Heart rate variability

Analysis of variations in the instantaneous heart rate time series using the beat-to-beat RR-intervals (the RR tachogram) is known as Heart Rate Variability (HRV) analysis [7,10]. HRV analysis has been shown to provide an assessment of cardiovascular disease [11]. The heart rate may be increased by slow acting sympathetic activity or decreased by fast acting parasympathetic (vagal) activity. The balance between the effects of the sympathetic and parasympathetic systems, the two opposite acting branches of the autonomic nervous system, is referred to as the sympathovagal balance and is believed to be reflected in the beat-to-beat changes of the cardiac cycle [7]. The heart rate is given by the reciprocal of the RR-interval in units of beats per minute. Spectral analysis of the RR tachogram is typically used to estimate the effect of the sympathetic and parasympathetic modulation of the RR-intervals. The two main frequency bands of interest are referred to as the Low-Frequency (LF) band (0.04 to 0.15 Hz) and the High-Frequency (HF) band (0.15 to 0.4 Hz) [10]. Sympathetic tone is believed to influence the LF component whereas both sympathetic and parasympathetic activity have an effect on the HF component [7]. The ratio of the power contained in the LF and HF components has been used as a measure of the sympathovagal balance [7,10]. Respiratory Sinus Arrhythmia (RSA) [12,13] is the name given to the oscillation in the RR tachogram due to parasympathetic activity which is synchronous with the respiratory cycle. The RSA oscillation manifests itself as a peak in the HF band of the spectrum. For example, 15 breaths per minute corresponds to a 4 second oscillation with a peak in the power spectrum at 0.25 Hz. A second peak is often found in the LF band of the spectrum at approximately 0.1 Hz. While the cause of this 10 second rhythm is strongly debated, one possible explanation is that it may be due to baroreflex regulation which creates the so-called Mayer waves in the blood pressure signal [14].