The Effect of Passive Heat Stress and Exercise-Induced Dehydration on the Compensatory Reserve During Simulated Hemorrhage

Abstract

Compensatory reserve represents the proportion of physiological responses engaged to compensate for reductions in central blood volume before the onset of decompensation. We hypothesized that compensatory reserve would be reduced by hyperthermia and exercise-induced dehydration, conditions often encountered on the battlefield. Twenty healthy males volunteered for two separate protocols during which they underwent lower-body negative pressure (LBNP) to hemodynamic decompensation (systolic blood pressure <80 mm Hg). During protocol #1, LBNP was performed following a passive increase in core temperature of ∼1.2°C (HT) or a normothermic time-control period (NT). During protocol #2, LBNP was performed following exercise during which: fluid losses were replaced (hydrated), fluid intake was restricted and exercise ended at the same increase in core temperature as hydrated (isothermic dehydrated), or fluid intake was restricted and exercise duration was the same as hydrated (time-match dehydrated). Compensatory reserve was estimated with the compensatory reserve index (CRI), a machine-learning algorithm that extracts features from continuous photoplethysmograph signals. Prior to LBNP, CRI was reduced by passive heating [NT: 0.87 (SD 0.09) vs. HT: 0.42 (SD 0.19) units, P <0.01] and exercise-induced dehydration [hydrated: 0.67 (SD 0.19) vs. isothermic dehydrated: 0.52 (SD 0.21) vs. time-match dehydrated: 0.47 (SD 0.25) units; P <0.01 vs. hydrated]. During subsequent LBNP, CRI decreased further and its rate of change was similar between conditions. CRI values at decompensation did not differ between conditions. These results suggest that passive heating and exercise-induced dehydration limit the body's physiological reserve to compensate for further reductions in central blood volume.

Conflict of interest statement

The authors report no conflicts of interest.

Figures

Fig. 1. Kaplan–Meier curves plotting the percent of individuals who tolerated a given cumulative stress index during simulated hemorrhage

The left panel presents data during the hyperthermic and normothermic conditions of protocol #1. The right panel presents data during the hydrated, isothermic dehydrated, and time-match dehydrated conditions of protocol #2. Significant (P ≤0.05) difference between the normothermic and hyperthermic curves (left panel) and between the hydrated and time-match dehydrated curves (right panel) based on Log-Rank (Mantel–Cox) test.

Fig. 2. Mean arterial pressure (left panel) and heart rate (right panel) during progressive lower body negative pressure (LBNP) to hemodynamic decompensation performed following whole-body passive heat stress (hyperthermic) or a normothermic time-control period (protocol #1)

The data are presented as mean ± 95% confidence intervals and were modeled using generalized estimating equations to account for differences in LBNP level at decompensation. Solid line, significantly different from baseline for normothermic. Dashed line, significantly different from baseline for hyperthermic.

Fig. 3. The compensatory reserve index during progressive lower body negative pressure (LBNP) to hemodynamic decompensation

The left panel presents data during the hyperthermic and normothermic conditions of protocol #1. The right panel presents data during the hydrated, isothermic dehydrated, and time-match dehydrated conditions of protocol #2. The data are presented as mean ± 95% confidence intervals and were modeled using generalized estimating equations to account for differences in LBNP level at decompensation. Solid line, significantly different from baseline for normothermic and hydrated. Dashed line, significantly different from baseline for hyperthermic and isothermic/time-match dehydrated.

Fig. 4. Receiver operating characteristic (ROC) curves with area under the curve (AUC) values and 95% confidence intervals for the compensatory reserve index during the normothermic and hyperthermic conditions of protocol #1 (left panel); and during the hydrated, isothermic dehydrated and time-match dehydrated conditions of protocol #2 (right panel)

Fig. 5. Individual compensatory reserve index tracings during progressive lower body negative pressure (LBNP) to hemodynamic decompensation performed following whole-body passive heat stress (HT) and a normothermic (NT) time-control period (protocol #1)

The dashed line indicates the start of LBNP.

Fig. 6. Mean arterial pressure (left panel) and heart rate (right panel) during progressive lower body negative pressure (LBNP) to hemodynamic decompensation during the hydrated, isothermic dehydrated, and time-match dehydrated conditions of protocol #2

The data are presented as mean ± 95% confidence intervals and were modeled using generalized estimating equations to account for differences in LBNP level at decompensation. Significantly different from baseline for hydrated (*), isothermic dehydrated (†), and time-match dehydrated (‡).

Fig. 7. Individual compensatory reserve index tracings during progressive lower body negative pressure (LBNP) to hemodynamic decompensation performed following exercise (protocol #2) during which: fluid losses were replaced (hydrated), fluid losses were not replaced and exercise lasted until the same increase in core temperature as hydrated (isothermic dehydrated), and fluid losses were not replaced and exercise lasted the same duration as hydrated (time-match dehydrated)

The dashed line indicates the start of LBNP.