Vectorcardiographic loop alignment and the measurement of morphologic beat-to-beat variability in noisy signals

The measurement of subtle morphologic beat-to-beat variability in the elect rocardiogram (ECG)/vectorcardiogram (VCG) is complicated by the presence of noise which is caused by, e.g., respiration and muscular activity. A metho d was recently presented which reduces the influence of such noise by perfo rming spatial and temporal alignment of VCG loops, The alignment is perform ed in terms of scaling, rotation and time synchronization of the loops. Usi ng an ECG simulation model based on propagation of action potentials in car diac tissue, the ability of the method to separate morphologic variability of physiological origin from respiratory activity was studied. Morphologic variability was created by introducing a random variation in action potenti al propagation between different compartments. The results indicate that th e separation of these two activities can be done accurately at low to moder ate noise levels (less than 10 mu V) At high noise levels, the estimation o f the rotation angles was found to break down in an abrupt manner. It was a lso shown that the breakdown noise level is strongly dependent on loop morp hology; a planar loop corresponds to a lower breakdown noise level than doe s a nonplanar loop.