Noninvasive assessment of Wedensky modulated signal-averaged electrocardiograms

Subthreshold stimulation without capture reduces the stimulation threshold and changes the action potential of subsequent suprathreshold stimulation, a phenomenon known as Wedensky modulation (WM). Patients with ventricular t achycardia (VT) inducible during electrophysiological testing(n = 47, mean age 63 +/- 13 years, 83% men), and healthy controls (n = 30, mean age 44 +/ - 16 years, 60% men) were subjected to transthoracic external subthreshold stimulation between surface precordial and left subscapular patch electrode s. Stimuli of 5, 10, 20, and 40 mn were delivered for 2 ms, in synchrony wi th, or 20 ms after, R wave detection. A total of 60-200 subthreshold stimul ated QRS complexes were averaged and compared with averaged nonstimulated c omplexes recorded during the same experimental session. To detect transient changes within the QRS complex, both signals were decomposed with 54 scale s of Morlet analyzing wavelets (central frequencies 40-250 Hz). Wavelet vec tor magnitude wets obtained for stimulated and nonstimulated complexes. The ir difference created a wavelet residuum (WR) that characterized WM numeric ally. The surface area of the three-dimensional envelope of WR was measured and statistically compared between VT patients and healthy controls. WR sh owed a significantly greater increase in the spectral power of the stimulat ed complex in healthy controls than in VT patients (P < 0.01). In conclusio n, (1) wavelet decomposition is a suitable tool to analyze WM, (2) WM in th e rate QRS complex is short, and (3) VT patients are less sensitive to Will , particularly at low subthreshold energies.