The authors appreciate Dr. Spodick's insight into our observations. As he notes, the truly novel aspect of this observation is the demonstration of a high-frequency band of heart rate variability whose origin differs from that of the phasic variation of vagal tone (classically described in the current literature on heart rate spectral analysis). Indeed, previous reports of frequency analysis of heart rate variability in patients with congestive heart failure have uniformly found that high-frequency variations in heart rate are markedly reduced. This was shown in our patient during periods in which pulsus alternans was not evident. Therefore, as we stated, this finding is clearly uncharacteristic in patients with congestive heart failure in whom pulsus alternans is not seen.

In the reports noted by Dr. Spodick, the variation in heart rate was an observation subordinate to the principal findings of the investigations and was represented in a classic time domain format [1, 2]. The spectral analysis technique used in our investigation is extremely sensitive to subtle variations in heart rate and blood pressure and uniquely displays these variations. This allows further insight into the mechanisms by which pulsus alternans is initiated and perpetuated.

The evolution of the medical literature demands the active participation of the reader, as Dr. Spodick shows. Indeed, several experts were kind enough to review our report before its submission, and they add to the expertise of those who participated in the journal peer review process. In our continued interest in this finding, we are indebted to Dr. Arthur Hollman, who directed us to the 1911 report of Sir Thomas Lewis who, in the section entitled The Electrocardiogram in Clinical Alternation, stated that ... alternation in R runs parallel with that of the pulse ... [3]. These observations similarly escaped the attention of the referees of Dr. Spodick's subsequent reports and further show the refinement of scientific understanding through this important dialogue.