Asymmetric properties of heart rate microstructure

The duration of each cardiac cycle is measured on ECG as the distance between the peaks of consecutive R waves (RR interval). Its inverse value corresponds to the heart rate (HR) changing in a beat-to-beat manner. HR accelerations are reflected as the shortenings of RR intervals while HR decelerations as the lengthening of RR intervals. HR asymmetry is a physiological phenomenon caused by an unequal input of HR decelerations and accelerations to the HR variability. Naturally occurring consecutive values of RR intervals create time series which are composed of acceleration and deceleration runs of different length. Some examples are a single HR acceleration, a pair of HR decelerations, a run consisting of five consecutive HR decelerations or a run composed of eight accelerations in a row. These runs make up the so-called heart rate microstructure that has asymmetric properties due to unequal contribution of acceleration and deceleration runs. Asymmetry of HR microstructure is a physiological finding in healthy people. However, the asymmetric properties of HR microstructure have been shown to significantly alter in some clinical conditions such as myocardial infarction, obstructive sleep apnea, chronic obturatory pulmonary disease or sepsis in infants. An abnormal HR microstructure has predictive value in survivors of myocardial infarction or patients with clinical indications for exercise treadmill stress test, e.g., for total mortality. In this review, we present and explain how the asymmetric properties of HR microstructure can be quantified, and summarise available data on the clinical and predictive value of this phenomenon and its analysis.