87 Effect of Body Warming on Blood Flow Distribution in Hypoxic Infant Rabbits

Newborn and small mammals, including human infants, respond to acute hypoxia (Hx) with a drop in body temperature (Tb). Yet, normal clinical practise is to maintain Tb within the normal range which may involve body warming. Previous experiments showed that warming hypoxic newborns reduced peripheral vascular resistance and increased cardiac output, suggesting that the normal redistribution of blood flow (BF) in Hx to the vital organs may be compromised.

To evaluate the effect of body warming in hypoxia on the distribution of BF to specific organs.

The fluorescent microspheres technique was used to compare the BF distribution in young rabbits (1 month age) during Hx (FIO₂=10%) with (n=9) and without (n=10) body warming. BF was measured in brain, GI tract, hindlimb muscle, diaphragm, skin, and kidneys, in normoxia (Nx) at 15°C ambient temperature (Ta), after 60 min of Hx at Ta = 15 °C, and again after a further 60 min of Hx at Ta =15°C or 27°C. A Ta of 27°C was sufficient to increase Tb to the Nx value in 40 min. Mean arterial blood pressure, oxygen consumption, hemoglobin O₂ saturation and blood gases were also measured. Statistical analysis was performed using a mixed factorial two-way ANOVA.

In Hx all animals dropped Tb (−1.8°C), and BF significantly increased to the brain (+50%), diaphragm (+76%) and hindlimb muscle (+28%), decreased to the GI tract (stomach: −25%, small intestine: −30%), and was unchanged in the remaining organs; these changes with Hx are similar to those reported by others. The increase in brain BF was sufficient to maintain O₂ supply at the Nx level. Re-warming significantly changed the hypoxic pattern of BF. Brain BF further increased by 100%, thereby elevating O₂ supply to a level 50% above the Nx value. BFincreased to the skin (+55%), and GI tract, however these increases were insufficient to return O₂ supply to the Nx level. BF decreased to hindlimb muscle and diaphragm; BF to the former decreased below even the Nx level (−30%), whereas BF to the diaphragm was still above the Nx value (+11%).

Warming alters the pattern of hypoxic BF redistribution such that BF is increased to the periphery. However, this increase did not come at the expense of brain BF. However, warming did not provide any great improvement in O₂ delivery. We conclude that warming may simply impose an additional demand on the cardiovascular system.