Pacing & Defibrillation

ECG

Library

News

Back

Oversensing and integrated bipolar mode - Evera XT DR

Patient


Male with ischaemic cardiomyopathy implanted with a dual-chamber defibrillator (Evera XT DR), hospitalised for pre-syncope; history of paroxysmal AF.


Trace

1- How would you describe the start of the interval plot?
The plot shows rapid, regular, atrial activity and a railroad-track appearance to the ventricular cycles, with a very short cycle (at the limit of the programmed blanking value) and a longer cycle.

2- How would you describe the end of the graph?
Over a few cycles, the ventricular rate becomes very rapid such that it is superimposed on the atrial rate.

3- What is your diagnosis?
The EGM confirms the existence of a regular atrial tachycardia with cycles of 220-230 ms and reveals an oversensing at the ventricular channel of a signal corresponding to atrial activity at the end of the cycle; the functioning of the ventricular adaptive sensing explains why previous atrial signals of equivalent amplitude are not sensed by the ventricular probe, the device being less sensitive at the beginning of the cycle and the sensitivity adapting to the amplitude of the QRS complex.

4- How do you explain the occurrence of pre-syncope?
The EGM shows a ventricular pause of around 2 seconds, which may explain the occurrence of presyncope; during this cycle, there is no spontaneous QRS complex; the sensitivity is therefore not readjusted, the device remains very sensitive and oversensing persists, explaining the absence of ventricular stimulation associated with the pause; the oversensing stops following the onset of a QRS complex (which readjusts the sensitivity level).

Take home message

  • This tracing is an example of oversensing of atrial activity by the ventricular channel during an episode of atrial tachycardia.
  • Oversensing is particularly concerning in pacing-dependent patients because 1) it can lead to ventricular pauses, 2) oversensing can be prolonged in the absence of spontaneous QRS complexes, with the sensitivity level remaining at a maximum and adapting to the amplitude of the oversensed signals, which are often small.
  • Oversensing of atrial depolarisation by the right ventricular lead is rare and occurs mainly in patients implanted with an integrated bipolar lead; in a patient in sinus rhythm, the right ventricular lead detects both atrial and ventricular depolarisation, as the PR interval is longer than the ventricular blanking; if the patient has complete atrioventricular block, oversensing of the P wave may inhibit ventricular pacing and cause asystole.
  • Oversensing of atrial depolarisation during atrial flutter or atrial tachycardia can cause both inappropriate therapies and asystole, if the patient is dependent.
  • The oversensing of atrial signals by the ventricular lead is more likely in 2 situations: 1) displacement of the right ventricular lead at the atrio-ventricular junction (coinciding with a drop in the measured R wave amplitude ); 2) positioning of an integrated bipolar lead close to the tricuspid annulus, with the distal coil straddling the valve (associated with a preserved R wave amplitude); 3) programming a Distal-Coil sensing vector with a bipolar lead whose coil is too close to the RA.
  • Atrial oversensing can also occur in more exceptional circumstances: 1) right ventricular lead inadvertently placed in the coronary sinus; 2) insulation defect in the atrial portion of the ventricular lead resulting in oversensing of atrial activity; 3) interaction between the atrial lead and the right ventricular lead, with the atrial lead coming into contact with the ventricular lead and generating a signal at the time of atrial systole.
  • Several solutions exist to avoid the occurrence of inappropriate therapies or ventricular pauses in a pacing-dependent patient: 1) reduce the ventricular sensitivity to prevent sensing of the additional signal linked to atrial activity; however, this programming modification is accompanied by an increased risk of undersensing VF, VF induction may be required to check that the induced VF is correctly detected with the new sensitivity value; 2) oversensing of atrial activity may require the defibrillation lead to be repositioned (new defibrillation lead if DF4 system or addition of a pace/sense lead if DF1 system).

X