LINEAR AND NONLINEAR PROPERTIES OF HEART-RATE CONTROL IN INFANTS AT RISK

The aim of this study was to test whether the heart rate (HR) control in infants at risk differs in comparison with healthy infants. Twelve former preterm infants suffering from bronchopulmonary dysplasia and 1 8 control infants, matched for their postconceptional age, were examin ed polygraphically during quiet and active sleep. HR, low-frequency (L F) power, high-frequency (HF) power, total power, and the ratio of LF to HF power (LF/HF) of the instantaneous HR spectra were calculated fo r linear analysis. The largest Lyapunov exponent (LLE) of the R-R inte rval time series was calculated to determine a nonlinear property of H R. Infants at risk had significantly lower LF power (median: 0.51 x 10 (-3) vs. 1.16 x 10(-3) Hz(2)) and lower LF/HF (median: 1.05 vs. 1.94) during quiet sleep. LLE was positive, revealing low-dimensional chaoti c behavior of HR control, and did not differ between both groups (medi an: quiet sleep, 0.05 bit/s vs. 0.06 bit/s; active sleep, 0.16 bit/s v s. 0.15 bit/s). Sleep state-related changes in spectral parameters and LLE were similar in both groups. In infants at risk, the lower LF/HF during quiet sleep can be interpreted in terms of changes in the rhyth mic components of the sympathovagal balance of the autonomic system, w hich is an expression of linear properties of HR control. Conversely, the lack of differences in LLE between both groups indicates similar n onlinear properties of the control system.