Electrical cardioversion, defibrillation and dual-chamber pacing
THE CONTEXT
Electrical cardioversion terminates arrhythmias by depolarising the myocardium. Defibrillation is primarily used for cardiac arrest due to ventricular fibrillation.
Electrical cardioversion
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Direct current (DC) cardioversion depolarises the myocardium, interrupts the arrhythmia and produces brief asystole followed by resumption of sinus rhythm.
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It is usually performed electively under general anaesthesia, using a 100–150 Joule shock.
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The shock is delivered immediately after the R wave. Delivery during ventricular repolarisation, on the T wave, may provoke ventricular fibrillation (VF).
Defibrillation
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Defibrillators deliver a high-energy unsynchronised shock for cardiac arrest due to VF, when precise discharge timing is not important.
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A biphasic shock reverses polarity midway through the shock, reducing the total energy required to depolarise the heart.
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For VF and other emergencies, the first and second shocks should be 150 Joules, followed by shocks of up to 200 Joules.
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Anaesthesia is unnecessary in this setting because the patient is unconscious.
Dual-chamber pacing
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Atrial and ventricular pacing produces narrow pacing spikes before the P wave and QRS complex.
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When spontaneous P waves occur, the pacemaker can sense or track them and pace the ventricle after an appropriate interval to maintain atrioventricular synchrony.