Respiratory sinus arrhythmia in the immediate post-exercise period: correlation with breathing-specific heart rate.

Background: Although the absolute values of pulmonary ventilation and cardiac output are similar, the designs of the respiratory and cardiovascular systems imply major differences in flow patterns, airflow being intermittent by comparison to the quasi-continuous pulmonary blood flow.

Purpose: We hypothesized that respiratory sinus arrhythmia (RSA, difference in heart rate (f) between inspiration and expiration, as percent of mean f) ameliorates the inevitable differences between air- and blood-flow patterns. Specifically, we hypothesized RSA to correlate more closely to the ratio between f and breathing frequency (f) (f/f "breathing-specific heart rate", a proxy for cardio-respiratory coupling) than to either f or f alone. Hence, we designed protocols to change independently f or f.

Methods: We measured RSA breath-by-breath in 145 young men and women during spontaneous breathing, breathing under cues at different f (to modify the denominator of f/f) and immediately post-exercise while breathing freely or by keeping f as at rest (to modify the nominator of f/f).

Results: RSA had no significant correlation with f, and a better correlation with f/f (r = 0.92) than with f alone (r = 0.75); the variance of the Y values of the f/f-RSA correlation was ~ half that of the f/RSA correlation (P < 0.002).

Conclusions: We propose that the f/f-RSA relationship reflects a central process that ameliorates gas exchange against the difference between air- and blood-flow patterns. The neurological mechanisms are still conjectural. Measurements of RSA could offer a glimpse of the degree of cardio-respiratory central compensation in face of the inequality between blood flow and airflow.