see also:

• ecg_avblock
• cardiac arrhythmias

• slow heart rate may be due to either:
  • sinus bradycardia (P waves present and are related to QRS complexes)
  • 2nd degree AV node block (P waves present but not always resulting in a QRS complex and thus rhythm may be irregular)
  • 3rd degree AV node block (P waves present but not related to QRS complexes which arise from another site and are regular)
  • sinus arrest with junctional or idioventricular rhythm (no P waves, regular QRS complexes)
    • retrograde P waves before or after QRS onset suggests AV nodal junctional rhythm - rate usually 40-60bpm
    • His bundle junctional rhythm - rate usually 35-45bpm
    • widened QRS suggests either:
      • junctional rhythm with intraventricular conduction defect, or,
      • idioventricular rhythm - widened QRS with rate usually 30-40bpm (may be 20-60bpm)
  • “slow AF” (no P waves and irregular QRS complexes)

• the most common acute causes of sinus bradycardia are a baroreceptor response or vasovagal response (eg. during insertion of an iv cannula in ED, tracheal suction) which are generally self-limiting although may cause syncope / near syncope and even a very prolonged sinus pause looking like asystole
• sinus bradycardia may be “normal” for those who exercise a lot and are very aerobically fit
• hypersensitive carotid sinus syndrome
  • cardioinhibitory
    • manifested by more than three seconds of ventricular asystole during stimulation of the carotid sinus, and results from an oversensitivity of the afferent carotid sinus nerves leading to the efferent vagal response
  • vasodepressor responses
• other reflex-mediated bradycardia:
  • diving reflex (cold water to the upper face)
  • oculocardiac reflex (trauma or pressure applied to the globe)
  • maxillofacial reflex (surgical manipulation of the maxillofacial or temperomandibular regions)
  • rectoprostatic massage
  • deglutition, micturition, and defecation syncope;
  • glossopharyngeal neuralgia
• sinus bradycardia and risk of atrioventricular block may be due to:
  • acute myocardial infarction (AMI/STEMI/NSTEMI) - 40% of cases of AMI have sinus brady
  • sick sinus syndrome / tachycardia-bradycardia syndrome
    • elderly
    • IHD
    • rheumatologic
    • oncologic
    • metabolic
    • infectious - myocarditis
    • structural
    • iatrogenic - radiation Rx;
    • amyloidosis
    • Lenegre’s disease
    • Lev’s disease
  • severe life threatening hyperkalaemia
    • suggested by “possible slow AF pattern”, flattening or loss of P waves, broadening of QRS, T wave abnormalities
  • hypermagnesemia
  • hypercalcaemia
  • excessive cardiac medications such as:
    • digoxin toxicity and other glycosides
    • beta adrenergic blockers
    • calcium channel blockers
  • other medications or toxins:
    • opioids, sedative-hypnotics, and alpha2-adrenergic agonists act via central nervous system-mediated mechanisms to produce bradycardia
    • cholinergic agents
    • organophosphates, cholinesterase inhibitors, lithium, phenothiazines, and cocaine
    • propylene glycol carrier of iv phenytoin
    • oseltamivir
    • cyanide poisoning
  • hypothermia - look for Osborne J waves, long QTc
    • see https://lifeinthefastlane.com/ecg-library/100-ecgs/quiz-ecg-010/
  • hypothyroidism, hypoadrenalism, hyperparathyroidism, and acromegaly
  • infections including typhoid (1st noted by Osler in 1898), influenza (noted by Oppenheimer in 1899¹⁾)
    • “fever with relative bradycardia” may occur with Gram negative intracellular infections such as typhoid, Legionnaire's disease, and pneumonia caused by Chlamydia sp ²⁾
  • in-utero fetal bradycardia (HR < 110 bpm) is a sign of fetal hypoxia and potentially imminent stillbirth
  • acute injury to the cervical cord resulting in disruption of the sympathetic fibers and unopposed vagal tone
  • stroke (CVA), subarachnoid haemorrhage (SAH), seizure (ictal bradycardia syndrome - esp. with temporal lobe seizures), and Guillain Barré syndrome
• bradycardia with rising BP and decreasing GCS is highly suggestive of Cushing's reflex due to severe, life threatening raised intracranial pressure (ICP)

• iv access
• cardiac monitor
• U&E, Ca, P
• correct any underlying cause such as hyperkalaemia

• defined as causing either:
  • acutely altered mental status
  • ischemic chest discomfort
  • acute heart failure
  • hypotension
    • could it be cyanide poisoning?
  • other signs of shock that persist despite adequate airway and breathing
  • see also cardiogenic shock
• iv atropine 0.5mg IV q3-5min is 1st line
  • avoid relying on atropine if high grade AV block (Mobitz II or 3rd Degree heart block)
• if no response then temporizing measures whilst awaiting transvenous temporary pacing such as either:
  • iv dopamine infusion
  • iv adrenaline / epinephrine infusion at 2-10 mcg/min
  • alternatively iv isoprenaline 2-10 mcg/min infusion may be considered but perhaps may cause more arrhythmias
  • transcutaneous pacing (TCP)

2010 AHA Guidelines