VT accurately detected by a single chamber defibrillator
Patient
This 48-year-old man received a Lumax 540 VR-T single chamber defibrillator in the context of ischemic cardiomyopathy with a depressed left ventricular ejection fraction. An event report (yellow color) was issued in the context of a classified VT1.
Main programmed settings
- VF zone (limit at 250 ms), VT1 zone (limit at 400 ms)
- 18/24 cycles in the VF zone and 26 cycles in the VT1 zone were needed for the diagnosis
- Maximum sensitivity programmed at 0.8 mV
- VF zone: single ATP one shot, followed by 8 shocks of maximum strength (40 J); VT1 zone: 5 bursts of ATP, followed by 3 ramps of ATP, followed by a single 10-J shock, followed by a single 20-J shock, followed by 6 shocks of maximum strength
- Effective discrimination in the VT zone (onset 20%, stability 24 ms)
- Pacing mode: VVI at 40 bpm
Trace
The 3 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, and 3) the right ventricular (RV) sensing channel.
- spontaneous rhythm;
- ventricular pair;
- sudden onset of tachycardia alternating in the beginning between the VT1 and the VF zones. The sudden onset was calculated on the basis of 8 sliding cycles, with the average of the last 4 RR intervals compared with the average of the next 4 RR intervals. The criterion was fulfilled when the difference between the 2 averages (61% in this case) exceeded the programmed sudden onset (20% in this patient);
- the morphology criterion is not included in the discrimination; the difference in morphology during the tachycardia versus spontaneous rhythm is visible on both the sensing and the high-voltage (shock) channel;
- the tachycardia stabilized. The stability criterion is fulfilled (rhythm sensed as stable) when, for a ventricular cycle, the difference between one RR interval and one of the 3 preceding RR intervals is shorter than the programmed value (24 ms in this patient). For this episode, the criterion was fulfilled, as the difference between 1 cycle and the 3 preceding RR never exceeded the 24-ms threshold. The 7-ms value shown in the table corresponds to the average difference among the last 4 cycles before the VT classification;
- classification of the VT1 episode. The average RR interval during the initial classification corresponds to the average of the 4 cycles preceding the classification;
- delivery of a burst of ATP (not visible);
- termination of the arrhythmia.
Comments
Single chamber defibrillators cannot measure the atrial rate. Biotronik defibrillators identify the origin of arrhythmias by analyzing the sudden versus gradual onset of the tachycardia and the stability versus instability of the rhythm. While neither criterion alone guarantees a flawless discrimination of all tachycardias, their specificity increases when they are combined. Depending on their settings, the sensitivity and specificity of each measurement can be increased or decreased. In this example, the sudden onset and regularity of the rhythm favored VT. Likewise, the variation in the morphology of the ventricular electrograms (which are not included in the device’s discrimination algorithm) was in favor of VT.
