Cardiopulmonary readjustments during graded immersion in water at 35 degrees C

Six normal male volunteers, aged 25 to 34, suspended vertically in a harness that allowed them to completely relax their postural muscles, were studied in four randomly ordered conditions, namely in air at 28 degrees C, and immersed in water at 35 degrees C to the level of the hips, the xiphoid, or the chin. In each situation, several variables were measured by noninvasive techniques. Cardiac output rose from 5.11 min-1 (air) to 8.31-min-1 (chin), the increase in each of the three steps being significant at the 0.001 level. Heart rate dropped from 76 to 68 min-1 (P less than 0.001) from air to xiphoid immersion, but appeared to rise again (P less than 0.02) during chest immersion. Functional residual capacity decreased marginally during lower limb submergence, and considerably in each of the following stages. Pulmonary capillary blood volume rose significantly only during abdomen immersion. The arterial-endtidal PCO2 difference was minimally reduced as water reached hip level and then remained steady. Mixed venous PO2 increased during abdomen submergence, and PVCO2, was unaltered throughout. Analysis of the step-to-step changes demonstrates that some variables are set by a combination of processes which may counteract each other, and explains the difference between results obtained by previous investigators.