NON-RESPIRATORY COMPONENTS OF HEART-RATE-VARIABILITY IN HEART-TRANSPLANT RECIPIENTS - EVIDENCE OF AUTONOMIC REINNERVATION

1. Although the high-frequency fluctuations in R-R interval (respirato ry sinus arrhythmia) observed in heart transplant recipients are not a reliable marker of reinnervation because of a previously shown direct mechanical effect of breathing, the presence of a non-respiration-rel ated low-frequency oscillation reflects rhythms generated outside the heart, and thus could be neurally mediated. 2. To evaluate the presenc e of reinnervation, the spontaneous variability in R-R interval was in vestigated, supine and after passive tilting, in 23 heart transplant r ecipients (age 43 years, range 23-64 years) and in 25 normotensive con trol subjects by autoregressive spectral analysis of low- and high-fre quency spontaneous fluctuations in R-R interval and respiration. The r esponse of R-R interval to amyl nitrite inhalation was also evaluated in five heart transplant recipients and eight control subjects. 3. Det ectable low-frequency oscillations, unrelated to respiration, were pre sent in 13/23 heart transplant recipients, particularly in those who w ere transplanted at least 20 months earlier (11/14). The natural logar ithm of the power of low-frequency fluctuations was markedly lower tha n in control subjects (0.75 +/- 0.21 versus 5.62 +/- 0.20 ms(2), P < 0 .001). The low-frequency but not the high-frequency fluctuations corre lated with time since transplantation (r = 0.44, P < 0.05). The subjec ts with low-frequency fluctuations showed a sudden decrease in R-R int erval with amyl nitrite linearly related to the decrease in mean blood pressure (r greater than or equal to 0.94). The slopes obtained in th ese heart transplant recipients were comparable (although of lower val ues) with those obtained in control subjects. The subjects with no low -frequency fluctuations showed no change in R-R interval despite a sim ilar reduction in blood pressure. 4. Twenty months after transplantati on signs of functional (reflex) reinnervation can be found in most hea rt transplant recipients.