Influence of temperature on the distribution of blood in humans as assessed by electrical impedance

Abstract

This study investigated whether ambient temperature influences the distribution of blood as indicated by electrical impedance. In ten supine humans, the room temperature was raised from 14 to 35°C. Skin temperature and blood flow on the thorax increased by 3.6 (SD 0.3)°C and 84 (SD 40)%, respectively, and by 9.8 (SD 1)°C and 115 (SD 45)%, respectively, on the extremities (P < 0.05). Cardiac output remained unchanged, ear temperature and heart rate became elevated, and the oesophageal temperature and mean arterial pressure decreased (P < 0.05). At five discrete frequencies (1.5, 5, 50, 100, 200 kHz) thoracic impedance was increased by 1.2 (SD 1) to 1.5 (SD 1) Ω (P < 0.05). In contrast, total body impedance was reduced by 16.4 (SD 5) Ω and leg impedance was reduced by 4.0 (SD 2) Ω, while an index of intracellular water within the thorax (the difference between the admittances at 100 kHz and 1.5 kHz) was decreased by 10 (SD 1) · 10⁻⁴ S (P < 0.05). The results would suggest that total body impedance is dominated by the impedance of the extremities. The increase in thoracic impedance and a decrease in leg impedance (as in total body impedance) could be explained by a redistribution of blood from the thorax to the extremities during heating. Such a translocation of blood was confirmed by a reduced impedance based index of intracellular water within the thorax.