double counting of the R wave
This 64-year-old man underwent implantation of a dual chamber Lumax 340 DR-T defibrillator in the context of ischemic cardiomyopathy with right bundle branch block and left anterior fascicular block; periodic EGM recording.
Four channels are present: 1) markers with time intervals, 2) shock channel (FF = far-field) between the ventricular lead coil and the pulse generator, 3) atrial sensing channel, 4) right ventricular (RV) sensing channel. EGMs were recorded for 30 seconds.
- Spontaneous sinus rhythm; oversensing of a supernumerary ventricular EGM, corresponding to double counting of the R wave at the end of the ventricular blanking period;
- Permanent oversensing; alternation of VS and VF cycles; the VF counter is never filled and no episode is recorded.
R wave double counting may occur when ventricular conduction is markedly slowed and the duration of the ventricular endocardial EGM is longer than the ventricular blanking period. With the new Lumax 740 defibrillators, the nominal post-ventricular blanking period has been extended to 110 ms. In this patient, the programmed ventricular blanking period was probably too short, and its lengthening without changes in the programmed sensitivity eliminated double counting while preserving the patient’s safety. The unchanged sensitivity guaranteed the proper sensing of ventricular arrhythmias, and the blanking period programmed at 120 ms did not preclude the flawless detection of ventricular VF, since such short physiological ventricular intervals are not observable.