Bradycardia in Adults

Topic Overview

Summary

Bradycardia is defined as a heart rate below 60 beats per minute (bpm) on resting electrocardiogram. While bradycardia can be a normal physiological finding in well-conditioned athletes and during sleep, pathological bradycardia results from dysfunction of the sinoatrial (SA) node, impaired atrioventricular (AV) conduction, or extrinsic factors such as medications, metabolic disturbances, or autonomic dysfunction. [1] The clinical significance depends on the presence of adverse features including hypotension, syncope, myocardial ischaemia, or heart failure, which determine the urgency of intervention. [2]

Sinus node dysfunction (previously termed "sick sinus syndrome") encompasses a spectrum of conditions involving abnormal impulse formation at the SA node, including sinus bradycardia, sinus arrest, sinoatrial exit block, and chronotropic incompetence. [3] Atrioventricular blocks represent disorders of impulse conduction from atria to ventricles, classified by degree and anatomical location. Second-degree AV block is subdivided into Mobitz type I (Wenckebach), characterized by progressive PR prolongation at the AV node level, and Mobitz type II, an infranodal conduction disturbance with higher risk of progression to complete heart block. [4]

The management paradigm hinges on distinguishing unstable (haemodynamically compromised) from stable bradycardia. Unstable bradycardia requires immediate treatment with atropine as first-line therapy, followed by transcutaneous or transvenous pacing if atropine fails or is contraindicated. [2] Stable symptomatic bradycardia necessitates identification and reversal of extrinsic causes, with consideration for permanent pacemaker implantation in cases of intrinsic conduction system disease. [5]

Key Facts

• Definition: Heart rate less than 60 bpm; symptomatic bradycardia requires correlation with clinical features
• Prevalence: Affects approximately 0.5-2% of the general population; higher in elderly (up to 5%) [3,24]
• Physiological bradycardia: Common in athletes (resting HR 40-60 bpm), during sleep, and with increased vagal tone
• Pathological causes: Sinus node dysfunction, AV block, drugs (beta-blockers, calcium channel blockers, digoxin), hypothyroidism, hyperkalaemia, hypothermia, myocardial infarction
• Adverse features: Hypotension (systolic BP less than 90 mmHg), altered mental status, syncope, chest pain/myocardial ischaemia, acute heart failure
• First-line emergency treatment: Atropine 500 mcg IV, repeat every 3-5 minutes (maximum 3 mg)
• Atropine limitations: Ineffective in complete heart block and infranodal blocks (Mobitz II, third-degree); may paradoxically worsen conduction
• Definitive treatment: Temporary pacing (transcutaneous/transvenous) for unstable patients; permanent pacemaker for symptomatic intrinsic conduction disease

Clinical Pearls

Atropine in Complete Heart Block: Atropine enhances AV nodal conduction but does not affect infranodal conduction. In complete heart block with a wide QRS escape rhythm (indicating infranodal block), atropine may increase atrial rate without improving ventricular rate, potentially worsening haemodynamics. Be prepared for pacing in these patients. [2]

Inferior MI and Bradycardia: The right coronary artery (RCA) supplies the SA node in 60% and AV node in 90% of individuals. Inferior myocardial infarction frequently causes bradycardia via vagal stimulation and ischaemia of conduction tissue. This bradycardia is often transient and responds to atropine, but may require temporary pacing if severe. [6]

Drug-Induced Bradycardia: Beta-blockers and calcium channel blockers (especially verapamil and diltiazem) are leading causes of symptomatic bradycardia. In severe poisoning, conventional therapies (atropine, calcium, glucagon) often fail. High-dose insulin euglycaemia therapy (insulin 1 unit/kg bolus, then 1-10 units/kg/hour with glucose supplementation) is first-line treatment for severe toxicity. [7]

Chronotropic Incompetence: Defined as failure to achieve 80% of age-predicted maximum heart rate (220 - age) during exercise. Present in approximately 50% of patients with sinus node dysfunction. Associated with reduced exercise tolerance despite normal resting heart rate. Exercise stress testing is essential when symptoms correlate with exertion. [3]

2:1 AV Block Dilemma: When every other P wave is blocked (2:1 conduction), distinguishing Mobitz I from Mobitz II can be challenging. Key differentiators: Mobitz II typically shows bundle branch block, wider QRS, and infranodal location. Carotid sinus massage may improve conduction in Mobitz I (unmasking Wenckebach pattern) but worsen Mobitz II. [4]

Why This Matters Clinically

Bradycardia with adverse features constitutes a medical emergency requiring immediate intervention to prevent cardiac arrest. The mortality associated with untreated complete heart block approaches 50% annually without pacemaker therapy. [8] Conversely, inappropriate treatment of physiological bradycardia can lead to unnecessary interventions and adverse outcomes. Rapid assessment of haemodynamic stability, accurate ECG diagnosis, and appropriate risk stratification are essential skills for acute care physicians. Permanent pacemaker implantation in appropriately selected patients dramatically reduces mortality and improves quality of life, with modern dual-chamber pacemakers reducing the incidence of atrial fibrillation and thromboembolic complications compared to single-chamber ventricular pacing. [5]

---

Visual Summary

Visual assets to be added:

• Advanced Life Support bradycardia algorithm (Resuscitation Council UK/ERC 2021)
• ECG examples: sinus bradycardia, first-degree AV block, Mobitz I, Mobitz II, third-degree heart block
• Anatomical illustration: cardiac conduction system with arterial blood supply
• Transcutaneous pacing pad placement (anterior-posterior and anterior-lateral)
• Decision tree: distinguishing physiological from pathological bradycardia
• Timeline: progression risk from Mobitz II to complete heart block
• Mechanism diagram: atropine action at muscarinic receptors
• Pacemaker modes and indications chart

---

Epidemiology

Incidence and Prevalence

Bradycardia is a common clinical finding with variable prevalence depending on the population studied and definition applied. Resting heart rate less than 60 bpm is present in approximately 15-25% of healthy young adults and trained athletes, representing physiological adaptation rather than pathology. [9]

Pathological bradycardia increases with age. Sinus node dysfunction occurs in approximately 1 in 600 cardiac patients over age 65, with incidence rising to 1 in 200 over age 75. [3] The prevalence of chronic AV block requiring pacemaker therapy is estimated at 0.04% in the general population, with higher rates in elderly individuals and those with structural heart disease. [5]

Approximately 50% of permanent pacemakers implanted in developed countries are for treatment of sinus node dysfunction, with AV block accounting for 35-40% and other indications comprising the remainder. [5,10,25] The annual incidence of pacemaker implantation ranges from 60-80 per million population in developed countries, translating to over 4,000 new pacemakers annually in the United Kingdom and 225,000 in the United States. [10,25]

Demographics and Risk Factors

Age: The most significant risk factor for bradyarrhythmias is advancing age, reflecting progressive degenerative fibrosis of the conduction system. The mean age at pacemaker implantation for sinus node dysfunction is 70-75 years. [3]

Sex: Males demonstrate slightly higher rates of AV block, while females have marginally higher rates of sinus node dysfunction, though differences are modest. [5]

Athletic Training: Endurance athletes commonly develop physiological sinus bradycardia (heart rate 40-60 bpm) and first-degree AV block due to enhanced vagal tone and cardiac remodeling. Prevalence of resting bradycardia exceeds 80% in elite endurance athletes. [9]

Cardiovascular Disease: Structural heart disease, including ischaemic heart disease, cardiomyopathy, myocarditis, and infiltrative diseases (sarcoidosis, amyloidosis), increases risk of conduction system disease. [11,26]

Genetic Factors: Familial forms of progressive cardiac conduction disease are recognized, most commonly due to mutations in SCN5A (sodium channel) and LMNA (lamin A/C) genes, typically with autosomal dominant inheritance. [12,27]

Causes and Aetiology

Bradycardia results from dysfunction at various levels of the cardiac conduction system or from extrinsic factors affecting normal conduction. A systematic approach categorizes causes as intrinsic (structural/degenerative) or extrinsic (reversible).

Intrinsic Cardiac Causes

Extrinsic (Reversible) Causes

---

Pathophysiology

Normal Cardiac Conduction

Understanding normal conduction is essential for comprehending bradyarrhythmia mechanisms. The cardiac action potential originates in the sinoatrial (SA) node, located at the junction of the superior vena cava and right atrium. SA node automaticity is determined by spontaneous diastolic depolarization (phase 4) mediated primarily by "funny current" (If) through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. [13]

The intrinsic firing rate of the SA node is approximately 100-110 bpm, modulated by autonomic tone to produce a resting heart rate of 60-100 bpm. Sympathetic stimulation (via beta-1 adrenergic receptors) increases SA node firing rate, while parasympathetic stimulation (via muscarinic M2 receptors) slows firing by increasing potassium efflux and reducing calcium influx. [13,28]

Following SA node depolarization, the electrical impulse propagates through atrial myocardium via preferential internodal pathways to reach the atrioventricular (AV) node, located in the inferior-posterior right atrium near the coronary sinus ostium. The AV node provides the only normal electrical connection between atria and ventricles and introduces a physiological delay (≈100 ms), allowing ventricular filling.

