Comparison of approximate entropy (ApEn) and spectral values of RR intervals for postinfarction patients versus healthy subjects. The curves are probability density functions calculated using the mean values and standard deviations for the values of the different variables. The abnormal values for the postinfarction patients are differentiated in each panel by means of a vertical line drawn at the 90% percentile of the values obtained from healthy subjects, which was defined as the normal range for each measure. The overlap is least for approximate entropy. LF = low frequency component; ln = natural logarithmic transformation of the absolute value; SDANN = standard deviation of RR intervals; VLF = very low frequency component.