Effects of passive heating on central blood volume and ventricular dimensions in humans

Abstract

Mixed findings regarding the effects of whole-body heat stress on central blood volume have been reported. This study evaluated the hypothesis that heat stress reduces central blood volume and alters blood volume distribution. Ten healthy experimental and seven healthy time control (i.e. non-heat stressed) subjects participated in this protocol. Changes in regional blood volume during heat stress and time control were estimated using technetium-99m labelled autologous red blood cells and gamma camera imaging. Whole-body heating increased internal temperature (> 1.0• C), cutaneous vascular conductance (approximately fivefold), and heart rate (52 ± 2 to 93 ± 4 beats min −1 ), while reducing central venous pressure (5.5 ± 07 to 0.2 ± 0.6 mmHg) accompanied by minor decreases in mean arterial pressure (all P < 0.05). The heat stress reduced the blood volume of the heart (18 ± 2%), heart plus central vasculature (17 ± 2%), thorax (14 ± 2%), inferior vena cava (23 ± 2%) and liver (23 ± 2%) (all P ≤ 0.005 relative to time control subjects). Radionuclide multiple-gated acquisition assessment revealed that heat stress did not significantly change left ventricular end-diastolic volume, while ventricular end-systolic volume was reduced by 24 ± 6% of pre-heat stress levels (P < 0.001 relative to time control subjects). Thus, heat stress increased left ventricular ejection fraction from 60 ± 1% to 68 ± 2% (P = 0.02). We conclude that heat stress shifts blood volume from thoracic and splanchnic regions presumably to aid in heat dissipation, while simultaneously increasing heart rate and ejection fraction.

Blood Flow Distribution during Heat Stress: Cerebral and Systemic Blood Flow

“…11,12 It is well established that heat stress modified the blood distribution of Q to the systemic circulation by an increase in cutaneous blood flow for thermoregulation. 4,[13][14][15] Similar to systemic circulation, the contribution of Q to CBF may be altered by heat stress. In the previous study, we have demonstrated that blood distribution in the brain (posterior and anterior CBF) was altered by several physiologic conditions, i.e., dynamic exercise 7 and orthostatic stress.…”

Compression leggings modestly affect cardiovascular but not cerebrovascular responses to heat and orthostatic stress in young and older adults

“…This displacement of blood away from the heart creates a regulatory problem, decreasing central blood volume, ventricular filling pressure, stroke volume and subsequently mean arterial blood pressure (MAP) (Crandall et al 2008). Mean arterial pressure can be maintained by increasing heart rate (HR) and myocardial contractility (and thus, Á Q), increasing peripheral vascular resistance, and limiting venous pooling (Wilson et al 2007;Crandall et al 2008). However, sustaining a higher HR under prolonged stress increases the risk of cardiac events, particularly in the elderly (Donaldson et al 2003).…”

Influences of Skin and Core Temperature on Cardiovascular Responses during Exercise

“…Vasodilation of cutaneous vessels decreases total peripheral resistance (TPR) and increases cutaneous vascular conductance (CVC). These changes reduce central blood volume (CBV) and lead to a 2 -6 mm Hg drop in central venous pressure (CVP) and a 5 -10 mm Hg decline in MAP [14,69]. Therefore, in response to large increases in SBF, cardiac output (Q), which is the product of stroke volume (SV) and heart rate (HR), must increase and vascular conductance in other vascular beds must decrease to prevent a decline in MAP and oxygen delivery to the brain [61,67,69].…”

“…High internal body temperature (T c ) can have profound effects on the cardiovascular system during resting conditions [12,14,67]. Furthermore, when T c is elevated above normal levels during exercise, the demands placed on the cardiovascular system can be extreme.…”