Pro-arrhythmogenic effect of a low output shock - Protecta XT CRT-D

Patient

Male implanted with a triple chamber defibrillator (Protecta XT CRT-D) for dilated cardiomyopathy.

Trace

1- What diagnosis is suggested by the interval plot?
The plot shows an episode of ventricular arrhythmia with acceleration into the VT zone; the first 3 bursts and the first 3 ramps do not terminate the arrhythmia; a low output shock (10 Joules) accelerates the arrhythmia into the VF zone; a maximum output shock terminates the arrhythmia.

2- What is your final diagnosis?
Initially there is a regular monomorphic ventricular tachycardia; the first 3 bursts (treatment 1) and the first 3 ramps (treatment 2) are ineffective; a first shock (treatment 3) accelerates the arrhythmia which degenerates into VF with extremely short cycles; a maximum output shock results in termination.

Take home message

The output of the first shock during VT can be programmed at maximum energy or at a lower energy (of the order of 10 to 15 Joules).
This trace shows the main drawback of programming a 10 Joule shock in the VT zone, namely the pro-arrhythmogenic risk.
Below a certain value, which varies depending on the patient and is directly linked to the defibrillation «threshold», not only can a shock prove ineffective, but it can also cause a monomorphic VT to accelerate and disorganise into a polymorphic arrhythmia, compromising the patient’s survival.
This tracing shows an extremely rapid, polymorphic arrhythmia induced by the first shock. Induced arrhythmias are often associated with very short ventricular cycle lengths; it is very rare to observe such rapid cycle lengths (in the order of 140 to 150 ms) in spontaneous arrhythmias.
There are a number of advantages to programming an initial shock of moderate output (10-15 Joules): 1) this output is very often sufficient to terminate an episode of VT; 2) the charge time for this output is very short, even if the few seconds saved compared to a maximum energy shock is not clinically important when the shock occurs after 3 burst sequences +/- 3 ramp sequences (more than one minute of arrhythmia); 3) battery consumption is lower for a shock at 10 Joules compared to 41 Joules, although battery wear is minimally affected if only a limited number of shocks are delivered
Other factors to be considered when programming shock output are: 1) despite the fact that most patients are still conscious during shocks delivered for VT, the pain due to the shock does not influence decisions surrounding the output of the first shock, since it is difficult to demonstrate a direct relationship between shock output and pain severity; 2) various studies have demonstrated the deleterious effects of device based shocks and their association with adverse outcomes; it therefore seems logical to assume a shock of 10-15 Joules may have fewer negative consequences than a shock of 41 Joules.

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