Effect of passive heat stress on arterial stiffness

Home-based heat therapy lowers blood pressure and improves endothelial function in older adults

Advancing age is associated with vascular dysfunction and hypertension, both of which increase cardiovascular event risk. Heat therapy has emerged as a novel intervention to improve cardiovascular health in various populations. Therefore, we tested the hypothesis that home-based lower body heat therapy would reduce blood pressure and improve endothelium-dependent vasodilation in older adults. Ambulatory blood pressure monitoring was performed in 19 older adults (67 ± 7 yr) before and after 8 wk of a sham intervention or heat therapy. Endothelium-dependent vasodilation of the superficial femoral artery was assessed via flow-mediated dilation. Participants were provided with a pair of tube-lined pants connected to a portable water circulator to perform the home-based sessions. Water temperature was set to 31°C for sham and 51°C for heat therapy, resulting in target skin temperatures of ∼33°C and ∼40°C, respectively. Participants were instructed to wear the pants 4 days/wk for 60 min each session. Adherence was 100% for both groups. Heat therapy reduced ambulatory daytime systolic blood pressure by Δ -5 ± 8 mmHg, but was unchanged for the sham group (Δ 1 ± 6 mmHg; P = 0.04). Likewise, heat therapy increased flow-mediated dilation (P = 0.02), whereas there was no change across time for the sham group (P = 0.5). These results combined with a strong adherence rate suggest that home-based lower body heat therapy could be an alternative nonpharmacological intervention to reduce blood pressure and improve vascular function, ultimately reducing long-term cardiovascular event risk in older adults.NEW & NOTEWORTHY Advancing age is associated with vascular dysfunction and hypertension, both of which increase cardiovascular event risk. This study determined that 8 wk of home-based lower body heat therapy reduced ambulatory daytime systolic blood pressure and increased flow-mediated dilation of the superficial femoral artery, outcomes not observed in the sham group. These improvements, coupled with 100% adherence among participants, suggest that home-based heat therapy is a pragmatic and effective strategy for improving cardiovascular health in older adults.

Meta-analysis of heat-induced changes in cardiac function from over 400 laboratory-based heat exposure studies

Heat waves are associated with increased fatalities from adverse cardiovascular events attributed to the negative effects of heat on cardiac function. However, scientific understanding of acute cardiac adjustments to heat has come primarily from laboratory experiments employing insulated and encapsulated heating modalities, most commonly water-perfused suits. We evaluated whether findings from those studies reflect cardiac responses during more natural exposures to hot ambient conditions simulated in climate-controlled chambers by synthesizing the findings from over 400 laboratory-based heat exposure studies (6858 participant-exposures) published between 1961-2024. Among all included studies, median (interquartile range) elevations in core temperature and heart rate from baseline to end-exposure were 0.9 (0.5-1.3)°C and 27 (15-40) beats/min. Multilevel mixed-effects meta-analyses revealed exacerbated elevations in heart rate, cardiac output, and rate pressure product (estimate of cardiac workload) and blunted falls in systolic pressure in participants heated via encapsulated modalities. Leveraging the large dataset, we also provide empirical estimates of body temperature and cardiovascular responses to a wide range of conditions experienced during heat waves. With rising global temperatures, ecologically-minded physiological research is needed to improve understanding of the effects of heat stress on cardiac responses and further the development of robust climate health models and evidence-based heat-health guidance.

The direct and urinary electrolyte-mediated effects of ambient temperature on population blood pressure: A causal mediation analysis

High ambient heat can directly influence blood pressure (BP) through the vasodilation of the skin vasculature and indirectly by affecting urinary volume and electrolyte levels. We evaluated the direct and urine electrolyte-mediated effects of ambient temperature on BP. We pooled 5,624 person-visit data from a community-based stepped-wedge randomized control trial in southwest coastal Bangladesh from December 2016 to May 2017. Same-day ambient temperature data from local weather stations were linked to participant BP and urine electrolytes using geo-locations of their residential addresses. We implemented causal mediation analyses using the product methods of coefficients with linear mixed models under the sequential ignorability assumption. Separate models were run for each urinary electrolyte mediator (sodium, potassium, calcium, and magnesium), followed by combined models to evaluate the natural direct and electrolyte-mediated indirect effects of temperature on BP. Models had participant-level random intercepts and were adjusted for age, sex, body mass index (BMI), religion, exercise, smoking status, sleep hours, alcohol consumption, urine creatinine, time trend, household assets, drinking water salinity, and seasonality. For the combined mediators (sodium, potassium, calcium, and magnesium), for every 5°C increase in average daily temperature: the direct effect on systolic BP was -1.42 (95 % CI: -1.94, -0.92) mmHg and urine sodium mediated effect was -0.12 (95 % CI: -0.20, -0.05) mmHg; while urine potassium mediated effect was 0.15 (95 % CI: 0.08, 0.25) mmHg; urine calcium-mediated effect 0.06 (95 % CI: 0.01, 0.12) mmHg; and urine magnesium mediated effect -0.00 (95 % CI: -0.03, 0.02) mmHg. We detected similar associations for diastolic BP, pulse pressure, and mean arterial pressure. We found a significant inverse direct effect of ambient temperature on BP compared to minimally mediated urine electrolyte effects. Further studies are needed to uncover the underlying mechanisms of ambient heat and BP associations and to describe the clinical consequences of these associations.

The multifaceted benefits of passive heat therapies for extending the healthspan: A comprehensive review with a focus on Finnish sauna

Passive heat therapy is characterized by exposure to a high environmental temperature for a brief period. There are several types of passive heat therapy which include hot tubs, Waon therapy, hydrotherapy, sanarium, steam baths, infrared saunas and Finnish saunas. The most commonly used and widely studied till date are the Finnish saunas, which are characterized by high temperatures (ranging from 80-100°C) and dry air with relative humidity varying from 10-20%. The goal of this review is to provide a summary of the current evidence on the impact of passive heat therapies particularly Finnish saunas on various health outcomes, while acknowledging the potential of these therapies to contribute to the extension of healthspan, based on their demonstrated health benefits and disease prevention capabilities. The Finnish saunas have the most consistent and robust evidence regarding health benefits and they have been shown to decrease the risk of health outcomes such as hypertension, cardiovascular disease, thromboembolism, dementia, and respiratory conditions; may improve the severity of musculoskeletal disorders, COVID-19, headache and flu, while also improving mental well-being, sleep, and longevity. Finnish saunas may also augment the beneficial effects of other protective lifestyle factors such as physical activity. The beneficial effects of passive heat therapies may be linked to their anti-inflammatory, cytoprotective and anti-oxidant properties and synergistic effects on neuroendocrine, circulatory, cardiovascular and immune function. Passive heat therapies, notably Finnish saunas, are emerging as potentially powerful and holistic strategies to promoting health and extending the healthspan in all populations.

