Low-amplitude shock in the VT zone

N° 18
Manufacturer Medtronic Device ICD Field Therapy

Patient implanted with a triple-chamber ICD (Protecta XT CRT-D) for ischemic cardiomyopathy; the recorded episode shows the limitations of the programming of a low-amplitude shock in the VT zone.

Graph and trace

The graph initially shows an 1:1 atrioventricular rhythm followed by a sudden acceleration of the ventricular rhythm in the VT zone; 3 bursts followed by 3 ramps are unsuccessful; an electrical shock of 10 Joules is delivered which does not terminate the arrhythmia but rather accelerates and disorganizes the latter; a maximum electrical shock of 35 Joules enables termination.

  1. the EGM reveals a ventricular tachycardia with atrioventricular dissociation;
  2. three successive bursts (fixed frequency) which prove unsuccessful;
  3. 3 ramps (increment between each stimulus; as can be seen, the pacing intervals cannot exceed 200 ms on the last pacing sequence (the last 4 stimuli are at fixed rate);
  4. the ensuing therapy is an electrical shock of 10 Joules;
  5. this small-amplitude shock is pro-arrhythmogenic with VF degradation (extremely rapid and polymorphic tachycardia);
  6. an electrical shock of 35 Joules is successful.

This tracing shows the main limitation of the programming of a 10 Joules shock in the VT zone and the pro-arrhythmogenic risk (concept of upper limit of vulnerability). Below a certain variable value depending on the patient and directly related to the "threshold" of defibrillation, a shock can not only be ineffective in terminating an arrhythmia but can also accelerate and disorganize a monomorphic VT into a polymorphic arrhythmia compromising the patient's prognosis in the short term. This tracing shows an extremely fast, polymorphic and very worrisome arrhythmia induced by the first shock. Induced arrhythmias are often associated with very short ventricular intervals; it is very rare to observe such rapid intervals (of the order of 140 to 150 ms) on spontaneous arrhythmias. This type of adverse reaction is relatively rare although constitutes a major limitation of this type of programming.