Anti-tachycardia pacing- A report

Re-entrant tachycardias are available vulnerable to ATP when the re-entry circuit is anatomically defined and there is an excitable gap between a circulating wave front and there is an excitable gap between a circulating wave front and its wake of refractoriness. Given these conditions, it is possible for a stimulated impulse to invade the circuit and create a bidirectional block. This will occur if the invading impulse proceeds in both directions in the circuit, clockwise and counter clockwise. In one directions, the invading impulse collides with and extinguishes the re-entering wave front , in the other direction, it is itself extinguished when it runs into the wake of refractoriness of the re-entering impulse. ATP cannot terminate VF or polymorphic VT. The usual technique of ATP is to deliver a train of stimuli at a somewhat faster rate than that of the tachycardia. A train of stimuli is more likely than a single stimulus to produce an impulse that penetrates the re-entrant circuit. With trains of stimuli, however, there is a risk that one stimulus will terminate a tachycardia and a subsequent one will restart the tachycardia. It may be necessary for the consultant to deliver literally dozens of trains of stimuli of different rates and durations, until finally a train is delivered that fortuitously ends with a pulse that extinguishes the tachycardia rather that re-inducing it. This process may take from ten seconds to a minute or more. ATP is usually used for VT’s that do not cause immediate hemodynamic compromise. Also, ATP often causes VT to accelerate or even degenerate into VF. For this reason, ATP must be used only when back up electrical defibrillation is available. The typical indication for ATP is for treatment of a recurrent re-entrant sustained VT which is refractory to drugs and which does not cause immediate hemodynamic compromise ATP would be used in preference to electrical cardio version, since patients much better tolerate the former. Most ICD’s have the capacity to deliver ATP as first line therapy for VT, thereby reducing the frequency of shocks. Direct current cardio version has the broadest spectrum of anti-arrhythmic activity of any therapy.