Accurate discrimination of ventricular tachycardia despite atrial undersensing

Patient

This 60-year-old man received a Lumax 340 DR-T dual chamber defibrillator for the management of episodes of VT in the context of dilated cardiomyopathy. An event report (yellow color) was issued in the context of a classified VT2.

Main programmed settings

VF zone (300 ms limit), VT2 zone (370 ms limit), VT1 zone (430 ms limit)
8/12 cycles in the VF zone, 16 cycles in the VT2 zone and 26 cycles in the VT1 zone were needed for the diagnosis
Maximum sensitivity programmed at 0.8 mV
VF zone: ATP one shot, followed by a single 30-J shock, followed by 7 shocks of maximum strength (40 J); VT2 zone: 3 bursts of ATP, followed by 3 ramps of ATP, followed by a single 10-J shock, followed by a single 24-J shock, followed by 6 shocks of maximum strength; VT1 zone: monitor only
Effective discrimination in the VT2 zone (SMART discrimination)
Pacing mode: DDD at 60 bpm

Trace

Remote tracing

The 4 channels available are 1) the markers with the time intervals, 2) the shock channel (FF = far field) between the coil of the RV lead and the pulse generator, 3) the atrial (A) sensing channel, and 4) the right ventricular (RV) sensing channel.

spontaneous rhythm in the atria and the ventricles;
tachycardia initially detected in the VT1 zone;
atrioventricular dissociation; the atrial activity is visible on the atrial electrogram channel, however without markers because it fell in the post-ventricular sensing atrial blanking period;
the atrial activity is more clearly visible;
VT2 classification after 16 classified cycles; discrimination in the PP>RR arm; average PP and RR during the initial classifications = 683 and 314 ms, respectively;
burst of ATP not visible;
successful burst of ATP and termination of the arrhythmia;
end of episode after 12 VP or VS cycles.

Comments

The first discrimination level of the SMART algorithm is a comparative analysis of the atrial and ventricular rhythms. In this patient, the episode was accurately diagnosed (VT) despite an error in the count of the atrial electrograms (P wave in the post ventricular sensing atrial blanking period). An accurate sensing at the level of both chambers is indispensable for the proper function of this dual chamber discrimination algorithm. A dislodgement of the atrial lead, a crosstalk with oversensing of the R wave in the atrium, undersensing of the atrial activity due to low-voltage activity in atrial fibrillation, and the occurrence of atrial activity in the post ventricular sensing atrial blanking period can be associated with inaccurate dual chamber discrimination. The post ventricular sensing atrial blanking period must be programmed long enough to prevent crosstalk, though not too long in order to preserve an accurate count of the atrial electrograms. It is noteworthy that the atrial cycles falling in the PVARP (Ars) were included in the atrial rate count.

Pacing & Defibrillation

ECG

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