RESPIRATORY SINUS ARRHYTHMIA IN HUMANS - AN OBLIGATORY ROLE FOR VAGALFEEDBACK FROM THE LUNGS

Respiratory sinus arrhythmia (RSA) is used as a noninvasive measure of vagal cardiac input, but its causative mechanisms in humans remain un determined. We compared the RSA of five lung-denervated double-lung tr ansplant patients with intact hearts to six normal (N) control subject s, five heart-denervated patients, and two liver transplant patients a t matched tidal volumes (VT's) and breathing frequencies. In N and liv er transplant subjects, RSA was significant during eupnea and increase d two- to threefold with increasing VT and inspiratory effort. In hear t- and lung-denervated subjects, RSA at eupnea was significant but was only 53% of that in N subjects and was not respondent to changing VT, inspiratory effort, or breathing frequency. We also compared the RSA of N subjects during voluntary (active) and passive positive pressure ventilation at normocapnia. RSA was reduced from 11 +/- 2.2 beats/min during active ventilation to 5.4 +/- 0.8 beats/min during PPV. We conc lude that vagal feedback from pulmonary stretch receptors is obligator y for the generation of a neurally mediated RSA in awake humans at nor mal and raised levels of VT and respiratory motor output. In intact hu mans, we also hypothesize an important effect for nonpulmonary central and/or peripheral modulation of RSA. It is likely that the key mechan isms for neurally mediated RSA in unanesthetized humans are mutually d ependent.