REVIEW ARTICLE Heart rate variability: a review U. Rajendra Acharya Æ K. Paul Joseph Æ N. Kannathal Æ Choo Min Lim Æ Jasjit S. Suri Received: 19 October 2005 / Accepted: 10 October 2006 / Published online: 17 November 2006 Ó International Federation for Medical and Biological Engineering 2006 Abstract Heart rate variability (HRV) is a reliable reflection of the many physiological factors modulating the normal rhythm of the heart. In fact, they provide a powerful means of observing the interplay between the sympathetic and parasympathetic nervous systems. It shows that the structure generating the signal is not only simply linear, but also involves nonlinear contri- butions. Heart rate (HR) is a nonstationary signal; its variation may contain indicators of current disease, or warnings about impending cardiac diseases. The indi- cators may be present at all times or may occur at random—during certain intervals of the day. It is strenuous and time consuming to study and pinpoint abnormalities in voluminous data collected over sev- eral hours. Hence, HR variation analysis (instanta- neous HR against time axis) has become a popular noninvasive tool for assessing the activities of the autonomic nervous system. Computer based analytical tools for in-depth study of data over daylong intervals can be very useful in diagnostics. Therefore, the HRV signal parameters, extracted and analyzed using com- puters, are highly useful in diagnostics. In this paper, we have discussed the various applications of HRV and different linear, frequency domain, wavelet domain, nonlinear techniques used for the analysis of the HRV. Keywords Heart rate variability  Autonomic nervous system  Poincare plot  Surrogate data  ANOVA test  Phase space plot  Correlation dimension  Lyapunov exponent  Approximate entropy  Sample entropy  Hurst exponent  Wavelet transform  Recurrent plot 1 Introduction Heart rate variability (HRV), the variation over time of the period between consecutive heartbeats, is pre- dominantly dependent on the extrinsic regulation of the heart rate (HR). HRV is thought to reflect the heart’s ability to adapt to changing circumstances by detecting and quickly responding to unpredictable stimuli. HRV analysis is the ability to assess overall cardiac health and the state of the autonomic nervous system (ANS) responsible for regulating cardiac activity. HRV is a useful signal for understanding the status of the ANS. HRV refers to the variations in the beat intervals or correspondingly in the instantaneous HR. The normal variability in HR is due to autonomic neural regulation of the heart and the circulatory sys- tem [111]. The balancing action of the sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) branches of the ANS controls the HR. Increased SNS or diminished PNS activity results in cardio-acceleration. Conversely, a low SNS activity or a high PNS activity causes cardio-deceleration. The degree of variability in the HR provides information U. Rajendra Acharya (&)  N. Kannathal  C. M. Lim Department of ECE, Ngee Ann Polytechnic, 535 Clementi Road, Singapore, Singapore 599 489 e-mail: aru@np.edu.sg K. Paul Joseph Electrical Engineering, National Institute of Technology Calicut, Calilcut 673601 Kerala, India J. S. Suri Idaho’s Biomedical Research Institute, ID, USA J. S. Suri Biomedical Technologies Inc., Westminster, CO, USA 123 Med Bio Eng Comput (2006) 44:1031–1051 DOI 10.1007/s11517-006-0119-0