Quantitative Thermal Stimulation Using Therapeutic Ultrasound to Improve Cerebral Blood Flow and Reduce Vascular Stiffness

It is important to improve cerebrovascular health before the occurrence of cerebrovascular disease, as it has various aftereffects and a high recurrence rate, even with appropriate treatment. Various medical recommendations for preventing cerebrovascular diseases have been introduced, including smoking cessation, exercise, and diet. However, the effectiveness of these methods varies greatly from person to person, and their effects cannot be confirmed unless they are practiced over a long period. Therefore, there is a growing need to develop more quantitative methods that are applicable to the public to promote cerebrovascular health. Thus, in this study, we aimed to develop noninvasive and quantitative thermal stimulation techniques using ultrasound to improve cerebrovascular health and prevent cerebrovascular diseases. This study included 27 healthy adults in their 20s (14 males, 13 females). Thermal stimulation using therapeutic ultrasound at a frequency of 3 MHz was applied to the right sternocleidomastoid muscle in the supine posture for 2 min at four intensities (2.4, 5.1, 7.2, and 10.2 W/cm2). Diagnostic ultrasound was used to measure the peak systolic velocity (PSV), heart rate (HR), and pulse wave velocity (PWV) in the right common carotid artery (CCA), and the physiological changes were compared between intervention intensities. Compared to pre-intervention (preI), the PSV showed a significant increase during intervention (durI) at intensities of 7.2 W/cm2 and 10.2 W/cm2 (p = 0.010 and p = 0.021, respectively). Additionally, PWV showed a significant decrease for post-intervention (postI) at 7.2 W/cm2 and 10.2 W/cm2 (p = 0.036 and p = 0.035, respectively). However, the HR showed no significant differences at any of the intensities. The results demonstrate that an intervention at 3 MHz with an intensity of 7.2 W/cm2 or more can substantially increase cerebral blood flow and reduce arterial stiffness. Therefore, the use of therapeutic ultrasound of appropriate intensity is expected to improve the cerebral blood flow and reduce vascular stiffness to maintain cerebral blood flow at a certain level, which is closely related to the prevention and treatment of cerebrovascular diseases, thereby improving cerebrovascular health.

The acute effect of whole-body heat therapy on peripheral and cerebral vascular reactivity in Black and White females

Among females in the U.S., Black females suffer the most from cardiovascular disease and stroke. While the reasons for this disparity are multifactorial, vascular dysfunction likely contributes. Chronic whole-body heat therapy (WBHT) improves vascular function, but few studies have examined its acute effect on peripheral or cerebral vascular function, which may help elucidate chronic adaptative mechanisms. Furthermore, no studies have investigated this effect in Black females. We hypothesized that Black females would have lower peripheral and cerebral vascular function relative to White females and that one session of WBHT would mitigate these differences. Eighteen young, healthy Black (n = 9; 21 ± 3 yr; BMI: 24.7 ± 4.5 kg/m2) and White (n = 9; 27 ± 3 yr; BMI: 24.8 ± 4.1 kg/m2) females underwent one 60 min session of WBHT (49 °C water via a tube-lined suit). Pre- and 45 min post-testing measures included post-occlusive forearm reactive hyperemia (peripheral microvascular function, RH), brachial artery flow-mediated dilation (peripheral macrovascular function, FMD), and cerebrovascular reactivity (CVR) to hypercapnia. Prior to WBHT, there were no differences in RH, FMD, or CVR (p > 0.05 for all). WBHT improved peak RH in both groups (main effect of WBHT: 79.6 ± 20.1 cm/s to 95.9 ± 30.0 cm/s; p = 0.004, g = 0.787) but not Δ blood velocity (p > 0.05 for both groups). WBHT improved FMD in both groups (6.2 ± 3.4 % to 8.8 ± 3.7 %; p = 0.016, g = 0.618) but had no effect on CVR in either group (p = 0.077). These data indicate that one session of WBHT acutely improves peripheral micro- and macrovascular but not cerebral vascular function in Black and White females.

Does the Combination of Finnish Sauna Bathing and Other Lifestyle Factors Confer Additional Health Benefits? A Review of the Evidence

Sauna bathing, a tradition deeply rooted in the Finnish culture, has been used for thousands of years for leisure, relaxation, and wellness. Sauna bathing is linked with substantial health benefits beyond its use for leisure and relaxation. Several observational and interventional studies suggest that regular or frequent sauna bathing reduces the incidence of vascular and nonvascular diseases, such as hypertension, cardiovascular disease, dementia, and respiratory conditions; may improve the severity of conditions such as musculoskeletal disorders, COVID-19, headache, and influenza; and increases the life span. The beneficial effects of sauna bathing on adverse outcomes have been linked to its blood pressure-reducing, anti-inflammatory, antioxidant, cytoprotective, and stress-reducing properties and its synergistic effect on neuroendocrine, circulatory, cardiovascular, and immune function. Evidence suggests that frequent sauna bathing is an emerging protective risk factor that may augment the beneficial effects of other protective risk or lifestyle factors, such as physical activity and cardiorespiratory fitness, or attenuate or offset the adverse effects of other risk factors, such as high blood pressure, systemic inflammation, and low socioeconomic status. This review summarizes the available epidemiologic and interventional evidence linking the combined effects of Finnish sauna bathing and other risk factors on vascular outcomes including cardiovascular disease and intermediate cardiovascular phenotypes, nonvascular outcomes, and mortality. We also discuss the mechanistic pathways underlying the joint contributions of Finnish sauna bathing and other risk factors on health outcomes, the public health and clinical implications of the findings, gaps in the existing evidence base, and future directions.

Radial artery pulse wave velocity: a new characterization technique and the instabilities associated with the respiratory phase and breath-holding

Objective. Pulse wave velocity (PWV) is a key diagnostic parameter of the cardiovascular system's state. However, approaches aimed at PWV characterization often suffer from inevitable drawbacks. Statistical results demonstrating how closely PWV in the radial artery (RA) and the respiration phase correlate, as well as RA PWV evolution during breath-holding (BH), have not yet been presented in the literature. The aims of this study are (a) to propose a simple robust technique for measuring RA PWV, (b) to reveal the phase relation between the RA PWV and spontaneous breathing, and (c) to disclose the influence of BH on the RA PWV.Approach.The high-resolution remote breathing monitoring method Sorption-Enhanced Infrared Thermography (SEIRT) and the new technique aimed at measuring RA PWV described in this paper were used synchronously, and their measurement data were processed simultaneously.Main results. Spontaneous breathing leaves a synchronous 'trace' on the RA PWV. The close linear correlation of the respiration phase and the phase of concomitant RA PWV changes is statistically confirmed in five tested people (Pearson's r is of the order of 0.5-0.8, P < 0.05). The BH appreciably affects the RA PWV. A phenomenon showing that the RA PWV is not indifferent to hypoxia is observed for the first time.Significance.The proposed technique for RA PWV characterization has high prospects in biomedical diagnostics. The presented pilot study deserves attention in the context of the mutual interplay between respiratory and cardiovascular systems. It may also be useful in cases where peripheral pulse wave propagation helps assess respiratory function.

