No abstract

This study compares three different pacing system analyzers, which measure impedance at different points during an impulse, with measurements telemetered from implanted pacemakers from four different manufacturers. Measurements were obtained at the time of implantation in a group of 103 patients. The measurements obtained by these different methods differ significantly from each other; the later during the impulse the impedance is measured, the higher in general its value. Interpretation of impedance values should take into account the technique of measurement.

Two cases in which a DDD pacemaker failed to track P waves due to repetitive activation of the automatic postventricular atrial refractory period (PVARP) extension are reported. In one case, the cycle was initiated by a premature ventricular contraction (PVC); in the other, by touching the unipolar pacemaker with a needle while suturing. We demonstrated that chest wall stimulation can be used to induce cycles of repetitive automatic PVARP extension and failure to track P waves. We demonstrated in vitro that touching a unipolar pacemaker with a needle can generate a signal sufficient to mimic a QRS complex, and initiate a cycle of repetitive P wave undersensing.

A case in which apparent undersensing was caused by detection of low amplitude impedance-measuring pulses emitted by a thoracic impedance-sensing rate-modulated pacemaker is described. The detection of these high frequency pulses was interpreted as "noise" by the pacemaker and led to asynchronous pacing in the interference reversion mode. Increasing the sensitivity failed to correct the problem. Decreasing the sensitivity eliminated oversensing of the low amplitude pulses and the apparent undersensing. Recognition of this phenomenon is important to prevent a misdiagnosis of undersensing.