1. Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Correspondence to: Chi-Sang Poon¹ Correspondence and requests for materials should be addressed to C.-S.P. (e-mail: Email: cpoon@mit.edu).

Abstract

The electrical properties of the mammalian heart undergo many complex transitions in normal and diseased states^{1, 2, 3, 4, 5, 6, 7}. It has been proposed that the normal heartbeat may display complex nonlinear dynamics, including deterministic chaos⁸,⁹, and that such cardiac chaos may be a useful physiological marker for the diagnosis^{10, 11, 12} and management¹³,¹⁴ of certain heart trouble. However, it is not clear whether the heartbeat series of healthy and diseased hearts are chaotic or stochastic^{15, 16, 17}, or whether cardiac chaos represents normal or abnormal behaviour¹⁸. Here we have used a highly sensitive technique, which is robust to random noise, to detect chaos¹⁹. We analysed the electrocardiograms from a group of healthy subjects and those with severe congestive heart failure (CHF), a clinical condition associated with a high risk of sudden death. The short-term variations of beat-to-beat interval exhibited strongly and consistently chaotic behaviour in all healthy subjects, but were frequently interrupted by periods of seemingly non-chaotic fluctuations in patients with CHF. Chaotic dynamics in the CHF data, even when discernible, exhibited a high degree of random variability over time, suggesting a weaker form of chaos. These findings suggest that cardiac chaos is prevalent in healthy heart, and a decrease in such chaos may be indicative of CHF.