INFLUENCE OF FORCED RESPIRATION ON NONLINEAR DYNAMICS IN HEART-RATE-VARIABILITY

Although it is doubtful whether the normal sinus rhythm can be describ ed as low-dimensional chaos, there is evidence for inherent nonlinear dynamics and determinism in time series of consecutive R-R intervals. However, the physiological origin for these nonlinearities is unknown. The aim of this study was to test whether the known nonlinear input f rom spontaneous respiration is a source for the nonlinearities in hear t rate variability. Twelve healthy subjects were examined in supine po sition with 3-h electrocardiogram recordings during both spontaneous a nd forced respiration in accordance with a metronome set to 12 min(-1) . Nonlinear dynamics were measured as the correlation dimension and th e nonlinear prediction error. Complexity expressed as correlation dime nsion was unchanged from normal respiration, 9.1 +/- 0.5, compared wit h forced respiration, 9.3 +/- 0.6. Also, nonlinear determinism express ed as the nonlinear prediction error did not differ between spontaneou s respiration, 32.3 +/- 3.4 ms, and forced respiration, 31.9 +/- 5.7. It is concluded that the origin of the nonlinear dynamics in heart rat e variability is not a nonlinear input from the respiration into the c ardiovascular oscillator. Additional studies are needed to elucidate t he mechanisms behind the nonlinear dynamics in heart rate variability.