Transfer function analysis of autonomic regulation. II. Respiratory sinus arrhythmia.

An efficient new technique was developed to investigate heart rate control at all physiologically relevant frequencies by using respiratory activity as a frequency probe of the autonomic nervous response. The transfer function from respiratory activity to heart rate was determined during 6-min periods in which the respiratory rate was voluntarily controlled in a predetermined but erratic fashion. Changes in posture were used to manipulate autonomic balance. Respiratory sinus arrhythmia was determined to be a frequency-dependent phenomenon with the magnitude and phase characteristics of a low-pass filter. In agreement with previous work, at typical respiratory frequencies (greater than 0.15 Hz) increases in heart rate occurred simultaneously with the onset of inspiratory activity; however, at frequencies less than 0.15 Hz the phase relationship was quite different, such that increases in heart rate preceded inspiration. Between 0.15 and 0.45 Hz, the transfer magnitude was consistently lower while the subjects were in the upright posture than when in the supine posture, but below 0.15 Hz, it was equal in both postures. A model for respiratory modulation of heart rate, based on the atrial rate response characteristics determined in the companion paper [Am. J. Physiol. 256 (Heart Circ. Physiol. 25): H142-H152, 1989], suggests that the magnitude and phase characteristics of the subjects in the supine and upright postures differ because of relatively increased sympathetic outflow in the upright posture. A precise and efficient characterization of respiratory sinus arrhythmia can yield considerable insight into the autonomic regulation of the heart.