Hemodynamic Responses Upon the Decision to Initiate Thermoregulatory Behavior in Humans
Authors: Schlader Zachary J , Sarker Suman , Mundel Toby , Coleman Gregory L , Chapman Christopher L , Sackett James R , Johnson Blair D 
Published: November 2023

Cold or heat exposure presents a hyper‐ or hypo‐tensive challenge that can strain cardiovascular function. It is not known if young healthy adults initiate thermoregulatory behavior prior to experiencing thermal induced hemodynamic changes that can challenge normal cardiovascular function. This study tested the hypothesis that thermoregulatory behavior in young healthy adults is initiated prior to hyper‐ or hypo‐ tension and subsequent cardiovascular strain.

Ten healthy young subjects (age: 26 ± 4 y, 4 females) wearing shorts and a t‐shirt underwent a 90 min behavioral test in which they voluntarily moved between a warm (39.9 ± 0.2°C, RH: 20 ± 0%) and a cool (16.9 ± 1.4°C. RH: 49 ± 3%) room when they felt ‘too warm’ or ‘too cool’. Blood pressure (Finometer), heart rate (ECG), and mean skin and rectal temperatures were measured continually. Stroke volume was estimated from the blood pressure waveform using Model flow. Cardiac output and total peripheral resistance were calculated from the Fick equation, while stroke work (stroke volume mean arterial pressure), rate pressure product (systolic blood pressure heart rate), and cardiac work (cardiac output mean arterial pressure) were calculated as indices of cardiac strain. Data are reported as a 30 s average immediately prior to moving from cool to warm (C to W) or warm to cool (W to C). Data were averaged across behaviors for a given subject (total behaviors/subject: 9 ± 3) and are reported as a change from the pretest baseline.

Mean skin temperature was different at C to W (33.5 ± 0.7°C) compared to W to C (36.1 ± 0.5°C, P<0.01), while rectal temperature did not differ at these points (C to W: 36.8 ± 0.2°C, W to C: 36.7 ± 0.2°C, P=0.17). Mean arterial pressure differed from baseline at C to W (by +7 ± 4 mmHg, P<0.01) and W to C (by −5 ± 5 mmHg, P=0.02). Cardiac output was not different between baseline and C to W (−0.1 ± 0.4 L/min, P=0.56) or W to C (+0.4 ± 0.4 L/min, P=0.10), but differed between C to W and W to C (by 0.5 ± 0.5 L/min, P=0.02). Heart rate and stroke volume did not differ between baseline and C to W (3 ± 4 bpm, 1.0 ± 3.6 mL/beat, P≥0.11). Heart rate was higher than baseline at W to C (by 5 ± 4 bpm, P=0.01), which was also different from C to W (by 8 ± 4 bpm, P<0.01), but stroke volume was not different between baseline and W to C (−0.2 ± 4.3 mL/beat, P=0.87). Total peripheral resistance and stroke work was different between baseline and C to W (by +1.9 ± 1.1 L/min/mmHg, +643 ± 609 mL mmHg, P≤0.03) and W to C (by −2.4 ± 2.7 L/min/mmHg, −392 ± 433 mL mmHg, P≤0.058), while rate pressure product and cardiac work did not differ between baseline and C to W (+119 ± 773 mmHg beat, +21 ± 55 L/min mmHg, P≥0.59) or W to C (−169 ± 899 mmHg beat, +5 ± 49 L/min mmHg, P≥0.91).

These data indicate that the initiation of thermoregulatory behavior in young healthy adults is preceded by moderate hyper‐ and hypo‐ tensive responses that may strain aspects of cardiovascular function, particularly in those populations with an already compromised cardiovascular system.