Heart Rate and Body Temperature Evolution in an Interval Program of Passive Heat Acclimation at High Temperatures (100 ± 2 °C) in a Sauna

Heat exposure provokes stress on the human body. If it remains constant, it leads to adaptations such as heat acclimation. This study aims to observe the evolution of heart rate (HR), core temperature (Tcore), and skin temperature (Tskin) in an intervallic program of exposure to extreme heat. Twenty-nine healthy male volunteers were divided into a control group (CG; n = 14) and an experimental group (EG; n = 15). EG experienced nine sessions (S) of intervallic exposure to high temperatures (100 ± 2 °C), whereas CG was exposed to ambient temperatures (22 ± 2 °C). HR, Tskin, and Tcore were monitored in S1, 4, 5, 8, and 9. An important increase in HR occurred in the S4 compared to the rest (p < 0.05) in EG. A lower HR was discovered in S8 and S9 compared to S4 and in S9 in relation to S1 (p < 0.05) in EG. EG experiences a gradual decrease in Tcore and Tskin, which was detected throughout the assessments, although it was only significant in the S8 and S9 (p < 0.05). Interval exposure to heat at 100 ± 2 °C elicits stress on the human organism, fundamentally increasing Tcore, Tskin, and FC. This recurring stress in the full program caused a drop in the thermoregulatory response as an adaptation or acclimation to heat.

Acute heat exposure improves microvascular function in skeletal muscle of aged adults

Acute heat exposure improves microvascular function in aged adults as assessed using reactive hyperemia. The cutaneous and skeletal muscle microcirculations are thought to contribute to this response, but this has never been confirmed due to the methodological challenges associated with differentiating blood flow between these vascular beds. We hypothesized that acute hot water immersion would improve endothelial-dependent, but not endothelial-independent vasodilation in the microcirculation of the vastus lateralis muscle in healthy aged adults. Participants (70 ± 5 yr) were immersed for 60 min in thermoneutral (36°C) or hot (40°C) water. Ninety minutes following immersion, skeletal muscle microdialysis was used to bypass the cutaneous circulation and directly assess endothelial-dependent and endothelial-independent vasodilation by measuring the local hyperemic response to graded infusions of acetylcholine (ACh, 27.5 and 55.0 mM) and sodium nitroprusside (SNP, 21 and 42 mM), respectively. The hyperemic response to 27.5 mM ACh did not differ between thermal conditions (P = 0.9). However, the hyperemic response to 55.0 mM ACh was increased with prior hot water immersion (thermoneutral immersion, 43.9 ± 23.2 mL/min/100 g vs. hot water immersion, 66.5 ± 25.5 mL/min/100 g; P < 0.01). Similarly, the hyperemic response to 21 mM SNP did not differ between thermal conditions (P = 0.3) but was increased following hot water immersion with the infusion of 42 mM SNP (thermoneutral immersion, 48.8 ± 25.6 mL/min/100 g vs. hot water immersion, 90.7 ± 53.5 mL/min/100 g; P < 0.01). These data suggest that acute heat exposure improves microvascular function in skeletal muscle of aged humans.NEW & NOTEWORTHY Acute heat exposure improves microvascular function in aged adults as assessed using reactive hyperemia. The cutaneous and skeletal muscle microcirculations are thought to contribute to this response, but this has never been confirmed due to the methodological challenges associated with differentiating blood flow between these vascular beds. Using the microdialysis technique to bypass the cutaneous circulation, we demonstrated that heat exposure improves endothelial-dependent and endothelial-independent vasodilation in the microcirculation of skeletal muscle in aged humans.

Infrared sauna as exercise-mimetic? Physiological responses to infrared sauna vs exercise in healthy women: A randomized controlled crossover trial

BACKGROUND

Passive heat therapies have been reported to have similar effects on the cardiovascular system as exercise. Studies supporting these findings in healthy populations have predominantly been done with men using warm water immersions or traditional saunas, rather than newer infrared-based saunas.

OBJECTIVE

To explore short-term thermal and cardiovascular responses in women using an infrared sauna as compared to moderate-intensity exercise.

STUDY DESIGN

Randomized controlled crossover trial with balanced allocations.

SETTING

Brisbane, Australia (August 2019 - March 2020) PARTICIPANTS: Ten healthy women (36 ± 9 years) INTERVENTIONS: 45 min of resting, infrared sauna or indoor bicycling PRIMARY OUTCOME MEASURES: tympanic/skin temperatures; respiratory rate; blood pressure; arterial stiffness; heart rate variability RESULTS: Tympanic temperatures were elevated during infrared sauna as compared to both control (mean diff = +1.05 ^oC ± SEM 0.12 ^oC, 95% C.I.: 0.73 - 1.36, p < 0.0005) and exercise (mean diff = +0.79 ^oC ± SEM 0.12 ^oC, 95% C.I.: 0.49 - 1.08, p < 0.0005). Respiratory rates were higher during exercise as compared to both control (mean diff = +7.66 ± SEM 1.37, 95% C.I.: 4.09 - 11.23, p < 0.0005) and infrared sauna (mean diff = +6.66 ± SEM 1.33, 95% C.I.: 3.20 - 10.11, p < 0.0005). No significant differences in non-invasive measures of blood pressure, arterial stiffness or heart rate variability were detected between any of the interventions.

CONCLUSIONS

These findings suggest the physiological effects of infrared sauna bathing are underpinned by thermoregulatory-induced responses, more so than exercise-mimetic cardiorespiratory or cardiovascular activations.

Regional thermal hyperemia in the human leg: Evidence of the importance of thermosensitive mechanisms in the control of the peripheral circulation

Hyperthermia is thought to increase limb blood flow through the activation of thermosensitive mechanisms within the limb vasculature, but the precise vascular locus in which hyperthermia modulates perfusion remains elusive. We tested the hypothesis that local temperature-sensitive mechanisms alter limb hemodynamics by regulating microvascular blood flow. Temperature and oxygenation profiles and leg hemodynamics of the common (CFA), superficial (SFA) and profunda (PFA) femoral arteries, and popliteal artery (POA) of the experimental and control legs were measured in healthy participants during: (1) 3 h of whole leg heating (WLH) followed by 3 h of recovery (n = 9); (2) 1 h of upper leg heating (ULH) followed by 30 min of cooling and 1 h ULH bout (n = 8); and (3) 1 h of lower leg heating (LLH) (n = 8). WLH increased experimental leg temperature by 4.2 ± 1.2ºC and blood flow in CFA, SFA, PFA, and POA by ≥3-fold, while the core temperature essentially remained stable. Upper and lower leg blood flow increased exponentially in response to leg temperature and then declined during recovery. ULH and LLH similarly increased the corresponding segmental leg temperature, blood flow, and tissue oxygenation without affecting these responses in the non-heated leg segment, or perfusion pressure and conduit artery diameter across all vessels. Findings demonstrate that whole leg hyperthermia induces profound and sustained elevations in upper and lower limb blood flow and that segmental hyperthermia matches the regional thermal hyperemia without causing thermal or hemodynamic alterations in the non-heated limb segment. These observations support the notion that heat-activated thermosensitive mechanisms in microcirculation regulate limb tissue perfusion during hyperthermia.

