Suppression of demand pacemakers due to spurious signals generated by inactive endocardial electrodes.

We report a case of oversensing observed in a patient with a VVI pacemaker. This phenomenon was provoked by an inactive lead whose tip was in close apposition with the tip of the stimulating electrode. Potentials of large amplitude arose from the contact between these two catheters, causing pacer suppression. These false signals have been revealed both by intracavitary recordings and by standard ECG. In the latter they assumed a P-like shape. Withdrawal of the temporary lead restored normal pacing.