Inhibition of a ventricular synchronous pacemaker.

Noncompetitive ventricular synchronous pacemakers (VVT) have been available since 1965. Most presently available and earlier models have a single, fixed pulse-to-pulse interval divided into a total refractory period of 350 to 400 msec., followed by a sensitive period. During the refractory period the pacemaker will not respond to QSR complexes or electrical signals, but a pacer stimulus will be produced during the sensitive period. The generator cannot be inhibited under any circumstances. The Cordis Omni-Sctocor has a refractory period and two sensitive periods, during one of which the generator is inhibited. Unlike other ventricular synchronous pacers, the Omni-Ectocor is capable of inhibition by single or multiple ventricular or other signals, of slowing or increase of the stimulus formation rate which depends on the timing of the premature ventricular contractions, and of inhibition by a run of ventricular tachycardia or by electromagnetic interference. The rate of interference producing these effects is a function of the automatic rate to which the generator is programmed. All of these factors must be considered for proper interpretation of the ECG.