Sinus Bradycardia

In sinus bradycardia the heart rate is below 60 bpm and there is a P wave before every QRS complex. The PR interval is constant. Causes are listed on the Context page.

Clinical Features

• Sinus bradycardia may be normal in healthy young adults, especially athletes.
• It may also be normal in those taking an appropriate dose of beta-blockers. In these cases, patients will be asymptomatic.
• It is considered pathological if it results in decreased cardiac output causing any or a combination of the following symptoms:
  • Pre-syncope
  • Syncope
  • Nausea
  • Breathlessness
  • Weakness or fatigue

Management [1]

In the absence of hypoxia and/or shock, the first line treatment for all symptomatic bradycardias that are unlikely to resolve spontaneously and expeditiously (e.g. in the setting of posterior or inferior MI or vagal syncope) is intravenous atropine 0.5 mg, initially. Consideration must also be given to the underlying cause.

Some specific causes of sinus bradycardia:

Hypothermia

A specific cause of sinus bradycardia which must not be overlooked in the elderly is hypothermia. The ECG may contain ‘J waves’ (see image). ‘J’ waves occur immediately after the QRS complex; their size may be proportional to the degree of hypothermia and they resolve with successful rewarming.

This ECG shows a hypothermic patient demonstrating ‘J waves’ Click the image to see a larger version.

Drugs [2]

Deliberate or iatrogenic drug overdose with AV nodal blocking agents, digoxin, organophosphates and other agents may require pharmacological treatment in addition to pacing. Individual drugs may cause other symptoms and signs (see below and the relevant toxicology sessions).

Beta Blockers

Atropine is unlikely to work.  Beta agonists such as adrenaline and isoprenaline are effective as infusions. Adrenaline is preferred due to its short half-life. There is anecdotal evidence for the use of high dose intravenous glucagon.

Calcium channel antagonists

Again atropine is unlikely to be effective. Intravenous fluid loading and intravenous calcium are the mainstays of treatment followed by an adrenaline infusion if required.

Digoxin

Normally causes a slowing of pre-existing atrial fibrillation. Occasionally a regularisation of the rhythm occurs (similar to a nodal bradycardia). Toxicity is worsened by hypokalemia which should be corrected if present. In severe cases an antidote is available (Fab fragments of digoxin antibodies).

Organophosphates

Bradycardia is part of a cholinergic syndrome. Large doses of Atropine are required titrated to a point when the patients mouth is dry.

Head injury

Significant head injury with raised intracranial pressure is associated with sinus bradycardia and raised blood pressure (Cushings Response); it is an ominous sign and is caused by brain stem compression.

Learning bite

Bradycardia and hypertension in a patient with a head injury is an ominous sign and is associated with significantly raised intracranial pressure.