From the AV node, the impulse proceeds through the bundle of His, which penetrates the fibrous annulus and divides into left and right bundle branches. The left bundle branch further subdivides into anterior and posterior fascicles. The bundle branches terminate in the Purkinje fiber network, enabling rapid and coordinated ventricular depolarization. [13]

Blood Supply to Conduction System

Understanding the arterial supply to conduction tissue explains patterns of bradycardia in myocardial infarction:

• SA Node: Supplied by the SA nodal artery, arising from the right coronary artery (RCA) in 60% of individuals and left circumflex artery (LCx) in 40%. [6]
• AV Node: Supplied by the AV nodal artery, arising from the RCA in 90% (dominant right coronary system) and LCx in 10% (dominant left circumflex system). [6]
• Bundle of His and Proximal Bundle Branches: Dual blood supply from AV nodal artery and septal perforators of left anterior descending artery (LAD), providing relative protection from ischaemia. [6]

Clinical Correlation: Inferior myocardial infarction (typically RCA occlusion) commonly causes SA and/or AV node dysfunction via direct ischaemia and vagal stimulation (Bezold-Jarisch reflex). This bradycardia is often transient and atropine-responsive. Anterior MI (LAD occlusion) rarely causes AV block but when present indicates extensive septal infarction with infranodal block, associated with poor prognosis. [6]

Mechanisms of Bradycardia

Bradycardia arises through three primary mechanisms: decreased SA node automaticity, impaired AV conduction, or enhanced parasympathetic (vagal) tone.

Sinus Node Dysfunction

Sinus node dysfunction encompasses several electrocardiographic manifestations:

1. Sinus Bradycardia: Slow but regular SA node firing (less than 60 bpm); P wave morphology and PR interval remain normal.

2. Sinus Pause/Arrest: Failure of SA node impulse formation resulting in absence of P waves for > 2-3 seconds. The pause is not a multiple of the baseline PP interval (distinguishing it from sinoatrial exit block).

3. Sinoatrial Exit Block: SA node fires normally but impulse fails to exit to atrial myocardium. The pause duration is a multiple of the baseline PP interval.

4. Chronotropic Incompetence: Inappropriate heart rate response to physiological demands, particularly exercise. Defined as failure to achieve ≥80% of age-predicted maximum heart rate (220 - age) during maximal exercise testing. Present in approximately 50% of sinus node dysfunction cases. [3]

5. Bradycardia-Tachycardia Syndrome: Alternating bradyarrhythmias (typically sinus bradycardia or pauses) and tachyarrhythmias (most commonly atrial fibrillation). Represents advanced sinus node disease with both depressed automaticity and atrial remodeling predisposing to arrhythmias. [3]

Pathological Substrate: Most sinus node dysfunction in elderly patients results from idiopathic degenerative fibrosis (Lenègre-Lev disease), characterized by progressive replacement of nodal tissue and atrial myocardium with fibrous and fatty tissue. [3]

Atrioventricular Blocks

AV blocks are classified by severity (first, second, or third degree) and anatomical site (AV nodal versus infranodal).

First-Degree AV Block: Prolonged PR interval > 200 ms (5 small squares) with 1:1 AV conduction. Represents delayed conduction through the AV node (80% of cases) or less commonly the His-Purkinje system. Generally benign but may progress to higher-degree block. [4]

Second-Degree AV Block - Mobitz Type I (Wenckebach):

• Mechanism: Progressive prolongation of AV nodal conduction until an impulse fails to conduct (blocked P wave).
• ECG Features: Progressive PR interval prolongation followed by a non-conducted P wave; the PR interval after the blocked beat is shorter than the PR immediately before the block; RR interval progressively shortens before the dropped beat; the pause containing the blocked P wave is less than twice the preceding RR interval.
• Anatomical Site: AV node (97% of cases).
• Significance: Generally benign when occurring at the AV node; often transient; may result from increased vagal tone or medications; rarely progresses to complete heart block. [4]

Second-Degree AV Block - Mobitz Type II:

• Mechanism: Intermittent failure of infranodal conduction without preceding PR prolongation.
• ECG Features: Fixed PR interval with sudden non-conducted P waves; often associated with bundle branch block (QRS ≥120 ms); may occur with fixed ratios (2:1, 3:1).
• Anatomical Site: His-Purkinje system (infranodal).
• Significance: High risk (35-50% annually) of progression to complete heart block and sudden death; requires permanent pacemaker regardless of symptoms. [4,5]

Third-Degree (Complete) AV Block:

• Mechanism: Complete failure of AV conduction; atria and ventricles depolarize independently.
• ECG Features: No relationship between P waves and QRS complexes (AV dissociation); atrial rate exceeds ventricular rate; escape rhythm determines QRS morphology and rate.
• Escape Rhythms:
  • "AV nodal escape (40-60 bpm): Narrow QRS, relatively stable, better haemodynamic tolerance."
  • "Ventricular escape (20-40 bpm): Wide QRS, less reliable, poor haemodynamic tolerance, risk of asystole."
• Significance: Symptomatic complete heart block requires permanent pacemaker; even asymptomatic cases warrant pacing due to risk of sudden death. [5]

2:1 AV Block - Diagnostic Challenge

When every other P wave is blocked, determining Mobitz I versus II is impossible without additional information:

• Favour Mobitz I (AV nodal): Narrow QRS complex, improvement with atropine or exercise, occurrence during vagal stimulation or with AV nodal blocking drugs.
• Favour Mobitz II (infranodal): Bundle branch block pattern (wide QRS), worsening with atropine, associated with anterior MI or extensive conduction disease.
• Electrophysiology Study: May be required for definitive localization if clinically important. [4]

Autonomic Influences

The autonomic nervous system profoundly influences heart rate:

Parasympathetic (Vagal) Effects:

• Mediated by acetylcholine acting on muscarinic M2 receptors.
• Increases potassium conductance (IKACh current), hyperpolarizing SA and AV nodal cells.
• Reduces If ("funny current") and calcium influx.
• Predominates at rest, particularly during sleep and in athletes.
• Vagal tone explains many cases of asymptomatic sinus bradycardia and transient AV block. [13]

Sympathetic Effects:

• Mediated by norepinephrine acting on beta-1 adrenergic receptors.
• Increases If current, calcium influx, and rate of diastolic depolarization.
• Increases SA node firing rate and AV node conduction velocity.
• Predominates during exercise, stress, and illness. [13]

Clinical Implications: Enhanced vagal tone explains bradycardia in:

• Well-conditioned athletes (resting bradycardia 40-60 bpm)
• Vasovagal syncope (sudden vagal surge)
• Inferior MI (Bezold-Jarisch reflex)
• Carotid sinus hypersensitivity
• Hypothyroidism (reduced metabolic demand and cardiac sympathetic responsiveness)

Molecular and Cellular Mechanisms

Recent advances have elucidated genetic and molecular causes of bradyarrhythmias:

Ion Channelopathies:

• Loss-of-function mutations in SCN5A (encoding cardiac sodium channel Nav1.5) cause progressive cardiac conduction disease with varying penetrance. [12]
• Mutations in HCN4 (encoding If channel) cause sinus node dysfunction with or without atrial fibrillation. [12]

Structural Protein Defects:

• LMNA mutations (encoding lamin A/C) cause dilated cardiomyopathy with early and progressive AV block, requiring pacemaker at young age. [12]

Autoimmune Mechanisms:

• Maternal anti-Ro (SSA) and anti-La (SSB) antibodies cross the placenta and damage fetal conduction tissue, causing congenital complete heart block in approximately 2% of exposed fetuses. [14,29]

---

Clinical Presentation

Symptom Spectrum

The clinical presentation of bradycardia ranges from asymptomatic incidental findings to life-threatening cardiovascular collapse. Symptom severity correlates with absolute heart rate, rate of onset, presence of underlying cardiovascular disease, and adequacy of compensatory mechanisms.

Asymptomatic Bradycardia

Many patients with chronic bradycardia, particularly sinus bradycardia and first-degree AV block, remain asymptomatic due to adequate stroke volume compensation. Athletes routinely tolerate resting heart rates of 40-50 bpm without symptoms. Asymptomatic patients do not require treatment but warrant evaluation to exclude reversible causes and monitor for progression. [9]

Cerebral Hypoperfusion Symptoms

The brain's high metabolic demand and limited oxygen reserves make it vulnerable to reduced cardiac output:

• Fatigue and Weakness: Most common symptoms; often subtle and attributed to aging or deconditioning. Present in 60-70% of symptomatic sinus node dysfunction. [3]
• Lightheadedness and Dizziness: Chronic mild cerebral hypoperfusion; occurs with exertion in chronotropic incompetence.
• Presyncope: Sensation of impending loss of consciousness; "graying out" of vision; necessitates sitting or lying down.
• Syncope: Transient loss of consciousness with spontaneous recovery. Occurs in approximately 50% of patients with symptomatic sinus node dysfunction. High-risk feature requiring urgent evaluation. [3]
• Cognitive Impairment: Chronic bradycardia may contribute to cognitive decline in elderly patients, potentially reversible with pacing. [15,30]

Cardiovascular Symptoms

• Dyspnoea: Reduced cardiac output during exertion causes breathlessness despite normal ventricular function.
• Exercise Intolerance: Inability to increase heart rate appropriately limits cardiac output augmentation during exertion (chronotropic incompetence).
• Chest Pain/Angina: Reduced coronary perfusion pressure combined with increased ventricular filling pressures may provoke angina even without obstructive coronary disease.
• Heart Failure: Chronic severe bradycardia may cause or exacerbate heart failure ("bradycardia-induced cardiomyopathy"), which may improve with pacing. [16,31]

Other Symptoms

• Palpitations: Paradoxical symptom in bradycardia-tachycardia syndrome, where patients experience both slow and fast heart rhythms.
• Oliguria: Reduced renal perfusion in severe bradycardia.
• Confusion/Altered Mental Status: Acute severe bradycardia; red flag requiring immediate intervention.

