Oversensing of the P waves
Patient
This case is even rarer, namely P-wave oversensing by the defibrillation lead. This is a routine follow-up examination in an asymptomatic patient implanted with an integrated bipolar
defibrillation lead, in which the shock electrode of the ventricular defibrillation lead is involved in the ventricular detection of the defibrillator. « Non-sustained » tachycardias are recorded in
the device memory.
The tachogram exhibits a characteristic triple railroad track pattern with RR, PR, and RP intervals.
Comments
The EGM reveals the cause of this oversensing.
- The concordance between the ventricular markers and the visible P waves can readily be seen on the ventricular electrogram, the latter of which correspond well with the P
waves of the atrial electrogram.
- For this integrated bipolar defibrillation lead, this phenomenon is related to a positioning of the shock electrode of the defibrillator ventricular lead that is too close to the tricuspid valve, and thus able to « pick up » the atrial signals.
- This risk is all the more significant if the right ventricle is small, and the distal end of the defibrillation lead is placed in the anterior or infundibular septal area. The proximal part of the shock electrode is therefore straddling the tricuspid ring.
- Here again, in the event of sinus acceleration and of atrial rhythm disturbances, false VT/VF diagnoses may occur and trigger dangerous inappropriate therapies.
- This problem can be solved by decreasing the ventricular sensitivity, but at the risk of under-detection of ventricular fibrillation. If the sensitivity is decreased, we recommend performing a VF induction to verify the proper VF detection quality in this downgraded configuration.
Take home message
When placing a ventricular defibrillation lead, the absence of oversensing of signals other than QRS, P-waves and T-waves should be controlled, which would thus require a repositioning of
the defibrillation lead, which is difficult to perform later during the follow-up.
