1/f fluctuation of heart rate in postoperative and brain-dead patients

The power spectra of heart rate fluctuations in normal subjects show that they are inversely proportional to the frequency [1] and are thus called 1/f fluctuations. Such 1/f fluctuations are a well known phenomenon that occurs when an electric current passes through a vacuum tube and a semiconductor. In biology, 1/f fluctuations have been observed in cellular membranes [2], brain waves [3], and during concentration modulation of action potentials [4]. The l/f fluctuations of heart rate have been used as a novel index of autonomic function and it provides a qualitative estimate of the state of the cardiovascular control system. Short-term analyses of power spectra such as the 256 beats of RR intervals or during a 1 hour period have shown that low-frequency heart rate variability (i.e., less than 0.1 Hz) is under both sympathetic and parasympathetic nervous control, whereas higher frequency variability is under parasym pathetic control [5-9]. Recently, analyses of heart rate variability from 24-hour electrocardiogram recordings are reported as development of digital holter electrocardiograph [10-12]. Nonrandom heart rate variability has also been studied using long-term monitoring. Such variability can be analyzed in two ways, by evaluating the signal in the time or in the frequency domain. In the time domain, the heart rate in normal subjects shows a sinusoidal rhythm called a circadian rhythm, which has a period of 20 h to 28 h. However, spectra are more frequently calculated in the frequency domain. A few studies have analyzed the rhythms and fluctuations in postoperative patients [13, 14]. In this study, the heart rates of patients who had been admitted to an intensive care unit (ICU) were studied, and the variability in both the time domain and frequency domain were analyzed and evaluated to differentiate between survivors and no survivors, including brain-dead patients [15].