The Combined Effects of Temperature and Posture on Regional Blood Flow and Haemodynamics

Under simultaneous ambient temperature and postural stressors, integrated regional blood flow responses are required to maintain blood pressure homeostasis. The aim of the present study was to assess the effect of ambient temperature and body posture on regional regulation of microvascular blood flow, specifically in the arms and legs.
Participants (N=11) attended two sessions in which they experienced transient ambient conditions, in a climatic chamber. During each 60-min trial, ambient temperature was varied between 15°C and 40°C and the participants were either standing or in a supine position; relative humidity in the chamber was maintained at 25.9 (6.6) %. Laser doppler flowmetry of the forearm (SkBFarm) and calf (SkBFcalf), and haemodynamic responses (heart rate, stroke volume, cardiac output, blood pressure), were measured continuously. Analyses of heart rate variability and wavelet transform were also conducted.
SkBFarm increased significantly at higher ambient temperatures (p=0.003), but not SkBFcalf. The standing posture caused lower overall SkBF in both regions throughout the protocol, regardless of temperature (p<0.001). With the exception of cardiac output, all haemodynamic variables were significantly altered as a result of changes in the ambient temperature and postural condition (all p<0.05), indicating that higher ambient temperatures and the standing condition elicited higher strain on the cardiovascular system. Mechanistic analyses identified greater sympathetic nerve activation, and higher calf myogenic activation at peak temperatures, in the standing condition.
Mechanistically and functionally, arm vasculature responds to modulation from both thermoregulation and baroreceptor activity. The legs, meanwhile, are more sensitive and strongly innervated by baroreflex regulatory mechanisms.