Key result
Delaying cold water immersion by 20 or 40 minutes for exertional heat strain led to incomplete restoration of cardiac autonomic control compared to a 5-minute delay (interaction P≤0.005).
Why the study?
Immediate cold water immersion is gold-standard for exertional heatstroke, but field treatment is often delayed, and its effect on cardiac autonomic control was unclear.
Does a delay in cold water immersion treatment affect the cardiac autonomic control of exertionally heat-strained individuals?
Does a delay in cold water immersion treatment affect the cardiac autonomic control of exertionally heat-strained individuals?
p-value: p=≤0.005
Delayed cold water immersion for exertional heat strain leads to only partial restoration of cardiac autonomic control, highlighting the need for continued cardiac monitoring after core temperature normalizes.
Delayed cold water immersion yields only partial autonomic recovery after heat strain; leaves open optimal timing for full HRV restoration in future trials.
NEW FINDINGS: What is the central question of this study? Does a delay in cold water immersion treatment affect the cardiac autonomic control of exertionally heat-strained individuals? What is the main finding and its importance? Cold water immersion is effective for treating exertionally heat-strained individuals even when treatment is commenced with a significant delay. However, that treatment delay leads to only partial/transient restoration of cardiac autonomic control. Therefore, we recommend that exertional heatstroke patients are continuously monitored for several hours even after core temperature has returned to normal values. ABSTRACT: Immediate cold water immersion (CWI) is the gold-standard treatment for exertional heatstroke. In the field, however, treatment is often delayed, primarily owing to a delayed paramedic response and/or inaccurate diagnosis. We examined the effect of treatment (reduction of rectal temperature to 37.5°C) delays of 5 (short), 20 (moderate) and 40 (prolonged) min on cardiac autonomic control [as assessed via heart rate variability (HRV)] in eight exertionally heat-strained (40.0°C rectal temperature) individuals. Eleven HRV indices were computed that have been described commonly in the literature and characterize almost all known domains of the variability and complexity of the cardiopulmonary system. We found that the cardiac autonomic control (as assessed via HRV) of exertionally heat-strained individuals was significantly affected by the amount of time it took for the CWI treatment to be applied. Six out of 11 HRV indices studied, from all variability domains, displayed strong (P ≤ 0.005) time × delay interaction effects. Moreover, the number of significantly (P ≤ 0.005) abnormal (i.e. different from the short delay) HRV indices more than doubled (seven versus 15) from the moderate delay to the prolonged delay. Finally, our results demonstrated that a CWI treatment applied with delays of 20 and, primarily, 40 min did not lead to a full restoration of cardiac autonomic control of exertionally heat-strained individuals. In conclusion, this study supports CWI for treating exertionally heat-strained individuals even when applied with prolonged delay, but it highlights the importance of continued cardiac monitoring of patients who have suffered exertional heatstroke for several hours after restoration of core temperature to normal.
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Flouris et al. (2019) studied Exertional heat strain (n=8). Delayed cold water immersion (20 and 40 min) vs. Short delay (5 min) was evaluated on Cardiac autonomic control (assessed via 11 heart rate variability indices) (p=≤0.005). Delaying cold water immersion by 20 or 40 minutes for exertional heat strain led to incomplete restoration of cardiac autonomic control compared to a 5-minute delay (interaction P≤0.005).
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