Cardiovascular Responses to Simultaneous Diving and Muscle Metaboreflex Activation

Area of Science:

• Physiology
• Cardiovascular Research
• Exercise Physiology

Background:

• Investigating the interplay between the muscle metaboreflex (MM) and the diving reflex (DR) during simultaneous activation is crucial for understanding physiological responses.
• Previous research has not fully elucidated the extent to which the DR can modulate or override the cardiovascular effects of the MM, particularly under mild exercise conditions.

Purpose of the Study:

• To assess hemodynamic changes, including stroke volume (SV), heart rate (HR), and cardiac output (CO), during the simultaneous activation of the muscle metaboreflex (MM) and diving reflex (DR).
• To test the hypothesis that the diving reflex (DR) can overwhelm the muscle metaboreflex (MM) when exercise intensity is mild.

Main Methods:

• Ten trained divers participated in a laboratory study involving four randomized conditions: postexercise muscle ischemia (PEMI), control exercise recovery, DR (breath-hold with face immersion), and PEMI-DR (simultaneous activation).
• Hemodynamic parameters (SV, HR, CO) were measured using impedance cardiography throughout the protocol.
• The protocol included handgrip exercise followed by either recovery, breath-hold with face immersion, or postexercise muscle ischemia with breath-hold and face immersion.

Main Results:

• At the end of apnea, heart rate (HR) decreased significantly more in the DR and PEMI-DR conditions compared to the PEMI condition (-40.8% and -40.3% vs. -9.1%, respectively).
• Stroke volume (SV) increase was less pronounced during the DR and PEMI-DR conditions (+32.4% and +21.7%) compared to PEMI (+6.0%).
• Consequently, cardiac output (CO) significantly decreased during the DR and PEMI-DR conditions (-23.0% and -29.0%) compared to PEMI (-1.4%).

Conclusions:

• The results support the hypothesis that the diving reflex (DR) can overwhelm the muscle metaboreflex (MM) in trained divers under the studied laboratory conditions.
• The significant reduction in cardiac output during simultaneous DR and MM activation highlights the potent cardiovascular modulatory effects of the diving reflex.