Long R-R intervals (prevented by pacing) may be associated with escape ventricular tachycardia (often heralded by frequent ventricular ectopic beats). Treatment
Pacing may treat re-entrant tachyarrhythmias, involving adjacent pathways. Insertion of an electrical stimulus into the circuit ('capture') may extinguish the arrhythmia. Capture requires the cardiac cycle to be scanned with electrical impulses in one of three ways.
1. Underdrive pacing, which is best employed when cycle frequency is below 150 beats/min. With pacing slightly slower than the native tachycardia, a single extra stimulus may 'break into' the tachycardia in 50 per cent of cases.
2. Single stimulus: the pacing box inserts (on demand) a single stimulus 10 per cent earlier than the anticipated next beat (i.e. 90 per cent of the R-R interval or 'cycle length'), and at progressively earlier stages if unsuccessful.
3. Overdrive pacing: this is generally better with tachycardias over 150 beats/min. Pacing slightly faster than the native tachycardia is more successful than underdriving by reducing the refractory period of the tissue between the pacing wire and the re-entry circuit, thus helping the impulse to 'break into' the circuit. However, it is more likely to accelerate the tachycardia. If specialist equipment is used, each burst may terminate with an extra beat of slightly shorter cycle length.
Antitachycardia pacing, for which an external defibrillator must be readily available, may be used to treat ventricular tachycardia, atrioventricular nodal re-entrant tachycardia, atrioventricular re-entrant tachycardia, and atrial flutter. Pacing is applied to the affected chamber, although overdrive atrial pacing can be used to treat 'slow' ventricular tachycardia, with burst pacing delivered until a ventricular capture beat is seen. Incremental rates or repetition after atropine (to improve atrioventricular conduction if high pacing rates are needed) can be used. In terminating supraventricular tachyarrhythmias, rapid pacing (400-800 beats/min) can be used to cause atrial fibrillation which usually reverts rapidly to sinus rhythm.
Antitachycardia pacing is indicated under the following circumstances.
1. The arrhythmia is due to a drug effect (e.g. antidepressants/antidysrhythmics), and adding further pharmacological agents may have unpredictable effects.
2. The arrhythmia is recurrent, necessitating repeated external cardioversion.
3. The arrhythmia is torsade de pointes.
4. The arrhythmia is still present despite pharmcotherapy.
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