How should I diagnose a heart block?

Diagnosis of Heart Block

The diagnosis of heart block is established by 12-lead electrocardiography, which identifies the specific type of atrioventricular conduction abnormality based on PR interval duration, P-wave conduction patterns, and QRS morphology. 1

Initial ECG Assessment

• Obtain a 12-lead ECG immediately in any patient presenting with syncope, presyncope, bradycardia, or unexplained hemodynamic instability to identify the presence and degree of AV block. 1
• Continuous cardiac monitoring with pulse oximetry and frequent blood pressure checks should be initiated while completing the diagnostic evaluation to detect acute deterioration. 1, 2
• Assess the patient's hemodynamic status by evaluating for altered mental status, syncope, chest pain, acute heart failure, hypotension, or signs of shock—these findings determine urgency of intervention. 1, 2

First-Degree AV Block

• First-degree AV block is defined by a PR interval >200 ms with 1:1 AV conduction where every P wave conducts to the ventricles; this represents conduction delay rather than true block. 1, 3
• When the QRS is narrow (<120 ms), the delay typically occurs at the AV node level; when the QRS is wide (≥120 ms), delay may be in the AV node or His-Purkinje system. 3
• First-degree AV block is generally benign and does not require pacing unless the PR interval exceeds 300 ms and causes symptoms similar to pacemaker syndrome. 1, 4

Second-Degree AV Block

Mobitz Type I (Wenckebach)

• Mobitz Type I is characterized by progressive PR interval prolongation before a nonconducted P wave, with inconstant PR intervals before and after the blocked beat. 1, 3
• The block typically occurs at the AV node level and is usually benign, especially when occurring nocturnally or with sinus slowing. 1, 3
• Asymptomatic patients with nocturnal type I second-degree AV block do not require electrophysiologic study or pacing. 1

Mobitz Type II

• Mobitz Type II is defined by constant PR intervals before and after a nonconducted P wave, with periodic single blocked P waves (excluding 2:1 block). 1, 3
• The block typically occurs in the His-Purkinje system, especially when the QRS is wide (≥120 ms), and carries a high risk of progression to complete heart block. 1, 3, 5
• Symptomatic patients with suspected His-Purkinje block require electrophysiologic study when the diagnosis has not been established by ECG alone. 1

2:1 AV Block

• 2:1 AV block is defined by every other P wave conducting to the ventricles at a constant (or near-constant) atrial rate <100 bpm. 1
• 2:1 AV block cannot be classified as Mobitz Type I or Type II because there is only one PR interval to examine before the blocked P wave; the site of block must be inferred from QRS width and associated rhythms. 6
• A narrow QRS complex suggests AV nodal block, while a wide QRS complex suggests His-Purkinje block in 80% of cases outside of acute myocardial infarction. 6

Advanced (High-Grade) AV Block

• Advanced AV block is defined by ≥2 consecutive P waves at a constant physiologic rate that do not conduct to the ventricles, with evidence of some residual AV conduction. 1
• This pattern indicates severe conduction system disease and carries a high risk of progression to complete heart block. 3

Third-Degree (Complete) AV Block

• Third-degree AV block is defined by complete absence of AV conduction, with no P waves conducting to the ventricles, resulting in AV dissociation. 1, 5
• The diagnosis can be made even in atrial fibrillation: look for a slow (<50 bpm), regular ventricular rhythm that is dissociated from the fibrillatory atrial activity. 2
• QRS morphology determines the anatomic level of block and prognosis: narrow QRS (40-60 bpm escape) suggests AV nodal block with a more stable junctional escape rhythm, while wide QRS (20-40 bpm escape) indicates infranodal His-Purkinje block with higher risk of asystole. 1, 5, 7

Bundle Branch Blocks and Fascicular Blocks

Right Bundle Branch Block (RBBB)

• Complete RBBB is defined by QRS duration ≥120 ms with rsr′, rsR′, or rSR′ pattern in V1-V2, S wave duration greater than R wave or >40 ms in leads I and V6, and R peak time >50 ms in V1. 1
• Incomplete RBBB has the same morphology criteria but QRS duration 110-119 ms. 1

Left Bundle Branch Block (LBBB)

• Complete LBBB is defined by QRS duration ≥120 ms with broad notched or slurred R wave in leads I, aVL, V5, and V6, absent Q waves in leads I, V5, and V6, and R peak time >60 ms in V5-V6. 1
• LBBB is very rare in otherwise healthy individuals and is a strong ECG marker of underlying structural cardiovascular disease, often occurring years before structural changes are detectable. 1
• Demonstration of complete bundle branch block or hemiblock in an athlete should lead to cardiological work-up including exercise testing, 24-hour ECG, and imaging for underlying pathological causes. 1

Left Anterior Fascicular Block

• Left anterior fascicular block is defined by QRS duration <120 ms with frontal plane axis between −45° and −90° and qR pattern in lead aVL. 1
• The estimated prevalence in the general population under age 40 is 0.5-1.0%, similar to the athletic population. 1

Diagnostic Adjuncts

Exercise Testing

• Exercise testing is useful to assess chronotropic response in patients with profound sinus bradycardia (heart rate ≤30 bpm) or markedly prolonged PR interval (≥400 ms). 1
• If the heart rate increases appropriately and the PR interval normalizes with exertion in an asymptomatic athlete, no further testing is necessary; conversely, further evaluation is required if the response is inadequate or symptoms are present. 1

Electrophysiologic Study

• Electrophysiologic study is indicated (Class I) for symptomatic patients with syncope or near-syncope in whom His-Purkinje block is suspected but has not been established by ECG. 1
• Electrophysiologic study is also indicated for patients with second- or third-degree AV block treated with a pacemaker who remain symptomatic, in whom ventricular tachyarrhythmia is suspected. 1
• Electrophysiologic study is NOT indicated (Class III) when symptoms and AV block are already correlated by electrocardiography, or in asymptomatic patients with transient AV block associated with sinus slowing. 1

Ambulatory ECG Monitoring

• 24-hour Holter monitoring should be performed to assess for intermittent high-grade AV block, complex ventricular arrhythmias, or to correlate symptoms with rhythm disturbances. 1
• Modern small, leadless ambulatory recorders allow for longer monitoring during training and competition to exclude complex arrhythmias. 1

Critical Pitfalls to Avoid

• Do not assume first-degree AV block is always benign: extreme PR prolongation (>300 ms) can cause symptoms similar to pacemaker syndrome and may warrant pacing. 4
• Do not classify 2:1 AV block as "Mobitz Type I" or "Mobitz Type II": this violates accepted definitions and requires assessment of QRS width and associated rhythms to determine the site of block. 6
• Do not use atropine for suspected His-Purkinje disease: it may increase the degree of AV block and is ineffective for infranodal conduction abnormalities. 6
• Do not overlook complete heart block in atrial fibrillation: the absence of P waves does not preclude the diagnosis—look for a slow, regular ventricular rhythm dissociated from fibrillatory activity. 2
• Do not assume third-degree AV block is benign based on age alone: even in young adults around age 40, thorough evaluation for reversible causes and definitive treatment are warranted. 5