[Title page]

Microvolt T-wave alternans (TWA), a subtle every-other-beat fluctuation of the electrocardiographic (ECG) T-wave amplitude, is a risk index for the occurrence of malignant ventricular arrhythmias. It is well-known that TWA increases with HR so that TWA is often analyzed during exercise. Since TWA during recovery is usually not evaluated, it is not clear if there is a HR-TWA one-to-one correspondence, or if TWA depends on how such HR was reached. Thus, the aim of the present study was to investigate TWA during both HR-increasing exercise and HR-decreasing recovery. To this aim, our HR adaptive match filter (AMF) for automatic TWA identification was applied to ECG recordings of 266 patients with implanted cardio-defibrillator (ICD). HR varied from 70 bpm to 125 bpm during exercise and vice versa during recovery. Our results indicate that TWA increased (from 17 μV to 51 μV) with increasing HR during exercise, and decreased (from 28 μV to 17 μV) with decreasing HR during recovery. However, such TWA changes did not occur symmetrically, but TWA was significantly lower during exercise than during recovery at low HRs (17-18 μV vs. 20-25 μV for HR<;100 bpm; P<;0.05), and vice versa at high HRs (41-51 μV vs. 28 μV for HR>100 bpm; P<;10-6). Thus, there is not a HR-TWA one-to-one correspondence but the value of TWA at a specific HR depends on how such HR was reached.