Organ blood flow and O2 transport during hypothermia (27°C) and rewarming in a pig model

Abstract

New findings: What is the central question of this study? Absence of hypothermia-induced cardiac arrest is a strong predictor for a favourable outcome after rewarming. Nevertheless, detailed knowledge of preferences in organ blood flow during rewarming with spontaneous circulation is largely unknown. What is the main finding and its importance? In a porcine model of accidental hypothermia, we find, despite a significantly reduced cardiac output during rewarming, normal blood flow and O₂ supply in vital organs owing to patency of adequate physiological compensatory responses. In critical care medicine, active rewarming must aim at supporting the spontaneous circulation and maintaining spontaneous autonomous vascular control.

Abstract: The absence of hypothermia-induced cardiac arrest is one of the strongest predictors for a favourable outcome after rewarming from accidental hypothermia. We studied temperature-dependent changes in organ blood flow and O₂ delivery ( $D_{O_2}$ ) in a porcine model with spontaneous circulation during 3 h of hypothermia at 27°C followed by rewarming. Anaesthetized pigs (n = 16, weighing 20-29 kg) were randomly assigned to one of two groups: (i) hypothermia/rewarming (n = 10), immersion cooled to 27°C and maintained for 3 h before being rewarmed by pleural lavage; and (ii) time-matched normothermic (38°C) control animals (n = 6), immersed for 6.5 h, the last 2 h with pleural lavage. Regional blood flow was measured using a neutron-labelled microsphere technique. Simultaneous measurements of $D_{O_2}$ and O₂ consumption ( ${\dot{V}}_{O_2}$ ) were made. During hypothermia, there was a reduction in organ blood flow, ${\dot{V}}_{O_2}$ and $D_{O_2}$ . After rewarming, there was a 40% reduction in stroke volume and cardiac output, causing a global reduction in $D_{O_2}$ ; nevertheless, blood flow to the brain, heart, stomach and small intestine returned to prehypothermic values. Blood flow in the liver and kidneys was significantly reduced. Cerebral $D_{O_2}$ and ${\dot{V}}_{O_2}$ returned to control values. After hypothermia and rewarming there is a significant lowering of $D_{O_2}$ owing to heart failure. However, compensatory mechanisms preserve O₂ transport, blood flow and ${\dot{V}}_{O_2}$ in most organs. Nevertheless, these results indicate that hypothermia-induced heart failure requires therapeutic intervention.

Keywords: accidental hypothermia; organ blood flow; oxygen transport; pleural lavage.