Augmentation Index Predicts the Sweat Volume in Young Runners

Sweating during exercise is regulated by objective parameters, body weight, and endothelial function, among other factors. However, the relationship between vascular arterial stiffness and sweat volume in young adults remains unclear. This study aimed to identify hemodynamic parameters before exercise that can predict sweat volume during exercise, and post-exercise parameters that can be predicted by the sweat volume. Eighty-nine young healthy subjects (aged 21.9 ± 1.7 years, 51 males) were recruited to each perform a 3-km run on a treadmill. Demographic and anthropometric data were collected and hemodynamic data were obtained, including heart rate, blood pressure and pulse wave analysis using non-invasive tonometry. Sweat volume was defined as pre-exercise body weight minus post-exercise body weight. Post-exercise hemodynamic parameters were also collected. Sweat volume was significantly associated with gender, body surface area (BSA) (b = 0.288, p = 0.010), peripheral systolic blood pressure (SBP), peripheral and central pulse pressure (PP), and was inversely associated with augmentation index at an HR of 75 beats/min (AIx@HR75) (b = -0.005, p = 0.019) and ejection duration. While BSA appeared to predict central PP (B = 19.271, p ≤ 0.001), central PP plus AIx@HR75 further predicted sweat volume (B = 0.008, p = 0.025; B = -0.009, p = 0.003 respectively). Sweat volume was associated with peripheral SBP change (B = -17.560, p = 0.031). Sweat volume during a 3-km run appears to be influenced by hemodynamic parameters, including vascular arterial stiffness and central pulse pressure. Results of the present study suggest that vascular arterial stiffness likely regulates sweat volume during exercise.

Improvements in vascular function in response to acute lower limb heating in young healthy males and females

Regular exposure to passive heat stress improves vascular function, but the optimal heating prescription remains undefined. Local limb heating is more feasible than whole body heating, but the evidence demonstrating its efficacy is lacking. The purpose of this study was to determine whether acute improvements in vascular function can be achieved with lower limb heating in 16 young healthy individuals (8 female, 8 male). In separate visits, participants underwent 45 min of ankle- and knee-level hot water immersion (45°C). A subset of seven participants also participated in a time-control visit. Endothelial function was assessed through simultaneous brachial and superficial femoral artery flow-mediated dilation (FMD) tests. Macrovascular function was quantified by %FMD, whereas microvascular function was quantified by vascular conductance during reactive hyperemia. Arterial stiffness was assessed through carotid-femoral and femoral-foot pulse wave velocity (PWV). Plasma concentrations of interleukin-6 and extracellular heat shock protein-72 (eHSP72) were used as indicators of inflammation. Our findings showed that 45 min of lower limb heating-regardless of condition-acutely improved upper limb macrovascular endothelial function (i.e., brachial %FMD; Pre: 4.6 ± 1.7 vs. Post: 5.4 ± 2.0%; P = 0.004) and lower limb arterial stiffness (i.e., femoral-foot PWV; Pre: 8.4 ± 1.2 vs. Post: 7.7 ± 1.1 m/s; P = 0.011). However, only knee-level heating increased upper limb microvascular function (i.e., brachial peak vascular conductance; Pre: 6.3 ± 2.7 vs. Post: 7.8 ± 3.5 mL/min ⋅ mmHg; P ≤ 0.050) and plasma eHSP72 concentration (Pre: 12.4 ± 9.4 vs. Post: 14.8 ± 9.8 ng/mL; P ≤ 0.050). These findings show that local lower limb heating acutely improves vascular function in younger individuals, with knee-level heating improving more outcome measures.NEW & NOTEWORTHY This study demonstrates that lower limb hot water immersion is an effective strategy for acutely improving vascular function in young, healthy males and females, thereby encouraging the development of accessible modes of heat therapy for vascular health.

Acute Effects of Short-Term Warm Water Immersion on Arterial Stiffness and Central Hemodynamics

Warm water immersion (WWI) has a potentially favorable effect on vascular health. However, the effects of short-term WWI on vascular function and central hemodynamics remain unclear. The present study aimed to determine the acute effects of short-term WWI on arterial stiffness and central hemodynamics in healthy men. Ten healthy men (27–57 years, 44 ± 12 years of mean age) underwent 5-min WWI (40–41°C) at the heart level. Systemic hemodynamics and tympanic temperature were monitored during WWI. Furthermore, pulse wave velocity (PWV) and aortic hemodynamics were measured before and 10 min after WWI. Cardiac output (CO) (via the Modelflow method) increased (P = 0.037), whereas tympanic temperature did not change (P = 0.879) during WWI. After 5-min WWI, heart rate (HR) and brachial diastolic blood pressure (BP) were significantly decreased. Aortic and leg PWV were decreased by 7.5 and 3.1%, respectively (P = 0.006 and P = 0.040). Femoral arterial blood flow was increased by 45.9% (P = 0.002), and leg vascular resistance was decreased by 29.1% (P < 0.001). Regarding central hemodynamic variables (estimated by general transfer function), aortic BP and augmentation index (AIx) did not change significantly, but the subendocardial viability ratio (SEVR), an index of coronary perfusion, was increased (P = 0.049). Our results indicate that a short-term WWI acutely improves aortic and peripheral arterial stiffness and coronary perfusion. Further studies to determine the interaction between the residual effect of a single bout of short-term WWI and chronic change (e.g., adaptation) in arterial stiffness and central hemodynamics are needed.

Noninvasive Measurement of Time-Varying Arterial Wall Elastance Using a Single-Frequency Vibration Approach

The arterial wall elastance is an important indicator of arterial stiffness and a kind of manifestation associated with vessel-related disease. The time-varying arterial wall elastances can be measured using a multiple-frequency vibration approach according to the Voigt and Maxwell model. However, such a method needs extensive calculation time and its operating steps are very complex. Thus, the aim of this study is to propose a simple and easy method for assessing the time-varying arterial wall elastances with the single-frequency vibration approach. This method was developed according to the simplified Voigt and Maxwell model. Thus, the arterial wall elastance measured using this method was compared with the elastance measured using the multiple-frequency vibration approach. In the single-frequency vibration approach, a moving probe of a vibrator was induced with a radial displacement of 0.15 mm and a 40 Hz frequency. The tip of the probe directly contacted the wall of a superficial radial artery, resulting in the arterial wall moving 0.15 mm radially. A force sensor attached to the probe was used to detect the reactive force exerted by the radial arterial wall. According to Voigt and Maxwell model, the wall elastance (E_single) was calculated from the ratio of the measured reactive force to the peak deflection of the displacement. The wall elastances (E_multiple) measured by the multiple-frequency vibration approach were used as the reference to validate the performance of the single-frequency approach. Twenty-eight healthy subjects were recruited in the study. Individual wall elastances of the radial artery were determined with the multiple-frequency and the single-frequency approaches at room temperature (25 °C), after 5 min of cold stress (4 °C), and after 5 min of hot stress (42 °C). We found that the time-varying E_single curves were very close to the time-varying E_multiple curves. Meanwhile, there was a regression line (E_single = 0.019 + 0.91 E_multiple, standard error of the estimate (SEE) = 0.0295, p < 0.0001) with a high correlation coefficient (0.995) between E_single and E_multiple. Furthermore, from the Bland-Altman plot, good precision and agreement between the two approaches were demonstrated. In summary, the proposed approach with a single-frequency vibrator and a force sensor showed its feasibility for measuring time-varying wall elastances.

