A shock to restore a viable rhythm - Claria Quad CRT-D

Patient

Male implanted with a triple-chamber defibrillator (Claria Quad CRT-D) for severe dilated cardiomyopathy; hospitalised for loss of consciousness with a shock from the device.

Trace

1- What diagnosis is suggested by the interval plot?
The plot shows an episode compatible with a ventricular arrhythmia detected in the VF zone; the shock is delivered after 15 seconds and seems to terminate the arrhythmia.

2- What is your final diagnosis?
The tracing shows a very rapid, polymorphic, ventricular arrhythmia (atrioventricular dissociation, ventricular rhythm faster than the atrial rhythm), detected in the VF zone; the initial VF counter is filled following 30 out of 40 cycles in the VF zone, triggering charging of the capacitors (VS markers); at the end of charging, there is confirmation of arrhythmia persistence (2 fast cycles out of 5) and the shock is delivered.

3-What is the effect of the shock?
The shock is effective and terminates the arrhythmia.

Take home message

For MedtronicTM defibrillators, a maximum of 6 shocks can be delivered for a single episode in the VF zone.
Although the output of each shock can be programmed independently, it is usual to program a maximum output shock (35 Joules) for shocks 2 to 6; on the other hand, the output of the first shock can either be programmed at the device’s maximum output, at a value 10 Joules lower (25 Joules), or at a lower output tested during an induction procedure.
Programming a medium-output first shock (between 15 and 20 Joules) reduces the charge time and the delay between the onset of arrhythmia and delivery of the shock, and may in certain specific cases, reduce the risk of loss of consciousness.
The choice of output for the first shock in the VF zone therefore represents a compromise: medium output may be sufficient to stop VF after a short charge time, but if this fails, the second shock of maximum energy will be delivered following a longer duration in VF; high energy from the outset is more effective for VF, but at the cost of a longer initial charge time.

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