CIRCADIAN ASPECTS OF APPARENT CORRELATION DIMENSION IN HUMAN HEART-RATE DYNAMICS

The purpose of this study was to examine changes in complexity of card iac dynamics over 24 h. With use of Holter monitoring, 27 24-h electro cardiogram recordings were obtained from 15 healthy subjects. For each recording, the apparent dimension (D-A) was calculated for consecutiv e sections of 500 heartbeats. These were used to determine nighttime a nd daytime dimension (D-A) and D-Ad, respectively) as well as the diff erence between D-An and D-Ad (Delta D-A). Mean 24-h D-A, D-An, and D-A d were 5.9 +/- 0.3, 6.3 +/- 0.5, and 5.6 +/- 0.6, respectively. D-Ac w as significantly higher than D-Ad (P < 0.001), with a mean Delta D-A o f 0.6 +/- 0.7. Furthermore, 67% of Delta D-A values were significantly different from zero at the 0.05 level. The results show that dimensio n analysis may be applied to heart rate dynamics to reveal circadian d ifferences of heart rate complexity. We suggest that the decreased com plexity during daytime may result from the synchronization of physiolo gical functions. The increase in complexity at night would then corres pond to an uncoupling of these functions during the regenerative perio d.