Repeated misclassifications of tachycardia by an implantable cardiac defibrillator

Figures

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  Fig. 1. Misinterpretation of tachycardia by the ICD. Starting from the top, 1 = atrial electrogram, 2 = ventricular electrogram, 3 = atrial marker channel (number indicates a morphology score), 4 = ventricular marker channel (T indicates tachycardia and F indicates fibrillation), 5 = atrial marker channel (refractory period and AA interval), 6 = ventricular marker channel (AV interval, absolute refractory period, and VV interval). Paper speed was 25 mm/s. A: Tachycardia showed cycle lengths (CL) of 290–320 m s. The ventriculo-atrial (VA) interval was 40 m s. B: The atrial electrogram (A) fell into the postventricuar atrial blanking (PVAB). There were 2 atrial electrograms detected (AS) that were interpreted as VT based on the number of the ventricular electrogram (V) greater than A. Anti-tachycardia ventricular pacing with a constant CL of 270 m s was initiated but the AA interval was not affected. Upon termination of pacing, the tachycardia resulted in 2:1 AV conduction. C: Every other A fell into the PVAB and it was interpreted as sinus rhythm at 98 bpm, leading to initiation of biventricular pacing. The sensed AV delay was set at 100 m s. The tachycardia CL was not affected during…

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  Fig. 2. A and B had the same format as in Fig. 1. Paper speed was 25 mm/s. A. Spontaneous termination of tachycardia. There was short VA interval, fixed VA interval despite tachycardia CL fluctuation (VA linking), and termination with the atrial electrogram. This is consistent with AV node reentry. The perfect morphology match correctly interpreted it as SVT. B. Another tachycardia due to AV node reentry. Note that the morphology score showed a poor match, suggestive of a different QRS morphology during tachycardia. This was misclassified as VT and antitachycardia pacing was initiated. C. The AA interval was 340 m s, which remained the same during ventricular pacing with a CL of 280 m s, and abruptly terminated with the second from the last pacing (N-1) without atrial capture. The last ventricular pacing (N) showed intact VA conduction. Rapid ventricular pacing induced a 3-beat ventricular tachycardia before sinus tachycardia was restored.

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  Fig. 3. Proposed termination mechanism of tachycardia by ventricular pacing shown in Fig. 2. Rapid ventricular pacing (N-3 and N-2) did not affect the tachycardia until the second from the last pacing (N-1) abruptly terminated it with VA block. The last pacing (N) resulted in intact VA conduction via the presumed fast pathway. This suggests that the ventricular stimulus (N-1) entered the tachycardia circuit and terminated it within the AV node. Thus, AV node is part of the tachycardia circuit.