Unmasking of slow intrinsic ventricular excitation by magnetic inhibition of R-wave inhibited demand pacemakers.

In patients with permanent pacemakers and slow intrinsic ventricular excitation, the pacemaker rhythm dominates ventricular excitation. Diagnostic considerations may require that the electrocardiographic morphology of the intrinsic beats be known, such as during the exclusion of the myocardial infarction. If the pacemaker is a Cordis R wave inhibited (Stanicor) type, it is possible to allow the intrinsic rhythm to emerge by external magnetic inhibition of the pacemaker. Such inhibition can be achieved by rotating a magnet back and forth through a 180 degree arc with the magnetized poles on the surface of the chest wall above the pacing unit. Successful pacemaker inhibition by the magnet will abolish the pacemaker rhythm. It is concluded that magnetic inhibition of R wave inhibited pacemakers may be a useful diagnostic tool in patients with Stanicor pacemaker rhythms.