Multiscale time irreversibility of heartbeat at rest and during aerobic exercise

The multiscale time irreversibility (MTI) involves the lack of consistency in the properties of a time series if one reverses the reading direction along the time. To analyze the RR time series at rest and during aerobic exercise through the MTI, both in healthy people and cardiac patients. The heartbeat signal was recorded beat to beat for 15 min at rest and 15 min while pedalling on a static bicycle in 10 healthy and active men (age 26.5 +/- 3.3 years; height 179.3 +/- 6.6 cm; weight 80.4 +/- 11.8 kg) and 10 cardiac patients (age 61.1 +/- 4.7 years, height 165.3 +/- 5.3 cm; weight 86.9 +/- 11.1 kg). The MTI was calculated through the asymmetry index (AI), defined as the sum of the values of asymmetry obtained for each scale from 1 to 10. The AI decreases significantly in healthy subjects from 0.51 +/- 0.28 at rest to 0.28 +/- 0.24 during exercise (P = 0.01) but not in cardiac patients (-0.2204 +/- 0.5097 at rest and 0.0848 +/- 0.1200 during exercise; P = 0.07). MTI distinguish adequately the four experimental situations because it can be considered as an index of the internal property of the signal in contrast to linear methods which are highly sensitive to external influences over the heart rhythm, particularly sympathetic and parasympathetic stimuli.