Non-invasive Wedensky modulation within the QRS complex.

To investigate non-invasively induced Wedensky modulation, 2ms pulses of 5, 20 and 40mA were delivered between precordial and subscapular patches synchronously with the ORS complex. Wavelet vector magnitude was obtained for averaged modulated and non-modulated complexes. The surface area of a 3D-envelope of their difference (WSR) was compared in 59 patients with an uncomplicated follow-up after myocardial infarction (MI) (42 men, 64.3+/-9.1 years), in 30 patients with ischaemic heart disease and a history of ventricular tachycardia/fibrillation (VT/VF) (29 men, 63.1+/-9.8 years), and in 53 healthy subjects (control) (22 men, 56.6+/-10.1 years). Reproducibility of the assessment was tested by computing relative errors in a sub-population of 30 VT/VF patients and 47 controls. Wedensky modulation parameters differed significantly between control, MI and VT/VF subjects. In 10 ms post-modulation windows, the following WSR values were obtained: controls: 1184+/-496 (5mA), 1553+/-838 (20 mA) and 2092+/-1488 (40 mA); VT/VF: 861+/-412 (5mA), 1134+/-636 (20 mA) and 1320+/-1036 (40 mA); MI: 1305+/-885 (5mA) and 1779+/-1169 (20 mA). With all modulating energies used, the VT/VF patients differed significantly from both the controls and MI patients; control patients against VT/VF patients: p<0.004 (5 mA), p<0.01 (20 mA) and p<0.001 (40 mA); VT/VF patients against MI patients: p<0.02 (5mA), p<0.01 (20 mA); control patients against MI patients: all p=NS. The reproducibility assessment showed an acceptable stability of Wedensky modulation parameters. This study demonstrated that wavelet decomposition detects non-invasive Wedensky modulation within the QRS complex, and VT/VF patients are less sensitive to Wedensky modulation than control and MI patients.

T-wave morphology differences between patients with and without arrhythmic complication of ischemic heart disease.

The study investigated the differences in T-wave morphology between normal controls, patients with an uncomplicated follow-up after a myocardial infarction (MI), and patients with ischaemic heart disease and a history of ventricular tachycardia/fibrillation (VT/VF). The study population consisted of 164 healthy patients (age 53.4 +/- 18.7 years old, 80 women), 123 VT/VF patients (age 63.8 +/- 10.1 years old, 15 women), and 196 MI patients (age 59.2 +/- 10.0 years old, 23 women). In all patients, supine resting signal-averaged orthogonal electrocardiograms were obtained. After singular value decomposition of electrocardiogram signal, 2 T-wave morphology descriptors were calculated: total cosine R to T describing the global angle between repolarisation and depolarisation loops, and percentage of loop area expressing the irregularity of the T-wave loop (a more irregular wave results in a lower percentage of loop area value). Both parameters were practically uncorrelated (Controls: r = - .106, MI r = .161, and VT/VF r = .173) and different between individual groups of patients: total cosine R to T (Control vs. MI: P = 4.3 x 10(-8), Control vs. VT/VF: P = 2.7 x 10(-16), MI vs. VT/VF: P = 3.1 x 10(-6)), percentage of loop area (Control vs. MI: P = 0.07, Control vs. VT/VF: P = 1.1 x 10(-8), MI vs. VT/VF: P = 2.9 x 10(-5), all nonparametric Mann-Whitney test). The comparisons of cumulative histograms also revealed significant differences between all three groups for both parameters (Kruskal-Wallis ANOVA test). Thus, these numerical descriptors of T-wave morphology are powerful indicators of arrhythmic complications among patients with ischaemic heart disease. They also differentiate between patients with stable uncomplicated ischaemic heart disease and healthy controls.

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 tachycardia (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 electrodes. Stimuli of 5, 10, 20, and 40 mA were delivered for 2 ms, in synchrony with, or 20 ms after, R wave detection. A total of 60-200 subthreshold stimulated QRS complexes were averaged and compared with averaged nonstimulated complexes recorded during the same experimental session. To detect transient changes within the QRS complex, both signals were decomposed with 54 scales of Morlet analyzing wavelets (central frequencies 40-250 Hz). Wavelet vector magnitude was obtained for stimulated and nonstimulated complexes. Their difference created a wavelet residuum (WR) that characterized WM numerically. The surface area of the three-dimensional envelope of WR was measured and statistically compared between VT patients and healthy controls. WR showed a significantly greater increase in the spectral power of the stimulated complex in healthy controls than in VT patients (P < 0.01). In conclusion, (1) wavelet decomposition is a suitable tool to analyze WM, (2) WM in the late QRS complex is short, and (3) VT patients are less sensitive to WM, particularly at low subthreshold energies.