Is sauna bathing protective of sudden cardiac death? A review of the evidence

Sudden cardiac death (SCD) is a global public health burden accounting for 15-20% of all deaths. Though established atherosclerotic risk factors explain a large proportion of the risk of SCD, these factors are often absent in a large proportion of SCD victims and the pathogenesis of SCD is still not fully established. It therefore appears that additional factors may be involved. Sauna bathing is a traditional Finnish activity that is mainly used for the purposes of relaxation and pleasure. Beyond its use for these purposes, sauna bathing has been linked with several health benefits. Emerging evidence suggests that sauna bathing is associated with reduced risk of adverse cardiovascular (CV) disease (CVD) and non-CVD outcomes as well as mortality. A number of reports have linked sauna bathing with reduced or increased risk of SCD, but the evidence is uncertain. This review summarizes available studies linking sauna bathing with SCD, the postulated mechanistic pathways underlying these associations, outlines areas of outstanding uncertainty, and the implications for prevention. We employed a comprehensive search for observational studies, randomized controlled trials (RCTs), and non-RCTs from MEDLINE and Embase since their inception until March 2019. Observational data suggest that regular sauna bathing is associated with a substantial risk reduction in SCD. Furthermore, the data suggest that a combination of regular physical activity and sauna baths confers substantial risk reduction for SCD compared with either modality alone. Few reports have linked sauna baths with SCDs, but these single case incidents have been attributed to the effects of dehydration, hypotension, and cardiac arrhythmias due to a combination of sauna exposure and alcohol consumption. Sauna bathing is generally safe for most healthy people and even among patients with stable CVD, if used sensibly and with caution. Plausible pathways underlying the protective effect of sauna bathing on SCD may be linked to the impact on CV function via reduced arterial stiffness, decreases in inflammation and oxidative stress, stabilization of the autonomic nervous system, beneficial changes in circulating lipid profiles and other CVD risk markers, and lowering of systemic blood pressure. Sauna is a potential novel tool to promote SCD prevention in addition to other known means, being an enjoyable way to take care of general health and well-being.

Free-living, continuous hypo-hydration, and cardiovascular response to exercise in a heated environment

Chronic dehydration (DEH) and heat stress combined with poor cardiovascular (CV) health may influence physiological responses to exercise. We examined the effects of free-living induced hypo-hydration on physiological responses to exercise in a heated environment and whether resting CV health is related to these changes. Participants (N = 16, 20.6 ± 1.2 years) were randomized to 3 days of voluntary fluid restriction (DEH) or intake (hydration [HYD]) followed by an exercise bout. CV health was assessed by flow-mediated dilation (FMD), pulse wave analysis, and heart rate variability (HRV). HYD was assessed by weight, urine color, and specific gravity (USG). Exercise trials were conducted in a heated environment (30.3 ± 0.8°C, 27.4 ± 7.4% RH) on a cycle ergometer for 30 min. Heart rate (HR), weighted skin (T_sk ) and mean body temperature (T_b ) and skin blood flow (SBF) were assessed during exercise. Pre-exercise weight (P < 0.005), urine color, and USG (P < 0.001) were different in between trials. HR was greater in DEH (153 ± 26 bpm) versus HYD (144 ± 23 bpm, P = 0.02) after exercise. No group differences were found, but a time interaction P < 0.001) for all temperature responses and time-by-trial interaction for T_re (P < 0.01) and T_sk (P < 0.001) was observed. Greater changes in T_re (P = 0.02) and T_sk (P < 0.01) were associated with increased FMD. Free-living, continuous DEH alters weight, blood, and urine markers of HYD as well as HR response during exercise. Resting CV health was related to increased change in T_re and T_sk , suggesting CV health plays a role in the mechanism of heat dissipation when DEH even in college-age men and women.

Arterial stiffness during whole-body passive heat stress in healthy older adults

We tested the hypothesis that whole-body passive heat stress reduces arterial stiffness in older adults. At preheat stress (baseline) and when core temperature was elevated by 0.6 ± 0.2°C (mild) and 1.2 ± 0.3°C (moderate), arterial stiffness was measured in eight healthy younger (26 ± 5 years) and eight healthy older (70 ± 4 years) adults in the supine position. Arterial stiffness was estimated from carotid-to-femoral pulse wave velocity (cfPWV, applanation tonometry). cfPWV was higher at baseline in older adults (8.8 ± 2.3 m/sec vs. 5.6 ± 0.9 m/sec, P < 0.01) and this difference was maintained throughout passive heat stress (P < 0.01). cfPWV did not change (P ≥ 0.49) with passive heat stress in either younger (at moderate heat stress: 6.0 ± 1.0 m/sec) or older (at moderate heat stress: 8.5 ± 1.6 m/sec) adults. However, the influence of baseline cfPWV on the change in cfPWV during mild (r = -0.66, P = 0.04) and moderate (r = -0.87, P < 0.01) heat stress were inversely related in older adults, and the strength of these relations was not statistically different (P = 0.08). In younger adults, the influence of baseline cfPWV on the change in cfPWV during mild heat stress was also inversely related (r = -0.79, P = 0.01), while the strength of this relation was attenuated at moderate heat stress (r = -0.24, P = 0.30). Changes in arterial stiffness during passive heat stress in adults aged ≥65 year are likely dependent on the magnitude of baseline arterial stiffness and not necessarily age.

Effect of heat stress on vascular outcomes in humans

In addition to its role as an environmental stressor, scientists have recently demonstrated the potential for heat to be a therapy for improving or mitigating declines in arterial health. Many studies at both ends of the scientific controls spectrum (tightly controlled, experimental vs. practical) have demonstrated the beneficial effects of heating on microvascular function (e.g., reactive hyperemia, cutaneous vascular conductance); endothelial function (e.g., flow-mediated dilation); and arterial stiffness (e.g., pulse-wave velocity, compliance, β-stiffness index). It is important to note that findings of beneficial effects are not unanimous, likely owing to the varied methodology in both heating protocols and assessments of outcome measures. Mechanisms of action for the effects of both acute and chronic heating are also understudied. Heat science is a very promising area of human physiology research, as it has the potential to contribute to approaches addressing the global cardiovascular disease burden, particularly in aging and at risk populations, and those for whom exercise is not feasible or recommended.

The effect of passive heat stress on distress andself-control in male smokers and non-smokers

In the current study, we tested the effects of core body temperature increases (e.g. heat stress) on affect, self-reported physical discomfort, and subsequent self-control in male smokers and nonsmokers using a novel passive heat stress paradigm, within a distress tolerance framework. Twenty-eight men (14 smokers), completed both heat stress and control sessions in randomized order. Results revealed that increases in core body temperature were associated with increased anxiety, irritability, and body discomfort as well as decreased happiness, with stronger effects for smokers. Smokers and nonsmokers both evidenced less self-control during the heat session and did not differ on this measure, nor on a measure of interoceptive sensitivity. The current study indicates that heat stress is a viable method for studying distress tolerance in men, and suggests the value in examining dynamic changes in self-control as a function of distress. Implications will be discussed for distress tolerance in general and smokers in specific.

