Normothermic central hypovolemia tolerance reflects hyperthermic tolerance

Abstract

To test the hypothesis that those who are highly tolerant to lower body negative pressure (LBNP) while normothermic are also highly tolerant to this challenge while hyperthermic. Sixty pairs of normothermic and hyperthermic LBNP tests to pre-syncope were evaluated. LBNP tolerance was quantified via the cumulative stress index (CSI), which is calculated as the sum of the product of the LBNP level and the duration of each level until test termination (i.e., 20mmHg×3min+30mmHg×3min, etc.). CSI was compared between normothermic and hyperthermic trials. Internal and skin temperatures, heart rate, and arterial pressure were measured throughout. Hyperthermia reduced (P<0.001) CSI from 997±437 to 303±213mmHgmin. There was a positive correlation between normothermic and hyperthermic LBNP tolerance (R (2)=0.38; P<0.001). As a secondary analysis, the 20 trials with the highest LBNP tolerance while normothermic were identified (indicated as the HIGH group; CSI 1,467±356mmHgmin), as were the 20 trials with the lowest normothermic tolerance (indicated as the LOW group; CSI 565±166mmHgmin; P<0.001 between groups). While hyperthermia unanimously reduced CSI in both HIGH and LOW groups, in this hyperthermic condition CSI was ~threefold higher in the HIGH group (474±226mmHgmin) relative to the LOW group (160±115mmHgmin; P<0.001). LBNP tolerance while hyperthermic is related to normothermic tolerance and, associated with this finding, those who have a high LBNP tolerance while normothermic remain relatively tolerant when hyperthermic.