CardiologyTier 1Disease (DEADMAN)

Cardiac arrhythmias - sinus node dysfunction

Description

Sick sinus syndrome (SSS) - spectrum of intrinsic sinoatrial node dysfunction
Encompasses sinus bradycardia, sinus pauses/arrest, sinoatrial exit block, and tachy-brady syndrome (alternating bradycardia with paroxysmal AF/atrial tachyarrhythmia)

Intrinsic - idiopathic fibrosis/degeneration of the sinoatrial node and surrounding atrial tissue (age-related, commonest), infiltrative disease (amyloid, sarcoid), ischaemia (SA nodal artery), post-surgical/ablation scarring, genetic (rare channelopathies in younger patients)
Extrinsic (reversible - exclude before pacing): drugs (beta-blockers, non-dihydropyridine CCBs, digoxin, antiarrhythmics), hypothyroidism, hyperkalaemia, hypothermia, raised ICP, high vagal tone (athletes, obstructive sleep apnoea), inferior MI (transient)
Same fibrotic atrial substrate that impairs the sinus node also predisposes to AF - explaining the tachy-brady overlap

Correlate symptoms with rhythm - the essential step

Symptoms: dizziness, presyncope/syncope, fatigue, exercise intolerance, palpitations (tachy-brady)
ECG/rhythm monitoring must capture symptoms concurrently with the bradyarrhythmia - asymptomatic bradycardia found incidentally does not by itself warrant pacing

Diagnostic patterns

Persistent sinus bradycardia inappropriate to physiological state
Sinus pauses >3 seconds (awake, not during sleep/high vagal tone)
Chronotropic incompetence - failure to appropriately increase heart rate with exertion - a common under-recognised cause of exertional fatigue
Tachy-brady syndrome - paroxysmal AF/flutter terminating in a long pause

Monitoring strategy by symptom frequency

Frequent symptoms - Holter (24-48h)
Infrequent symptoms - extended ambulatory monitor (2-4 weeks) or implantable loop recorder
Exercise testing if chronotropic incompetence suspected

Exclude reversible causes before attributing to intrinsic SSS

Review medication list, TFT, electrolytes, consider ischaemia if acute presentation

By presenting problem

### Reversible cause identified

Treat/withdraw the cause first (stop/reduce the offending drug, correct hypothyroidism/electrolytes) - pacing not needed if bradycardia resolves

### Symptomatic bradycardia, no reversible cause -> permanent pacemaker

Class I indication: documented symptomatic sinus bradycardia or pauses, with symptoms directly correlated to the rhythm
Chronotropic incompetence causing exertional symptoms - pacemaker with rate-responsive programming

### Tachy-brady syndrome

Pacing addresses the bradycardia component and permits safe use of rate/rhythm-control drugs (beta-blocker, digoxin) for the tachyarrhythmia that would otherwise be limited by bradycardia
Anticoagulation per usual AF risk stratification (CHA2DS2-VASc) regardless of pacing decision - paroxysmal AF carries the same stroke risk as sustained AF

### Asymptomatic bradycardia

Observe - pacing not indicated in the absence of correlated symptoms, regardless of how impressive the ECG looks

### Acute/reversible bradycardia causing haemodynamic compromise

Atropine, isoprenaline, or transcutaneous/transvenous temporary pacing as a bridge while the reversible cause is treated

Atrial fibrillation/flutter - shared atrial substrate, defines tachy-brady syndrome
Falls and fall-related injury in the elderly
Amyloidosis/infiltrative cardiomyopathy - consider if SSS occurs with conduction disease and LV hypertrophy pattern

Progressive - intrinsic SSS rarely improves and typically worsens over years
Untreated symptomatic disease - recurrent syncope (fall/injury risk), progressive fatigue/functional decline
With appropriate pacing, prognosis is generally good and reflects the underlying/associated cardiac disease rather than the sinus node dysfunction itself
Pacing improves symptoms but does not reduce mortality in isolated SSS - the indication is symptom control, not survival

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