Cardiovascular Responses to Simultaneous Diving and Muscle Metaboreflex Activation

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.