Time reversibility in short-term heart period variability

We propose a simple normalized index (i.e. the percentage of positive variations between two successive samples, PV%) and a procedure based on surrogate data to detect irreversible time series. This index was applied to short term heart period variability series (∼300 cardiac beats) derived from 10 young healthy humans at rest (R), during head-up tilt (T) and controlled respiration at 10, 15 and 20 breaths/minute (R10, R15 and R20). We found that heart period variability is time irreversible in 30%, 60%, 60%, 60% and 40% of the subjects at R, T, R10, R15 and R20 respectively. Mostly during T, R10, R15 and R20 the values of PV% indicate the presence of bradycardic runs (i.e. sequences characterized by heart period lengthening) lasting less than tachycardic ones, thus indicating a specific asymmetry of the temporal features present in the heart period variability series.