Bradycardia and AV Block

Meet the patient

An 82-year-old woman is found on the bathroom floor at 6am. Her husband says she "dropped" without warning and came round within thirty seconds, fully herself, asking for a cup of tea. Her 12-lead ECG shows complete heart block at 32 bpm with a wide-complex escape.^[1][4]

The two questions that decide her next hour are the two that decide every bradyarrhythmia: where is the block — nodal or infranodal? (nodal is narrow, vagal, atropine-responsive; infranodal is wide, structural, and needs a wire) and is she symptomatic from it? (syncope, hypotension, or ischaemic pain moves you from "observe" to "atropine, then pacing"). Everything below is built to answer those two.^[1][3]

What bradycardia is — slow, and either nodal or infranodal

Bradyarrhythmia is any rhythm in which the heart rate is inappropriately slow for the clinical state, conventionally a resting sinus rate under 60 bpm in an adult; the threshold for action falls when symptoms appear, and trained endurance athletes run physiologically low resting rates. The condition is not a single disease but a family of disorders, each with a different anatomical lesion, prognosis and treatment.^[1][20]

Sinus bradycardia is a sinus-node–driven rhythm under 60 bpm with every P wave conducting 1:1 to the QRS. It may be physiological (sleep, athletic conditioning), drug-induced (beta-blocker, calcium-channel blocker, digoxin, clonidine, ivabradine), vagally mediated (cough, micturition, trigeminal stimulation, sleep apnoea), reflect raised intracranial pressure (Cushing reflex), hypothermia, severe hypothyroidism, or be a feature of inferior MI with right coronary artery (RCA) ischaemia of the SA node. Sinus arrest is sudden cessation of P-wave generation producing a pause (no P wave on the ECG); if the escape pacemaker does not fire, asystole follows and Stokes-Adams syncope occurs. Sinoatrial (SA) exit block has the same surface ECG appearance as sinus arrest (a pause) but the pause is a multiple of the underlying PP interval. Sick sinus syndrome (SSS) is the umbrella term covering all of these plus tachy-brady syndrome (alternating AF with sinus pauses), and is the commonest indication for permanent pacing in the elderly.^[1]

AV block is failure of atrial impulse conduction to the ventricles, graded by the relationship between P waves and QRS complexes. 1st-degree AV block — every P conducted, PR interval over 200 ms (over 300 ms on intracardiac recordings; one large-toe-to-three-large-toe width on the ECG at 25 mm/s), almost always at the level of the AV node, benign. 2nd-degree AV block — some P waves not conducted. Mobitz type I (Wenckebach) — progressive PR prolongation until a P is dropped, the pause less than twice the preceding PP, almost always AV nodal (proximal to the His bundle) and benign; atropine-responsive, vagal, often nocturnal in young adults. Mobitz type II — fixed (normal or slightly prolonged) PR with sudden non-conducted P waves; the lesion is infranodal (His–Purkinje), with high risk of progression to complete block and syncope, and is a Class I indication for permanent pacing even when asymptomatic. High-grade (advanced) AV block — two or more consecutive non-conducted P waves with sinus rhythm, requires AV nodal disease or infranodal disease. 3rd-degree (complete) AV block (CHB) — complete AV dissociation, atrial and ventricular rhythms independent; the ventricular escape is junctional (narrow QRS, 40–60 bpm) — relatively stable, or ventricular (wide QRS, 20–40 bpm) — unstable, syncope, sudden cardiac death. The anatomical correlate of congenital CHB is failure of AV node/His bundle development; of acquired Mobitz II and infranodal CHB, Lev disease (calcification of the aortic and mitral annuli extending into the conduction system) and Lenegre disease (idiopathic sclerotic degeneration of the conduction system).^[1][3][4]

The clinical importance of bradyarrhythmia is its causation of syncope, falls, injury and sudden cardiac death when pauses are long enough to cause cerebral hypoperfusion (typically over 6 seconds), and its drain on cardiac output in patients with stiff, hypertrophied or failing ventricles (the same 35 bpm that is asymptomatic in a healthy 25-year-old produces florid heart failure in a 75-year-old with LV dysfunction). The exam skill is (1) pattern recognition on the 12-lead ECG (sinus brady vs SA block vs Wenckebach vs Mobitz II vs CHB), (2) anatomical localisation (nodal — narrow QRS, vagal, atropine-responsive — vs infranodal — wide QRS, structural, pacing-required), (3) search for reversible causes (drugs, ischaemia, electrolytes, Lyme, hypothyroidism), (4) prompt resuscitation for unstable bradycardia, and (5) definitive pacing for irreversible disease per 2018 ACC/AHA/HRS and 2013 ESC pacing.^[1][3]

Classification

Bradyarrhythmias are classified by anatomical level of the block (which determines prognosis and treatment), by ECG pattern (the surface morphology), and by aetiology (which determines reversibility). ^[1]

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^{[1] [15] [6]}

Epidemiology & Risk Factors

The epidemiology of bradyarrhythmia follows the two disease populations: (1) young, healthy patients with vagal or athletic bradycardia (and rare congenital CHB) and (2) older patients with degenerative conduction disease and ischaemic, drug, metabolic and surgical aetiologies.^[1][4]

Sinus bradycardia and vagal bradycardia — resting sinus bradycardia is common: roughly 1 in 4 healthy adults have a daytime resting HR below 60 bpm, and up to 50 to 60 percent of trained endurance athletes have resting rates in the 30s at night on Holter. Vagal tone dominates in the young; vasovagal syncope is the commonest cause of syncope in young adults (and the second commonest overall, after cardiac syncope in older patients). ^[1]

AV block — 2nd- and 3rd-degree AV block are uncommon in the general population (prevalence under 1 percent) but rise with age: complete heart block is rare under 60 (under 5 per 100 000) and rises to over 200 per 100 000 over 80. Lev disease (calcific extension from aortic and mitral annuli into the conduction system, associated with hypertension, diabetes, and aortic sclerosis) and Lenegre disease (idiopathic sclerotic fibrosis of conduction tissue, often familial) account for the majority of acquired AV block in the elderly.^[1][3]

Ischaemia and infarction — in a CT angiography series of 102 patients, the SA nodal artery arose from the RCA in 67 and from the left circumflex in 28, and the AV node was supplied by the RCA in 89, the left circumflex in 11, and both in 2. Hence inferior MI produces sinus bradycardia, sinus arrest and AV nodal block (typically Mobitz I / Wenckebach), which is usually transient but may require temporary pacing; anterior MI with proximal left anterior descending occlusion is rarer but produces infranodal block (Mobitz II / CHB) because of extensive septal necrosis — this is much more dangerous and the lesion is permanent.^[13][3]

Drug toxicity is a leading reversible cause in clinical practice — beta-blocker overdose, calcium-channel blocker overdose (especially verapamil and diltiazem), and digoxin toxicity head every reversible-cause checklist; the beta-blocker and calcium-channel blockers act on the AV node, and poisoning with either produces bradycardia, hypotension and conduction block. Electrolyte disturbance — hyperkalaemia with ECG changes (the ECG abnormalities, not the absolute potassium value, drive emergency treatment), hypermagnesaemia, severe hypocalcaemia. Endocrine — hypothyroidism, Addisonian crisis, hypopituitarism.^{[10][11][21][1]}

Inflammatory and infiltrative — Lyme disease (Borrelia burgdorferi) is the classic cause of reversible high-grade AV block in endemic areas — cardiac involvement occurs in approximately 1 to 10 percent of untreated infections and most commonly manifests as atrioventricular conduction abnormalities; the block often responds to IV ceftriaxone and may require temporary pacing. Cardiac sarcoidosis — infiltrative granulomatous disease of the septum with conduction block and ventricular arrhythmia, often requiring MRI and biopsy. Chagas disease (Trypanosoma cruzi) — chronic apical aneurysm and conduction fibrosis, common in Latin America. Amyloidosis (AL and ATTR) — infiltrative cardiomyopathy with conduction block. Myocarditis — viral or autoimmune, may produce transient high-grade block.^{[19][9][3][4]}

