Sinus Bradycardia

Sinus rhythm with a resting heart rate of <60bpm in adults (or below the normal range for children).

Background

  • Sinus bradycardia is not uncommon cause of slow heart rate.
  • It can generally be split into three categories
    • Physiological (the classic example is in athletes who can have a resting heart rate of <60bpm naturally)
    • Pathological (see causes below)
    • In the context of old age and sick sinus syndrome / sinus node dysfunction

Causes (Pathological)

  • Drug induced (common) e.g. beta-blockers; calcium channel blockers; digoxin; amiodarone; clonidine; verapamil
  • Hypoxia; Hypothyroidism; Hypothermia; Hypoadrenalism; Hyperkalaemia
  • Acute MI/ischaemia
  • High vagal tone or vagal stimulation

Presentation

Management

  • Manage the underlying problem (this may require an ABCDE approach)
  • If concerned about heart rate in the acute setting; atropine may be given
  • In the chronic setting, if the patient is frequently symptomatic and no underlying cause has been identified (or management is not possible), consider internal pacing.

A note on sick sinus syndrome

  • Definition: Syndrome of signs (mainly ECG changes) which suggest dysfunction of the sinus node causing an inability to properly generate cardiac impulses
  • Causes: Most commonly caused by idiopathic fibrosis of the sinus node (occurs mainly in old age).  Other causes include cardiac disease (ischaemia/MI; pericarditis; rheumatic heart disease); collagen vascular disorders (amyloidosis; sarcoidosis; haemochromatosis); Friedrich’s ataxia; muscular dystrophy;
  • Changes on ECG: sinus bradycardia; sinus exit block; sinus arrest; tachy-brady syndrome
  • Presentation: Often aymptomatic but can present as above in sinus bradycardia.  Symptoms can also be of irritability, forgetfulness, palpitations, angina, vague GI symptoms, fatigue, nocturnal wakefulness.
  • Treatment: A pacemaker may be indicated in symptomatic patients.

A note about SA block and SA arrest

SA Block

  • Not commonly clinically relevant but may be a feature of SSS or other atrial disease
  • Usually normal sinus node function (except in SSS) but failure of the conducting pathways in the atria to the AV node.  As with AV block, there are 3 types
    1. First degree- delay between the impulse generation and transmission to the atrium (this is rarely symptomatic on its own (atrial contraction is often just an extra help with ventricular filling) and is not detectable on ECG)
    2. Second degree
      1. Type 1 (Wenckebach)- Progressive lengthening of the interval between impulse generation and transmission, resulting in eventual failure of transmission
        1. With SA block, this shows on ECG as closer P waves, eventually grouping of P-QRS, and finally a dropped beat.
      2. Type 2- Intermittent dropped P wave with a constant interval
        1. I.e. same as Mobitz II BUT no p waves
        2. Type_II_S-A_block
      3. Third degree SA block- complete absence of p waves BUT can have normal QRS – junctional rhythm; or wide QRS
        1. Indistinguishable from sinus arrest

Sinus arrest

  • No P waves- if the heart doesn’t use another pacemaker (ventricular, junctional or otherwise) then this will cause complete cardiac arrest.
  • Fortunately, this is often only transient.  However, it may be recurrent, and atrial pacing is usually required.
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