Physical Examination Findings

Vital Signs

• Heart Rate: less than 60 bpm by definition; severe bradycardia less than 40 bpm
• Blood Pressure: May be normal (compensated), low (decompensated), or elevated (hypertensive patients on rate-limiting drugs)
• Respiratory Rate: May increase compensatorily in low cardiac output states
• Oxygen Saturation: Usually normal unless pulmonary oedema develops

Cardiovascular Examination

Pulse Characteristics:

• Rate: Slow, measure for full 60 seconds to detect irregularities
• Rhythm: Regular in sinus bradycardia and complete heart block with stable escape; irregular in sinus node dysfunction with pauses, or complete block with irregular escape
• Volume: May be increased (large stroke volume) in chronic compensated bradycardia; reduced in acute decompensated bradycardia

Jugular Venous Pressure (JVP):

• Cannon A Waves: Intermittent prominent jugular pulsations occurring when right atrium contracts against a closed tricuspid valve. Pathognomonic of AV dissociation (complete heart block, ventricular tachycardia). Irregular cannon waves suggest complete heart block; regular cannon waves suggest ventricular pacing or junctional rhythm. [17]
• Elevated JVP: May indicate heart failure secondary to bradycardia or concomitant cardiac disease

Cardiac Auscultation:

• Variable S1 Intensity: In complete heart block, S1 intensity varies depending on the relationship between atrial and ventricular contraction. When the atria contract just before the ventricles (short PR interval), S1 is loud; when contraction timing is suboptimal, S1 is soft. [17]
• S3/S4 Gallop: May indicate heart failure or ventricular dysfunction

Signs of Haemodynamic Compromise:

• Hypotension: Systolic BP less than 90 mmHg or > 30 mmHg drop from baseline
• Shock: Cold peripheries, prolonged capillary refill time (> 2 seconds), pallor, diaphoresis
• Pulmonary Oedema: Bilateral crackles, indicating acute heart failure

Neurological Examination

• Level of Consciousness: Alert versus confused/drowsy versus unresponsive
• Focal Neurological Signs: Exclude stroke as cause or consequence of syncope

General Examination - Clues to Aetiology

• Hypothyroidism: Bradycardia, hypothermia, delayed relaxation of reflexes, dry skin, non-pitting oedema (myxoedema), hair loss
• Hypothermia: Core temperature less than 35°C; shivering, confusion, J waves (Osborn waves) on ECG
• Hyperkalaemia: Muscle weakness, absent reflexes; ECG shows tall peaked T waves, widened QRS
• Increased Intracranial Pressure: Bradycardia + hypertension + irregular respirations = Cushing's triad; examine for papilloedema

Adverse Features - Immediate Risk Stratification

The Resuscitation Council UK Advanced Life Support guidelines define adverse features indicating haemodynamically unstable bradycardia requiring urgent treatment: [2]

Presence of any adverse feature indicates unstable bradycardia requiring immediate atropine and preparation for pacing. [2]

Red Flags - High-Risk Features

Beyond acute adverse features, certain findings indicate high risk of progression or sudden death, necessitating urgent specialist evaluation and likely pacemaker implantation:

• Mobitz Type II AV Block: 35-50% annual risk of progression to complete heart block [4,5]
• Complete (Third-Degree) Heart Block: Unreliable escape rhythm; risk of asystole
• Ventricular Pause ≥3 Seconds: Indicates severe conduction disease
• Symptomatic Sinus Node Dysfunction: Syncope or presyncope correlated with bradycardia
• Wide-Complex Escape Rhythm: Indicates infranodal escape with poor reliability
• Bradycardia in Acute MI: May indicate extensive infarction (anterior MI with AV block) or require temporary pacing
• Drug Toxicity with Haemodynamic Compromise: Beta-blocker or calcium channel blocker overdose may require advanced therapies beyond atropine

---

Clinical Examination

Systematic Approach

A structured examination identifies bradycardia severity, haemodynamic consequences, and aetiological clues.

Initial Assessment (ABCDE Approach)

Airway: Patent and maintained

Breathing:

• Respiratory rate (normal 12-20/min; tachypnoea may indicate compensation or pulmonary oedema)
• Oxygen saturation
• Auscultation: bilateral air entry, crackles suggesting heart failure

Circulation:

• Heart rate and rhythm (palpate for 60 seconds)
• Blood pressure (compare to baseline if known)
• Capillary refill time (less than 2 seconds normal)
• Peripheral perfusion (warm versus cold extremities)

Disability:

• Conscious level (AVPU or GCS)
• Blood glucose (exclude hypoglycaemia)

Exposure:

• Temperature (exclude hypothermia)
• Signs of trauma from syncope

Focused Cardiovascular Assessment

Jugular Venous Pulse:

• Position patient at 45 degrees
• Identify JVP pulsations (versus carotid pulsations)
• Assess JVP height (normal less than 3 cm above sternal angle)
• Observe for cannon A waves (irregular giant A waves = complete heart block)

Precordial Palpation:

• Apex beat position and character
• Parasternal heave (right ventricular hypertrophy)
• Thrills (palpable murmurs)

Auscultation:

• Heart sounds: intensity and variation of S1 (variable in complete heart block), presence of S3/S4
• Murmurs: aortic stenosis (radiation to carotids), mitral regurgitation (radiation to axilla)
• Pericardial rub (myopericarditis)

Peripheral Pulses:

• Radial pulse: rate, rhythm, character, symmetry
• Femoral pulses: assess for radiofemoral delay (aortic coarctation)

Examination for Underlying Causes

Thyroid Status:

• Palpate thyroid gland: goiter, nodules
• Signs of hypothyroidism: bradycardia, hypothermia, dry skin, periorbital oedema, delayed relaxation of ankle reflex, pretibial myxoedema (rare)

Medication Review:

• Review drug chart/medication list
• Specific attention to: beta-blockers, calcium channel blockers, digoxin, amiodarone, ivabradine, cholinesterase inhibitors

Neurological Assessment (if syncope occurred):

• Focal deficits suggesting stroke
• Signs of raised intracranial pressure (papilloedema, Cushing's triad)

Musculoskeletal Assessment (if syncope resulted in fall):

• Injuries: fractures, lacerations, head trauma

---

Investigations

Electrocardiogram (ECG) - Essential First Investigation

A 12-lead ECG is mandatory in all patients with bradycardia. It provides diagnosis, risk stratification, and guides management.

Systematic ECG Interpretation in Bradycardia

1. Rate: Calculate ventricular rate (300/number of large squares between R waves, or count QRS complexes in 10 seconds × 6)

2. Rhythm: Identify P waves and their relationship to QRS complexes

3. PR Interval: Measure from start of P wave to start of QRS (normal 120-200 ms / 3-5 small squares)

4. QRS Duration: Narrow (less than 120 ms) suggests supraventricular rhythm; wide (≥120 ms) suggests ventricular origin or bundle branch block

5. P:QRS Relationship: 1:1 (normal or first-degree block), intermittent conduction (second-degree block), or AV dissociation (third-degree block)

ECG Patterns in Bradycardia

Additional ECG Features to Assess

QRS Morphology:

• Bundle branch block patterns increase likelihood of infranodal conduction disease (Mobitz II, complete heart block with ventricular escape)

J Waves (Osborn Waves): Positive deflection at J point (QRS-ST junction); seen in hypothermia

Hyperkalaemia: Tall peaked T waves, widened QRS, flattened/absent P waves, sine wave pattern (severe)

Ischaemia/Infarction: ST-segment elevation/depression, T wave inversion, Q waves

Other Arrhythmias: Atrial fibrillation with slow ventricular response (irregularly irregular rhythm, absent P waves), sinus arrhythmia (beat-to-beat variation in PP interval, normal in young patients)

Blood Tests

Essential Initial Tests

Conditional Tests

Cardiac Monitoring

Continuous ECG Monitoring (Telemetry)

• Indication: All patients with symptomatic bradycardia, high-degree AV block, or adverse features
• Purpose: Detect pauses, assess rhythm variability, monitor for progression
• Duration: Until definitive treatment (pacing) or reversible cause corrected

Ambulatory ECG Monitoring

24-48 Hour Holter Monitor:

• Indication: Symptoms occurring daily; assessment of heart rate variability; quantify burden of bradycardia or pauses
• Interpretation: Symptomatic bradycardia requires correlation between symptoms (patient diary) and rhythm abnormalities

Extended Ambulatory Monitoring (7-30 days):

• Indication: Less frequent symptoms (weekly)
• Types: Patch monitors, event recorders

Implantable Loop Recorder (ILR):

• Indication: Infrequent symptoms (monthly) where diagnosis is critical, particularly unexplained syncope
• Duration: Up to 3 years continuous monitoring
• Evidence: Reveals diagnosis in 50-60% of unexplained syncope cases; cost-effective compared to repeated investigations [18]

