Conditions

Bradycardia

Continuous ECG monitoring helps track heart rhythm

Bradycardia is defined as a resting heart rate below 60 beats per minute. In physiologically trained individuals — particularly athletes — resting bradycardia reflects enhanced vagal tone and is entirely normal. Pathological bradycardia arises when the heart's electrical conduction system fails to generate or conduct impulses at a rate sufficient to maintain adequate cardiac output, producing symptoms and, in severe cases, haemodynamic compromise. Accurate identification of the underlying mechanism guides management and the decision regarding pacemaker implantation.

Types & Causes

• Sinus node dysfunction (sick sinus syndrome) encompasses a spectrum of sinus node abnormalities — sinus bradycardia, sinus pauses, sinoatrial exit block, and chronotropic incompetence (failure of heart rate to increase appropriately with exertion). Tachy-brady syndrome — alternating episodes of bradycardia and supraventricular tachycardia, most commonly atrial fibrillation — is a common and clinically significant variant that complicates anticoagulation and rate-control decisions.
• Atrioventricular (AV) block is classified by severity. First-degree AV block involves prolonged PR interval without dropped beats and is typically benign. Second-degree AV block — Mobitz type I (Wenckebach) and Mobitz type II — involves intermittent failure of conduction; Mobitz II carries higher risk of progression to complete block and often requires pacing. Third-degree (complete) AV block is a complete dissociation between atrial and ventricular activity, producing a slow escape rhythm, haemodynamic instability, and syncope — requiring urgent pacemaker implantation.
• Reversible causes include hypothyroidism, electrolyte disturbances (hyperkalaemia, hyponatraemia), medications (beta-blockers, calcium channel blockers, digoxin, amiodarone), Lyme carditis, and increased vagal tone from inferior myocardial infarction. Identifying and correcting reversible causes may avoid the need for permanent pacing.

Symptoms

Symptomatic bradycardia presents with fatigue, reduced exercise tolerance, exertional breathlessness, lightheadedness, presyncope, and syncope. Chronotropic incompetence — where heart rate fails to rise appropriately during physical activity — produces disproportionate exertional fatigue and breathlessness that may be attributed to deconditioning without cardiac investigation. Cognitive impairment and memory difficulties can develop in patients with sustained cerebral hypoperfusion from severe bradycardia. Asymptomatic bradycardia detected incidentally requires careful evaluation to determine whether it is physiological or pathological.

Diagnosis

A resting 12-lead ECG identifies sinus bradycardia, conduction abnormalities, and AV block patterns present at the time of recording. Ambulatory Holter monitoring over 24–48 hours captures intermittent bradycardia, pauses, and tachy-brady cycles. Exercise stress testing assesses chronotropic response — a blunted heart rate rise below 80% of age-predicted maximum confirms chronotropic incompetence. Echocardiography evaluates underlying structural heart disease and left ventricular function. Thyroid function, electrolytes, and medication review are performed in all patients. Electrophysiology study (EPS) assesses sinus node recovery time and His-Purkinje conduction in patients where non-invasive testing is inconclusive and pacing is being considered.

Treatment

• Reversible causes are corrected first — medication adjustment, thyroid replacement, electrolyte normalisation, or treatment of underlying infection can restore normal conduction without the need for pacing.
• Permanent pacemaker implantation is indicated for symptomatic bradycardia due to sinus node dysfunction or AV block not attributable to a reversible cause, and for high-degree or complete AV block regardless of symptoms. Device selection is individualised: single-chamber pacing is appropriate for selected patients with permanent atrial fibrillation; dual-chamber pacing preserves AV synchrony and is preferred for most patients with sinus node dysfunction. Leadless pacemakers — implanted directly into the right ventricle via catheter without transvenous leads — are an increasingly used option for patients at high infection risk or with vascular access limitations. His bundle pacing and left bundle branch area pacing are physiological pacing modalities that activate the ventricles via the native conduction system, preserving more natural ventricular synchrony compared to conventional right ventricular apical pacing — increasingly preferred in patients with preserved LV function to reduce the long-term risk of pacing-induced cardiomyopathy.

Experiencing fatigue, dizziness, or diagnosed with a slow heart rate? Dr. Peter Chang consults at Paragon Medical Centre, Orchard Road. Book a specialist assessment today.