• there are 3 reasons why a shock may be delivered following the detection of an arrhythmia in the VT zone: 1) the electric shock is the first therapy programmed in the VT zone; 2) previous therapies (anti-tachycardia stimulation bursts) have proved ineffective; 3) an anti-tachycardia pacing burst accelerates the tachycardia into the VF zone, as in this example.
• acceleration of VT and degeneration into VF are well-described complications following an anti-tachycardia pacing sequence, with an incidence of approximately 1 to 5% ; this justifies the absolute necessity of programming shocks as a back-up in the VF zone; the efficacy and safety of anti-tachycardia pacing are inversely related; a greater number of tachycardias can be terminated with a more aggressive protocol (shorter coupling intervals, more cycles per sequence, more sequences) but with an increased risk of acceleration
• anti-tachycardia pacing can 1) accelerate a tachycardia with the same morphology as the previous one, 2) accelerate a tachycardia with a different morphology, or 3) cause an organized tachycardia to degenerate into VF.

This figure shows how the anti-tachycardia pacing sequence accelerates a monomorphic VT at 200 beats/minute into a very rapid ventricular flutter and then into VF.