees C elicited carotid artery contraction at high concentrations (toxic levels) but at any concentration during elevated temperature (39-43 degrees C). Ethanol potentiated the effect of hyperthermia-induced vasoconstriction and reduced cerebral perfusion as shown by radionuclide imaging. The synergistic effect of ethanol and hyperthermia may induce heat stroke and brain damage.

Exertional heat stroke in competitive athletes

Exertional heat stroke (EHS) is a serious medical condition that can have a tragic outcome if proper assessment and treatment are not initiated rapidly. This article focuses on critical misconceptions that pertain to the prevention, recognition, and treatment of EHS, including 1) the randomness of EHS cases, 2) the role of nutritional supplements in EHS, 3) temperature assessment, 4) onset of EHS and the possible lucid interval, 5) rapid cooling, and 6) return to play. Exploration of these topics will enhance the medical care regarding EHS.

Thermal reactions of blood vessels in vascular stroke and heatstroke

Research on the pathophysiology and treatment of brain damage with special focus on thermal vascular responses is the subject of this minireview. Interruption of cerebral blood supply by vascular obstruction, temporary cardiac arrest or hyperthermia causes a sudden attack of vascular stroke or heatstroke with serious consequences. It may not induce immediate cell death, but can precipitate a complex biochemical cascade leading to a delayed neuronal loss. When testing thermal vasomotor responses by stepwise cooling of isolated carotid arteries, a temperature-proportional dilatation was observed while heating induced the opposite response: a marked vasoconstriction. General hyperthermia with an increased oxygen demand combined with a reduction of blood supply therefore is a serious consequence. At the cellular level an important mechanism involving hyperthermia is the temperature-dependent regulation of K(+) channel tone of vascular smooth muscle. Further, their inhibition through temperature elevation causes vasoconstriction. In heatstroke, which can induce platelet aggregation and the release of the vasoconstrictor serotonin, arterial cooling attenuates this response. General hypothermia is induced to prevent or attenuate neurological damage in stroke. The procedure is not without serious side effects. Therefore, rapid institution of selective brain cooling has been considered in adults and in infants with postpartum encephalopathy.