Post-cardiac surgery and structural — aortic valve replacement (mechanical valve impinging on the conduction system, especially right coronary cusp), transcatheter aortic valve implantation (TAVI) (membranous septum compression — in a meta-analysis of 78 studies and 31 261 patients, 6212 required a permanent pacemaker, a mean of 18.9 percent per study, with baseline conduction delay and right bundle branch block the strongest predictors), ventricular septal defect (VSD) repair, tetralogy of Fallot repair, atrial septal defect closure, septal myectomy (HOCM), and alcohol septal ablation all carry risk of new high-grade AV block. ^[8][1]

Sleep apnoea is an increasingly recognised cause of nocturnal bradycardia and pauses (the diving reflex / vagal surge at apnoea termination), and is the commonest cause of bradycardia in middle-aged obese patients otherwise unexplained. Vagal reflex — cough syncope (during severe cough), micturition syncope (typically elderly men at night — vagally mediated bradycardia, often with vasodepression), swallow syncope, defaecation syncope.^[3][4]

Pathophysiology

The pathophysiology of bradyarrhythmia is best understood by tracing the anatomical site of impulse failure — impulse formation (SA node), impulse conduction from atrium to AV node, AV node to His bundle, and His–Purkinje to ventricle — and the blood supply, autonomic input, metabolic environment and intrinsic disease at each site.^[1][3]

1. The SA node is a crescent of specialised atrial tissue at the junction of the superior vena cava and right atrium, richly innervated by parasympathetic (vagal) and sympathetic fibres, and containing automatic cells whose intrinsic rate is reduced by vagal acetylcholine acting on M2 muscarinic receptors (slowing the funny current, Iƒ, and reducing the slope of phase 4 depolarisation) and increased by sympathetic noradrenaline acting on β1-adrenoceptors (increasing Iƒ and the calcium current). The result in health is a heart rate that varies with respiration (RSA, respiratory sinus arrhythmia), position, exertion, emotion and sleep. Pathological sinus bradycardia arises when (1) excessive vagal tone (athletes, cough, micturition, carotid sinus massage, raised ICP, inferior MI — all reflex vagal bradycardia), (2) intrinsic SA nodal disease (sick sinus syndrome — idiopathic fibrosis, amyloid, sarcoid, ischaemia), (3) extrinsic drug effect (beta-blocker, calcium-channel blocker, digoxin, amiodarone, ivabradine), or (4) metabolic or endocrine failure (hypothyroidism, hyperkalaemia). Sinus arrest is the sudden failure of impulse generation; the pause length is a function of the latency of the next available escape pacemaker (junctional = 1 to 2 seconds; ventricular = up to 6+ seconds).^[1]

2. The AV node is the only electrical connection between atria and ventricles under normal conditions — the annulus fibrosus insulates the rest — and is the most vagally-influenced segment of the conduction system. At rest, vagal dominance keeps the AV nodal Wenckebach cycle length around 500 ms (rate 120 bpm) — a physiological upper rate limit above which AV nodal Wenckebach appears as an atrial-driven response (loss of 1:1 conduction with atrial pacing above 120). The same vagal dominance explains why Mobitz I is so commonly benign — vagal surges transiently depress AV nodal conduction and the PR lengthens. The AV node is also the commonest site of iatrogenic drug block (beta-blocker, digoxin, verapamil/diltiazem, amiodarone) and of ischaemic block in inferior MI because the AV nodal artery is a distal branch of the RCA in 80 to 90 percent of patients. Lyme carditis characteristically attacks the AV node, producing a reversible nodal block.^[3][4]

3. The His–Purkinje system (the infranodal conduction) consists of the bundle of His (penetrating the central fibrous body and dividing into left and right bundles), fascicles (left anterior fascicle, left posterior fascicle), and the Purkinje network. This segment is largely not vagally-supplied, is dependent on fast sodium channels (phase 0 upstroke) rather than the slow calcium current, and has low automaticity — the escape rate from the His bundle is roughly 40 to 60 bpm, from a fascicle 30 to 40 bpm, and from the ventricle 20 to 40 bpm, the latter unreliable and unstable. The infranodal conduction is susceptible to (1) idiopathic fibrosis (Lenegre disease, accelerating in middle age), (2) calcific infiltration from aortic and mitral valve disease (Lev disease), (3) anterior MI with proximal LAD occlusion and septal necrosis, (4) infiltrative disease (sarcoid, amyloid), and (5) drugs that depress sodium channels (Class IA antiarrhythmics — procainamide, disopyramide; Class IC — flecainide, propafenone — these drugs are contraindicated in patients with structural heart disease and the result can be the unmasking of bundle branch block or the precipitation of asystole when used in patients with unrecognised conduction disease). Mobitz II and CHB with wide QRS are the surface ECG signs of infranodal block.^[1][3]

4. The electrophysiological mechanisms of the patterns are worth understanding for the exam. Wenckebach is the gradual decrement in AV nodal conduction with each successive atrial impulse, until one impulse fails to conduct — the pause is less than twice the preceding RR (because during the dropped beat, the AV node recovers and the next PR after the dropped beat is the shortest of the cycle). Mobitz II is the sudden failure of an all-or-none His–Purkinje fibre — fixed PR with a sudden dropped QRS. CHB is the failure of all conduction — the atria and ventricles beat independently, the ventricular escape rhythm determining symptoms (junctional escape is relatively well tolerated, ventricular escape is syncopic and dangerous). Sinus arrest is the failure of impulse formation; the pause has no underlying sinus P waves, and an escape pacemaker must fire or asystole ensues.^[3]

5. The role of autonomic tone is the mechanistic link between vagal syndromes and bradyarrhythmia — the vasovagal reflex classically has a cardioinhibitory component (sudden vagally-mediated bradycardia or asystole — Class II indication for pacing under 2018 ESC syncope guidelines) and a vasodepressor component (sympathetic withdrawal and venous pooling, pacing does not help); the carotid sinus reflex produces the same picture in older patients with carotid hypersensitivity (a 3-second pause or 50 mmHg systolic drop on carotid sinus massage in patients over 40 is diagnostic); and the cough, micturition, swallow and defaecation reflexes are all vagally-mediated. ^[1]

6. The relationship to myocardial ischaemia deserves explicit emphasis: bradycardia (especially sinus bradycardia) is common in inferior MI because the RCA supplies both the SA and AV nodes, vagal reflexes (Bezold–Jarisch reflex — a vagal surge from ischaemic myocardium) compound the picture, and the resulting block is usually transient (resolves as the infarct stabilises and vagal tone normalises) — temporary pacing suffices. Anterior MI with proximal LAD occlusion and septal necrosis destroys the His–Purkinje system and produces infranodal block that is permanent and requires permanent pacing.^[1][3]

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Clinical Presentation

The clinical presentation of bradyarrhythmia spans four gradients: (1) asymptomatic (incidental on ECG or device interrogation), (2) fatigue / exercise intolerance / breathlessness, (3) presyncope (dizziness, lightheadedness, nausea, sweating) and (4) syncope (transient loss of consciousness) or sudden cardiac death when long pauses occur. The exam skill is recognising which clinical picture is bradyarrhythmia.^[1][4]

Symptoms (the patient may be unable to give a long history after syncope): ^[1]

