The aim of the study was to analyse the left ventricular contraction pattern in left bundle branch block (LBBB), to create experimentally a comparable pattern in animals and to relate this to haemodynamic measurements. In 20 normal subjects and 16 patients with LBBB without coronary heart disease we performed computer-assisted segmental left ventricular wall motion analysis during various systolic periods using two-dimensional echocardiography. The normal subjects showed on average a uniform shortening of all segments in systole; in patients with LBBB, however, asynchronous contractions of various types and intensities were found. Examination of the contraction pattern of each LBBB patient within the confidence range of the normal subjects showed that in 94% there was an abnormally small shortening of one of the sectors at one time in the second part of systole, and in 74% in the region of the interventricular septum. A "septum index" showed significant differences (p less than 0.0025) between LBBB patients and normal subjects. By right ventricular stimulation of the apex (RVA) and the outflow tract (RVOT) we simulated these contraction patterns in 6 dogs. With RVA stimulation the left ventricular contraction pattern was nearly physiological, while with RVOT stimulation the septum movement was paradoxical. With RVA stimulation cardiac output measured by thermodilution was higher (3.45 vs. 3.11 l/min, p less than 0.002) and the left ventricular end-diastolic pressure lower (7.0 vs. 8.0 mm Hg, p less than 0.002) than on RVOT stimulation; aortic pressure and the first derivative of left ventricular pressure did not differ significantly.(ABSTRACT TRUNCATED AT 250 WORDS)