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

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.

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

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

RSA had no significant correlation with f_H, and a better correlation with f_H/f_R (r² = 0.92) than with f_R alone (r² = 0.75); the variance of the Y values of the f_H/f_R-RSA correlation was ~ half that of the f_R/RSA correlation (P < 0.002).

We propose that the f_H/f_R-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.