Relationship between cardioventilatory coupling and respiratory sinus arrhythmia.

Recent studies have suggested a role for respiratory sinus arrhythmia in improving pulmonary gas transfer. However, the physiological role of cardioventilatory coupling is unknown. We have tested the hypothesis that cardioventilatory coupling aligns heart beats at positions of the ventilatory cycle where they are maximally affected by respiratory sinus arrhythmia (RSA). In 15 anaesthetized, spontaneously breathing subjects, we recorded continuously the timing of inspiration and the ECG. We then plotted the change in RR interval which occurred, as a result of RSA, after onset of inspiration (RSA plot). By comparing these RSA plots with the distribution of heart beats during the ventilatory cycle, we were able to examine the positioning of heart beats relative to acceleration of heart rate as a result of RSA. In all but two subjects we observed that during periods of strong cardioventilatory coupling, heart beats occurred at positions in the ventilatory cycle where they were maximally affected by RSA. In the presence of strong coupling, the shortest possible RR interval occurred during late inspiration, the longest possible RR interval occurred immediately before the start of inspiration. We suggest that cardioventilatory coupling may have a physiological role in optimizing RSA, perhaps to improve cardiopulmonary performance during sleep.