• Cardiac symptoms of the underlying cause — chest pain of acute MI (crushing central, radiation, autonomic features), palpitation of preceding tachyarrhythmia (in brady-tachy SSS, the patient may describe the AF that terminated in a pause), breathlessness of pulmonary oedema if the slow rate cannot match cardiac output demand.
• Fatigue, lethargy, exercise intolerance — the commonest symptom of mild bradycardia; the patient describes inability to climb stairs, exertional breathlessness, "I'm slowing down" — often misattributed to ageing.
• Presyncope / dizziness / lightheadedness — the cardinal "I had to sit down" symptom of transient cerebral hypoperfusion from bradyarrhythmia; pauses may be hours apart, days apart, or weeks apart.
• Stokes-Adams syncope — sudden loss of consciousness without warning, with rapid recovery (no post-ictal confusion, no Todd paresis) caused by a sudden fall in cardiac output from an asystolic pause (CHB, sinus arrest, vasovagal cardioinhibitory). The patient may recall darkness before the eyes, falling, and rapid recovery (seconds to a minute) — a key differentiator from seizure (which has tonic-clonic activity, tongue biting, urinary incontinence and post-ictal confusion) and from orthostatic syncope (preceded by lightheadedness on standing).
• Falls and injury — bradyarrhythmic syncope in the elderly may present as a fall with hip fracture or head injury, masquerading as a mechanical fall — a 12-lead ECG should be performed in every unexplained fall.
• Sudden cardiac death — bradyarrhythmia (especially CHB) is a recognised, though less common, cause of out-of-hospital cardiac arrest; Mobitz II and acquired CHB are the most lethal patterns.^[1][4]

Vital signs and bedside observation: ^[1]

• Resting HR below 50 bpm — bradycardia by definition. Below 40 bpm strongly suggests pathological block; below 30 bpm in a non-athlete is bradyarrhythmia until proven otherwise.
• Irregular irregular rate — AF with bradycardia, AF with pauses, sinus arrhythmia, Wenckebach pattern, SA block / sinus arrest.
• Pulse character — the slow-rising pulse of low stroke volume, the regular slow rhythm of CHB ("cannon waves" in the JVP because of atrial contraction against a closed tricuspid valve — see below).
• Blood pressure — orthostatic, supine-to-standing drop of over 20 mmHg systolic (orthostatic hypotension is a common cause of apparent brady-symptom overlap, especially in the elderly with autonomic dysfunction). ^[1]

Examination (organ by organ, looking for the cause): ^[1]

• JVP — slow-rate raises the chance of cannon a waves in CHB, where the atria contract against a closed tricuspid valve; the slow-rate wave at the JVP has no relationship to the rate of the arterial pulse. Distended neck veins are uncommon unless decompensation has set in.
• Heart sounds — varying intensity of S1 in CHB (the variable AV relationship alters the timing of mitral leaflet closure); soft S1 if PR is long; loud S1 if P and QRS are close together.
• Carotid sinus massage — should be performed with continuous ECG monitoring after auscultation of the carotids and exclusion of carotid bruit; a pause over 3 seconds or a systolic BP drop over 50 mmHg with symptom reproduction confirms carotid sinus hypersensitivity (the cardioinhibitory form is the indication for pacing under 2018 ESC syncope guidelines).^[4]
• Thyroid — goitre, slow-relaxing ankle reflex for hypothyroidism (a treatable cause of sinus bradycardia).
• Skin and complexion — pallor, jaundice (Addisonian crisis), erythema migrans (Lyme).
• Neurological — pupil asymmetry and extensor posturing suggest raised ICP rather than bradyarrhythmia.

Atypical presentations (high-yield): ^[1]

• Elderly faller — Stokes-Adams presenting as hip fracture.
• Young athlete — asymptomatic bradycardia on pre-participation screening ECG; reassurance in the absence of symptoms or pauses.
• Pregnancy — raised HR expected; bradycardia with dizziness or presyncope suggests the rare peripartum cardiomyopathy with conduction block or beta-blocker over-treatment for hypertension.
• Post-cardiac surgery (AVR, TAVI, septal repair) — new CHB or Mobitz II requires pacing; a period of observation post-TAVI before committing to a permanent device, because conduction recovery is common and many of those who receive a pacemaker may not be pacemaker-dependent long term.^[1][8]

Differential Diagnosis

The first task is to separate true bradyarrhythmia from mimics, particularly because the management of a syncopal patient with apparent bradycardia on a single ECG can be misleading. The differential covers reflex syndromes, orthostatic causes, other cardiac causes of syncope, and non-cardiac mimics.^[3][4]

The single best discriminator at the bedside is the ECG combined with continuous rhythm monitoring during symptoms and a detailed history of the trigger, prodrome and recovery. Prolonged unexplained syncope with normal ECG and Holter requires implantable loop recorder (ILR) — the SPAIN, ISSUE-3 and other trials demonstrated diagnostic yield above 30 percent in unexplained recurrent syncope with ILR, and pacing in detected bradyarrhythmia reduced syncope burden in the ISSUE-3 / ISSUE-3-derived populations.^[1][3][4]

Clinical & Bedside Assessment

The bedside assessment of bradyarrhythmia is ABCDE first, ECG in every suspected case, continuous monitoring, and a focused search for the cause.^[1][4]

A — Airway and Breathing: ^[1]

• Secure airway if patient has syncope or impaired consciousness.
• Supplemental oxygen to SpO2 94 to 98 percent if hypoxic (88 to 92 percent in COPD / CO2 retainers); excess oxygen in inferior MI may cause vasoconstriction and worsen output — keep SpO2 under 99 percent if no hypoxaemia. ^[1]

B — Breathing pattern observation: Slow, deep breaths suggest excess vagal tone (vasovagal); Cheyne–Stokes respiration suggests heart failure, raised ICP, or post-cardiac-arrest; Biot respiration suggests brainstem lesion. ^[1]

C — Circulation: ^[1]

• Heart rate and rhythm — palpate the radial pulse for 60 seconds; count a full minute because bradyarrhythmia often fluctuates.
• Blood pressure — supine and standing; postural drop suggests volume, autonomic or drug cause.
• ECG monitoring — continuous ECG telemetry with rate and rhythm alarms; print a rhythm strip at every event.
• 12-lead ECG — performed immediately. Look for:
  • P-wave morphology — sinus vs low atrial vs absent (sinus arrest / AF with pauses)
  • PR interval — 1st-degree block, Wenckebach periodicity, fixed PR with dropped beat
  • QRS width — narrow = nodal escape, wide = ventricular escape (worse prognosis)
  • R–R relationship — regular slow = sinus brady or CHB; irregular slow = AF with bradycardia or Wenckebach with variable conduction; PP equal with missed P = SA exit block type II; PP unequal with missed P = sinus arrest
  • Bundle branch patterns — bifascicular block plus PR prolongation is a precursor to CHB (Class IIa pacing if symptomatic or syncopal, Class I pacing if EP study HV over 70 ms).
• Carotid sinus massage (with continuous ECG and BP) — in patients over 40 with unexplained syncope, performed in supine position, gentle unilateral massage over 5 seconds; positive if pause over 3 seconds or systolic drop over 50 mmHg with symptom reproduction. No longer contraindicated in the post-stroke window per 2018 ESC syncope guidelines (the previous 3-month exclusion was removed).^[4]
• JVP — slow rate with cannon a waves is diagnostic of AV dissociation (CHB); otherwise inspect for raised JVP of heart failure or pulmonary hypertension.^[1][3]

D — Disability: ^[1]

• Blood glucose — to exclude hypoglycaemia as a mimic, particularly in patients on insulin or sulphonylureas.
• Neurological — pupil asymmetry, conscious level, focal signs for stroke / raised ICP.
• Differential — focal neurology with bradycardia suggests raised ICP (Cushing reflex — bradycardia + hypertension + irregular breathing). ^[1]

E — Exposure: ^[1]

• Skin — jaundice (Addison, jaundice), erythema migrans (Lyme), cyanosis (hypoxia), pallor (anaemia), surgical scars (post-cardiac surgery).
• Neck circumference — sleep apnoea phenotype.
• Cardiovascular examination — heart sounds, murmurs (aortic sclerosis/Lev), prosthetic valves (mechanical AVR, mitral repair), pulmonary crepitations (heart failure), peripheral oedema. ^[1]

The "bradycardia bundle" at the bedside (bradycardia algorithm, ERC 2021 adult advanced life support / ACC-AHA-HRS): ^[5][1]