Cardiovascular and Other Health Benefits of Sauna Bathing: A Review of the Evidence

Sauna bathing, an activity that has been a tradition in Finland for thousands of years and mainly used for the purposes of pleasure and relaxation, is becoming increasingly popular in many other populations. Emerging evidence suggests that beyond its use for pleasure, sauna bathing may be linked to several health benefits, which include reduction in the risk of vascular diseases such as high blood pressure, cardiovascular disease, and neurocognitive diseases; nonvascular conditions such as pulmonary diseases; mortality; as well as amelioration of conditions such as arthritis, headache, and flu. The beneficial effects of sauna bathing on these outcomes have been linked to its effect on circulatory, cardiovascular, and immune functions. It has been postulated that regular sauna bathing may improve cardiovascular function via improved endothelium-dependent dilatation, reduced arterial stiffness, modulation of the autonomic nervous system, beneficial changes in circulating lipid profiles, and lowering of systemic blood pressure. This review summarizes the available epidemiological, experimental, and interventional evidence linking Finnish sauna bathing and its effects on cardiovascular outcomes and other disease conditions on the basis of a comprehensive search for observational studies, randomized controlled trials, and non-randomized controlled trials from MEDLINE and EMBASE from their inception until February 24, 2018. An overview of the postulated biological mechanisms underlying the associations between sauna bathing and its health benefits, areas of outstanding uncertainty, and implications for clinical practice is also provided.

Effects of heat and cold on health, with special reference to Finnish sauna bathing

Environmental stress such as extremely warm or cold temperature is often considered a challenge to human health and body homeostasis. However, the human body can adapt relatively well to heat and cold environments, and recent studies have also elucidated that particularly heat stress might be even highly beneficial for human health. Consequently, the aim of the present brief review is first to discuss general cardiovascular and other responses to acute heat stress, followed by a review of beneficial effects of Finnish sauna bathing on general and cardiovascular health and mortality as well as dementia and Alzheimer's disease risk. Plausible mechanisms included are improved endothelial and microvascular function, reduced blood pressure and arterial stiffness, and possibly increased angiogenesis in humans, which are likely to mediate the health benefits of sauna bathing. In addition to heat exposure with physiological adaptations, cold stress-induced physiological responses and brown fat activation on health are also discussed. This is important to take into consideration, as sauna bathing is frequently associated with cooling periods in cold(er) environments, but their combination remains poorly investigated. We finally propose, therefore, that possible additive effects of heat- and cold-stress-induced adaptations and effects on health would be worthy of further investigation.

Cardiovascular health effects following exposure of human volunteers during fire extinction exercises

BACKGROUND

Firefighters have increased risk of cardiovascular disease and of sudden death from coronary heart disease on duty while suppressing fires. This study investigated the effect of firefighting activities, using appropriate personal protective equipment (PPE), on biomarkers of cardiovascular effects in young conscripts training to become firefighters.

METHODS

Healthy conscripts (n = 43) who participated in a rescue educational course for firefighting were enrolled in the study. The exposure period consisted of a three-day training course where the conscripts participated in various firefighting exercises in a constructed firehouse and flashover container. The subjects were instructed to extinguish fires of either wood or wood with electrical cords and mattresses. The exposure to particulate matter (PM) was assessed at various locations and personal exposure was assessed by portable PM samplers and urinary excretion of 1-hydroxypyrene. Cardiovascular measurements included microvascular function and heart rate variability (HRV).

RESULTS

The subjects were primarily exposed to PM in bystander positions, whereas self-contained breathing apparatus effectively abolished pulmonary exposure. Firefighting training was associated with elevated urinary excretion of 1-hydroxypyrene (105%, 95% CI: 52; 157%), increased body temperature, decreased microvascular function (-18%, 95% CI: -26; -9%) and altered HRV. There was no difference in cardiovascular measurements for the two types of fires.

CONCLUSION

Observations from this fire extinction training show that PM exposure mainly occurs in situations where firefighters removed the self-contained breathing apparatus. Altered cardiovascular disease endpoints after the firefighting exercise period were most likely due to complex effects from PM exposure, physical exhaustion and increased core body temperature.

Lower-limb hot-water immersion acutely induces beneficial hemodynamic and cardiovascular responses in peripheral arterial disease and healthy, elderly controls

Passive heat induces beneficial perfusion profiles, provides substantive cardiovascular strain, and reduces blood pressure, thereby holding potential for healthy and cardiovascular disease populations. The aim of this study was to assess acute responses to passive heat via lower-limb, hot-water immersion in patients with peripheral arterial disease (PAD) and healthy, elderly controls. Eleven patients with PAD (age 71 ± 6 yr, 7 male, 4 female) and 10 controls (age 72 ± 7 yr, 8 male, 2 female) underwent hot-water immersion (30-min waist-level immersion in 42.1 ± 0.6°C water). Before, during, and following immersion, brachial and popliteal artery diameter, blood flow, and shear stress were assessed using duplex ultrasound. Lower-limb perfusion was measured also using venous occlusion plethysmography and near-infrared spectroscopy. During immersion, shear rate increased (P < 0.0001) comparably between groups in the popliteal artery (controls: +183 ± 26%; PAD: +258 ± 54%) and brachial artery (controls: +117 ± 24%; PAD: +107 ± 32%). Lower-limb blood flow increased significantly in both groups, as measured from duplex ultrasound (>200%), plethysmography (>100%), and spectroscopy, while central and peripheral pulse-wave velocity decreased in both groups. Mean arterial blood pressure was reduced by 22 ± 9 mmHg (main effect P < 0.0001, interaction P = 0.60) during immersion, and remained 7 ± 7 mmHg lower 3 h afterward. In PAD, popliteal shear profiles and claudication both compared favorably with those measured immediately following symptom-limited walking. A 30-min hot-water immersion is a practical means of delivering heat therapy to PAD patients and healthy, elderly individuals to induce appreciable systemic (chronotropic and blood pressure lowering) and hemodynamic (upper and lower-limb perfusion and shear rate increases) responses.

