Power spectrum analysis of heart rate variability in human cardiac transplant recipients.

The study by Sands et all reported promising results with the mathematical technique of power spectrum analysis of heart rate variability to detect graft rejection. We were surprised, however, to learn that they did not find spectral peaks in transplant recipients. In a similar study,2 we detected peaks at the so-called Mayer wave frequency (0.1 Hz) as well as at the frequency of respiratory sinus arrhythmia (RSA). Furthermore, we found that the RSA peak had better repeatability at two weeks (r=0.81) than the Mayer wave frequency and was significantly lower during rejection periods (p=0.039). We suspect that part of the difference in our findings may be attributable to the different techniques used. Specifically, Sands et al used a smoothing technique that, in effect, created a low-pass filter. Although this is acceptable, this often results in large, low-frequency spectral peaks that may "swamp" or interfere with small, higher-frequency peaks such as the RSA.3 To avoid this problem, we high-pass filtered the data at approximately 0.04 Hz to remove the low-frequency drift or aperiodic trend. This technique may also introduce spurious peaks near the corner frequency of the filter. However, we believe it highly unlikely that this filtering procedure affected the RSA peaks because the RSA frequencies were sufficiently higher. Furthermore , one of our subjects demonstrated clear, unequivocal Cheyne-Stokes respirations that modulated the heart rate and were clearly evident on the tachogram.4,5 We agree with Sands et al that this technique is of potential benefit to transplant recipients and that larger, more-controlled studies are warranted. Cohen RJ: Power spectrum analysis of heart rate variability in human cardiac transplant recipients. MM, Porges SW: Use of power spectral analysis of respiratory sinus arrhythmia to detect graft rejection. The effect of heart transplantation on Cheyne-Stokes respiration associated with congestive heart failure. Reply Drs. Zbilut and Lawless report finding 0.1-Hz heart rate fluctuations as well as respiratory sinus arrhythmia in heart transplant recipients, which differs from our characterization of heart rate fluctuations in transplant patients as broad-band.' They attribute these differences to our signal processing methodology , claiming that spectral smoothing might cause low-frequency power to spill over into the respiratory frequency range and obscure the true variability in that range. Furthermore , they report that their measure of variability, peak spectral power of respiratory sinus arrhythmia (PSP-RSA), declined during rejection episodes, whereas our measure of variability (total, 0.02-2.0-Hz power) increased during rejection episodes. We …