Mechanisms and significance of pacemaker-induced extracardiac sound

The frequency of pacemaker-induced extracardiac sound was studied in 26 patients with transvenous pacemakers and 2 with transthoracic units; mechanisms of this sound are described in 3 patients. In each case the extracardiac sound was associated with skeletal muscle contraction. In Case 1 intercostal muscle contraction was located at the site of a previously spliced transthoracic electrode wire, thereby suggesting stimulus leak. In Case 2 pacemaker-induced extracardiac sound developed at the site of an indifferent unipolar electrode, and the intensity of the sound was related to the output of the pacemaker generator. Pacemaker-induced extracardiac sound was detected in a third patient in whom a bipolar transvenous electrode was in apparent good position on routine chest roentgenogram. Right heart angiograms demonstrated that the catheter tip had perforated the right ventricular apex. The intensity of the sound appeared to be determined by the proximity of the electrode tip to the skeletal muscle as well as by the strength of the electrical stimulus.