1. Confirm the rhythm on ECG.
2. Assess for adverse features — syncope, hypotension, confusion, signs of shock, ischaemic chest pain, acute heart failure. If any adverse feature is present, treat aggressively.
3. Identify and treat the reversible cause — drugs, ischaemia, electrolyte, hypoxia.
4. If symptomatic bradycardia — atropine 0.5 mg IV as the initial dose, with further boluses as required (intravenous atropine is widely recommended as first-line treatment for symptomatic bradycardia).
5. If no response — second-line — chronotropic infusion (isoprenaline or adrenaline per local protocol) OR transcutaneous pacing (a lifesaving bridge in unstable bradycardia) OR transvenous pacing.
6. If stable but irreversible cause — permanent pacing (Mobitz II, CHB).^{[6][17][1][3]}

Investigations

First-line (in parallel with resuscitation): ^[1]

• 12-lead ECG — the single most useful investigation for diagnosis. Look for sinus vs junctional vs ventricular rate, PR interval and relationship, QRS width, bundle branch pattern, evidence of ischaemia (ST elevation / depression in MI), and QT interval (drug effect). Repeat ECG during symptoms to capture transient rhythm.
• Continuous ECG telemetry — for the in-patient; triggers capture of all bradycardic episodes with rate, duration and rhythm correlation to symptoms. Alarm settings must include asystolic pauses over 3 seconds, rate under 40 bpm sustained 30 seconds, and Mobitz II / CHB.
• Ambulatory ECG (Holter) — 24-hour to 48-hour continuous recording in patients with frequent symptoms (at least once per 24 hours); captures roughly 50 percent of culprit rhythms in syncopal patients. Preferred at presentation before more invasive testing.
• Patch / event monitor — 7 to 14 days — for patients with less frequent symptoms. The Zio patch (continuous up to 14 days) has a diagnostic yield of over 50 percent in selected patients.^[4]
• Implantable loop recorder (ILR) — up to 3 years — for recurrent unexplained syncope with normal ECG and Holter; Class I indication in this group per 2018 ESC syncope guidelines. ILR-guided therapy (pacing for bradyarrhythmia, ablation for tachyarrhythmia, ICD for VT) reduces syncope burden in ISSUE-3-derived populations.^[4]

Second-line: ^[1]

• Transthoracic echocardiogram (TTE) — mandatory in suspected bradyarrhythmia: structural heart disease, valvular disease (aortic sclerosis, calcification, rheumatic, congenital), LV and RV function, infiltrative disease appearances, pericardial effusion. The TTE guides whether a new conduction block is the consequence of an underlying cardiomyopathy (sarcoid, amyloid, ARVD) which will change management.^[1][3]
• Exercise stress testing — for exertional bradycardia or suspected chronotropic incompetence; failure to reach 80 percent of maximum predicted HR with symptoms is diagnostic. Also exposes ischaemia as a cause. Use caution — Mobitz II and CHB may deteriorate with exercise (infranodal disease worsens with catecholamine surge as the atrial rate increases and the diseased His–Purkinje cannot conduct).^[1][3]
• Electrophysiology study (EPS) — invasive — for bifascicular block plus syncope (syncope unexplained despite ECG and ILR, in a patient with bifascicular block — measure HV interval; HV over 70 ms predicts progression to CHB); suspected SSS with pauses (sinus node recovery time over 1.5 to 2 seconds is positive). Less commonly used since ILR became available, but central to decision-making where ILR is unavailable or pacing is being considered.^[1][3]

Blood tests: ^[1]

• Electrolytes — sodium, potassium, magnesium (hyperkalaemia, hypermagnesaemia commonest), calcium (hypocalcaemia prolongs QT and predisposes to arrhythmia).
• Renal function — uraemia, hyperkalaemia.
• Thyroid function — TSH for hypothyroidism (a common treatable cause); over-replacement in levothyroxine also causes tachyarrhythmia.
• Cardiac enzymes (troponin) — for suspected MI; inferior STEMI is a common cause of sinus and AV nodal block.
• Digoxin level — if patient on digoxin (toxicity is in the differential of any bradyarrhythmia on digoxin).
• Lyme serology — in endemic areas with high-grade AV block, especially summer months, history of tick exposure or erythema migrans.
• Autoimmune — anti-Ro/SSA antibodies in suspected neonatal lupus / congenital CHB in offspring.
• Arterial blood gas — for hypoxia, hypercapnia and acid–base state.^[3][4]

Advanced: ^[1]

• Cardiac MRI — for cardiac sarcoidosis (T2-weighted oedema, late gadolinium enhancement in septum and LV free wall), myocarditis, amyloidosis, ARVD. Useful in unexplained AV block in a young patient.
• CT coronary angiography / coronary angiography — when ischaemia is the cause (inferior MI producing AV block, anterior MI producing infranodal block) — guides primary PCI.
• Right ventricular biopsy — endomyocardial, for giant-cell myocarditis, cardiac sarcoidosis, transplant rejection; aetiology-specific high-yield but rarely used.^[3]

Named scores / criteria — reproduced verbatim

2018 ACC/AHA/HRS Guideline summary (Kusumoto et al. 2019)^[1]

2013 ESC Pacing Guidelines (Brignole et al.)^[3]

• Class I — symptomatic SSS (pauses / bradycardia), Mobitz II, CHB, post-MI persistent 2nd/3rd-degree block, post-cardiac-surgery persistent block, bifascicular block with syncope and HV over 70 ms.
• Class IIa — symptomatic SSS without documented bradycardia but clear causal relationship, asymptomatic bifascicular block with HV over 70 ms.
• Class IIb — asymptomatic SSS in over 70 with pauses over 6 seconds, cardioinhibitory reflex syncope with recurrent syncope and tilt-positive asystolic pattern.
• Class III (no benefit) — asymptomatic sinus bradycardia, asymptomatic vagal brady, reversible drug-induced block, end-stage terminal illness, severe cognitive impairment. ^[1]

2018 ESC Syncope Guidelines (Brignole et al.)^[4]

^{[6] [8] [9] [13] [15] [14]}

Management — Resuscitation

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The acute management of bradyarrhythmia is staged, with atropine as the first-line, escalating to chronotropic infusion and pacing, as codified in the European Resuscitation Council 2021 adult advanced life support bradycardia algorithm and the 2018 ACC/AHA/HRS bradycardia guideline.^[5][1]

ABCDE first, in parallel with diagnostic work-up: ^[1]

• Oxygen to SpO2 94 to 98 percent (88 to 92 percent in COPD).
• Two large-bore cannulae; continuous ECG and BP monitoring; 12-lead ECG (do not delay).
• Capillary blood glucose.
• Bedside echo if cause is unclear.
• Send bloods — FBC, U&E (potassium, magnesium, calcium), troponin, thyroid, digoxin level, Lyme serology, ABG. ^[1]

If the patient has adverse features — syncope, hypotension, confusion, ischaemic chest pain, acute heart failure — treat aggressively: ^[1]

• Atropine 0.5 mg IV as the initial dose — intravenous atropine is widely recommended as the first-line treatment for symptomatic bradycardia, and a 2025 scoping review found the practice of using 0.5 mg as the initial dose clinically reasonable, with no trial directly comparing 0.5 mg with 1 mg; none of the moderate-dose (0.5 mg to under 1 mg) studies reported worsening bradycardia. Give further boluses as required and reassess. Cardiac transplant recipients have completely denervated hearts and atropine is unreliable as a chronotropic agent in them (only about half of transplant recipients given atropine during stress testing reached target heart rate in one series) — pacing or infusion therapy is preferred.^[6][16][1]
• Second-line — if atropine fails or is contraindicated (Mobitz II, infranodal block, transplant recipient) — a chronotropic infusion (isoprenaline, or adrenaline/dopamine per local resuscitation protocol) titrated to heart rate and blood pressure. In poisoning-related bradycardia, catecholamines, vasopressors and high-dose insulin euglycaemic therapy are the specific adjuncts (see antidotes below).^[5][10]
• Transcutaneous pacing (TCP) — a lifesaving procedure for patients with certain types of unstable bradycardia while definitive pacing is arranged; set the pacemaker to demand mode and increase output until electrical capture is confirmed, with sedation and analgesia because chest-wall muscle contraction is uncomfortable. In a crossover trial the anteroposterior pad position captured at a mean 33 mA lower threshold than anterolateral (93 vs 126 mA) — guideline bodies favour the AP position. TCP is a bridge to definitive transvenous or permanent pacing.^[17]
• Transvenous pacing — placement of a pacing wire via the internal jugular or femoral vein into the right ventricle under fluoroscopy. Indicated when TCP fails to capture, when pacing is required for a prolonged period, or when pacemaker implantation is being deferred. Risks — pneumothorax, arterial puncture, RV perforation, infection; requires operator experience and fluoroscopy or blind placement.^[1]

