Table of contents
EKGs · AV Conduction Disorders
AV Conduction Diagnosis
Atrioventricular Conduction Disorders
Learning Objectives
- Use P waves, QRS complexes, and PR intervals to classify atrioventricular (AV) block.
- Distinguish first-degree AV block, Mobitz I, Mobitz II, high-grade block, and third-degree block.
- Recognize the limits of labeling 2:1 AV block from a short rhythm strip.
- Identify reversible causes and dangerous clinical features.
- Select immediate stabilization and definitive pacing strategies.
The conduction mental model
The sinus node initiates atrial depolarization. The impulse then passes through the AV node, His bundle, bundle branches, and Purkinje system. On the electrocardiogram (ECG), the PR interval represents atrial depolarization plus conduction to the ventricles. Normal AV conduction produces one QRS complex after every P wave with a consistent PR interval.
AV block is best understood as failed or delayed transmission:
- Delay without dropped beats: first-degree AV block.
- Intermittent failure: second-degree AV block.
- No atrial impulses conduct: third-degree, or complete, AV block.
The QRS width helps localize disease. A narrow QRS favors AV nodal or proximal disease, whereas a wide QRS raises concern for disease below the AV node in the His–Purkinje system. This is a clue, not an absolute rule.
The conduction balls track the P wave across atrial paths, pause at the AV node during the PR segment, accelerate during QRS, and settle during the T wave.
A repeatable ECG approach
Before naming the block, answer five questions:
- Are P waves present and regular?
- Are QRS complexes present and regular?
- Does every P wave conduct to a QRS complex?
- For conducted beats, is the PR interval fixed or changing?
- Is the QRS narrow or wide?
Then classify the pattern:
- Every P conducts, PR longer than 200 ms: first-degree AV block.
- Progressive PR prolongation before a dropped QRS: Mobitz I.
- Constant PR intervals with an abruptly dropped QRS: Mobitz II.
- At least two consecutive nonconducted P waves: high-grade AV block.
- No stable P-to-QRS relationship: third-degree AV block.
Do not diagnose Mobitz II unless the sinus rate is stable, the PR intervals surrounding the blocked P wave are unchanged, and concealed extrasystoles or vagal slowing do not better explain the pattern.
First-degree AV block
First-degree AV block is delayed conduction of every atrial impulse, producing a PR interval longer than 200 ms with no dropped QRS complexes. The term “block” is imperfect because conduction is delayed rather than interrupted.
Common settings include increased vagal tone, AV nodal–blocking medications, inferior myocardial infarction, myocarditis, and degenerative conduction disease. Most patients are asymptomatic. Marked PR prolongation can impair atrioventricular timing and produce exertional intolerance, fatigue, or symptoms resembling pacemaker syndrome.
Recognition: one P wave precedes every QRS, and the prolonged PR interval remains constant.
Second-degree AV block
Mobitz I: Wenckebach
Mobitz I usually reflects progressive AV nodal conduction delay. The PR interval lengthens beat by beat until a P wave fails to conduct; the cycle then resets. Grouped beating is common, and the R–R intervals often shorten before the pause.
Mobitz I may occur with increased vagal tone, sleep, inferior ischemia, or AV nodal–blocking drugs. A narrow QRS and improvement with exercise or atropine support nodal localization. Symptomatic or exercise-induced patterns still require careful evaluation.
Mobitz II
Mobitz II produces abrupt nonconducted P waves without progressive PR prolongation. Conducted beats have stable PR intervals. The lesion is usually within or below the His bundle, often with a wide QRS or bundle-branch block. Because infranodal disease can progress unpredictably, acquired Mobitz II not explained by a reversible or physiologic cause is a pacing indication even without symptoms.
The 2:1 and high-grade problem
In 2:1 AV block, every other P wave is blocked. Because only one PR interval is visible before each dropped beat, the strip cannot demonstrate progressive prolongation or prove constancy across two consecutive conducted beats. Report 2:1 AV block, then use QRS width, response to exercise or atropine, ambulatory monitoring, and electrophysiologic testing when needed to estimate the level of block.
High-grade AV block means at least two consecutive P waves fail to conduct while some AV conduction remains. Treat wide-QRS 2:1 block or high-grade block as potentially infranodal until evaluated.
Common mimics of Mobitz II
- A blocked premature atrial contraction hidden in the preceding T wave.
- Vagally mediated block accompanied by sinus slowing.
- Nonconducted atrial bigeminy.
- Atrial flutter with fixed or variable conduction.
- Recording artifact or an obscured QRS complex.
