Although the high-frequency fluctuations in R-R interval (respiratory 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-related low-frequency oscillation reflects rhythms generated outside the heart, and thus could be neurally mediated. 2. To evaluate the presence of reinnervation, the spontaneous variability in R-R interval was investigated, supine and after passive tilting, in 23 heart transplant recipients (age 43 years, range 23-64 years) and in 25 normotensive control subjects by autoregressive spectral analysis of low- and high-frequency spontaneous fluctuations in R-R interval and respiration. The response of R-R interval to amyl nitrite inhalation was also evaluated in five heart transplant recipients and eight control subjects. 3. Detectable low-frequency oscillations, unrelated to respiration, were present in 13/23 heart transplant recipients, particularly in those who were transplanted at least 20 months earlier (11/14). The natural logarithm of the power of low-frequency fluctuations was markedly lower than in control subjects (0.75 +/- 0.21 versus 5.62 +/- 0.20 ms2, P < 0.001). The low-frequency but not the high-frequency fluctuations correlated with time since transplantation (r = 0.44, P < 0.05). The subjects with low-frequency fluctuations showed a sudden decrease in R-R interval with amyl nitrite linearly related to the decrease in mean blood pressure (r > or = 0.94). The slopes obtained in these heart transplant recipients were comparable (although of lower values) with those obtained in control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)