Cardiac arrest with bradyarrhythmia: if the patient is in asystolic or severe bradycardic arrest, follow the ALS algorithm (ERC 2021 adult advanced life support) — adrenaline per protocol dosing cycles, identify and treat the reversible cause (the 4 H's: hypoxia, hypo/hyperkalaemia, hypovolaemia, hypothermia, plus the 4 T's: thrombosis, tamponade, tension pneumothorax, toxins).^[5]

Adjunctive therapy in MI-related bradyarrhythmia: ^[1]

• Inferior MI with AV block — reperfusion is the definitive therapy; sinus and nodal blocks are usually transient and recover with reperfusion. Temporary pacing for symptomatic bradycardia (atropine as first-line, transvenous pacing for atropine non-response).^[13]
• Anterior MI with infranodal block — urgent reperfusion (primary PCI); block is usually permanent because the septal necrosis destroys the His–Purkinje system — permanent pacing required.^[1]

Management — Definitive & Stepwise

The definitive management of bradyarrhythmia is cause-specific and staged: identify and treat the cause, support the patient through the acute phase, and implant a permanent pacemaker when the cause is irreversible and the bradyarrhythmia is clinically significant.^[1][3]

Step 1 — Identify and treat the reversible cause (within minutes to hours)

The first priority is to search for a reversible cause because successful treatment obviates the need for permanent pacing. ^[1]

• Drug toxicity — withhold the offending drug (beta-blocker, calcium-channel blocker, digoxin). For digoxin toxicity use digoxin immune Fab (DigiFab/DigiBind) — the specific antidote for life-threatening or potentially life-threatening digoxin toxicity or overdose; supportive care with atropine and pacing as required. For beta-blocker overdose, high-dose glucagon (an accepted antidote with significant haemodynamic effect regardless of beta-blockade) plus catecholamines/vasopressors and high-dose insulin euglycaemic therapy, with IV lipid emulsion and VA-ECMO for refractory cases. For calcium-channel blocker overdose, IV calcium (gluconate or chloride) plus high-dose insulin therapy and vasopressors. Temporary overdrive pacing has a role in poisoning-related arrhythmia.^{[12][18][10][11]}
• Ischaemia — emergency reperfusion (primary PCI for STEMI, fibrinolysis if PCI is unavailable). Bradyarrhythmia in inferior MI is usually self-limiting (resolves as the RCA territory becomes reperfused or the vagal surge subsides); temporary pacing as required.^[13]
• Lyme carditis — IV ceftriaxone 2 g every day (or 1 g every 12 hours) for 2 to 3 weeks — the recommended intravenous regimen for carditis-stage Lyme disease; oral regimens (doxycycline, amoxicillin) serve earlier stages. Temporary pacing may be required, but the block is typically reversible with treatment and permanent pacing is rarely needed.^[9][19]
• Hypothyroidism — levothyroxine replacement (low starting dose in the elderly, titrated against thyroid function); bradycardia resolves over weeks.
• Obstructive sleep apnoea — CPAP reduces nocturnal bradycardia and improves oxygenation.
• Vagal syndromes — vasovagal: non-pharmacological (education, salt, hydration, counter-pressure manoeuvres, head-up tilt sleeping); carotid sinus hypersensitivity: dual-chamber pacing for the cardioinhibitory form over age 40.

Step 2 — Pacemaker implantation (the definitive pacing therapy) — if the cause is irreversible

Class I permanent pacing indications (per 2018 ACC/AHA/HRS and 2013 ESC guidelines):^[1][3]

• Symptomatic sinus node dysfunction including the brady-tachy form of SSS.
• Acquired Mobitz II AV block — even when asymptomatic.
• Acquired complete heart block — even when asymptomatic.
• Post-MI persistent second- or third-degree AV block.
• Post-cardiac-surgery or post-TAVI persistent block.
• Asymptomatic bifascicular block with syncope and HV over 70 ms on EP study.^[1][3]

Pacemaker codes (NBG; NASPE/BPEG Generic): ^[1]

• VVI — ventricular pacing, ventricular sensing, inhibited mode (single-chamber ventricular). Used when atrial sensing is unnecessary (chronic AF with bradycardia).
• AAI — atrial pacing, atrial sensing, inhibited mode. Used in isolated sinus node dysfunction with intact AV conduction.
• DDD — dual-chamber pacing and sensing, with atrial tracking (P-synchronous ventricular pacing) and rate-responsive modes. The standard for sinus node dysfunction and AV block in sinus rhythm (preserves AV synchrony, reduces pacemaker-syndrome, improves exercise capacity).^[1][3]

Choosing the device (the practical algorithm):^[3]

Step 3 — Adjunctive therapies and chronic management

• Anticoagulation in brady-tachy SSS / paroxysmal AF — per CHA2DS2-VASc score, no different from other AF populations.
• Driving and occupational restrictions — symptomatic bradyarrhythmia mandates driving cessation in many jurisdictions; permanent pacemaker restores licence eligibility after a defined period of freedom from syncope.
• Sport and exercise — paced patients with stable restoration of normal heart rate and no evidence of underlying structural disease can usually return to recreational sport; competitive collision sport is restricted in some devices because of the lead vulnerability.
• Follow-up — device clinic review at 1 week, 1 month, every 6 to 12 months, with remote monitoring for the majority of devices. Battery end-of-life (EOL) and lead failure are anticipated complications. ^[1]

Stepwise Management Protocol

The European Resuscitation Council 2021 adult advanced life support bradycardia pathway is the bedside decision-tree that brings the resuscitation, cause identification and definitive therapy together.^[5][1]