Exercise-induced heat stress disrupts the shear-dilatory relationship

NEW FINDINGS

What is the central question of this study? Although heat stress is known to increase cardiovascular strain, no study, to date, had explored the potential impact of exercise-induced heat stress on vascular function. What is the main finding and its importance? We found that acute exercise tended to reduce flow-mediated dilatation (FMD), owing in part to reduced reactive hyperaemia/shear stimulus; thus, when FMD is normalized to shear no postexercise deficit exists. Exercise-induced heat stress increased reactive hyperaemia, shear rate, coupled with a sustained FMD postexercise, suggests that exercise-induced heat stress increases the amount of shear stimulus to elicit a similar response, indicating reduced vascular responsiveness, or reserve, which might increase cardiovascular susceptibility. Heat stress increases cardiovascular strain and is of particular concern in occupations, such as firefighting, in which individuals are required to perform strenuous work while wearing personal protective equipment. Sudden cardiac events are associated with strenuous activity and are the leading cause of duty-related death among firefighters, accounting for ∼50% of duty-related fatalities per year. Understanding the acute effects of exercise-induced heat stress (EIHS) on vascular endothelial function may provide insight into the mechanisms precipitating acute coronary events in firefighters. The purpose of this study, therefore, was to determine the effects of EIHS on vascular endothelial function. Using a balanced crossover design, 12 healthy men performed 100 min of moderate-intensity, intermittent exercise with and without EIHS (personal protective equipment or cooling vest, respectively). Measurements of flow-mediated dilatation (FMD), reactive hyperaemia and shear rate area under the curve (SR_AUC ) were performed pre- and postexercise. During EIHS, core temperature was significantly higher (38 ± 0.1 versus 37 ± 0.1°C). Postexercise FMD tended to be suppressed in both conditions, but was not different from pre-exercise. Reactive hyperaemia was reduced after no-EIHS but increased after EIHS. Thus, normalizing FMD to the shear stimulus (FMD/SR_AUC ) revealed a significant reduction in FMD after EIHS only (pre-exercise 0.15 ± 0.04 and 0.13 ± 0.02 s^-1 versus postexercise, 0.13 ± 0.02 and 0.06 ± 0.02 s^-1 , no-EIHS and EIHS, respectively). We conclude that moderate heat stress superimposed on moderate-intensity exercise resulted in reduced vascular endothelial function. This heat stress-induced alteration in the shear-dilatory relationship may relate to the increased risk of acute coronary events associated with activities that combine physical exertion and heat stress (i.e. firefighting).

Regular Exercise Reduces Endothelial Cortical Stiffness in Western Diet-Fed Female Mice

We recently showed that Western diet-induced obesity and insulin resistance promotes endothelial cortical stiffness in young female mice. Herein, we tested the hypothesis that regular aerobic exercise would attenuate the development of endothelial and whole artery stiffness in female Western diet-fed mice. Four-week-old C57BL/6 mice were randomized into sedentary (ie, caged confined, n=6) or regular exercise (ie, access to running wheels, n=7) conditions for 16 weeks. Exercise training improved glucose tolerance in the absence of changes in body weight and body composition. Compared with sedentary mice, exercise-trained mice exhibited reduced endothelial cortical stiffness in aortic explants (sedentary 11.9±1.7 kPa versus exercise 5.5±1.0 kPa; P<0.05), as assessed by atomic force microscopy. This effect of exercise was not accompanied by changes in aortic pulse wave velocity (P>0.05), an in vivo measure of aortic stiffness. In comparison, exercise reduced femoral artery stiffness in isolated pressurized arteries and led to an increase in femoral internal artery diameter and wall cross-sectional area (P<0.05), indicative of outward hypertrophic remodeling. These effects of exercise were associated with an increase in femoral artery elastin content and increased number of fenestrae in the internal elastic lamina (P<0.05). Collectively, these data demonstrate for the first time that the aortic endothelium is highly plastic and, thus, amenable to reductions in stiffness with regular aerobic exercise in the absence of changes in in vivo whole aortic stiffness. Comparatively, the same level of exercise caused destiffening effects in peripheral muscular arteries, such as the femoral artery, that perfuse the working limbs.

Passive heat therapy improves endothelial function, arterial stiffness and blood pressure in sedentary humans

KEY POINTS

A recent 30 year prospective study showed that lifelong sauna use reduces cardiovascular-related and all-cause mortality; however, the specific cardiovascular adaptations that cause this chronic protection are currently unknown. We investigated the effects of 8 weeks of repeated hot water immersion ('heat therapy') on various biomarkers of cardiovascular health in young, sedentary humans. We showed that, relative to a sham group which participated in thermoneutral water immersion, heat therapy increased flow-mediated dilatation, reduced arterial stiffness, reduced mean arterial and diastolic blood pressure, and reduced carotid intima media thickness, with changes all on par or greater than what is typically observed in sedentary subjects with exercise training. Our results show for the first time that heat therapy has widespread and robust effects on vascular function, and as such, could be a viable treatment option for improving cardiovascular health in a variety of patient populations, particularly those with limited exercise tolerance and/or capabilities.

ABSTRACT

The majority of cardiovascular diseases are characterized by disorders of the arteries, predominantly caused by endothelial dysfunction and arterial stiffening. Intermittent hot water immersion ('heat therapy') results in elevations in core temperature and changes in cardiovascular haemodynamics, such as cardiac output and vascular shear stress, that are similar to exercise, and thus may provide an alternative means of improving health which could be utilized by patients with low exercise tolerance and/or capabilities. We sought to comprehensively assess the effects of 8 weeks of heat therapy on biomarkers of vascular function in young, sedentary subjects. Twenty young, sedentary subjects were assigned to participate in 8 weeks (4-5 times per week) of heat therapy (n = 10; immersion in a 40.5°C bath sufficient to maintain rectal temperature ≥ 38.5°C for 60 min per session) or thermoneutral water immersion (n = 10; sham). Eight weeks of heat therapy increased flow-mediated dilatation from 5.6 ± 0.3 to 10.9 ± 1.0% (P < 0.01) and superficial femoral dynamic arterial compliance from 0.06 ± 0.01 to 0.09 ±0.01 mm(2)  mmHg(-1) (P = 0.03), and reduced (i.e. improved) aortic pulse wave velocity from 7.1 ± 0.3 to 6.1 ± 0.3 m s(-1) (P = 0.03), carotid intima media thickness from 0.43 ± 0.01 to 0.37 ± 0.01 mm (P < 0.001), and mean arterial blood pressure from 83 ± 1 to 78 ± 2 mmHg (P = 0.02). No changes were observed in the sham group or for carotid arterial compliance, superficial femoral intima media thickness or endothelium-independent dilatation. Heat therapy improved endothelium-dependent dilatation, arterial stiffness, intima media thickness and blood pressure, indicating improved cardiovascular health. These data suggest heat therapy may provide a simple and effective tool for improving cardiovascular health in various populations.

Thermotherapy reduces blood pressure and circulating endothelin-1 concentration and enhances leg blood flow in patients with symptomatic peripheral artery disease

Leg thermotherapy (TT) application reduces blood pressure (BP) and increases both limb blood flow and circulating levels of anti-inflammatory mediators in healthy, young humans and animals. The purpose of the present study was to determine the impact of TT application using a water-circulating garment on leg and systemic hemodynamics and on the concentrations of circulating cytokines and vasoactive mediators in patients with symptomatic peripheral artery disease (PAD). Sixteen patients with PAD and intermittent claudication (age: 63 ± 9 yr) completed three experimental sessions in a randomized order: TT, control intervention, and one exercise testing session. The garment was perfused with 48°C water for 90 min in the TT session and with 33°C water in the control intervention. A subset of 10 patients also underwent a protocol for the measurement of blood flow in the popliteal artery during 90 min of TT using phase-contrast MRI. Compared with the control intervention, TT promoted a significant reduction in systolic (∼11 mmHg) and diastolic (∼6 mmHg) BP (P < 0.05) that persisted for nearly 2 h after the end of the treatment. The serum concentration of endothelin-1 (ET-1) was significantly lower 30 min after exposure to TT (Control: 2.3 ± 0.1 vs. TT: 1.9 ± 0.09 pg/ml, P = 0.026). In addition, TT induced a marked increase in peak blood flow velocity (∼68%), average velocity (∼76%), and average blood flow (∼102%) in the popliteal artery (P < 0.01). These findings indicate that TT is a practical and effective strategy to reduce BP and circulating ET-1 concentration and enhance leg blood flow in patients with PAD.

