Effects of whole body heat stress on sublingual microcirculation in healthy humans

Purpose To assess the effect of whole body heat stress on sublingual microcirculation. Methods Fourteen apparently healthy subjects participated in the study. Passive body heating was performed by immersing the subjects up to the waist in a water bath at 44 °C continuously until a rectally obtained core temperature of 39.5 °C was reached. Systemic hemodynamic parameters and sublingual microcirculation were evaluated and recorded before heating, immediately after heating, and 1 h after heating. Results The subjects showed very high physiological stress and significantly increased noradrenaline and prolactin concentrations in the blood. Whole body heating resulted in significantly increased oxygen uptake, heart rate, and cardiac output. One hour after heating, heart rate remained increased, but cardiac output almost returned to baseline. Mean arterial pressure significantly decreased after heating and remained decreased for at least 1 h. There was no significant difference in the microvascular flow index and proportion of perfused vessels of small vessels at the end of heating and 1 h after heating, in comparison with baseline variables. However, functional capillary density and total vessel density of small vessels significantly increased at the end of heating (10.8 ± 2.4 vs. 11.7 ± 2.0 1/mm and 19.5 ± 3.5 vs. 22.2 ± 3.3 mm/mm 2 , p < 0.05, respectively) and remained increased 1 h after heating. Conclusion Whole body heat stress increases sublingual functional capillary density, oxygen consumption, and cardiac output.