Subtypes & Scenarios

• Inferior MI with AV block — in a 102-patient CT angiography series the SA nodal artery arose from the RCA in 67 and the AV node was RCA-supplied in 89. Sinus bradycardia, sinus arrest and nodal block (Mobitz I) are common with inferior occlusion. Usually transient — syncope with new AV block should prompt scrutiny of the inferior leads and early reperfusion; temporary pacing for symptomatic or hypotensive patients.^[13][3]
• Anterior MI with new AV block — proximal LAD occlusion produces septal necrosis and infranodal block (Mobitz II, CHB). Mortality high because of the size of MI. Permanent pacing required.^[1][3]
• Lyme carditis — Borrelia burgdorferi produces reversible high-grade AV block in endemic areas (cardiac involvement in roughly 1 to 10 percent of untreated infections); admit for monitoring and IV ceftriaxone 2 g every day for 2 to 3 weeks; temporary pacing suffices in most.^[19][9]
• Post-cardiac surgery — particularly aortic valve replacement (mechanical valve impact on conduction tissue), TAVI (mean 18.9 percent per study require a permanent pacemaker in a 31 261-patient meta-analysis; baseline RBBB and conduction delay predict it), ventricular septal defect repair, tetralogy of Fallot repair, septal myectomy / alcohol septal ablation. Observe before committing to a permanent device because conduction recovery is common.^[8][3]
• Sleep apnoea — central or obstructive sleep apnoea produces nocturnal bradycardia and pauses at apnoea termination (diving reflex). CPAP reduces nocturnal bradycardia and improves symptoms.^[4]
• Athletic bradycardia — trained endurance athletes have low resting sinus rates with sinus rhythm; enhanced vagal tone can produce marked sinus bradycardia and even prolonged ventricular pauses with AV block, as in a reported marathon runner with 6.6 s vagal pauses — investigation is driven by symptoms, pause length and structural assessment rather than the rate alone.^[20][1]
• Reflex syncope over 40 — in the double-blind VPS-II trial, pacing did not reduce the risk of recurrent syncope in vasovagal syncope (42 percent vs 33 percent recurrence; relative risk reduction 30 percent with CI crossing zero), so pacing is not first-line. In ISSUE-3, highly selected over-40 patients with documented 3 s or longer asystole had a 57 percent relative reduction in syncope recurrence with dual-chamber pacing (25 percent vs 57 percent at 2 years).^[14][15]
• Drug-induced — beta-blocker, calcium-channel blocker, digoxin; withhold drug, supportive care; Fab for digoxin; glucagon for beta-blocker; IV calcium with high-dose insulin for CCB; pacing as bridge if needed.^[12][18][11]
• Carotid sinus hypersensitivity — cardioinhibitory, vasodepressor or mixed. Dual-chamber pacing for the cardioinhibitory form with recurrent syncope in older patients.^[4]
• Infiltrative disease — cardiac sarcoidosis (septal LGE on MRI, granulomatous inflammation, ventricular arrhythmia; can mimic idiopathic AV block in a young patient — consider MRI with gadolinium and FDG-PET), amyloidosis (AL or ATTR with low voltage ECG despite increased wall thickness), haemochromatosis (rare, often with diabetes and skin pigmentation).^[3][4]
• Hyperkalaemia — produces ECG changes that drive emergency treatment (a prehospital protocol treats presumed hyperkalaemia with ECG changes using calcium, sodium bicarbonate and salbutamol with cardiac monitoring; in-hospital regimens add insulin/dextrose and renal replacement therapy for refractory cases).^[21]
• Post-cardiac transplant — the heart is completely denervated after transplantation and atropine is unreliable as a chronotropic agent (only about half of recipients given atropine during stress testing reached target heart rate); infusion therapy or pacing is preferred if bradycardic.^[16]
• Congenital CHB — autoimmune maternal anti-Ro/SSA antibodies (neonatal lupus); pacing if symptomatic, resting rate under 50, exercise intolerance, pauses, ventricular dysfunction. Up to two-thirds require pacing in childhood or early adulthood.^[3]

Complications & Pitfalls

Cardiac complications: ^[1]

• Stokes-Adams syncope — sudden asystolic pause producing cerebral hypoperfusion; injury from falls; rare sudden cardiac death if prolonged.
• Heart failure — chronic bradycardia produces LV dilatation from low output; pacing restores output (and reverse remodels).
• Progression to higher-grade block — Mobitz II has the potential to progress to complete heart block and can lead to death if unrecognized; Mobitz I may also be temporary or permanent depending on the impairment of the conduction system.^[7]
• Tachy-brady syndrome — paroxysmal AF on a background of SSS, with long sinus pauses on AF termination; high stroke risk; anticoagulation required.
• Pacemaker complications — pneumothorax, lead dislodgement, infection (early and late — pocket erosion, endocarditis), lead fracture, venous thrombosis, pacemaker syndrome (AV dyssynchrony in VVI pacing, manifests as fatigue, hypotension, pulsations in the neck — converted to DDD or reduced ventricular pacing). ^[1]

Pitfalls (errors that kill): ^[1]

• Mistaking Mobitz II for Mobitz I — narrow QRS with Wenckebach is nodal; wide QRS Mobitz II is infranodal; Mobitz II has the potential to progress to complete heart block and can lead to death if unrecognized, so it carries the pacing obligation. Always look for the QRS width and the presence or absence of progressive PR prolongation.^[7][1][3]
• Atropine as the answer to everything — atropine is first-line for symptomatic bradycardia, but the optimal dose is unproven (no RCT comparing 0.5 mg with 1 mg) and none of the moderate-dose studies reported worsening bradycardia; escalate to infusion and pacing rather than relying on repeated boluses in high-grade or infranodal block.^[6][17]
• Misdiagnosing vasovagal syncope as AV block — the prodrome (warmth, nausea, sweating, blurred vision) and trigger are absent in AV block; carotid sinus massage and tilt testing can distinguish but pacing is not first-line for vasovagal — VPS-II showed no reduction in recurrent syncope with pacing.^[4][14]
• Misdiagnosing seizure as Stokes-Adams syncope — Stokes-Adams has rapid recovery without post-ictal state; seizure has tonic-clonic activity, tongue biting, urinary incontinence, post-ictal confusion; an ECG and a witness description usually establish the diagnosis. Always do an ECG in unexplained loss of consciousness.^[1][4]
• Forgetting Lyme in summer bradycardia — endemic areas, history of tick exposure / erythema migrans; IV ceftriaxone 2 g daily and temporary pacing; permanent pacing rarely needed.^[9][19]
• Treating hyperkalaemia with atropine alone — atropine does not correct hyperkalaemia; treatment is calcium (with sodium bicarbonate and salbutamol per emergency protocols), insulin/dextrose and renal replacement therapy for refractory cases.^[21]
• Pacing the elderly patient with cognitive impairment without careful discussion — the benefit of pacing in advanced dementia is limited; involve the patient and family in the decision.^[3][4]

Prognosis & Disposition

Prognosis depends on the underlying pattern and cause. Asymptomatic sinus bradycardia in a young athlete is benign; Mobitz II and complete heart block carry the risk of progression to asystole and death if unrecognized, and in anterior MI the prognosis is driven by the size of the infarct — pacing corrects the block but does not restore lost myocardium.^{[7] [1]}

Predictors of adverse outcome: age over 75, LVeF under 35, bundle branch block with syncope, HV over 70 ms on EP study, persistent Mobitz II, CHB with wide-complex escape, non-reversible cause, delay to pacing, recurrent syncope, falls and injury, underlying structural heart disease (LV dysfunction, valvular, infiltrative). ^[1]

Disposition: ^[1]

• Critical care / coronary care unit — symptomatic bradycardia with adverse features, Mobitz II, CHB (especially with wide-complex escape or haemodynamic instability), post-MI bradyarrhythmia, post-cardiac-surgery, post-TAVI new block in the first 24 to 48 hours.
• Telemetry ward — stable sinus bradycardia, Mobitz I without symptoms, post-pacemaker insertion.
• Outpatient — asymptomatic sinus bradycardia, asymptomatic 1st-degree AV block, asymptomatic Mobitz I, bradycardia in a known athlete.
• Specialist pacing clinic follow-up — every 6 to 12 months with remote monitoring; battery end-of-life identified 6 to 12 months in advance to allow elective generator replacement before emergency.^[1][3]

Special Populations

• Athletes — trained endurance athletes have physiologically low resting heart rates, and enhanced vagal tone can produce marked sinus bradycardia and even prolonged pauses; investigation is triggered by symptoms, pauses, Mobitz II or higher-grade block, or syncope rather than the rate alone.^[20][1]
• Elderly (over 75) — high prevalence of SSS and AV block (Lev / Lenegre); falls the commonest presentation. Driving licence cessation / restoration rules vary by jurisdiction. Careful shared-decision making for pacing in cognitive impairment and end-stage disease.^[3]
• Pregnancy — heart rate rises by 10 to 15 bpm in pregnancy; persistent bradycardia is unusual and warrants investigation. Lidocaine, sotalol, amiodarone are problematic. Pacing is safe (radiation-shielded implantation) when required.^[3]
• Paediatric — congenital CHB (maternal anti-Ro/SSA antibodies), post-cardiac-surgery for congenital heart disease (VSD, ToF, TGA repair), long QT with bradycardia-dependent torsades. Pacing when symptomatic or resting HR below 50 bpm; epicardial leads in small children because of size constraints.^[3]
• Cardiac transplant — the heart is completely denervated; atropine is unreliable as a chronotropic agent (only about half of recipients given atropine during stress testing reached target heart rate); pacing is preferred if bradycardic.^[16]
• Bifascicular block with syncope — Class IIa pacing if EP study HV over 70 ms; ILR if EP study inconclusive; many patients have a bradyarrhythmic mechanism identified by ILR within 2 years.^[1][3]

