Bradycardia

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Bradycardia is strictly defined in adults as a pulse rate below 60 beats per minute (bpm). However, few individuals are symptomatic unless the heart rate is below 50 bpm.[1] A resting bradycardia can be normal - for example, in trained athletes. Absolute bradycardia is defined as below 40 bpm. Physiologically, heart rate can vary in normal adults from 40 bpm up to 180 bpm. However, a relative bradycardia may be greater than 60 bpm if that rate is too slow for the haemodynamic requirements of the patient.

Children tend to have a higher resting pulse rate than adults and generally the smaller they are, the faster the heart rate. Hence, in a neonate, bradycardia may be defined as a rate below 100 bpm.

It is impossible to give meaningful figures on incidence and prevalence. In most young people bradycardia is physiological and represents athletic training. The incidence of pathological bradycardia rises with age as the underlying causes become more frequent.

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Bradycardia may be asymptomatic but can present with syncope, fatigue or dizziness. Ischaemic chest pain, Stokes-Adams attacks, hypoxic seizures, congestive heart failure, cardiovascular collapse and sudden cardiac death may occur, depending on the underlying cause of bradycardia.[2]

History

  • Is there any history of chest pain or collapse?
  • Is there any shortness of breath on exertion? It is important to ask about exercise and tolerance of exercise. An athlete with a resting bradycardia will raise heart rate and cardiac output to meet demand. Anyone who can sustain even moderate exercise without undue distress is most unlikely to have a pathological cause.
  • Is the patient aware of heart rate? Is it always slow or just at times? Are there any 'thumps' in the chest, extra or missed beats?
  • Is there a history of faintness, dizziness, nausea or chronic fatigue?
  • It is important to ask about medication, as drugs are an important cause of bradycardia.

Examination

  • Does the patient look well or are there signs of inadequate cardiac output - for example, cold peripheries, peripheral cyanosis or features of congestive cardiac failure?
  • Examination of jugular venous pressure (JVP) may reveal:
    • Elevation of the JVP in heart failure.
    • Cannon waves, which may be seen in complete atrioventricular (AV) dissociation. This is an intermittent very high jugular pulse as the atrium contracts against a closed tricuspid valve.
  • Careful examination of the pulse and count for at least 30 seconds (the slower the rate, the longer the duration). An athlete may show sinus arrhythmia but this is rare beyond 30.
  • Is there any shift in the cardiac apex?

Sinus bradycardia may be physiological with a high resting vagal tone, eg in athletes or the diving reflex.

Sinus bradycardia

The causes of a pathological sinus bradycardia include:

Ectopic atrial rhythm or wandering atrial pacemaker

  • Ectopic atrial foci, if ≥3 foci = wandering atrial pacemaker.
  • Different P-wave morphologies ± PR duration variations.
  • Not normally clinically significant.

Sinoatrial (SA) block or sinus exit block

  • SA node fails to produce an impulse or not conducted to atria.
  • ECG typically shows absent P waves with escape rhythm.
    • Junctional - narrow complexes at 45-60 bpm.
    • Ventricular - wide complexes at 30-45 bpm.
  • Usual causes:
    • Ischaemia.
    • Hyperkalaemia.
    • Excess vagal tone.
    • Negative chronotropes.
  • Treat if symptomatic.

Sick sinus syndrome

See also separate Sick Sinus Syndrome article.

  • Tachycardia-bradycardia syndrome (characterised by bursts of atrial tachycardia interspersed with periods of bradycardia; paroxysmal atrial flutter or fibrillation may also occur).[2]
  • Causes:
  • ECG may show SA block, sinus or atrial bradycardia with bursts of atrial tachycardia (usually atrial fibrillation).
  • Management:
    • Treat any symptomatic acute arrhythmia.
    • Most require a permanent pacemaker for the bradycardia component ± anti-arrhythmic medication (eg digoxin or verapamil) to suppress tachycardias.

