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Bradycardia
Essentials
- Identify physiological bradycardia and determine the underlying cause of symptomatic bradycardia.
- Stop (pause) medications that slow down the heart rate (digitalis, beta blockers, verapamil, diltiazem, cholinergic drugs for dementia) when symptomatic bradycardia is detected.
- Refer for further cardiac investigations all patients in whom bradycardia has caused disturbances of consciousness or other strong cardiac symptoms.
General
- Asymptomatic bradycardia in a healthy heart is benign and usually does not require further investigations or treatment.
- In cardiac diseases, bradycardia may cause faintness or disturbances of consciousness (presyncope), or lead to worsening of cardiac insufficiency.
- The cause of bradycardia (< 50 beats/min) may be benign sinus bradycardia, dysfunction of the sinus node or an atrioventricular conduction disturbance.
- An erroneous impression of bradycardia may be caused by a pulse deficit if the heart rate is assessed only by palpation of the pulse.
- In patients with atrial fibrillation or ectopic beats, the weakest beats are often not felt; for example, bigeminy at 80 bpm may be felt as only half of this rate by palpation.
Sinus bradycardia
- In sinus bradycardia, the heart rate is below 50 and the shape of the P wave is normal.
- Physiological conditions, general diseases or temporary or chronic heart disease can cause sinus bradycardia.
- Physical fitness, e.g. in endurance athletes
- Vagal reflexes associated with nausea, vomiting, coughing
- Medications (e.g., beta blockers, digoxin, verapamil, diltiazem, antiarrhythmic agents, drugs for dementia)
- Hypothyroidism
- Sinus bradycardia may cause, through a compensatory mechanism, extrasystolias. In such cases the treatment of extrasystolias with medication that slows down heart rate, such as beta blockers, may make the situation worse than it was.
Bradyarrhythmias
- There is a disturbance either in the activation of the sinus node (sinus arrest), in the conduction of the impulse to the atria (sinoatrial block), or in the conduction from atria to the ventricles (atrioventricular block).
- Sick sinus syndrome (SSS) manifests most often as sinus bradycardia, restrained physiological increase in the heart rate, breaks in the function of the sinus node Sick Sinus Syndrome and often also as tachyarrhythmias of atrial origin (so-called brady-tachy syndrome).
- The patients are sensitive to drugs that slow down the function of the sinoatrial node.
- The cause behind an atrioventricular conduction disorder (AV block) occurring in an adult is the degeneration or damage of the conduction pathway.
- In first-degree atrioventricular block, the PQ interval is prolonged (> 0.2 s). The block will not cause bradycardia if the sinus node functions normally.
- In Mobitz I (Wenckebach) type second degree atrioventricular block, the PQ interval is progressively prolonged until a single P-wave fails to conduct. It may occur during rest particularly at nighttime also in completely healthy persons without any symptoms.
- In Mobitz II type atrioventricular block, a P wave fails to conduct without the preceding prolongation. The prognosis is worse than in the block of Mobitz I type.
- In third degree atrioventricular block (complete block), the P waves are not conducted to the ventricles at all. The heart rate in complete block is 20 to 60 bpm depending on the origin of the escape rhythm.
- Certain syndromes and diseases are often associated with disturbances in the function of both the sinoatrial node and the atrioventricular node. These include, for example,
- vasodepressive syndrome
- hypersensitivity of the carotid sinus
- increased intracranial pressure
- hypothermia
- acute phase of a myocardial infarction, especially in posterobasal myocardial infarction.
Diagnostics
- The cornerstone in the diagnostics of bradycardia is an ECG recorded during symptoms.
- 12-lead ECG is sufficient if it can be acquired during symptoms.
- Holter monitoring is a good method for revealing functional disturbances of the sinoatrial node, and it may also reveal a momentary atrioventricular block.
- A normal Holter record does not exclude bradycardia, if there were no symptoms during the monitoring.
- An implantable ECG monitor may be indicated, if bradycardia is associated with disturbances of consciousness (see Syncope Syncope: Causes and Investigations).
- If bradycardia causes symptoms, an echocardiography should be performed once to rule out heart diseases and, if indicated, also an exercise stress test may be performed. An electrophysiological examination is rarely indicated because of bradycardia.
- Reflex bradycardia may be detected in a tilt test.
Treatment
- The rule of thumb is that symptomless bradycardia requires no treatment.
- In symptomatic bradycardia, the need for treatment depends on the primary diseases of the patient, on the haemodynamic disturbance caused by bradycardia, and on the subjective sensations of the patient.
- In mild functional disorders of the sinoatrial node Sick Sinus Syndrome and in first degree and, when occurring at rest, in Mobitz I type second degree atrioventricular block, the discontinuation of medications slowing down sinus frequency or the atrioventricular conduction is often sufficient treatment.
- In Mobitz type II second-degree atrioventricular block and in complete block the cause often is in the distal conduction pathway, and the disorder will not be corrected by discontinuation of medication that causes bradycardia.
- First aid in severe bradycardia
- Atropine 0.5 mg i.v., repeated at 5-min intervals, is the first-line therapy for acute bradycardia (caused by vagotonia).
- If needed, also isoprenaline-infusion can be used.
- The need for temporary pacing is assessed individually depending on the severity of the patient's symptoms Cardiac Pacemakers and Monitoring Their Function.
- All medications causing bradycardia are discontinued and in case of drug poisoning activated charcoal is administered.
- In digoxin poisoning, a specific antidote is used.
- The effect of calcium-channel blockers may be reversed with calcium gluconate.
- There is no effective long-term pharmaceutical treatment for symptomatic bradycardia, so the implantation of a permanent pacemaker is required.
- After the first aid, patients suffering from severe functional disorder of the sinoatrial node or from Mobitz II type or complete atrioventricular block are referred for more specific cardiologic examinations to assess the need of a permanent pacemaker Cardiac Pacemakers and Monitoring Their Function.
- A higher degree AV block is rare in previously healthy young persons, and it usually calls for careful aetiological investigations and e.g. exclusion of cardiac sarcoidosis Sarcoidosis.
- If permanent pacemaker treatment seems indicated, it is advisable to avoid temporary pacing but to directly implant the permanent pacemaker in order to prevent infections and other complications.