Evidence, Guidelines & Regional Differences

Key guidelines: ^[1]

• 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay (Kusumoto et al., JACC 2019 — PMID 30412709). The most modern North American guideline on bradyarrhythmia.^[1]
• 2013 ESC Guidelines on Cardiac Pacing and Cardiac Resynchronization Therapy (Brignole et al., Europace 2013 — PMID 23801827) — the European reference for pacing indications.^[3]
• 2018 ESC Guidelines for the Diagnosis and Management of Syncope (Brignole et al., EHJ PMID 29562304) — covers the reflex syncope (vasovagal, CSS), orthostatic hypotension and the indications for pacing in reflex syncope (carotid sinus massage criteria, ILR use, ISS).^[4]
• European Resuscitation Council 2021 Adult Advanced Life Support guidelines (Soar et al., Resuscitation 2021 — PMID 33773825) — the source of the bedside acute bradycardia algorithm (atropine first-line, then chronotropic infusion and pacing).^[5]

Landmark trials and statements every exam candidate should know: ^[1]

• VPS-II trial (Connolly et al., JAMA 2003 — PMID 12734133) — in a double-blind randomised trial, pacing therapy did not reduce the risk of recurrent syncope in patients with recurrent severe vasovagal syncope (42 percent vs 33 percent recurrence; 30 percent relative risk reduction with CI crossing zero) — pacing is not first-line for vasovagal syncope.^[14]
• ISSUE-3 (Brignole et al., Circulation 2012 — PMID 22565936) — dual-chamber pacing produced a 57 percent relative reduction in syncope recurrence (25 percent vs 57 percent at 2 years) in patients aged 40 or over with 3 s or longer documented asystole — pacing works only in this highly selected cardioinhibitory group.^[15]
• SPAIN trial — ILR-guided therapy reduced syncope recurrence in unexplained syncope with normal ECG.
• His bundle pacing / conduction system pacing — observational data demonstrate preserved LV function and improved outcomes compared to RV apical pacing; increasingly adopted as an alternative to RV apical or biventricular pacing in AV block with normal LVEF.
• DAVID trial — single-chamber ICD (VVI backup at 40 bpm) versus dual-chamber ICD with pacing mode — DDDR pacing produced more heart failure hospitalisations (ventricular desynchrony and unnecessary ventricular pacing) — informs the minimisation of RV pacing and use of MVP / AAIR→DDD mode-switching algorithms.
• COMPANION, CARE-HF — biventricular pacing (CRT) reduced mortality and hospitalisation in LBBB with LV dysfunction — informs CRT as Class I in bradyarrhythmia with reduced LVEF and bundle branch block.^[3]

Regional deltas: ^[1]

• US / North America (ACC/AHA/HRS) — 2018 bradycardia guideline is the primary reference; antiarrhythmic drug choices influenced by FDA labelling; His bundle pacing increasingly used in major centres.
• UK / Europe (ESC, Resuscitation Council UK) — 2013 ESC and 2018 ESC syncope as primary; ILR-first pathway for unexplained recurrent syncope; pacing for cardioinhibitory CSS over 40 well established; ILR for older patients with unexplained falls has economic and clinical justification in NHS commissioning.
• Asia-Pacific / Low- and middle-income — pacing access remains a challenge; single-chamber VVI pacemakers commonest in resource-limited settings; pacing rates lower than in high-income countries, particularly for indications like CSS where the ILR-and-pacing pathway requires infrastructure.^[3][4]

Current controversies: (1) role of ILR in older fallers (pilot data suggest up to 50 percent of unexplained falls are bradyarrhythmic); (2) conduction system pacing (His bundle / LBB area) as a replacement for RV apical pacing; (3) pacing for vasovagal syncope — ISSUE-3 is suggestive but not definitive; (4) risk-stratification after TAVI for new LBBB / conduction block; (5) digital ECG and AI-driven detection of conduction abnormalities. ^[1]

Exam Pearls

• Bradyarrhythmia = HR below 60 bpm adult; the symptomatic threshold is lower, and trained endurance athletes run physiologically low resting rates. ^[1][20]
• Two master categories — sinus node dysfunction (sinus brady, sinus arrest, SA block, sick sinus syndrome) and AV block — graded 1st, 2nd-degree Mobitz I (Wenckebach, nodal, atropine-responsive, benign), 2nd-degree Mobitz II (infranodal, Class I pacing even when asymptomatic, can progress to complete heart block), 3rd-degree / complete (AV dissociation, Class I pacing).^[7][1]
• Nodal vs infranodal rule — Mobitz I = nodal = narrow QRS = atropine-responsive = usually benign. Mobitz II = infranodal = wide QRS = pacing-required, even when asymptomatic. ^[7][1][3]
• Acute symptomatic bradycardia — atropine 0.5 mg IV initial dose with repeat boluses as required (first-line, widely recommended; optimal dose unproven); if inadequate — chronotropic infusion or transcutaneous pacing (lifesaving bridge in unstable bradycardia); transvenous pacing for definitive; permanent pacing (VVI/AAI/DDD) for irreversible disease.^[6][17]
• Causes — inferior MI (RCA supplies AV node), drug toxicity (beta-blocker, CCB, digoxin), age-related fibrosis (Lev / Lenegre), electrolytes (hyperkalaemia, hypermagnesaemia), Lyme, Chagas, sarcoid, amyloid, post-cardiac surgery (AVR, TAVI, septal repair), vagal (athletes, sleep apnoea, cough, micturition). ^[1][3]
• Inferior MI with AV block — the AV node is RCA-supplied in most patients (89 of 102 on CT angiography), vagal mechanism (Bezold–Jarisch), usually transient, temporary pacing if symptomatic, atropine often effective (nodal block). Permanent pacing usually not required.^[13]
• Anterior MI with new infranodal block (Mobitz II or CHB) — proximal LAD occlusion and septal necrosis, permanent pacing required.
• Mobitz II and 3rd-degree AV block = pacing (permanent), Class I indication.
• Temporary pacing if reversible cause (drug toxicity, acute MI, post-cardiac surgery, Lyme, myocarditis).
• Stokes-Adams syncope — sudden syncope without warning, rapid recovery, no post-ictal confusion, often with a long asystolic pause, contrasted with seizure (tonic-clonic, post-ictal, tongue-biting) and orthostatic (prodrome, standing).^[1][4]
• 2018 ACC/AHA/HRS Class I pacing — symptomatic SSS (and brady-tachy), Mobitz II, complete heart block, post-MI persistent block, post-cardiac surgery / TAVI persistent block. ^[1]
• Choice of device — VVI for AF + bradycardia; AAI for sinus brady with intact AV node; DDD for sinus-node dysfunction and AV block in sinus rhythm (preserves AV synchrony, reduces AF, improves exercise capacity); CRT for LBBB with LV dysfunction; rate-drop pacing for reflex syncope.^[1][3]

Exam application bank (NEET-PG / INICET)

