Key result
Diving reflex overrides muscle metaboreflex, cutting heart rate by ~40% vs. metaboreflex alone.
Why the study?
The hemodynamic changes during simultaneous activation of muscle metaboreflex and diving reflex were not well characterized in trained divers.
Does simultaneous activation of the diving reflex and muscle metaboreflex alter hemodynamic responses compared to either reflex alone in trained divers?
RCT (n=10)
Randomly assigned crossover
No
Does simultaneous activation of the diving reflex and muscle metaboreflex alter hemodynamic responses compared to either reflex alone in trained divers?
In trained divers, the diving reflex overrides the muscle metaboreflex during mild exercise, leading to decreased heart rate and cardiac output.
May warrant caution during breath-hold exercise; leaves open clinical translation in cardiovascular disease.
Background: The aim of study was to assess hemodynamic changes during the simultaneous activation of muscle metaboreflex (MM) and diving reflex (DR) in a laboratory setting. We hypothesized that as long as the exercise intensity is mild DR can overwhelm the MM. Methods: Ten trained divers underwent all four phases (randomly assigned) of the following protocol. (A) Postexercise muscle ischemia session (PEMI): 3 min of resting followed by 3 min of handgrip at 30% of maximum force, followed immediately by 3 min of PEMI on the same arm induced by inflating a sphygmomanometer. Three minutes of recovery was further allowed after the cuff was deflated for a total of 6 min of recovery. (B) Control exercise recovery session: the same rest-exercise protocol used for A followed by 6 min of recovery without inflation. (C) DR session: the same rest-exercise protocol used for A followed by 1 min of breath-hold (BH) with face immersion in cold water. (D) PEMI-DR session: the same protocol used for A with 60 s of BH with face immersion in cold water during the first minute of PEMI. Stroke volume (SV), heart rate (HR), and cardiac output (CO) were collected by means of an impedance method. Results: At the end of apnea, HR was decreased in condition C and D with respect to A (−40.8 and −40.3%, respectively vs. −9.1%; p < 0.05). Since SV increase was less pronounced at the same time point (C = +32.4 and D = +21.7% vs. A = +6.0; p < 0.05), CO significantly decreased during C and D with respect to A (−23 and −29.0 vs. −1.4%, respectively; p < 0.05). Conclusion: Results addressed the hypothesis that DR overcame the MM in our setting.
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Giacomo et al. (2021) conducted an RCT in Healthy trained divers (n=10). Simultaneous diving reflex and muscle metaboreflex activation (PEMI-DR) vs. Postexercise muscle ischemia (PEMI) was evaluated on Heart rate change at the end of apnea (p=<0.05). Simultaneous activation of the diving reflex and muscle metaboreflex significantly decreased heart rate by 40.3% and cardiac output by 29.0% compared to metaboreflex activation alone, demonstrating that the diving reflex overrides the metaboreflex.
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