Non-linear cardiac dynamics and morning dip: an unsound circadian rhythm

The frequency of sudden cardiac death increases in the morning. The relatio nship between decreased complexity of heart rate dynamics and sudden cardia c death has been documented. An understanding of circadian variation in the complexity of cardiac dynamics may be important to predict and prevent sud den cardiac death. Dynamic 24-h electrocardiographic recordings were obtain ed from 30 healthy ambulant subjects aged 41-50 years, and the digitized da ta were partitioned into sections of 30 min duration. For each section, fou r indexes obtained from separate algorithms of non-linear dynamics of the R R interval - modified correlation dimension, Lyapunov exponent, approximate entropy, and fractal dimension - were calculated. Normalized low- (0.04-0. 15 hertz) and high-frequency (>0.15 hertz) components were also calculated. All four indexes of non-linear dynamics showed a remarkably similar circad ian rhythm: a prominent morning dip preceded by a steep decline during the late night, a recovery during the evening and a peak around midnight. In th e morning, the low-frequency component rose rapidly with concomitant reduct ion in the high-frequency component. The complexity of cardiac dynamics dec reases significantly in the morning, and this may contribute to the ominous ly increased rate of cardiac death in the morning hours.