Exercise Stress Testing

Indication: Symptoms with exertion; suspected chronotropic incompetence

Protocol: Standard Bruce protocol or modified protocols for elderly/limited mobility

Interpretation:

• Chronotropic Incompetence: Failure to achieve ≥80% of age-predicted maximum heart rate (220 - age) despite maximal effort. Present in 50% of sinus node dysfunction. Indicates need for rate-responsive pacemaker. [3]
• Exercise-Induced AV Block: Development of second- or third-degree AV block during exercise suggests infranodal disease; requires pacemaker

Caution: Contraindicated in acute MI, unstable angina, severe symptomatic bradycardia (perform under controlled environment with pacing capability)

Echocardiography

Transthoracic Echocardiography (TTE):

• Indication: Structural heart disease assessment, ventricular function, valvular disease
• Findings:
  • "Left ventricular ejection fraction (LVEF): Reduced in bradycardia-induced cardiomyopathy (may improve with pacing) [16]"
  • "Regional wall motion abnormalities: Suggest ischaemic heart disease"
  • "Valve disease: Aortic stenosis, endocarditis (aortic root abscess causing AV block)"
  • "Infiltrative disease: Increased wall thickness in amyloidosis, sarcoidosis"
  • "Pericardial effusion: Hypothyroidism, myopericarditis"

Coronary Angiography

Indication:

• Acute coronary syndrome with bradycardia
• Persistent AV block post-MI (assess for revascularization)
• Angina with bradycardia

Findings: Coronary stenosis/occlusion; RCA occlusion in inferior MI may explain bradycardia

Electrophysiology Study (EPS)

Indications:

• Differentiate Mobitz I from Mobitz II when ECG equivocal (especially 2:1 block)
• Localize level of block (AV nodal versus infranodal) when pacemaker decision uncertain
• Assess sinus node function (sinus node recovery time, sinoatrial conduction time) when diagnosis unclear
• Suspected carotid sinus hypersensitivity (carotid sinus massage during EP study)

Interpretation:

• His bundle electrogram differentiates AV nodal (prolonged AH interval) from infranodal (prolonged HV interval or block below His)
• Sinus node recovery time (SNRT) > 1500 ms or corrected SNRT > 550 ms indicates sinus node dysfunction

Other Specialized Investigations

Tilt Table Testing: Vasovagal syncope; orthostatic hypotension; differentiate reflex syncope (bradycardic versus hypotensive phenotype) to guide therapy [1]

Carotid Sinus Massage: Carotid sinus hypersensitivity (ventricular pause > 3 seconds or systolic BP drop > 50 mmHg); requires ECG and BP monitoring; contraindicated in carotid stenosis

Cardiac MRI:

• Sarcoidosis: Late gadolinium enhancement (LGE) identifies myocardial involvement and conduction disease risk
• Myocarditis: Edema, LGE patterns

Genetic Testing: Young patients with conduction disease, family history of sudden death, or syndromic features (SCN5A, HCN4, LMNA mutations)

---

Classification & Staging

Anatomical Classification of AV Block

Understanding the anatomical site of block informs prognosis and management:

Functional Classification

Reversible Bradycardia:

• Causes: Medications, electrolyte abnormalities, hypothyroidism, hypothermia, vagal stimulation, acute ischaemia
• Management: Correct underlying cause; supportive care; avoid permanent pacemaker unless intrinsic disease persists after correction

Intrinsic (Irreversible) Conduction Disease:

• Causes: Degenerative fibrosis, structural heart disease, infiltrative disease, genetic channelopathies
• Management: Permanent pacemaker if symptomatic or high-risk features

Haemodynamic Classification

Stable Bradycardia:

• No adverse features present
• Adequate perfusion and consciousness
• Management: Identify and treat underlying cause; monitor; consider pacemaker for symptomatic intrinsic disease

Unstable Bradycardia (Adverse Features Present):

• Shock (hypotension, poor perfusion)
• Syncope
• Myocardial ischaemia
• Heart failure
• Management: Immediate atropine; prepare for pacing; treat underlying cause [2]

Risk Stratification

Low Risk (Observation, Treat Reversible Causes):

• Asymptomatic sinus bradycardia
• Asymptomatic first-degree AV block
• Physiological bradycardia (athletes, sleep)

Moderate Risk (Close Monitoring, Specialist Referral):

• Symptomatic sinus bradycardia (non-urgent symptoms: fatigue, exercise intolerance)
• Mobitz I second-degree AV block (especially if symptomatic)
• Asymptomatic Mobitz II (debate exists; most recommend pacemaker)

High Risk (Urgent/Emergent Intervention):

• Symptomatic Mobitz II
• Any complete heart block
• Symptomatic sinus node dysfunction with syncope
• Ventricular pauses ≥3 seconds with symptoms
• Bradycardia with adverse features
• Anterior MI with new bundle branch block or high-degree AV block

---

Management

Management of bradycardia is stratified by haemodynamic stability, presence of reversible causes, and risk of progression or sudden death.

Initial Assessment and Resuscitation

All patients presenting with bradycardia require immediate assessment using the ABCDE approach:

1. Airway: Ensure patency
2. Breathing: Administer oxygen if hypoxic (SpO₂ less than 94% or less than 88% in COPD)
3. Circulation: Establish IV access; continuous ECG monitoring; obtain 12-lead ECG
4. Disability: Assess conscious level
5. Exposure: Measure core temperature; assess for trauma (if syncope occurred)

Obtain focused history (if patient stable):

• Symptom onset and duration
• Medication history (especially rate-limiting drugs)
• Cardiac history (MI, heart failure, structural disease)
• Systemic symptoms (hypothyroidism, infection)

Management of Unstable Bradycardia (Adverse Features Present)

The presence of ANY adverse feature (shock, syncope, myocardial ischaemia, heart failure) indicates haemodynamically unstable bradycardia requiring immediate treatment according to the Advanced Life Support bradycardia algorithm. [2]

Step 1: Atropine

Dose: 500 micrograms (0.5 mg) intravenous bolus

Repeat: Every 3-5 minutes to maximum total dose of 3 mg (6 doses)

Mechanism: Competitive antagonist of muscarinic M2 receptors; reduces vagal tone; increases SA node automaticity and AV node conduction velocity [19]

Onset: 1-2 minutes; peak effect 3-4 minutes

Efficacy:

• Effective in sinus bradycardia and AV nodal blocks (Mobitz I, first-degree)
• Ineffective or harmful in infranodal blocks (Mobitz II, complete heart block with wide QRS escape)
• Success rate approximately 60-70% in unstable bradycardia overall [2]

Limitations and Adverse Effects:

• Paradoxical Bradycardia: Low doses (less than 0.5 mg) may cause transient worsening via central vagal stimulation; always use ≥500 mcg doses
• Ineffective in Infranodal Block: Atropine increases atrial rate but does not improve infranodal conduction; may worsen haemodynamics by increasing atrial rate without increasing ventricular rate
• Adverse Effects: Tachycardia, dry mouth, urinary retention, confusion (elderly), mydriasis, blurred vision
• Contraindications: Acute angle-closure glaucoma (relative); myasthenia gravis (relative)

Clinical Pearls:

• If atropine fails after 1-2 doses in complete heart block, proceed directly to pacing rather than continuing to maximum dose
• Atropine ineffectiveness suggests infranodal block requiring pacing
• Have pacing equipment ready before administering atropine in suspected infranodal block

Step 2: Interim Measures (While Awaiting Pacing)

If atropine is ineffective or unsuitable, use interim pharmacological measures to maintain cardiac output while arranging pacing:

Isoprenaline (Isoproterenol) Infusion:

• Dose: 5 micrograms/minute IV infusion, titrate to response (usual range 2-10 mcg/min)
• Mechanism: Non-selective beta-agonist; increases SA node automaticity, AV conduction, and ventricular contractility
• Caution: May cause tachyarrhythmias, myocardial ischaemia; increase myocardial oxygen demand; use lowest effective dose

Adrenaline (Epinephrine) Infusion:

• Dose: 2-10 micrograms/minute IV infusion (start low, titrate)
• Mechanism: Alpha and beta agonist; increases heart rate, contractility, and vascular tone
• Preparation: 1 mg adrenaline in 100 mL normal saline = 10 mcg/mL concentration
• Advantages: Provides both chronotropic and vasopressor support
• Disadvantages: Increases myocardial oxygen consumption; may provoke arrhythmias

Dopamine Infusion:

• Dose: 2-10 micrograms/kg/minute IV infusion
• Mechanism: Dose-dependent effects; primarily beta-1 agonist at 2-10 mcg/kg/min
• Note: Less commonly used than adrenaline in UK/European practice

Aminophylline:

• Dose: 100-200 mg IV bolus slowly
• Mechanism: Phosphodiesterase inhibitor; adenosine antagonist; increases heart rate
• Evidence: Limited; case reports suggest benefit in refractory bradycardia
• Adverse Effects: Nausea, tremor, arrhythmias

These are temporizing measures only; definitive pacing must be arranged urgently.

Step 3: Transcutaneous Pacing (TCP)

External cardiac pacing delivered via adhesive electrodes placed on the chest wall.

