Double counting of the R wave and electrical shock
Tracing
Manufacturer Abbott
Device ICD
Field Sensing
N° 13
Patient
A 58-year-old man suffering from advanced, idiopathic, dilated cardiomyopathy received an Atlas triple chamber CRT-D connected to a right atrial, a right ventricular defibrillation lead implanted at the base of the septum, and a left, lateral ventricular lead. He was seen in consultation after receiving a shock at rest, without prior symptoms or sensation of tachycardia.
Main programmed parameters
- VF zone at 214 bpm, VT zone at 150 bpm
- 12 cycles in each the VF and the VT zones were needed for the diagnosis
- Maximum sensitivity programmed at 0.3 mV
- VF zone: six 36-J shocks (maximum strength); VT zone: 6 bursts of ATP followed by a single 7-J shock, followed by a single 20-J shock, followed by 2 shocks of maximal strength
- DDDR pacing mode at 70 bpm; DDI post-shock pacing mode at 60 bpm
Graph and trace
Narrative
Episode diagnosed as VF treated with a maximum 36-J shock (double coil impedance of 36 Ohms).
Tracing
1: 1:1 tachycardia (1 atrial for 1 ventricular event) at 120 bpm. The atrial and ventricular EGM are nearly simultaneous, consistent with sinus tachycardia, atrial tachycardia or VT with retrograde conduction;
2: Two ventricular EGM were sensed for the same ventricular complex, the first classified as VS and the second unclassified (-) at the end of the ventricular blanking period. The first sensed ventricular electrogram was simultaneous with the atrial activity. It is not clear whether the atrial signal was oversensed in the ventricle, or whether a very wide and fragmented ventricular EGM was double counted. The atrial channel shows the mark of a ventricular EGM that was not sensed;
3: DDI episode pacing mode after 4 T or F cycles. Return of sinus rhythm was not diagnosed later on the tracing because F or T classified cycles occurred between the VS;
4: Increasingly frequent double counting;
5: Detection of a VF episode (12 F classified cycles) and onset of the capacitors charge;
6: Nearly incessant sensing of 2 EGM per cycle during the charge; end of charge and confirmation before the shock on the second cycle after the charge has ended;
7: 36-J shock;
8: Profound slowing of the atrial rate, eliminating sinus tachycardia as the initial diagnosis. The differential diagnosis between atrial tachycardia and VT remains unclear. Acceleration of the ventricular rate. It is noteworthy that the ventricular EGM are visible, though were not sensed, on the atrial channel. Diagnosis of return of sinus rhythm.
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The initial 1:1 tachycardia might have been sinus tachycardia, atrial tachycardia, junctional tachycardia, or VT with retrograde conduction. The 120 bpm rate at rest was inconsistent with sinus or junctional tachycardia. Oversensing occurred when the atrial and ventricular EGM were nearly simultaneous. Each ventricular cycle was sensed twice. It may be challenging to differentiate oversensing of atrial activity by the ventricular channel from double counting of the R wave. It seems probable that this double counting on the ventricular channel corresponded to the summation of the atrial EGM and a very wide ventricular EGM.