The purpose of this study was to evaluate the usefulness of phase analysis in detecting the altered activation sequence induced by different pacing modes. Radionuclide ventriculography and planar gated blood pool scintigraphy were performed at rest in 56 patients with different pacemakers. This method permitted us to localize the pacemaker impulse site in the right ventricle and its diffusion in the heart. In patients with VVI pacemaker, this technique showed an evident asynchronism of contraction and relaxation of each ventricle and the standard deviation of phase angle (sigma), calculated by computer, is greater during pacing than sinus rhythm for left (LV) and right (RV) ventricles (LV sigma: 17 degrees +/- 4 vs 11 degrees +/- 3, less than 0.001; RV sigma: 31 degrees +/- 7 vs 14 degrees +/- 4, P less than 0.001). In the patients with VVI rate responsive pacemakers, the LV sigma changed from 18.5 +/- 3 under pacing to 11 degrees +/- 3 in sinus rhythm, P less than 0.001, while the RV sigma changed from 30 degrees +/- 8 to 14 degrees +/- 4, P less than 0.001. Instead in the patients with DDD pacemakers, the LV sigma changed from 15.5 degrees +/- 2 under pacing to 11 degrees +/- 3 in sinus rhythm, P less than 0.05, while the RV sigma changed from 29.1 degrees +/- 6 to 14 degrees +/- 4, P less than 0.001.(ABSTRACT TRUNCATED AT 250 WORDS)