Low-amplitude shock and acceleration of arrhythmia - Incepta CRT

• the amplitude of the first shock in the VT zone can be programmed at maximum energy or at a lower amplitude (in the order of 10 to 15 Joules)
• there are a number of advantages to programming a first shock of moderate amplitude (10-15 Joules): 1) this amplitude is very often sufficient to terminate a VT episode; 2) the charge time for this amplitude is very short, even if the few seconds difference in comparison to a maximum amplitude shock may not be clinically important when the shock occurs after 3 burst +/- 3 ramp sequences (more than one minute of arrhythmia); 3) power consumption is lower for a shock at 10 Joules versus 41 Joules, even if battery usage is only slightly affected if a limited number of shocks are delivered; 4) even though for most VT episodes the shock is delivered while the patient is still conscious, the painful nature of the shock contributes minimally to decision-making regarding the amplitude of the first shock, since it is difficult to demonstrate a direct relationship between the amplitude of the shock delivered and the amplitude of the pain caused; 5) various studies have demonstrated the deleterious nature of an electric shock and its association with an adverse prognosis ; it therefore seems logical to think that a 10-15 Joule shock will have fewer negative consequences than a 41 Joule shock, and it seems advisable to choose the least traumatic therapy possible.
• this tracing shows the main drawback of programming a low-amplitude shock (11 Joules) in the VT zone: pro-arrhythmogenic risk and acceleration of the arrhythmia (concept of upper limit of vulnerability); in this patient, the shock has the opposite effect to what was intended; below a certain value, which varies according to the patient and is directly linked to the defibrillation «threshold», not only can a shock prove ineffective in terminating an arrhythmia, but it can also accelerate and cause degeneration of a monomorphic VT into a polymorphic arrhythmia, compromising the patient’s short-term prognosis.

This figure shows how a medium-amplitude shock accelerates a monomorphic VT at 160 beats/minute into a very rapid ventricular flutter.