One-line answer

Bradyarrhythmia in adults means a resting heart rate below 60 bpm (the clinical threshold for action drops when symptoms appear, and trained endurance athletes run physiologically low resting rates). The two master categories are sinus node dysfunction (sinus bradycardia, sinus arrest, sinoatrial block, sick sinus syndrome) and atrioventricular (AV) block — 1st degree (PR over 200 ms, benign), 2nd degree Mobitz I (Wenckebach, usually benign, AV nodal, atropine-responsive), 2nd degree Mobitz II (infranodal, potential to progress to complete heart block, Class I indication for pacing even when asymptomatic), and 3rd degree (complete heart block, AV dissociation, Class I pacing). Common aetiologies include age-related fibrosis and calcification (Lev and Lenegre disease), inferior MI (the AV node is RCA-supplied in most patients), drug toxicity (beta-blocker, calcium-channel blocker, digoxin), hyperkalaemia, Lyme carditis and post-cardiac surgery including TAVI (mean 18.9 percent per study require a permanent pacemaker). Acute management: atropine 0.5 mg IV initial dose first-line for symptomatic bradycardia, then chronotropic infusion and transcutaneous/transvenous pacing; permanent pacing (VVI/AAI/DDD) for irreversible Mobitz II and complete heart block. Lyme carditis — IV ceftriaxone 2 g every day for 2 to 3 weeks; pacing for vasovagal syncope is not first-line (VPS-II) but cuts recurrence by 57 percent in selected over-40s with 3 s or longer documented asystole (ISSUE-3).^{[1][7][13][8][6][17][9][14][15]}

Worked stems (answer without another resource)

Stem 1 — Classic presentation. Map symptoms to mechanism; name the first investigation and first treatment step with dose/route if drug therapy is standard. ^[1]

Stem 2 — Unstable / complicated. List red flags that force immediate resuscitation, theatre, ICU, antidote, or reperfusion — and what you do in the first 15 minutes. ^[1]

Stem 3 — Atypical group. Elderly, pregnancy, child, or immunocompromised: how presentation and thresholds change. ^[1]

Stem 4 — Differential trap. Name the three closest mimics and one discriminator for each. ^[1]

Stem 5 — Disposition. Who goes home with safety-netting, who is admitted, who needs HDU/ICU/theatre, and what follow-up is mandatory. ^[1]

Rapid viva checklist

1. Definition + classification
2. Pathophysiology chain
3. Bedside signs / criteria
4. Score with exact components (if any)
5. Emergency bundle
6. Definitive therapy with doses
7. Complications of disease and of treatment
8. Special populations
9. Guideline/trial name if classic
10. Three exam traps

Coverage self-check

If you cannot answer any stem above from this page alone, re-read the matching section — the page is intended to be self-sufficient for final-prof and NEET-PG/INICET questions on Bradycardia and AV Block.

The mantra, and the ward-round test

^{[1] [3]}

The mantra: Mobitz I is nodal and forgivable; Mobitz II is infranodal and unforgiving — the first gets atropine and observation, the second gets a pacemaker even when the patient feels well.^[1][3]

Ward-round test — three stems, thirty seconds each

Stem 1 — the asymptomatic Mobitz II (answer)ShowHide

A 68-year-old man is admitted with a community-acquired pneumonia. A routine ECG shows 2-to-1 AV block with a fixed PR and a wide QRS. He denies syncope, dizziness, or chest pain. The junior wants to discharge him once the pneumonia settles. What is the right plan? Model: This is 2nd-degree Mobitz II block — fixed PR with sudden non-conducted P waves, a wide QRS marking it as infranodal. Mobitz II is a Class I indication for permanent pacing even when the patient is asymptomatic, because it has the potential to progress to complete heart block and can lead to death if unrecognized. Atropine will not help (the block is below the AV node). Admit for cardiac monitoring, pacing work-up, and permanent pacemaker implantation once the acute infection is treated. Do not discharge to "observe".^[7][1][3]

Stem 2 — the symptomatic bradycardia that ignores atropine (answer)ShowHide

A 75-year-old on metoprolol and diltiazem for hypertension presents with syncope, heart rate 34 bpm, blood pressure 78/50, wide-complex escape. You give repeated atropine boluses with no change. What next, and why did atropine fail? Model: Atropine failed because the block is infranodal (wide-complex escape) — atropine speeds AV-nodal conduction, not the His–Purkinje system. He is symptomatic and unstable, so escalate the bradycardia algorithm: chronotropic infusion and transcutaneous pacing (the lifesaving bridge in unstable bradycardia) on the way to transvenous and then permanent pacing. Treat the overdose in parallel — IV calcium plus high-dose insulin for the calcium-channel blocker and glucagon for the beta-blocker — but do not delay pacing for the antidote.^{[6][17][11][18]}

Stem 3 — high-grade AV block in summer (answer)ShowHide

A 34-year-old man from a Lyme-endemic region presents in July with new high-grade AV block, a wide-complex escape at 38 bpm, and a history of a spreading red rash three weeks ago after a hiking trip. What is the diagnosis, and what is the management? Model: This is Lyme carditis — Borrelia burgdorferi causing reversible high-grade AV block (cardiac involvement occurs in about 1 to 10 percent of untreated infections and most commonly manifests as AV conduction abnormalities). The block can be rapidly progressive, so admit for cardiac monitoring. Give IV ceftriaxone 2 g every day for 2 to 3 weeks (the recommended regimen for carditis-stage Lyme disease), and use temporary pacing (transcutaneous or transvenous) if he is symptomatic or the escape is unstable. Permanent pacing is rarely required — the block typically resolves with antibiotic treatment. The teaching point: a young patient with new high-grade AV block in an endemic area is Lyme carditis until proven otherwise, and is usually reversible.^[19][9]

References20ShowHide

1. [1]Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society J Am Coll Cardiol, 2019.PMID 30412709
2. [3]Brignole M, Auricchio A, Baron-Esquivias G, et al. 2013 ESC guidelines on cardiac pacing and cardiac resynchronization therapy: the task force on cardiac pacing and resynchronization therapy of the European Society of Cardiology (ESC). Developed in collaboration with the European Heart Rhythm Association (EHRA) Europace, 2013.PMID 23801827
3. [4]Brignole M, Moya A, de Lange FJ, et al. 2018 ESC Guidelines for the diagnosis and management of syncope Eur Heart J, 2018.PMID 29562304
4. [5]Soar J, Böttiger BW, Carli P, et al. European Resuscitation Council Guidelines 2021: Adult advanced life support Resuscitation, 2021.PMID 33773825
5. [6]Yokose M, Sangawa M, Shiomi H, et al. Initial Dose of Intravenous Atropine for Patients With Symptomatic Bradycardia - A Scoping Review Circ Rep, 2025.PMID 41216404
6. [7]Mangi MA, Jones WM, Mansour MK, et al. Second-Degree Atrioventricular Block StatPearls, 2026.PMID 29493981
7. [8]Ullah W, Zahid S, Zaidi SR, et al. Predictors of Permanent Pacemaker Implantation in Patients Undergoing Transcatheter Aortic Valve Replacement - A Systematic Review and Meta-Analysis J Am Heart Assoc, 2021.PMID 34259045
8. [9]Sigal LH Current recommendations for the treatment of Lyme disease Drugs, 1992.PMID 1379147
9. [10]Rotella JA, Greene SL, Koutsogiannis Z, et al. Treatment for beta-blocker poisoning: a systematic review Clin Toxicol (Phila), 2020.PMID 32310006
10. [11]Baid H, Kaeley N, Singh S, et al. Treatment Modalities in Calcium Channel Blocker Overdose: A Systematic Review Cureus, 2023.PMID 37664357
11. [12]Wei S, Niu MT, Dores GM Adverse Events Associated with Use of Digoxin Immune Fab Reported to the US Food and Drug Administration Adverse Event Reporting System, 1986-2019 Drugs Real World Outcomes, 2021.PMID 33721285
12. [13]Saremi F, Abolhoda A, Ashikyan O, et al. Arterial supply to sinuatrial and atrioventricular nodes: imaging with multidetector CT Radiology, 2008.PMID 18024438
13. [14]Connolly SJ, Sheldon R, Thorpe KE, et al. Pacemaker therapy for prevention of syncope in patients with recurrent severe vasovagal syncope: Second Vasovagal Pacemaker Study (VPS II): a randomized trial JAMA, 2003.PMID 12734133
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