Indications:

• Symptomatic bradycardia unresponsive to atropine
• Bridge to transvenous pacing
• Immediately available in cardiac arrest settings

Equipment:

• Defibrillator with pacing capability
• Large adhesive pacing electrodes

Technique:

1. Electrode Placement: Two options:

   • Anterior-Posterior: Anterior electrode over left precordium (V2-V4 position); posterior electrode on back between scapulae
   • Anterior-Lateral: Anterior electrode over sternum; lateral electrode at left mid-axillary line (V6 position)
   • Anterior-posterior positioning generally provides better capture with lower currents

2. Initial Settings:

   • Pacing Rate: 60-80 bpm (higher if severe bradycardia or cardiac arrest)
   • Pacing Current: Start at minimum output

3. Achieve Capture:

   • Gradually increase current (mA) until electrical and mechanical capture achieved
   • Electrical Capture: Pacing spike followed by wide QRS complex on monitor
   • Mechanical Capture: Palpable pulse corresponding to paced rate (essential to confirm)
   • Typical capture threshold: 50-100 mA (range 40-150 mA)
   • Set output 5-10 mA above capture threshold to ensure consistent capture

4. Analgesia/Sedation:

   • TCP is painful; provide analgesia and sedation if patient conscious
   • Options: Morphine 2.5-5 mg IV + midazolam 1-2 mg IV (titrate to comfort)
   • Caution with sedation in haemodynamically unstable patients

Complications and Limitations:

• Pain/Discomfort: Most common; requires analgesia/sedation
• Failure to Capture: Obesity, large breasts, COPD (hyperinflated lungs), pericardial effusion
• Skin Burns: Prolonged use (> 30-60 minutes); not suitable for definitive long-term pacing
• Myocardial Damage: Theoretical risk with prolonged high-output pacing

Clinical Pearls:

• Always confirm mechanical capture by palpating pulse; ECG alone can be misleading (muscle stimulation artifact may mimic capture)
• TCP is a bridge to transvenous pacing; arrange transvenous pacing urgently
• In cardiac arrest, apply pacing pads early but prioritize high-quality CPR

Step 4: Transvenous Pacing (TVP)

Temporary pacing wire inserted via central venous access (typically right internal jugular or left subclavian vein) and advanced to right ventricular apex under fluoroscopic or echocardiographic guidance.

Indications:

• Unstable bradycardia requiring prolonged pacing (> 1-2 hours)
• Failed transcutaneous pacing
• Bridge to permanent pacemaker
• Reversible causes expected to resolve (e.g., drug toxicity, Lyme carditis, inferior MI with AV block)

Procedure:

• Central venous access (sheath introducer)
• Pacing wire advanced under fluoroscopy to RV apex
• Position confirmed by:
  • "Fluoroscopy: Wire tip at RV apex"
  • "Electrical testing: R wave amplitude > 6 mV; pacing threshold less than 1.0 mA at 0.5 ms pulse width"
  • "Chest X-ray: Post-procedure to confirm position and exclude pneumothorax"

Settings:

• Pacing Mode: Usually VVI (ventricular demand pacing)
• Rate: 60-80 bpm
• Output: 2-3 times pacing threshold to ensure consistent capture
• Sensitivity: Adjust to sense native beats and avoid inappropriate pacing

Complications:

• Procedural: Bleeding, pneumothorax (2-5%), arterial puncture, air embolism
• Device-Related: Lead displacement (most common), perforation, infection, thrombosis

Maintenance:

• Check pacing thresholds daily (may increase over days)
• Secure wire and introducer sheath carefully
• Limit duration (typically less than 7 days) due to infection risk
• Decision re: permanent pacemaker versus removal once reversible cause treated

Step 5: Specialist Referral and Definitive Management

All patients with unstable bradycardia require urgent cardiology consultation for:

• Evaluation for permanent pacemaker implantation
• Management of underlying cause (e.g., PCI for acute MI)
• Intensive care management if haemodynamically unstable despite pacing

Management of Stable Bradycardia

Patients without adverse features require systematic evaluation and risk-stratified management.

Step 1: Identify and Treat Reversible Causes

Medication Review:

• Culprit Drugs: Beta-blockers, calcium channel blockers (verapamil, diltiazem), digoxin, amiodarone, sotalol, ivabradine, clonidine, lithium, antipsychotics (clozapine), cholinesterase inhibitors (donepezil, rivastigmine)
• Action: Stop or reduce dose of offending medication if safe to do so; avoid abrupt cessation of beta-blockers (rebound ischaemia risk)

Metabolic Correction:

• Hypothyroidism: Levothyroxine replacement; bradycardia typically resolves over weeks
• Hyperkalaemia: Treat according to severity (calcium gluconate, insulin-glucose, salbutamol nebulizer, dialysis if severe)
• Hypothermia: Gradual rewarming; avoid atropine and pacing until core temperature > 30°C unless life-threatening bradycardia

Treat Underlying Cardiac Conditions:

• Acute MI: Reperfusion therapy (PCI or thrombolysis); bradycardia from inferior MI often resolves with reperfusion
• Myocarditis/Endocarditis: Antimicrobial therapy; temporary pacing if AV block develops
• Lyme Carditis: Doxycycline 100 mg BD or ceftriaxone 2 g IV daily; AV block typically resolves within 1-6 weeks; temporary pacing if symptomatic [20]

Step 2: Risk Stratification and Pacemaker Evaluation

Indications for Permanent Pacemaker Implantation:

The 2021 European Society of Cardiology guidelines and 2018 ACC/AHA/HRS guidelines provide comprehensive indications for pacemaker implantation. [5,21]

Class I Indications (Definite benefit; treatment recommended):

Class IIa Indications (Benefit likely outweighs risk; reasonable to perform):

• Asymptomatic sinus bradycardia less than 40 bpm awake with unclear cause
• First-degree AV block with symptoms and no other explanation, if PR interval very prolonged (> 300 ms) causing suboptimal AV synchrony
• Mobitz I second-degree AV block with symptoms

Class IIb Indications (Benefit uncertain; may be considered):

• Asymptomatic Mobitz I at level of His-Purkinje system (infranodal)
• Syncope of unknown etiology with incidental finding of bifascicular block

Class III Indications (No benefit or potentially harmful; not recommended):

• Asymptomatic sinus bradycardia (including athletes)
• Asymptomatic first-degree AV block
• Mobitz I AV block during sleep or in athletes
• Drug-induced bradycardia when drug can be safely discontinued

Step 3: Pacemaker Mode Selection

Modern pacemakers are described using a standardized 3-5 letter code; the first three letters are most clinically relevant:

Common Pacing Modes:

VVI/VVIR:

• Ventricular pacing, ventricular sensing, inhibited by sensed events
• "R" indicates rate-responsive (adjusts rate based on activity)
• Indications: Permanent atrial fibrillation with bradycardia; patients with limited life expectancy or those unsuitable for atrial lead
• Disadvantages: No AV synchrony; "pacemaker syndrome" (fatigue, dyspnoea, hypotension from loss of atrial kick); higher risk of atrial fibrillation

AAI/AAIR:

• Atrial pacing, atrial sensing, inhibited
• Indications: Sinus node dysfunction with intact AV conduction
• Advantages: Maintains AV synchrony; physiological
• Disadvantages: Risk of developing AV block over time (5-10% over 5 years); not commonly used in elderly

DDD/DDDR:

• Dual-chamber pacing and sensing; inhibited and triggered response
• Indications: AV block (most common); sinus node dysfunction with potential for AV block
• Advantages: Maintains AV synchrony; physiological; adapts to sinus rhythm when present
• Most commonly implanted mode (60-70% of pacemakers) [5]

Rate-Responsive Pacing:

• Indicated in chronotropic incompetence
• Sensors detect physical activity (accelerometer) or minute ventilation
• Automatically increase pacing rate during exercise

His Bundle Pacing (HBP) and Left Bundle Branch Area Pacing (LBBAP):

• Newer physiological pacing techniques
• Paces intrinsic conduction system rather than RV apex
• Avoids adverse effects of RV pacing
• Technically challenging; emerging as alternative to conventional pacing [22]

Step 4: Post-Pacemaker Management

Immediate Post-Implant:

• Chest X-ray to confirm lead position and exclude pneumothorax
• ECG to verify pacing function
• Wound care and observation for haematoma

Follow-Up:

• Initial check at 4-6 weeks post-implant (wound healing, threshold testing, lead parameters)
• Remote monitoring or in-person checks every 3-12 months
• Battery longevity: typically 7-12 years depending on pacing burden
• MRI safety: modern pacemakers are generally MRI-conditional (check manufacturer specifications)

Patient Education:

• Avoid contact sports and repetitive overhead arm movements for 4-6 weeks
• Carry pacemaker identification card
• Report symptoms: dizziness, syncope, palpitations, hiccups (diaphragmatic stimulation), wound issues
• Device interrogation before surgical procedures (electrocautery precautions)

Specific Clinical Scenarios

Bradycardia in Acute Myocardial Infarction

Inferior MI (RCA Occlusion):

• Common (20-40% of inferior MIs develop bradycardia or AV block)
• Mechanisms: Vagal stimulation (Bezold-Jarisch reflex) + ischaemia of SA/AV node
• Type: Usually AV nodal (Mobitz I or transient complete heart block with narrow QRS escape)
• Management: Often transient (resolves within 24-48 hours); atropine usually effective; temporary pacing if unstable; permanent pacemaker rarely needed unless block persists > 2-3 weeks post-MI [6]

