A REALITY-BASED PARADIGM FOR HEART RATE VARIABILITY

Robert Sheldon MD PhD and Daniel Roach PhD
Cardiovascular Research Group, University of Calgary, Calgary, Alberta, Canada            

Measures of heart period variability are used as probes into the dynamics of the cardiovascular control system.  Results from spectral and nonlinear analyses are often interpreted in terms of long term, deterministic controls that are presumed to underlie heart rate variability.  However, analyses with nonlinear predictability, correlation dimension, and information scaling have shown that heart period sequences do not have the characteristics of signals with long-term determinism.  What then might be the origin of heart period variability?

We have shown that heart period variability is due to a linear concatenation of temporally localized events. In this paradigm, heart period sequences are simply sequences of transient and characteristically structured changes in heart period, much like sentences are composed of strings of words.  There would then be a lexicon of recurrent, similarly shaped transient structures, like words, and each word would have a specific physiologic basis.  We coin the word “lexon” to refer to the meaningful transient structures in heart period sequences.  The first lexon is the brief burst of tachycardia lasting about 25 seconds that occurs at the initiation of exercise. We have also shown that that smaller bursts are made by exercise with smaller muscles; that long range measures such as ULF and SDANN are due to chronotropic responsiveness to exertion; that low frequency heart rate variability is due to baroreceptor-mediated tracking by heart period of rippling blood pressure changes; that heart rate turbulence is a lexon that is inducible in the electrophysiologic laboratory, and that transient abrupt bradycardias are a type of alerting response.

Each of these lexons is quantitatively described; can be precisely located in a heart period sequence; can be induced under controlled conditions; has a characteristic physiology; and therefore illuminates temporally localized physiology from heart period recordings.