Tachygram explanation

1. Upon a 1:1 AV associated sinus rhythm, there is a series of ventricular events which are premature contractions since the atrial signals remain slow (blue crosses).
2. These ventricular cycles have two different coupling intervals, initiating a double railroad track appearance and indicating an intrinsic oversensing.

EGM

1. The EGM confirms the oversensing of cardiac events that are not QRS, but rather T waves (arrows) in this example. The diagnosis is made easy by comparing the markers with the ventricular electrogram. Because of constant repolarization times, the RR’ and R’R intervals are relatively fixed and therefore explain the double railroad track appearance of the tachogram.
2. In the event of sinus rate acceleration, this T-wave oversensing causes a false diagnosis of VT or even VF, and therefore possible inappropriate and dangerous therapies.
3. Fortunately, this phenomenon is extremely rare in Microport defibrillators. This is explained by the filters inherent to signal processing. At the time of implantation, changing the position of the ventricular lead may remove the phenomenon provided that all other implantation rules are followed. Shortly after implantation, a repositioning of the lead could be envisaged, but at the cost of a reoperation. Thereafter, there is only the reduction in sensitivity which can solve the problem, but at the risk of under-detection of ventricular arrhythmias. If this latter solution is applied, it is advisable to perform a VF induction to verify that fibrillation is correctly detected with a reduced ventricular sensitivity.

Message

Consequently, and at the time of implantation, the ventricular electrogram must be analysed concurrently with the measurement of the electrical parameters of the lead, and the lead must be repositioned when intrinsic oversensing is detected.