Anterior MI (LAD Occlusion):

• Less common (5-10%) but more serious
• Indicates extensive septal infarction affecting His-Purkinje system
• Type: Infranodal (Mobitz II or complete block with wide QRS escape)
• Prognosis: Poor (reflects large infarct size); high in-hospital mortality (50-80%)
• Management: Temporary pacing often required; permanent pacemaker indicated if block persists; prognosis determined by infarct size rather than conduction abnormality [6]

Reperfusion Bradycardia:

• Transient bradycardia during reperfusion (PCI or thrombolysis), especially inferior MI
• Usually benign and self-limiting
• May require brief atropine; rarely needs pacing

Beta-Blocker and Calcium Channel Blocker Toxicity

Overdose or excessive therapeutic dosing can cause severe bradycardia, hypotension, and cardiogenic shock. [7]

Clinical Features:

• Bradycardia (may be relative; heart rate 60-80 bpm but inadequate for degree of shock)
• Hypotension and shock
• Altered mental status
• Pulmonary oedema
• Hypoglycaemia (beta-blocker overdose; impaired gluconeogenesis)

Standard Treatments (Often Ineffective):

Atropine: Limited efficacy; may try 3 mg total but often fails

Calcium Chloride/Gluconate:

• Modest benefit in CCB overdose; little benefit in beta-blocker
• Dose: Calcium chloride 10% 10-20 mL IV (or calcium gluconate 10% 30-60 mL)
• Repeat doses or infusion (0.2-0.4 mL/kg/hour calcium chloride 10%)

Glucagon:

• Bypasses beta-receptors; activates adenylyl cyclase independently
• Dose: 5-10 mg IV bolus, then 2-10 mg/hour infusion
• Efficacy: Moderate chronotropic and inotropic effects in beta-blocker poisoning [7]
• Adverse effects: Nausea, vomiting, hypokalaemia
• Limited value in CCB poisoning

High-Dose Insulin Euglycaemia (HIE) Therapy (First-Line in Severe Toxicity):

• Mechanism: Improves myocardial contractility by enhancing carbohydrate utilization; positive inotrope and possibly chronotrope
• Indications: Severe beta-blocker or CCB poisoning with shock
• Regimen:
  1. Bolus: Regular insulin 1 unit/kg IV (with 50 mL 50% dextrose if glucose normal)
  2. Infusion: 1-10 units/kg/hour regular insulin IV
  3. Dextrose co-infusion: 0.5 g/kg/hour (adjust to maintain glucose 6-10 mmol/L)
  4. Potassium monitoring and replacement (insulin drives K⁺ intracellularly)
• Monitoring: Blood glucose every 15-30 minutes; potassium every 1-2 hours; continuous ECG and BP
• Evidence: Case series and animal studies show superiority over conventional therapies; no large RCTs [7]

Other Therapies:

• Intravenous Lipid Emulsion (ILE): Intralipid 20% for lipophilic beta-blockers (propranolol) and CCBs; dose: 1.5 mL/kg bolus, then 0.25 mL/kg/min infusion
• Vasopressors: Norepinephrine, epinephrine, or vasopressin for refractory hypotension
• Cardiac Pacing: Temporary pacing may not overcome severe myocardial depression
• Extracorporeal Membrane Oxygenation (ECMO): Case reports of successful bridge to recovery in refractory cardiogenic shock [7]

Bradycardia in Hypothyroidism

Hypothyroidism reduces metabolic rate, cardiac output, and chronotropic response. Sinus bradycardia is common; heart rate typically 50-60 bpm. [23]

Mechanism:

• Reduced beta-adrenergic receptor density and responsiveness
• Decreased cardiac contractility
• Pericardial effusion (in severe cases; rarely causes tamponade - "pseudotamponade" with echo signs but no haemodynamic compromise)

Management:

• Levothyroxine replacement; start low dose in elderly or cardiac disease (12.5-25 mcg daily, increase every 4-6 weeks)
• Bradycardia typically improves over weeks; no role for atropine or pacing in stable patients
• Emergency: Severe hypothyroidism (myxoedema coma) requires IV levothyroxine, hydrocortisone, supportive care; consider temporary pacing if unstable bradycardia

Bradycardia in Athletes

Endurance athletes commonly exhibit resting bradycardia (40-60 bpm), first-degree AV block, and Mobitz I AV block (especially during sleep) due to increased vagal tone and cardiac remodeling. [9]

Normal Findings in Athletes:

• Sinus bradycardia: Heart rate 40-60 bpm (sometimes less than 40 bpm)
• First-degree AV block: PR interval up to 300 ms
• Mobitz I AV block: Especially during sleep; disappears with exercise
• Junctional escape rhythm

Abnormal Findings Requiring Evaluation:

• Symptomatic bradycardia (syncope, presyncope, dyspnoea)
• Mobitz II AV block
• Complete heart block
• Sinus pauses > 3 seconds while awake
• Failure of heart rate to increase appropriately with exercise

Assessment:

• Exercise testing: Heart rate should normalize and increase appropriately; bradyarrhythmias should disappear
• If abnormal findings persist or symptoms present, treat as non-athlete

---

Complications

Complications of Bradycardia Itself

Haemodynamic Collapse and Cardiogenic Shock:

• Severe bradycardia (especially less than 40 bpm) reduces cardiac output
• Compensatory mechanisms (increased stroke volume, vasoconstriction) may fail
• Results in hypotension, end-organ hypoperfusion, shock

Syncope and Trauma:

• Sudden bradycardia or pauses cause cerebral hypoperfusion and loss of consciousness
• Risk of injury from falls: fractures (hip, wrist, skull), head trauma, lacerations
• Recurrent syncope severely impairs quality of life and independence

Bradycardia-Induced Ventricular Arrhythmias:

• Severe bradycardia prolongs QT interval
• Torsades de pointes (polymorphic VT) may occur
• Requires immediate pacing and correction of electrolytes (magnesium)

Heart Failure:

• Chronic severe bradycardia may cause "bradycardia-induced cardiomyopathy" with reduced ejection fraction
• Mechanism: Inadequate cardiac output despite compensatory increased stroke volume; neurohormonal activation
• Often reversible with pacing [16]

Thromboembolism:

• Sinus node dysfunction with bradycardia-tachycardia syndrome increases atrial fibrillation risk
• Atrial fibrillation carries stroke risk requiring anticoagulation

Sudden Cardiac Death:

• Untreated complete heart block: 50% annual mortality [8]
• Mobitz II AV block: 35-50% progression to complete block annually without pacing [4,5]
• Asystole from failure of escape rhythm

Complications of Treatment

Atropine Adverse Effects

• Tachycardia
• Dry mouth, urinary retention
• Confusion (especially elderly)
• Mydriasis and blurred vision
• Paradoxical bradycardia (doses less than 0.5 mg)
• Precipitation of acute angle-closure glaucoma (rare)

Transcutaneous Pacing Complications

• Pain and discomfort (most common; requires analgesia)
• Failure to capture (10-20% of cases)
• Skin burns (prolonged use)
• Muscle contractions interfering with ventilation

Transvenous Pacing Complications

Procedural (Related to Central Venous Access):

• Bleeding and haematoma (1-3%)
• Pneumothorax (2-5%; higher with subclavian approach)
• Arterial puncture (1-2%)
• Air embolism (rare)
• Thoracic duct injury (left-sided access)

Device-Related:

• Lead displacement (5-10%; most common complication)
• Ventricular perforation (1-2%; may cause pericardial effusion/tamponade)
• Infection (increases with duration; 2-5% after 7 days)
• Venous thrombosis
• Tricuspid valve damage

Arrhythmias:

• Ventricular ectopy during lead manipulation (common, usually benign)
• Ventricular tachycardia/fibrillation (rare)

Permanent Pacemaker Complications

Acute (Within 30 Days):

• Pocket haematoma (1-5%)
• Pneumothorax (1-2%)
• Lead displacement (1-3%)
• Infection (0.5-2%)
• Cardiac perforation (0.5-1%)

Long-Term:

• Lead fracture (0.5-1% per year)
• Infection (endocarditis, pocket infection; 0.5-1% over device lifetime)
• Tricuspid regurgitation (transvenous lead crossing valve)
• Pacemaker syndrome (fatigue, dyspnoea, hypotension in VVI pacing due to loss of AV synchrony; 10-20% of VVI pacemakers)
• Pacemaker-mediated tachycardia (endless loop tachycardia in DDD pacemakers)
• Ventricular pacing-induced cardiomyopathy (chronic RV pacing may reduce LVEF; consider His bundle or biventricular pacing if high pacing burden expected)

---

Prognosis & Outcomes

Physiological Bradycardia

• Excellent prognosis
• No intervention required
• Athletes: bradycardia reflects superior cardiovascular fitness

Reversible Causes

• Prognosis depends on underlying etiology
• Medication-induced: Excellent after drug cessation/adjustment
• Hypothyroidism: Bradycardia resolves with levothyroxine replacement
• Inferior MI with transient AV block: Usually resolves; good prognosis if infarct size small
• Lyme carditis: AV block typically resolves with antibiotics [20]

Sinus Node Dysfunction

Natural History Without Pacing:

• Variable; many patients have slow progression over years
• Syncope occurs in approximately 50% [3]
• Sudden death risk lower than in AV block but increased compared to general population

With Pacemaker:

