Period multupling-evidence for nonlinear behaviour of the canine heart

Although there has recently been considerable interest in applying the theory of nonlinear dynamics to the analysis of complex systems1,2, as yet applications of the theory to biological systems in vivo have been very limited. We report here evidence of nonlinear behaviour in the electrocardiogram and arterial blood pressure traces of the noradrenaline-treated dog. Noradrenaline produces variations in these traces that repeat themselves with regular periods of integral numbers of heart-beats (period multupling), an effect that resembles the ‘period-doubling’ and other ‘bifurcative’ behaviour3–5 observed when the driving frequency of a nonlinear oscillator is increased above a critical value6–9. The simplest type of periodic variation that we report is the so-called ‘electrical alternans’, which has long been known as one response of cardiac electrical activity to certain stresses and disease states10–13.