48. SINUS BRADYCARDIA

Sinus bradycardia is a heart rate less than 60 beat per minute in an adult. Common causes include increased vagal tone (traction on the eye or peritoneum, laparoscopy), drugs (narcotics, beta adrenergic blockers, calcium channel blockers, halothane, repeated doses of suxamethonium), hypoxaemia, hypothermia, hypothyroidism, disease of the sinus node (sick sinus syndrome), high spinal or epidural block and congenital heart block.

Sinus bradycardia may be well tolerated if it develops slowly. Sinus bradycardia that occurs suddenly may cause symptoms. With all patients, the anaesthetist must check that the patient is receiving oxygen and is ventilating well. Bradycardia is common in hypoxaemic arrest. Verify the bradycardia and assess its haemodynamic significance. (Check the blood pressure and feel a peripheral pulse)

Management

If the sinus bradycardia is not associated with any symptoms, monitor the patient closely. Look for and treat any cause of sinus bradycardia. Bradycardia during spinal or epidural anaesthesia should be treated even if the patient is asymptomatic.

If the sinus bradycardia is associated with minor symptoms (small decrease in blood pressure, nausea, vomiting, mild change in conscious state), treat the bradycardia and the cause. Initial drug treatment is atropine 0.5 mg repeated doses to a total of 3 mg. Other alternative drugs include adrenaline (epinephine), isoprenaline and ephedrine.

If the sinus bradycardia is associated with severe symptoms (severe hypotension, loss of consciousness, seizures), call for help, ensure the patient is receiving 100% oxygen, is ventilating well and that all anaesthetic drugs are turned off. Give adrenaline (epinephine) 0.1 mg repeated doses. If the bradycardia fails to respond to repeated adrenaline doses, consider giving isoprenaline or using transcutaneous cardiac pacing if available.

Sinus bradycardia due to a first-degree block or mobitz type 1 second-degree block is rarely symptomatic.

With a mobitz type one block there is a progressive increase in the delay between the P and the QRS complex, until a QRS complex is missed.

A mobitz type 2 second degree block is usually caused by myocardial infarction or chonic degeneration of the A-V conduction system and can progress unexpectedly to a third degree block. With a mobitz type 2 block there is intermittent failure of AV conduction with the loss of a QRS complex, without a progressive increase in the delay between the P and QRS complex.

With a third degree block there is total failure of the AV conduction. This is an unstable rhythm that is associated with severe bradycardia and periods of ventricular asystole.

Sick sinus syndrome shows alternating bradycardia and tachycardia. There may be periods of severe bradycardia or sinus arrest which may alternate with periods of supraventricular tachycardia (SVT) or AF. It usually occurs in elderly patients with ischaemic heart disease and may be precipitated by anaesthesia. Treatment requires cardiac pacing.