Electrical activation of ventricles and interventricular septum in hypertrophic obstructive cardiomyopathy.

In 10 cases with proven left ventricular outflow obstruction, we studied the time sequence of activation at the epicardial surface, in the left ventricular wall, and in the interventricular septum by means of epicardial exploration and intramural needle electrodes carrying 10 to 20 terminals, during surgical exposure. A variable delay (15-40 msec) was found in subendocardial activation of the anterior paraseptal left ventricular wall, probably caused by a block in the anterior division of the left bundle-branch. Epicardial excitation is additionally retarded by the increased diameter of the left ventricular wall because of hypertrophy. Conduction velocity in the left ventricular wall and in the interventricular septum was found to be approximately normal: +/-45 cm/sec; activation of the hypertrophic interventricular septum starts at normal times and proceeds mainly from left to right, with a smaller contribution from right to left; both fronts meet to the right of the middle of the interventricular septum; total septal activation time is prolonged because of hypertrophy.