• Excellent symptom relief (> 90% improvement in quality of life)
• Reduced syncope, improved exercise tolerance
• Dual-chamber pacing (DDD/DDDR) reduces atrial fibrillation incidence by 15-20% compared to VVI pacing [5]
• Normal life expectancy if no other cardiac disease

Atrioventricular Block

Mobitz I (Wenckebach):

• Generally benign course
• Rarely progresses to complete heart block (less than 5% over years)
• Pacemaker usually not required unless symptomatic

Mobitz II:

• High risk of progression to complete heart block: 35-50% annually [4,5]
• Risk of sudden death without pacing
• Pacemaker implantation dramatically improves prognosis (near-normal life expectancy)

Complete Heart Block:

Without Pacemaker:

• 1-year mortality: 50% [8]
• Sudden death from asystole or ventricular arrhythmias
• Syncope, heart failure, reduced quality of life

With Pacemaker:

• 1-year mortality: less than 5% (usually from comorbid conditions) [8]
• Survival approaches that of age-matched controls without heart block
• Excellent symptom relief
• Quality of life improvement significant

Bradycardia in Acute MI

Inferior MI:

• Bradycardia and AV block usually transient (24-72 hours)
• Prognosis determined by infarct size rather than bradycardia
• Permanent pacemaker rarely needed (less than 5%)

Anterior MI with AV Block:

• Indicates extensive infarction
• In-hospital mortality 50-80% [6]
• Survivors often require permanent pacemaker
• Poor long-term prognosis related to degree of LV dysfunction

Impact of Pacemaker Therapy on Outcomes

Multiple studies demonstrate benefits of pacemaker therapy:

Mortality Reduction: Compared to no pacing in symptomatic conduction disease, pacemaker implantation reduces mortality by 60-80% [8]

Quality of Life: Significant improvements in symptom scores, exercise capacity, and functional status [5]

Dual-Chamber vs. Ventricular Pacing: Dual-chamber (DDD) pacing reduces:

• Atrial fibrillation incidence by 15-20%
• Thromboembolic events by 20-25%
• Heart failure hospitalizations by 10-15%
• Pacemaker syndrome (eliminated)
• Overall mortality benefit modest but favours dual-chamber pacing [5]

Physiological Pacing (His bundle, LBBAP, or CRT when indicated) may further improve outcomes by avoiding adverse effects of chronic RV pacing [22]

---

Evidence & Guidelines

Key Guidelines

1. Glikson M, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. European Heart Journal. 2021;42(35):3427-3520. PMID: 34455427

   • Comprehensive European guidance on pacemaker indications, mode selection, and management
   • Class I, IIa, IIb, III recommendations with evidence levels

2. Kusumoto FM, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. Circulation. 2019;140(13):e382-e482. PMID: 30586772

   • North American guidelines; similar recommendations to ESC with minor variations

3. Resuscitation Council UK. Advanced Life Support Guidelines. 2021.

   • Emergency management algorithms for bradycardia
   • Defines adverse features and treatment pathways

4. Sorajja D, et al. 2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation (Bradycardia aspects). Circulation. 2024;149(1):e1-e156.

   • Management of bradycardia in context of AF and rate control

Landmark Trials and Key Evidence

Pacemaker Mode Selection:

5. MOST (Mode Selection Trial in Sinus-Node Dysfunction): 2002. Dual-chamber (DDDR) vs. ventricular (VVIR) pacing in sinus node dysfunction. Result: DDDR reduced atrial fibrillation by 21% but no mortality difference. Established DDD as preferred mode for quality of life. [PMID: 12446525]

6. CTOPP (Canadian Trial of Physiologic Pacing): 2000. Physiological pacing (DDD/AAI) vs. VVI. Result: 18% reduction in atrial fibrillation; trend toward reduced stroke and heart failure. [PMID: 11136931]

Bradycardia in Acute MI:

7. Behar S, et al. Prognostic significance of second-degree atrioventricular block in inferior wall acute myocardial infarction. American Journal of Cardiology. 1993;72(11):831-834. [PMID: 8213514]
   • Inferior MI with Mobitz I: Generally good prognosis
   • Anterior MI with AV block: High mortality (reflects infarct size)

Drug Toxicity Management:

8. Engebretsen KM, et al. High-dose insulin therapy in beta-blocker and calcium channel-blocker poisoning. Clinical Toxicology. 2011;49(4):277-283. PMID: 21563902
   • Review of HIE therapy; superior to conventional treatments in severe toxicity

Sinus Node Dysfunction:

9. Adan V, Crown LA. Diagnosis and treatment of sick sinus syndrome. American Family Physician. 2003;67(8):1725-1732. PMID: 12725451

   • Comprehensive review of sinus node dysfunction
   • Emphasizes correlation of symptoms with ECG findings for diagnosis

10. Hawks MK, Paul MLB, Malu OO. Sinus Node Dysfunction. American Family Physician. 2021;104(2):179-185. PMID: 34383451

    • Updated review including chronotropic incompetence definition and exercise testing

AV Block Classification:

11. Nelson WP. Diagnostic and Prognostic Implications of Surface Recordings from Patients with Atrioventricular Block. Cardiac Electrophysiology Clinics. 2016;8(1):25-35. PMID: 26920166
    • Detailed analysis of ECG diagnosis of AV blocks
    • Mobitz I vs. II differentiation

Syncope Management:

12. Brignole M, Rivasi G, et al. New insights in diagnostics and therapies in syncope: a novel approach to non-cardiac syncope. Heart. 2021;107(11):864-873. PMID: 33462120
    • Mechanism-specific therapy for reflex syncope
    • Bradycardic phenotype benefits from pacing

Implantable Loop Recorders:

13. Solbiati M, et al. Implantable loop recorder versus conventional diagnostic workup for unexplained recurrent syncope. Cochrane Database of Systematic Reviews. 2016;(4):CD011637.
    • ILR increases diagnostic yield in unexplained syncope to 50-60%

Atropine in Bradycardia:

14. Patel P, McLendon K, Preuss CV. Atropine. StatPearls. 2025. PMID: 29262018
    • Comprehensive review of atropine pharmacology and use in bradycardia

Temporary Pacing:

15. Dalia T, Amr BS. Pacemaker Indications. StatPearls. 2025. PMID: 29939600
    • Temporary and permanent pacing indications

Genetic Causes:

16. Wilde AAM, Bezzina CR. Genetics of cardiac arrhythmias. Heart. 2005;91(10):1352-1358.
    • SCN5A, HCN4, LMNA mutations causing bradyarrhythmias

Cardiac Physiology:

17. Mangrum JM, DiMarco JP. The evaluation and management of bradycardia. New England Journal of Medicine. 2000;342(10):703-709.
    • Classic review of bradycardia mechanisms and management

Lyme Carditis:

18. Forrester JD, Mead P. Third-degree heart block associated with lyme carditis: review of published cases. Clinical Infectious Diseases. 2014;59(5):622-626.
    • AV block in Lyme disease; usually resolves with antibiotic therapy

Hypothyroidism:

19. Baldwin C, Newman JD, Cho L, Mara P. Myxedema Heart and Pseudotamponade. Journal of the Endocrine Society. 2021;5(1):bvaa125. PMID: 33354637
    • Cardiovascular manifestations of severe hypothyroidism
    • Pseudotamponade phenomenon with large effusions

Calcium Channel Blocker Poisoning:

20. Graudins A, Lee HM, Druda D. Calcium channel antagonist and beta-blocker overdose: antidotes and adjunct therapies. British Journal of Clinical Pharmacology. 2016;81(3):453-461. PMID: 26344579
    • Comprehensive review of CCB/BB overdose management
    • High-dose insulin as first-line therapy

Physiological Pacing:

21. Vijayaraman P, et al. His Bundle Pacing. Journal of the American College of Cardiology. 2018;72(8):927-947.
    • Emerging physiological pacing techniques
    • Avoids adverse effects of RV pacing

Evidence Synthesis

Quality of Evidence:

• Emergency management of unstable bradycardia (atropine, pacing): Moderate quality (largely observational; RCTs difficult due to emergency nature)
• Permanent pacemaker indications: High quality (large observational cohorts; some RCTs for mode selection)
• Drug toxicity management (HIE): Low to moderate quality (case series, animal studies; no large RCTs)

Guideline Concordance: ESC and ACC/AHA/HRS guidelines are highly concordant with minor variations in Class IIa/IIb recommendations.

---

Patient & Family Information

What is Bradycardia?

Bradycardia means your heart is beating slower than normal – less than 60 beats per minute. For some people, especially athletes, this is completely normal. For others, a slow heart rate can cause symptoms or be a sign of a problem with the heart's electrical system.

What Causes Bradycardia?

Common causes include:

• Medications: Beta-blockers, calcium channel blockers, and other heart medications
• Heart conditions: Problems with the heart's natural pacemaker (SA node) or wiring (conduction system)
• Thyroid problems: An underactive thyroid gland slows everything down, including the heart
• Electrolyte imbalances: Abnormal potassium levels
• Normal aging: The heart's electrical system can wear out over time

What Symptoms Should I Watch For?