Third-degree AV block
In third-degree AV block, no atrial impulse conducts to the ventricles. P waves and QRS complexes therefore march independently. The atrial rate is usually faster than the ventricular escape rate, and the PR interval continually changes.
The conduction balls track the P wave across atrial paths, pause at the AV node during the PR segment, accelerate during QRS, and settle during the T wave.
Recognition features:
- Regular P–P intervals.
- Regular R–R intervals generated by an escape focus.
- No fixed relationship between P waves and QRS complexes.
- A narrow QRS when the escape focus is junctional; a broad, slower QRS when the escape focus is ventricular.
- Possible capture beats when intermittent conduction is present; true complete block has no conducted beats.
Symptoms range from fatigue and exercise intolerance to syncope, hypotension, ischemic chest discomfort, heart failure, or cardiac arrest. A slow, wide-complex escape rhythm is particularly unreliable.
Causes and evaluation
Look for reversible causes while defining the likely level of disease.
Important causes include:
- Medications or toxins: beta blockers, non-dihydropyridine calcium-channel blockers, digoxin, antiarrhythmics, and selected toxic exposures.
- Ischemia: inferior infarction often affects the AV node; anterior infarction more often signals extensive infranodal injury.
- Metabolic: hyperkalemia, hypoxia, hypothermia, hypothyroidism, and severe systemic illness.
- Inflammatory or infectious: myocarditis, cardiac sarcoidosis, Lyme carditis, and endocarditis with periannular extension.
- Structural or procedural: degeneration, cardiomyopathy, congenital disease, cardiac surgery, transcatheter aortic valve replacement, or catheter ablation.
Initial evaluation should include:
- Symptoms, vital signs, perfusion, and continuous rhythm monitoring.
- A 12-lead ECG and comparison with prior tracings.
- Medication, supplement, and exposure review.
- Electrolytes, renal function, and targeted testing for ischemic, endocrine, infectious, or inflammatory disease.
- Echocardiography when structural disease is suspected or newly identified conduction disease is significant.
- Ambulatory monitoring when symptoms are intermittent and no diagnostic rhythm has been captured.
Management
Management begins with the patient, not the rhythm label.
Unstable bradycardia
Signs of instability include hypotension, altered mental status, ischemic chest discomfort, acute heart failure, shock, or ongoing syncope. Provide airway and oxygen support as indicated, establish intravenous access, place pacing/defibrillator pads, and treat reversible causes.
Atropine may improve AV nodal block, but it is less reliable in Mobitz II, high-grade, or third-degree infranodal block. Do not delay transcutaneous pacing or vasoactive support when the patient is unstable. Confirm electrical and mechanical capture; transcutaneous pacing is a bridge to transvenous or permanent pacing or to reversal of the cause.
Definitive decisions
- First-degree AV block: treat reversible causes; pacing is not routine unless symptoms clearly result from marked AV dyssynchrony or another pacing indication exists.
- Mobitz I: correct reversible causes and correlate symptoms with the rhythm. Asymptomatic physiologic or nodal Wenckebach often requires observation rather than pacing.
- Mobitz II, high-grade, or third-degree AV block: permanent pacing is recommended when the block is acquired and not reversible or physiologic.
- Acute infarction, drug toxicity, infection, or metabolic disease: stabilize first, correct the cause, and reassess persistence before committing to permanent pacing when clinically appropriate.
Summary
- Start with the P-to-QRS relationship, then examine PR behavior and QRS width.
- First-degree AV block delays every impulse; Mobitz I progressively lengthens the PR interval before a dropped beat.
- Mobitz II abruptly drops a QRS despite stable conducted PR intervals and usually reflects infranodal disease.
- A 2:1 strip cannot by itself distinguish Mobitz I from Mobitz II.
- Third-degree AV block produces independent atrial and ventricular rhythms.
- Stabilize symptomatic patients, correct reversible causes, and do not delay pacing in unstable advanced block.
- Acquired Mobitz II, high-grade, and third-degree AV block generally require permanent pacing when not reversible or physiologic.
References
- 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. Link
- Glikson M, Nielsen JC, Kronborg MB, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. European Heart Journal. 2021. Link
- Barold SS, Herweg B. Mobitz type II second-degree atrioventricular block: a commonly overdiagnosed and misinterpreted arrhythmia. Frontiers in Cardiovascular Medicine. 2024. Link
- Alboni P, Holz A, Brignole M. Vagally mediated atrioventricular block: pathophysiology and diagnosis. Heart. 2013. Link
Last Edited 07/22/2026