T-wave alternans hysteresis on heart rate

Microvolt T-wave alternans (TWA) increases with heart rate (HR). Thus, TWA is usually analyzed during exercise. However, since TWA during recovery is usually not analyzed, it is not clear if TWA and HR are linked by a one-to-one correspondence, or if it does exist a TWA hysteresis on HR. To investigate such issue TWA was identified in ECG recordings of 266 patients with implanted cardio-defibrillator acquired during a bicycle ergometer test, which included a HR-increasing exercise and a HR-decreasing recovery, both characterized by a HR from 80 to 125 bpm. TWA was always found to have a positive association with HR but, at each HR, exercise TWA was typically different from recovery TWA. Specifically, TWA increased exponentially during exercise (fitting-exponential-curve correlation: ρ=0.99, P<;10-7) while decreased linearly during recovery (fitting-line correlation: ρ=0.94, P<;10-4). The two fitting curves crossed at about 115 bpm, so that for lower HRs (80-110 bpm) exercise TWA was significantly lower than recovery TWA (16-21 μV vs. 22-27 μV; P<;0.01), while for higher HRs (120-125 bpm) exercise TWA was significantly higher than recovery TWA (41-51μV vs. 28 μV; P<;10-6). Thus, it does exist a TWA hysteresis on HR since TWA does not depend only on the actual value of HR but also on such value being reached during exercise or recovery.