AV blocks

  • With all AV blocks additionally rule out:
  • First-degree:
    • Slow AV node conduction.
    • ECG: PR interval >200 ms.
    • All atrial impulses conducted to ventricles.
    • Benign.
  • Second-degree:
    Some atrial impulses are not conducted to ventricles.
    • Mobitz type I (Wenckebach):
      • AV node conduction defect.
      • ECG: repeated lengthening of PR until a P is not followed by a QRS complex.
      • Usually asymptomatic and treatment not required.
      • Treat if symptomatic or in the context of inferior myocardial infarction.
    • Mobitz type II:
      • Conduction defect below AV node.
      • Degenerative: Lev's disease (Lenègre-Lev syndrome), which is an acquired complete heart block due to idiopathic fibrosis and calcification of the electrical conduction system.
      • ECG: constant PR with intermittent QRS absence.
      • Risk of progression to third-degree block.
      • Atropine ineffective as block below AV node.
      • Permanent pacing is required if symptomatic.
    • 2:1 block:
      • ECG: 2 P waves for every QRS complex.
      • May occur in digoxin toxicity or ischaemia.
      • Needs further electrophysiological tests to determine treatment.
  • Third-degree:
    • AV dissociation.
    • Atrial impulses not conducted to ventricles.
    • ECG: both P waves and QRS escape complexes may be present, but occur independently.
    • Treat as below, although atropine is unlikely to be effective and a permanent pacemaker will be required.
    • Myocardial fibrosis is the most common cause.
    • Associations:
      • Inferior acute myocardial infarction.
      • Sick sinus syndrome.
      • Mobitz type II.
      • Second-degree block plus new bundle branch or fascicular block.

Consider:

  • A 12-lead ECG will indicate the true nature of the rhythm and possibly show ischaemic changes, myocardial infarction and other conduction defects like bundle branch block.
  • If the bradycardia is variable, ambulatory ECG monitoring may be necessary.
  • Ischaemic heart disease may require investigation.
  • Electrolytes, glucose, calcium, magnesium, thyroid function tests, and toxicology to exclude causes as above.
  • Rule out and treat any underlying causes of sinus bradycardia.
  • Treatment is indicated if significantly symptomatic (syncope, hypotension, cardiac failure).
  • Diagnose and treat myocardial ischaemia.
  • If symptomatic (often not until heart rate <40) treat as below.

Initial management

  • Resuscitation; see also the Resuscitation Council's website, listed under Document references below, for Bradycardia Algorithm.[3]
  • Intravenous access with cannula insertion.
  • Blood tests:
    • FBC.
    • U&Es.
    • LFTs, including albumin and total protein.
    • Blood glucose.
    • Calcium.
    • Creatinine.
    • Cardiac enzymes (troponin).
    • TFTs.
    • Digoxin level if appropriate.
  • ECG.
  • Treat the underlying cause if present (cease negative chronotrope, correct electrolytes, etc.).
  • Treat bradycardia only if there are signs and symptoms:
    • Syncope.
    • Systolic blood pressure <90 mm Hg.
    • Heart rate <40 bpm.
    • Ventricular arrhythmias requiring suppression.
    • Heart failure.
    • Reduced consciousness or cognitive function.
  • Drugs:
    • Atropine: 0.5-1.0 mg (0.02 mg/kg, minimum of 0.1 mg in a child) repeated up to 2 mg.
      Only useful if increased vagal tone, ie problem above AV node.
    • Isoprenaline: bolus 20-40 micrograms IV, infusion 0.5 micrograms/minute of 2 mg/100 ml normal saline.
      Caution: as a pure beta agonist, can cause beta-2 vasodilatation in muscle beds, leading to hypotension.
    • Adrenaline: infusion 2-10 micrograms/minute (ideally needs a central line); useful if hypotension is an issue.
    • Others:
      • Dopamine: 5-20 micrograms/kg/minute through central line.
      • Glucagon (betablocker or calcium-channel overdose).
      • Digoxin-specific antibody fragments (Digibind®) for digoxin toxicity.
  • Pacing may be required if drug therapy fails:
  • With excessive vagal tone it can be corrected either by ceasing the activity that caused excessive vagal tone or by giving a drug such as atropine.[3]
  • Hypothyroidism, hypothermia, raised intracranial pressure, etc. need treatment of the underlying condition.
  • Infective causes usually recover spontaneously.
  • Where heart block or sick sinus is the cause, implantation of a pacemaker may be required.
  • Appropriate treatment of any underlying cause, eg hypothyroidism.
  • Glycopyrrolate (antimuscarinic agent) is used in anaesthesia to prevent the bradycardia induced by cholinergic drugs such as the anticholinesterases.

Further reading & references

  1. Livingston MW et al; Sinus Bradycardia, eMedicine, Aug 2009
  2. Da Costa D, Brady WJ, Edhouse J; Bradycardias and atrioventricular conduction block. BMJ. 2002 Mar 2;324(7336):535-8.
  3. Adult bradycardia algorithm, Resuscitation Council UK (2010)

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Huw Thomas, Dr Richard Draper
Current Version:
Document ID:
621 (v22)
Last Checked:
18/02/2011
Next Review:
17/02/2016