Overdetection of atrial depolarization by the right ventricular lead is rare and is mainly observed in patients implanted with an integrated bipolar lead. In a patient in sinus rhythm, the right ventricular lead detects both atrial and ventricular depolarization, as the PR interval is longer than ventricular blanking. If the patient has complete atrioventricular block, overdetection of the P wave may inhibit ventricular pacing and cause asystole. Similarly, overdetection of atrial depolarization during atrial flutter or tachycardia can cause both inappropriate therapy and asystole if the patient is dependent.

Overdetection of the atrial signal occurs preferentially in two situations 1) displacement of the right ventricular lead at the atrioventricular junction (coinciding with a decrease in the measured R wave amplitude) 2) positioning of an integrated bipolar lead near the tricuspid annulus, with the distal coil straddling the valve (coinciding with preserved R wave amplitude). This observation is more common in patients with small cardiac cavities: children, hypertrophic cardiomyopathy.

Atrial overdetection can also occur in more unusual circumstances: 1) right ventricular lead unintentionally placed in the coronary sinus 2) insulation defect located in the atrial portion of the lead, causing overdetection of atrial activity 3) interaction between the atrial lead and the right ventricular lead, the atrial lead coming into contact with the ventricular lead and generating a signal at the time of atrial systole.

In a triple-chamber defibrillator, if the left ventricular lead recedes into the coronary sinus, it may detect atrial activity and cause a loss of left ventricular pacing on devices that have left ventricular lead detection.

Overdetection of the P wave results in an alternation between two signals of different morphology, an alternation between two cycles with a characteristic rail-like appearance on the graph (identical to that observed when the R wave is counted twice). The PR cycle is classified as VF, and the classification of the RP space depends on the heart rate, the programmed zones, and the manufacturer (frequently not classified in an Abbott defibrillator, which explains the high rate of inappropriate therapies with this type of device).

If P wave oversensing occurs in sinus rhythm, one possible strategy is to force atrial pacing using DDDR mode and an atrial overdrive algorithm to increase the heart rate and prevent sensitivity from reaching its maximum value. Ventricular blanking induced by atrial pacing can also prevent cross-listening. However, this solution is usually temporary and does not protect the patient from possible oversensing during supraventricular arrhythmia.

In most cases, P wave overdetection requires repositioning of the defibrillation lead (new defibrillation lead if using the DF4 system, or addition of a pacing/sensing lead if using the DF1 system).

To minimize the risk of P wave oversensing, it is important during implantation to ensure that the distal coil is completely located in the right ventricular cavity when an integrated bipolar lead is implanted.