This study aims to compare state-space characteristics of heart-rate variability signals (HRV) of mammals (rats and neonate and adult humans) and reptiles (lizards, Gallotia galloti) in relation to the parasympathetic control. HRV were derived from the RR intervals of digitized electrocardiograms. Experiments in animals were performed in control and under parasympathetic blockade with atropine. Recordings in humans were made in waking-resting state. HRV complexity was estimated from the correlation integral by calculating its correlation dimension (D2) after reconstructing the signal attractor in the state-space of the system. Nonlinear components in the HRV were detected using surrogate data method.

The D2 in neonate humans was smaller than in adult mammals and lizards. No differences were found among the D2 in adult humans, rats and lizards. Atropine decreased the D2 in rats but did not alter it in lizards. Recordings from adult mammals exhibited greater nonlinear behaviour than that of neonate humans and lizards. The respiratory sinus arrhythmia (RSA), that is parasympathetic mediated, seems to be the cause of the rats' HRV complexity. The lack of RSA in lizards could explain the invariability of their HRV complexity after atropine. The smaller value of the D2 in neonate humans against adults could be due to the neonate humans' lack of RSA during their first weeks of life. The high nonlinear behaviour of adult mammals' HRV, in contrast with what occurs in neonate humans and lizards, could be due to their modulation by the sympathetic nervous system.

Support: FIS.-PI052166; MCyT-BFI2002-01159; PI042005/005