Combining hypoxia with thermal stimuli in humans: physiological responses and potential sex differences

Abstract

Increasing prevalence of native lowlanders sojourning to high altitudes (>2,500 m) for recreational, occupational, military, and competitive reasons has generated increased interest in physiological responses to multistressor environments. Exposure to hypoxia poses recognized physiological challenges that are amplified during exercise and further complicated by environments that might include combinations of heat, cold, and high altitude. There is a sparsity of data examining integrated responses in varied combinations of environmental conditions, with even less known about potential sex differences. How this translates into performance, occupational, and health outcomes requires further investigation. Acute hypoxic exposure decreases arterial oxygen saturation, resulting in a reflex hypoxic ventilatory response and sympathoexcitation causing an increase in heart rate, myocardial contractility, and arterial blood pressure, to compensate for the decreased arterial oxygen saturation. Acute altitude exposure impairs exercise performance, for example, reduced time to exhaustion and slower time trials, largely owing to impairments in pulmonary gas exchange and peripheral delivery resulting in reduced V̇o_2max. This exacerbates with increasing altitude, as does the risk of developing acute mountain sickness and more serious altitude-related illnesses, but modulation of those risks with additional stressors is unclear. This review aims to summarize and evaluate current literature regarding cardiovascular, autonomic, and thermoregulatory responses to acute hypoxia, and how these may be affected by simultaneous thermal environmental challenges. There is minimal available information regarding sex as a biological variable in integrative responses to hypoxia or multistressor environments; we highlight these areas as current knowledge gaps and the need for future research.

Keywords: altitude; cardiovascular; hypobaric; multistressor; thermoregulation.

Figure 1.

A schematic of the major physiological responses to hypoxia (left), combined cold-hypoxia (left), and heat-hypoxia (right). Responses during combined thermal and hypoxic stimuli are in comparison with thermal (cold or hot) normoxic conditions unless stated, with increased (↑), decreased (↓), no change (↔), or an unknown (?) response. Physiological strain refers to the physiological responses during environmental stress and/or exercise, primarily heart rate and core body temperature, indicating the overall strain on the body and potential for heat illness. BAT, brown adipose tissue; Bf, blood flow; CO, cardiac output; f_R, respiratory frequency; GFR, glomerular filtration rate; ${\text{HCO}}_3^{-}$, bicarbonate ions; HR, heart rate; HVR, hypoxic ventilatory response; MAP, mean arterial pressure; SNA, sympathetic nervous system activity; SkBf, skin blood flow; SV, stroke volume; T_core, core body temperature; T_sk, skin temperature; VC, vasoconstriction; VD, vasodilation; V̇e, minute ventilation; V_T, tidal volume. The figure was created using BioRender.