Effect of passive heat stress on arterial stiffness in smokers versus non-smokers

In non-smokers, passive heat stress increases shear stress and vasodilation, decreasing arterial stiffness. Smokers, who reportedly have arterial dysfunction, may have similar improvements in arterial stiffness with passive heat stress. Therefore, we examined the effects of an acute bout of whole-body passive heat stress on arterial stiffness in smokers vs. non-smokers. Thirteen smokers (8.8 ± 5.5 [median = 6] cigarettes per day for > 4 years) and 13 non-smokers matched for age, mass, height, and exercise habits (27 ± 8 years; 78.8 ± 15.4 kg; 177.6 ± 6.7 cm) were passively heated to 1.5 °C core temperature (T C) increase. At baseline and each 0.5 °C T C increase, peripheral (pPWV) and central pulse wave velocity (cPWV) were measured via Doppler ultrasound. No differences existed between smokers and non-smokers for any variables (all p >  .05), except cPWV slightly increased from baseline (526.7 ± 81.7 cm · s(-1)) to 1.5 °C ΔT C (579.7 ± 69.8 cm · s(-1); p < 0.005), suggesting heat stress acutely increased central arterial stiffness. pPWV did not change with heating (grand mean: baseline = 691.9 ± 92.9 cm · s(-1); 1.5 °C ΔT C = 691.9 ± 79.5 cm · s(-1); p > 0.05). Changes in cPWV and pPWV during heating correlated (p < 0.05) with baseline PWV in smokers (cPWV: r = -0.59; pPWV: r = -0.62) and non-smokers (cPWV: r = -0.45; pPWV: r = -0.77). Independent of smoking status, baseline stiffness appears to mediate the magnitude of heating-induced changes in arterial stiffness.

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.

Vascular and central hemodynamic changes following exercise-induced heat stress

This study examined the effects of moderate exercise-induced heat stress (EIHS) on vascular function, central hemodynamic load and indices of coronary perfusion. Vascular-hemodynamic measures were collected in 12 healthy men (aged 22±3 years) pre and post 100 minutes of moderate, intermittent exercise in two randomized conditions: heat stress (HS; wearing firefighter personal protective equipment (PPE)), and no heat stress (NHS; wearing a cooling shirt and equivalent PPE weight). Aortic blood pressure, reflected wave pressure (Pb), systolic (SPTI) and diastolic pressure time-integral (DPTI), and aortic stiffness were assessed before and after each condition. SPTI was significantly greater, and DPTI and Pb were significantly lower for HS-post compared to NHS-post (p<0.05). Pulse wave velocity was not different between conditions. In conclusion, EIHS does not affect aortic stiffness, but increases indices of myocardial work and reduces indices of coronary perfusion which may be related to chronotropic responses to EIHS. The mismatch between oxygen demand and oxygen supply may increase cardiac vulnerability to ischemia during strenuous work in the heat.

Effect of local cooling on pro-inflammatory cytokines and blood flow of the skin under surface pressure in rats: feasibility study

The primary purpose of this feasibility study was to establish a correlation between pro-inflammatory cytokine accumulation and severity of tissue damage during local pressure with various temperatures. The secondary purpose was to compare skin blood flow patterns for assessing the efficacy of local cooling on reducing skin ischemia under surface pressure. Eight Sprague-Dawley rats were assigned to two protocols, including pressure with local cooling (Δt = -10 °C) and pressure with local heating (Δt = 10 °C). Pressure of 700 mmHg was applied to the right trochanter area of rats for 3 h. Skin perfusion quantified by laser Doppler flowmetry and TNF-∗ and IL-1β levels were measured. Our results showed that TNF-α concentrations were increased more significantly with local heating than with local cooling under pressure whereas IL-1β did not change. Our results support the notion that weight bearing soft tissue damage may be reduced through temperature modulation and that non-invasive perfusion measurements using laser Doppler flowmetry may be capable of assessing viability. Furthermore, these results show that perfusion response to loading pressure may be correlated with changes in local pro-inflammatory cytokines. These relationships may be relevant for the development of cooling technologies for reducing risk of pressure ulcers.

Heat acclimation increases arterial elasticity in young men

The major physiological adaptations that occur during heat acclimation (HA) are well documented. However, no studies have provided compelling evidence about the effect of HA on arterial elastic properties. The aim of this study was to examine the changes in large artery elasticity (LAE) and small artery elasticity (SAE) concomitant with HA and to determine the potential relationships among changes in arterial elasticity, baseline aerobic fitness level, and improvement in endurance capacity (EC). During 10-day HA, the subjects (n = 21) exercised daily on a treadmill for 110 min at an intensity of 55%–60% of peak oxygen uptake in a climatic chamber preset to 42 °C and 18% relative humidity. EC was tested in the heat before and after HA. Arterial elasticity was assessed by diastolic pulse wave analysis (HDI/Pulse Wave CR-2000) at baseline and after HA. Blood samples were drawn at baseline. After HA, there was a 17% increase in LAE (from 21.19 ± 4.72 mL·mm Hg−1 × 10 to 24.77 ± 5.91 mL·mm Hg−1 × 10, p < 0.05) and an 18% increase in SAE (from 9.32 ± 1.76 mL·mm Hg−1 × 100 to 10.98 ± 1.75 mL·mm Hg−1 × 100, p < 0.01). EC increased by 86% (from 88.62 ± 27.51 min to 161.95 ± 47.80 min, p < 0.001) as a result of HA. No significant associations were revealed between changes in arterial elasticity parameters and improvement in EC or baseline aerobic fitness level. We demonstrated, for the first time, that HA has a positive impact on the parameters of arterial elasticity. Further investigations are needed to determine the mechanisms underlying these changes and the potential relationships among arterial elasticity, aerobic fitness level, and EC.

Gender difference in the acute influence of a 2-hour run on arterial stiffness in trained runners

Arterial stiffness scores over a 5-h period following a 2-h run were measured in trained males (N = 8, age 39.3 ± 2.3 y) and females (N = 8, 35.8 ± 2.8 y). Subjects reported for two lab sessions (randomized, crossover design) from 7:30 am to 4:15 pm, separated by 1-2 weeks, and either rested or ran for 2 h on a treadmill at 75% VO(2 max) from 9:15 to 11:15 am. Augmentation index standardized to a heart rate of 75 bpm (AIx75), and carotid to femoral pulse wave velocity (PWV), were measured by applanation tonometry. Significant interaction effects were measured for AIx75 (P = 0.039) and PWV (P = 0.020), and compared with the rest condition, female runners experienced decreased AIx75 from 11:45 am to 3:15 pm, and in PWV at 11:45 am and 12:15 pm, in contrast to no change in the male runners. These data support a notable gender difference in arterial stiffness following a 2-h bout of running.