An method for analyzing irreversible apneic coma (IAC) for determining the presence of irreversible apneic coma (IAC) by analyzing the heart rate variability of a brain traumatic patient, thereby providing a physician a reference index to determine whether brain death has occurred. This method includes, at first, recording an electrocardiogram (ECG) from a subject. Then, analyzing R-R interval in said electrocardiogram (ECG), and plotting said R-R interval into Poincare plot, wherein the X coordinate in said Poincare plot represents R-R interval(n), and n is a 1.about.data number. Y coordinate in said Poincare plot represents RR(n+1). And, finally, quantifying said Poincare plot, and obtaining semi-major axis (SD1), semi-minor axis (SD2), and SD1/SD2 of said Poincare plot, as well as Poincare plot area.