Respiratory modulation of human autonomic function on Earth.

Area of Science:

• Neurophysiology
• Autonomic Nervous System Regulation

Background:

• Heartbeat interval fluctuations are typically linked to breathing and baroreflex activity.
• The precise mechanisms driving autonomic responses during altered breathing, such as apnea, remain incompletely understood.

Purpose of the Study:

• To investigate the baroreflex mediation of R-R interval fluctuations during normal breathing.
• To determine the inputs responsible for autonomic responses during apnea.
• To explore the role of central respiratory drive in autonomic control.

Main Methods:

• Studied healthy astronauts performing controlled breathing protocols (fixed, random, hyperventilation, apnea).
• Recorded electrocardiogram, arterial pressure, respiratory CO2, tidal volume, and muscle sympathetic nerve activity.
• Analyzed R-R interval fluctuations and autonomic responses under various breathing conditions.

Main Results:

• R-R interval fluctuations at normal breathing frequencies were not baroreflex-mediated and ceased during apnea.
• Apnea responses were not attributable to changes in chemoreceptor, baroreceptor, or pulmonary stretch receptor input.
• Muscle sympathetic nerve activity increased during apnea despite rising arterial pressure.

Conclusions:

• R-R interval fluctuations during normal breathing are unlikely baroreflex-mediated.
• Autonomic responses to apnea are primarily driven by central respiratory motoneurone activity.
• Findings challenge traditional models of autonomic control during respiratory challenges.