Inspiratory and expiratory sinus arrhythmia in healthy human

Breathing and ECG were recorded in 33 healthy human subjects at rest. The RR interval was labeled according to its occurrence in the respiratory phases: II (both R waves during inspiration), IE (first R wave in inspiration and following R wave in expiration), EE (both R waves during expiration), and EI (first R wave in expiration and following R wave in inspiration). For each subject, II and EE intervals were plotted versus normalized mean inspiratory duration and normalized mean expiratory duration, respectively, and correlation coefficients and slopes of regression lines were calculated. Statistical analyses were conducted to compare these slopes between and within individuals. These relationships appeared to be linear in most cases, although neither the decrease nor the increase in heart rate occurred at the same rate for all subjects. Overall, the slope associated with II intervals was not higher, in terms of absolute values, than the slope associated with EE intervals for the same subject. Our results underscore the difference between inspiratory and expiratory sinus arrhythmia and suggest that the effects of any stimulation presumed to induce changes in vagal cardiac activity should primarily be sought during expiration.