Bradyarrhythmias

Bradyarrhythmias are abnormal heart rhythms with a pulse rate of <60 beats per minute (bpm). This can be due to a variety of causes, including sinus node disease, atrioventricular disease, or toxins.

Although the definition of bradyarrhythmia (brady- = slow, arrhythmia = abnormal rhythm) is defined as a heart rate (HR) <60bpm, many patients remain asymptomatic until the HR falls well below this rate.

A bradyarrhythmia can either be physiological or pathological.

*Throughout this article, bradycardia (brady- = slow, cardia = heart) will be used synonymously and interchangeably with bradyarrhythmia.

Causes

Physiological Causes

In the normal heart, the resting heart rate (HR) ranges from 60-100 bpm, and this is determined by the pacemaker cells of the sinoatrial node (SAN), and the confluence of the sympathetic and the parasympathetic nervous systems – therefore, in normal physiology, an increase in vagal tone, or a decrease in sympathetic outflow, will cause a drive towards sinus bradycardia (a HR <60 bpm that is determined by the SAN).

Examples of physiological causes of an increase in vagal tone include:

• The diving reflex
• Athlete’s heart
• Vagal manoeuvres
• Valsalva manoeuvre
• Respiratory-induced sinus arrhythmia (expiration phase)

Pathological Causes

Aside from physiological causes of bradycardia, there are multiple pathological causes of bradycardia – these can be subdivided into cardiac, and non-cardiac causes.

Cardiac causes

Cardiac causes of bradyarrhythmias are usually inherent to abnormalities with impulse generation (i.e., sinus arrest) or signal conduction (i.e., complete heart block). The cardiac causes below will be explored in more detail in future articles, but the general risk factors for both SAN and AVN disease include cardiac ischaemia, myocarditis, and idiopathic fibrosis / sclerosis1.

Sinoatrial node causesAtrioventricular node causesOther
Sinus pause / arrestSecond degree AV block – Mobitz IIdioventricular rhythm
Sinoatrial exit blockSecond degree AV block – Mobitz IICardiac ischaemia
Sick sinus syndromeThird-degree AV blockMyocarditis
 Junctional rhythmIdiopathic fibrosis / sclerosis
 AV block in the context of atrial arrhythmia (i.e., atrial flutter with variable AV block) 
Non-cardiac causes

There is a range of non-cardiac causes of bradycardia, and, although not exhaustive, the mnemonic below can be used as a useful memory tool:

ABCDEFG HIT

Non-cardiac causesExamples
AutoimmuneSystemic lupus erythematosus2
Beta-blockersBisoprolol, Esmolol, Atenolol, Metoprolol, Propanolol
Calcium-channel blockersNon-dihydropyridines – verapamil, diltiazem
Drugs (other)Digoxin, alpha-2 agonists, amiodarone, GABA-receptor agonists / modulators
EndocrineHypothyroidism
Funny current inhibitorsIvabradine
GlucoseHypoglycaemia
Hyper/hypo –Hyperkalaemia, hypokalaemia, hypoxia, hypothermia
InfectionLyme disease, Salmonella typhi, Legionnaire’s disease
TraumaRaised intracranial pressure (Cushing’s reflex)

History

When taking a history, it is important to be comprehensive and to proceed in a structured manner, asking through the presenting complaint to the systems review. Focused features to always include in any cardiac history include:

Chest pain*
• Shortness of breath*
• Dizziness / syncope / light-headedness*
Palpitations*
• Smoking and alcohol
• Hypercholesterolaemia

*The above symptoms would be more concerning for cardiac disease if they occurred in conjunction with exercise.

Symptoms

Physiological bradycardia does not always cause symptoms, and patients are commonly asymptomatic. In pathological states, bradycardia alone is unlikely to be a patient’s presenting complaint, and is likely to be manifest as a sign of another underlying issue – however, if a patient is inappropriately bradycardic, there is the risk of under perfusing muscle groups and vital organs, and we could expect this to present as a variety of symptoms:

• Confusion
• Dizziness and light-headedness
• Difficulty thinking/memory deficit
• Chest pain/tightness
• Collapse/syncope
• Fatigue/tiredness
• Dyspnoea and shortness of breath

Examination – Signs

Examination of a patient with bradycardia should bear in the mind the concerns related to reduced cardiac output, and potential precipitating causes. Signs elicited may coincide with the symptoms a patient is experiencing. Common signs may include3:

• Raised jugular venous pressure
• Cannon-a waves in the jugular venous pulse (JVP)
• Peripheral cyanosis / Pallor
• Thyroid mass/goitre
• Hypothermia
• Features of increased intracranial pressure (associated with Cushing’s Triad)

Investigations

Diagnostic investigations for bradycardia, alongside a focused history and examination, should include4:

1st line investigationsFurther investigations
12 lead ECGEvent recorder / loop recorder
Full blood screen*Exercise testing
24hr Holter monitorTilt table testing
EchocardiogramLyme titres and urinary antigen test
Chest X-rayElectrophysiology testing

*In this context, a full blood screen would include an FBC, U+E, CRP, TFT, troponin, magnesium, glucose, and toxicology screens and drug levels (where suspected, i.e., digoxin)

Management

The management of bradycardia should first be determined by the clinical status of the patient. In any patient with haemodynamically instability, they should be managed as part of the Resuscitation Council UK’s guidelines5, as seen further below under ‘Life-threatening bradyarrhythmias’.

If a patient is haemodynamically stable, it should be remembered that bradycardia is usually a symptom of an underlying condition, and treatment will usually be guided by the underlying aetiology, where a pathological or iatrogenic cause is suspected (i.e., negatively chronotropic drugs). For example, if a patient appears to be bradycardic secondary to hypothyroidism in the absence of other causes, replacing thyroid hormones would be expected to normalise the heart rate. Alternatively, if a patient becomes bradycardic intraoperatively, secondary to sedation and anaesthesia, they can be treated with glycopyrrolate.

It is important to involve senior support early in patient management, especially if a cardiac cause is suspected, as this would need to be escalated to cardiology.

Life-threatening bradyarrhythmias

The Resuscitation Council UK Guidelines 2021 highlight that the management of bradycardia should be determined by ‘evidence of life-threatening signs’ (shock, syncope, myocardial ischemia, or heart failure), and also the ‘risk of asystole’.

As with any patient examination, an A to E approach should be used when assessing the patient with bradycardia – this will help identify any concerning features, and thus determine the appropriate management. The Resuscitation Council UK’s algorithm for managing life-threatening bradycardia can be seen below:

Resus Bradyarrthymia Guideline

Image obtained from Resuscitation Council UK via the URL: https://www.resus.org.uk/sites/default/files/2021-04/Bradycardia%20Algorithm%202021.pdf

References

  1. https://patient.info/doctor/bradycardia
  2. https://litfl.com/bradycardia-ddx/
  3. https://bestpractice.bmj.com/topics/en-gb/832/history-exam#keyFactors
  4. https://bestpractice.bmj.com/topics/en-gb/832/investigations#firstOrder
  5. https://www.resus.org.uk/library/2021-resuscitation-guidelines/adult-advanced-life-support-guidelines

Useful links

Author: Dr Steven Scholfield (FY2)

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