• Conversion of ventricular and supraventricular tachyarrhythmias

Contraindications

• When another treatment is better (e.g. pharmacological cardioversion) or there is acceptance of supraventricular arrhythmia with rate control. Seek advice from a cardiologist about the management of haemodynamically stable tachyarrhythmias before cardioversion.
• Digoxin toxicity
• Hypokalaemia (plasma potassium <3.5 mmol/L)
• Thyrotoxicosis if in atrial fibrillation (cardioversion unlikely to be successful without correction of thyrotoxicosis)

Potential complications

• Tachyarrhythmias:
• Ventricular: non-sustained VT (5%); sustained VT; VF (especially if the shock is delivered during the vulnerable phase of the cardiac cycle: its delivery should be synchronized with the QRS complex to avoid this)
• Atrial: SVT (30%), sinus tachycardia, AVNRT, atrial flutter
• Bradyarrhythmias (0.9%) – transient LBBB, high degree AV block, asystole. These may uncommonly require atropine or temporary pacing whether externally or transvenous. Risk factors: antiarrhythmic drugs.
• ST elevation and T wave changes. These are non-specific and by themselves do not indicate an acute coronary syndrome.
• Interference with settings of permanent pacemakers and implantable cardioverter-defibrillators (these should be checked post-cardioversion)
• Thromboembolism – pulmonary or systemic. More likely in atrial fibrillation or flutter. Incidence 5.3% if not anticoagulated, <1% if adequately anticoagulated (see Table 121.3). The cause can be dislodgement of exiting thrombus or more usually de novo thrombus due to atrial stunning.
• Complications of general anaesthesia/sedation
• Skin burns from shocks – consider prophylactic topical hydrocortisone or topical NSAID
• Myocardial necrosis – minimal necrosis, typically with higher energy levels. Usually asymptomatic with mild troponin or CK-MB rise.
• Myocardial dysfunction – global LVSD (stunning), atrial stunning
• Pulmonary oedema (rarely), transient hypotension (often fluid responsive)