In normal subjects the respiratory sinus arrhythmia (RSA) is considered an index of efferent vagal activity to the heart. The human transplanted heart is thought to be denervated and hence any fluctuation present in the RR variability can be either due to reinnervation or to represent the effect of a non autonomic activity, such as a direct effect of respiration on atrial stretch. We evaluated the presence of spontaneous fluctuations in RR interval by simultaneous power spectrum analysis of RR interval and respiration variability, using uni- and bivariate algorithms.
A small but clear ‘RSA’ was present in all transplant subjects, evidenced by a small fluctuation on the RR interval power spectrum, synchronous and coherent with the main component of the respiratory spectrum. Although the amplitude of this ‘RSA’ was in the range of 1-4% of that of controls its possible artifactual origin could be ruled out by absence of major changes in QRS morphology with respiration. No changes were observed during manoeuvres altering the sympatho-vagal balance (such as tilting), or known to produce a reflex brady- or tachycardia (neck suction or amyl nitrite inhalation) in subjects transplanted 3-20 months earlier. The only condition which modified the RSA in transplants was the increase in ventilation during physical exercise regardless of changes in sympathetic tone. This “RSA’ did not increase with time since transplantation (3-52 months).
These data suggest a non reflex mechanical effect on atrial stretch, from inspiratory increased venous return. In particular cases we have found that the respiratory effect extends also to the so-called low-frequency band, attributed to the sympathetic influence: the simultaneous acquisition and analysis of the respiratory signal is therefore essential for a correct interpretation of the spectral bands, even when the breathing is controlled by a metronome. However, in subjects who had their transplant for more than 20 months we often found low-frequency oscillations (LF) of non-respiratory origin; only in these subjects could amyl nitrite produce an immediate increase in heart rate. In the absence of other known alternative explanations, these LF should, unlike the RSA, be regarded as a sign of functional reinnervation in heart transplanted subjects.