Symptomatic bradycardia

What it is

Bradycardia with a pulse is assessed for its appropriateness to the clinical condition. The heart rate is typically under 50/min if there is a bradyarrhythmia, and what turns a slow rate into an emergency is not the rate itself but the harm it is causing.

The common bradycardias:

• Sinus bradycardia: a sinus rhythm under 60 beats/min with the SA node still the pacemaker, so the TP interval is prolonged and all waves are visible.
• Escape rhythms, from sinus arrest or exit block: SA node failure with no P wave, so an AV nodal escape rhythm takes over. The AV node rate is 40-60 bpm, against 90-100 bpm for the SA node.
• Conduction blocks: impaired AV conduction, often due to IHD, and often with an escape rhythm.

Causes and risk

Causes of sinus bradycardia:

• Non-pharmacological: normal during sleep, increased vagal tone such as in athletes, hypothyroidism.
• Pharmacological: beta-blockers, calcium-channel blockers, digoxin.

Sinus bradycardia is also normal at rest, in sleep and in elite athletes, and is pathological when SA node firing is reduced, as in IHD or old age, or when there is cardiomyopathy.

Donepezil, a cholinesterase inhibitor, is worth its own note. Through its cholinergic effect these medications have a vagotonic effect on the sinoatrial and atrioventricular nodes, and can cause bradycardia or heart block in patients with or without underlying cardiac conduction abnormalities. Syncope has been reported, and donepezil should be avoided in patients with syncope. Memantine, escitalopram, lisinopril and zolpidem are not associated with bradycardia.

Bradycardia is also one of the arrhythmias that complicate STEMI, along with AF, FV and AV block.