Many people with bradycardia have no symptoms. When symptoms occur, they may include:

• Fatigue and weakness: Feeling tired even after rest
• Dizziness or lightheadedness: Especially when standing up
• Fainting or near-fainting: Loss of consciousness or feeling like you might pass out
• Shortness of breath: Difficulty breathing, especially with activity
• Confusion: Feeling "foggy" or having trouble concentrating
• Chest discomfort: Chest pain or pressure

Seek immediate medical attention if you experience:

• Fainting or loss of consciousness
• Severe dizziness
• Chest pain
• Severe shortness of breath
• Confusion or altered mental state

How is Bradycardia Diagnosed?

Your doctor will:

• Take your pulse and blood pressure
• Review your medications
• Perform an electrocardiogram (ECG/EKG) to measure your heart's electrical activity
• Order blood tests to check thyroid function and electrolytes
• Possibly arrange a heart monitor to wear at home for 24 hours or longer if symptoms are intermittent

How is Bradycardia Treated?

Treatment depends on the cause and whether you have symptoms:

If caused by medication:

• Your doctor may adjust or stop the medication

If caused by thyroid or electrolyte problems:

• Treating the underlying condition usually fixes the slow heart rate

If caused by a problem with the heart's electrical system:

• Pacemaker: A small device implanted under the skin that helps maintain a normal heart rate. This is the most common treatment for symptomatic bradycardia that can't be fixed by treating an underlying cause.

What is a Pacemaker?

A pacemaker is a small device (about the size of a matchbox) placed under the skin near your collarbone. It has one or two thin wires (leads) that go into your heart. The pacemaker monitors your heart rate and sends electrical signals when needed to keep your heart beating at a healthy rate.

Pacemaker implantation:

• Usually done under local anesthesia
• Takes 1-2 hours
• Most people go home the same day or next day
• Full recovery in 4-6 weeks

Living with a pacemaker:

• Most activities are safe, including exercise
• Avoid contact sports that could damage the device
• Modern pacemakers are safe around most household electronics
• Carry a pacemaker identification card
• Regular check-ups (every 3-12 months) to make sure it's working properly
• Battery lasts 7-12 years; replacement is a minor procedure

What is the Outlook?

For physiological (normal) bradycardia: Excellent; no treatment needed

For bradycardia caused by medications or reversible conditions: Excellent once the cause is treated

For bradycardia requiring a pacemaker: Excellent; pacemakers are highly effective and safe. Most people return to normal activities and have a normal life expectancy.

Questions to Ask Your Doctor

• What is causing my bradycardia?
• Do I need treatment?
• If I need a pacemaker, what type will I get?
• What activities should I avoid?
• How often will I need check-ups?
• Are there any warning signs I should watch for?

Resources

• British Heart Foundation: www.bhf.org.uk – Information on heart rhythm problems and pacemakers
• Arrhythmia Alliance: www.heartrhythmalliance.org – Patient support and information
• NHS Information on Arrhythmias: www.nhs.uk/conditions/arrhythmia
• Cardiac Rhythm Alliance: Patient resources and support groups

---

References

Primary Guidelines

1. Brignole M, Rivasi G, Sutton R. New insights in diagnostics and therapies in syncope: a novel approach to non-cardiac syncope. Heart. 2021;107(11):864-873. PMID: 33462120

2. Resuscitation Council UK. Advanced Life Support Guidelines. 2021. Available from: https://www.resus.org.uk/

3. Hawks MK, Paul MLB, Malu OO. Sinus Node Dysfunction. Am Fam Physician. 2021;104(2):179-185. PMID: 34383451

4. Nelson WP. Diagnostic and Prognostic Implications of Surface Recordings from Patients with Atrioventricular Block. Card Electrophysiol Clin. 2016;8(1):25-35. PMID: 26920166

5. Glikson M, Nielsen JC, Kronborg MB, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. Eur Heart J. 2021;42(35):3427-3520. PMID: 34455427

6. Harikrishnan P, Gupta T, Palaniswamy C, et al. Complete Heart Block Complicating ST-Segment Elevation Myocardial Infarction: Temporal Trends and Association With Mortality. JACC Clin Electrophysiol. 2015;1(6):529-538.

7. Graudins A, Lee HM, Druda D. Calcium channel antagonist and beta-blocker overdose: antidotes and adjunct therapies. Br J Clin Pharmacol. 2016;81(3):453-461. PMID: 26344579

8. Edhag O, Swahn A. Prognosis of patients paced for chronic atrioventricular block. Acta Med Scand. 1976;200(6):457-463.

9. Sharma S, Drezner JA, Baggish A, et al. International recommendations for electrocardiographic interpretation in athletes. J Am Coll Cardiol. 2017;69(8):1057-1075.

10. Mond HG, Proclemer A. The 11th world survey of cardiac pacing and implantable cardioverter-defibrillators: calendar year 2009--a World Society of Arrhythmia's project. Pacing Clin Electrophysiol. 2011;34(8):1013-1027.

11. Zimetbaum PJ, Josephson ME. Evaluation of patients with bradycardia. In: Kasper DL, et al. Harrison's Principles of Internal Medicine. 20th ed. McGraw-Hill; 2018.

12. Wilde AAM, Bezzina CR. Genetics of cardiac arrhythmias. Heart. 2005;91(10):1352-1358.

13. DiFrancesco D. The role of the funny current in pacemaker activity. Circ Res. 2010;106(3):434-446.

14. Buyon JP, Clancy RM. Neonatal lupus: review of proposed pathogenesis and clinical data from the US-based Research Registry for Neonatal Lupus. Autoimmunity. 2003;36(1):41-50.

15. Dardiotis E, Hobson P, Weir D, et al. Clinical associations of pacemaker implantation in Parkinson's disease and dementia. J Neurol Sci. 2017;376:67-71.

16. Kiehl EL, Makki T, Kumar R, et al. Incidence and predictors of right ventricular pacing-induced cardiomyopathy in patients with complete atrioventricular block and preserved left ventricular systolic function. Heart Rhythm. 2016;13(12):2272-2278.

17. Adgey AAJ, Geddes JS, Webb SW, et al. Acute phase of myocardial infarction. Lancet. 1971;2(7732):501-504.

18. Solbiati M, Costantino G, Casazza G, et al. Implantable loop recorder versus conventional diagnostic workup for unexplained recurrent syncope. Cochrane Database Syst Rev. 2016;(4):CD011637.

19. Patel P, McLendon K, Preuss CV. Atropine. In: StatPearls. StatPearls Publishing; 2025. PMID: 29262018

20. Forrester JD, Mead P. Third-degree heart block associated with lyme carditis: review of published cases. Clin Infect Dis. 2014;59(5):622-626.

21. 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. Circulation. 2019;140(13):e382-e482. PMID: 30586772

22. Vijayaraman P, Naperkowski A, Ellenbogen KA, Dandamudi G. Electrophysiologic Insights Into Site of Atrioventricular Block: Lessons From Permanent His Bundle Pacing. JACC Clin Electrophysiol. 2015;1(6):571-581.

23. Baldwin C, Newman JD, Cho L, Mara P. Myxedema Heart and Pseudotamponade. J Endocr Soc. 2021;5(1):bvaa125. PMID: 33354637

24. Semelka M, Gera J, Usman S. Sick sinus syndrome: a review. Am Fam Physician. 2013;87(10):691-696. PMID: 23939447

25. Raatikainen MJ, Arnar DO, Zeppenfeld K, et al. Statistics on the use of cardiac electronic devices and electrophysiological procedures in the European Society of Cardiology countries: 2014 report from the European Heart Rhythm Association. Europace. 2015;17 Suppl 1:i1-i75. PMID: 25616426

26. Birnie DH, Sauer WH, Bogun F, et al. HRS expert consensus statement on the diagnosis and management of arrhythmias associated with cardiac sarcoidosis. Heart Rhythm. 2014;11(7):1305-1323. PMID: 24819193

27. Ackerman MJ, Priori SG, Willems S, et al. HRS/EHRA expert consensus statement on the state of genetic testing for the channelopathies and cardiomyopathies. Heart Rhythm. 2011;8(8):1308-1339. PMID: 21787999

28. Opthof T. The normal range and determinants of the intrinsic heart rate in man. Cardiovasc Res. 2000;45(1):177-184. PMID: 10728364

29. Friedman DM, Kim MY, Copel JA, et al. Utility of cardiac monitoring in fetuses at risk for congenital heart block: the PR Interval and Dexamethasone Evaluation (PRIDE) prospective study. Circulation. 2008;117(4):485-493. PMID: 18195175

30. Belmin J, Abderrhamane M, Medjahed S, et al. Variability of blood pressure response to orthostatism and reproducibility of the diagnosis of orthostatic hypotension in elderly subjects. J Gerontol A Biol Sci Med Sci. 2000;55(11):M667-M671. PMID: 11078097

31. Naegeli B, Kurz DJ, Koller D, Straumann E. Single-chamber ventricular pacing increases markers of left ventricular dysfunction compared with dual-chamber pacing. Europace. 2007;9(4):194-199. PMID: 17272325

---

Document Information:

• Total Lines: 1,485
• Total Citations: 30
• Last Updated: 2026-01-16
• Evidence Level: High (multiple high-quality guidelines and systematic reviews)
• Target Audience: Clinicians (Emergency Medicine, Cardiology, Acute Medicine, Internal Medicine) and Medical Students
• Examination Relevance: MRCP, MRCS, FRACP, FRCEM, MCEM, USMLE Step 2/3, PLAB, AMC