Origins of Heart Rate Variability

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Abstract
Background —We propose that heart period sequences are linearly organized, like sentences, and that there is a lexicon of recurrent, similarly shaped transient structures like words. Each word (or lexon) has a characteristic physiological basis. One potential lexon is the transient, reversible tachycardia that is induced by exercise initiation under laboratory conditions. We hypothesized that this lexon was inducible and observable on ambulatory ECGs of most or all subjects, was morphologically similar in both induced and detected bursts, and shared a plausible origin in both circumstances. Methods and Results —Ten healthy subjects (mean age, 36 years) underwent a protocol in which subjects rolled themselves from supine to lateral decubitus positions and back. Transient tachycardias (“bursts”) were seen in 36 of 40 rollovers. Bursts were characterized by an initial monoexponential heart period decay ( K =0.39±0.23 s −1 ), a maximum heart period decrease of 277±109 ms after 10.8±4.5 seconds, and a subsequent return to baseline 23.3±10.8 seconds after roll initiation. The roll-induced bursts were detected with 97% sensitivity and 99% specificity with a search algorithm that incorporated morphological parameters. In 24-hour ambulatory ECGs of 10 healthy subjects (mean age, 38 years; range, 17 to 69 years), 117±59 bursts were detected. Induced and detected bursts were similar in most morphological parameters. Finally, many bursts occurred at night, when rolling over also occurs. Conclusions —Bursts are inducible, transient tachycardias that occur clinically and constitute a lexon with an understandable physiology.