Bradycardias and Heart Blocks

Although a bradycardia is classically defined as a heart rate of less than 60 bpm, in the context of a failing left ventricle, a faster bradycardia than this may require treatment. Bradycardias may have an underlying cause such as hypothermia, severe head injury or poisoning. All patients should be given oxygen and IV access is needed (Figure RS. 10).

For the purposes of treatment sinus and junctional bradycardias (this includes first degree AV block) and Mobitz type I second degree AV block (Wenckebach phenomenon) are considered together (see below).

Sinus Bradycardia and Atrioventricular Block: Second Degree

Both rhythms are usually manifestations of increased vagal tone, though Mobitz type I block may be associated with inferior myocardial infarction and is often temporary. Patients with either of these forms of bradycardia should be assessed and if the rate is less than 40 bpm, there is haemodynamic compromise, or ventricular escape beats are seen, then atropine should be given (0.5 mg IV) and repeated until there is a response or a total of 3 mg has been injected. If symptomatic bradyarrhythmias persist then further measures are required. If the presenting bradycardia is diagnosed as being Mobitz type II (second degree) AV block or 2:1 AV block or there is complete (third degree) heart block with a broad QRS complex, there is a risk of asystole. Atropine should again be given, but only as a holding measure while help is sought for placement of a transvenous pacing wire. If the clinical situation deteriorates, external pacing is indicated. In the absence of such equipment, an isoprenaline infusion should be started but this may compromise an ischaemic left ventricle and increase infarct size because of the effect of isoprenaline in increasing myocardial oxygen demand.

Figure RS.10

Algorithm for bradycardia and heart blocks © ERC 96

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