If there is imminent cardiac arrest, call the arrest team and manage along standard lines (see Chapter 6).

If there is a reduced level of consciousness, severe pulmonary oedema, or the systolic BP is <90 mmHg:

• Record a 12-lead ECG (if possible) for later analysis.
• If the heart rate is >150/min, call an anaesthetist in preparation for DC cardioversion starting at 200J (Chapter 121).
• If the heart rate is <40/min, give atropine 0.6–1.2 mg IV, with further doses at 5-min intervals up to a total dose of 3 mg if the heart rate remains below 60/min. If the bradycardia is unresponsive or recurs, use an external cardiac pacing system or put in a temporary transvenous pacemaker (Chapter 119).

If the patient is haemodynamically stable, there is time to make a working diagnosis and plan management. Clinical assessment in summarized in Table 39.1 and urgent investigation in Table 39.2. Record a 12-lead electrocardiogram and a long rhythm strip. Further management is determined by the type of arrhythmia.

Regular broad complex tachycardia (see Chapter40):

• The diagnosis is usually ventricular tachycardia (VT). Haemodynamic stability does not exclude VT.
• If there is ischaemic heart disease or cardiomyopathy, the diagnosis is virtually always VT.
• Suspect diagnoses other than VT in young patients (age <40 years), or with known Wolff-Parkinson-White or bundle branch block. If there is doubt, assess the effect of adenosine (Table 42.3).

Irregular broad complex tachycardia (see Chapter41):

• This is likely to be atrial fibrillation with bundle branch block or, less commonly, pre-excited atrial fibrillation (Figure 41.1). The difference between the maximum and minimum instantaneous heart rates calculated from the shortest and longest RR intervals is usually >30/min.
• The differential diagnosis is polymorphic ventricular tachycardia. This is usually due to therapy with anti-arrhythmic and other drugs which prolong the QT interval (e.g. amiodarone, sotalol), especially in patients with hypokalaemia or hypomagnesaemia.

Regular narrow complex tachycardia (see Chapter42):

• The differential diagnosis is given in Table 39.4.
• Vagotonic manoeuvres increase AV block and may terminate the arrhythmia if it involves the AV node, or reveal atrial activity. Ask the patient to perform a Valsalva manoeuvre (attempting to blow the plunger from a 10 mL syringe, while semi-recumbent, is an effective method of generating the necessary intrathoracic pressure) or try carotid sinus massage. If vagotonic manoeuvres do not restore sinus rhythm, give adenosine (Table 42.3).
• Suspect atrial flutter with 2:1AV conduction rather than sinus tachycardia if the rate is around 150/min.
• Suspect atrial tachycardia or junctional tachycardia if digoxin toxicity is possible. Digoxin toxicity is likely if plasma digoxin level is >3.0ng/mL (>3.8nmol/L), especially if there is hypokalaemia (<3.5 mmol/L), hypomagnesaemia or hypercalcaemia. Systemic features include nausea, vomiting, diarrhoea and delirium.

Irregular narrow complex tachycardia (see Chapters42 and 43)

• The diagnosis is usually atrial fibrillation (AF).
• Other possibilities are sinus rhythm with frequent supraventricular extrasystoles or multifocal atrial tachycardia (the rhythm looks half-way between sinus and AF).

Bradycardia (rate <60/min) (see Chapter44)

• The differential diagnosis is given in Table 39.3. Look carefully at the PR interval and the relationship between the P wave and QRS complex
• A regular ventricular rate <50/min in a patient with atrial fibrillation indicates complete heart block (not ‘slow AF’); always consider digoxin toxicity.