P-wave oversensing
Generalities
Oversensing of the far-field atrial activation is uncommon but can be life-threatening in patients requiring ventricular pacing for complete atrioventricular block.
In patients with sinus rhythm and intrinsic ventricular activation, the RV lead both senses the atrial and ventricular activation, the PR interval being longer than the ventricular blanking period. There is a risk of spurious shocks delivered during sinus tachycardia.
In patients with sinus rhythm and complete atrio-ventricular block, the oversensing of the P-wave may inhibit ventricular pacing and lead to asystole.
Similarly, oversensing of atrial depolarizations during atrial flutter or atrial tachycardia is likely to cause spurious shocks and possible asystole if the patient's underlying rhythm is complete atrioventricular block.
Atrial oversensing usually traditionally occurs in 2 clinical situations 1) RV lead dislodgement to the atrioventricular junction (coincides with an acute decrease in the amplitude of the R wave) 2) when an integrated bipolar lead has been positioned near the tricuspid annulus, the RV distal coil crossing the valve (coincides with a stable, normal amplitude R -wave). This observation is more frequent in patients with small hearts or a small RV cavity: children, hypertrophic cardiomyopathy.
Atrial oversensing can be observed in other infrequent situations: 1) RV lead unintentionally implanted in the coronary sinus 2) RV lead insulation defective in the atrium allowing for sensing of atrial activity 3) Atrial lead-to RV lead interaction, the atrial lead contacting the RV lead during atrial systole and generating a signal sensed by the RV lead.
In CRT devices, the LV lead may migrate proximally toward the coronary sinus with an increased probability of sensing the atrial activity and of loss of LV pacing in devices that sense from the LV lead.
P-wave oversensing during sinus rhythm results in alternation of 2 ventricular cycle lengths with the same characteristic railroad track pattern on the graph as observed during R-wave double counting. The ventricular channel senses the atrial event inappropriately. The QRS following the P wave is then sensed as a second, very rapid, ventricular EGM and is classified in the VF zone. This second signal is sensed after ventricular blanking, the absolute refractory period triggered by the first ventricular sensing.
In case of oversensing during sinus rhythm, a possible strategy is to force atrial pacing using DDDR pacing and specific algorithms forcing atrial capture, to increase the heart rate, shorten the ventricular cycle length and prevent ventricular sensitivity from reaching its minimal value. The post-atrial paced ventricular blanking can also reduce the probability of atrial oversensing. This solution is most of the time temporary and do not protect the patient in case of oversensing of atrial arrhythmia.
In the majority of cases, consistent oversensing of spontaneous P waves requires lead revision (new high-voltage RV lead or implantation of an additional sensing/pacing RV lead in a DF1 system).
To minimize the probability of atrial oversensing, it is important during implantation to ensure that the distal coil lies entirely within the RV cavity. P-wave oversensing is very infrequent in adults when the RV lead is positioned at the apex.