Wavelet decomposition of Wedensky modulated electrocardiograms: Differences between patients with ventricular tachycardia and healthy volunteers

Subthreshold stimulation without capture reduces the stimulation threshold and changes the action potential of subsequent suprathreshold stimulation (Wedensky modulation). To investigate this phenomenon after transthoracic subthreshold stimulation, 2 ms pulse of 5-40 mA between surface precordial and subscapular patches were delivered synchronously with or 20 ms after R wave detection. A total of 60 to 200 subthreshold stimulated QRS complexes were averaged and compared with averaged non-stimulated complexes recorded during the same experimental session. Vector magnitude wavelet decompositions (53 scales of central frequencies 40-250 Hz) were obtained for both stimulated and non-stimulated complexes and their difference characterized the Wedensky modulation numerically. The surface area of the 3D envelope of the wavelet residuum was measured and was statistically compared in the VT pts and healthy controls. The test was performed in 47 pts with EP inducible VT (aged 63±13 yrs, 83% male) and in 30 healthy controls (aged 44±16 yrs, 60% male). The residuum showed an increase in the spectral power of the stimulated complex that was significantly more marked in healthy volunteers (p<0.01) than in VT patients. The study demonstrated that: (1) wavelet decomposition of signal averaged ECG is suitable tool to analyze Wedensky modulation, (2) Wedensky modulation in the late QRS complex is short, and (3) that VT patients are less sensitive to the Wedensky modulation especially at very low subthreshold energies.