| According to the nonlinear theory, the author has designed and conducted the PHCA (profound hy-pothermia and circulatory arrest) experiments, which are used to research for the nonlineardynamics of theheart. Through the analysis of the power spectra, the computation of the correlation dimension and the Lya-punov exponent to a large number of ECG (electrocardiogram) signals which are sampled on the several temper-ature platforms of the PHCA experiments, the following conclusions are shown: (1) The power spectra, thecorrelation dimension and the Lyapunov exponent to ECG signals reflect the whole dynamic characteristics of thehearts, which may be used to estimate healthy condition of the heart. (2) Under normal physiological conditionsthe cardiac activities are chaotic, while under pathologic conditions the cardiac activities approach regularity. |