Clinical Overview

Atrioventricular (AV) block is a delay or interruption in the transmission of an electrical impulse from the atria to the ventricles, caused by an anatomical or functional impairment somewhere in the conduction system between them (StatPearls, Atrioventricular Block, 2024). This dataset carries a bare “AVB” label as a general entry alongside four graded labels — First-Degree AV Block (1AVB), a generic Second-Degree AV Block (2AVB), the specifically Wenckebach/Mobitz I form of second-degree block (2AVB1), and Third-Degree AV Block (3AVB) — and records carrying this bare label rarely also carry one of the graded ones: of the 244 records filed under it, only 1 also carries the first-degree label and only 2 also carry the generic second-degree label, with none carrying the Wenckebach-specific or third-degree labels. [CLINICAL REVIEW NEEDED: the dataset does not document how it decided to file a given record under this bare AVB label rather than under one of the four graded labels, so a strip carrying this label should not be read as asserting which degree is present.] This page covers the concept spanning all degrees — where the delay sits and how complete it is — while the degree-specific criteria live on the family pages linked in the differential below.

Mechanistically, AV conduction can be slowed or blocked at more than one level, and the level plus the pattern of dropped beats is what separates the four grades from one another. A delay confined to the AV node itself, with every impulse still eventually reaching the ventricles, is first-degree block; intermittent, non-conducted beats define second-degree block, which splits further by mechanism — Mobitz I (Wenckebach) is a nodal delay that progressively worsens beat to beat until one impulse fails, while Mobitz II is a fixed, all-or-nothing failure typically seated lower in the His-Purkinje system; and a complete, sustained failure of every atrial impulse to reach the ventricles, with the atria and ventricles beating independently, is third-degree (complete) block (StatPearls, 2024; LITFL, AV block, 2024; AMBOSS, Atrioventricular Block, 2023). This page’s own scope is the shared mechanism — impairment somewhere along the AV conduction axis — rather than the site- and pattern-specific criteria that assign a particular record to one of the four grades.

Clinical significance varies enormously across that range, which is the main reason a strip filed under the bare label should be read cautiously rather than assumed benign. First-degree block and Mobitz I are usually well tolerated, cause little or no hemodynamic disturbance, and carry a comparatively low risk of progressing to a higher-degree block; Mobitz II, high-grade block, and third-degree block carry a materially higher risk of progressing to complete heart block or a slow escape rhythm inadequate to maintain cardiac output, and are treated as higher-acuity findings from the outset (LITFL, 2024; StatPearls, 2024). AV block as a category is not rare: a 2025 emergency-department screening study of 692 patients over age 40 found AV block on ECG in 7% of the cohort — well above a cited 0.6% nationwide Chinese screening rate — with age (odds ratio 2.8), ischemic heart disease (odds ratio 1.9), and diabetes mellitus (odds ratio 3.9) as independent predictors on multivariable analysis (Shibli et al., Journal of Cardiovascular Electrophysiology, 2025). [CLINICAL REVIEW NEEDED: that study’s elevated prevalence is drawn from a conflict-affected health system with reduced access to preventive cardiac care, so its absolute rate should not be generalized to other populations without that context.]

Symptoms track the same range as significance. First-degree block is typically asymptomatic and found incidentally on a routine strip. Higher-degree block can produce dyspnea, fatigue, lightheadedness, presyncope, or syncope from inadequate cardiac output, and a sudden, transient loss of consciousness caused specifically by a pause in ventricular activity — a Stokes-Adams attack — is classically associated with high-grade and third-degree block (AMBOSS, 2023; StatPearls, 2024).

Common causes and risk factors span the whole family rather than any one grade specifically: age-related fibrosis and sclerosis of the conduction system is the single most common cause, cited at roughly 40% of cases, followed by ischemic heart disease at roughly 20% (Merck Manual Professional Edition, 2024). Increased vagal tone, AV-nodal-blocking medications (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, amiodarone), myocarditis (including Lyme disease), infiltrative disease such as cardiac sarcoidosis, collagen vascular disease, electrolyte disturbances, congenital heart block from transplacental maternal autoantibodies, and injury from cardiac surgery or catheter-based procedures round out the list (StatPearls, 2024; Merck Manual Professional Edition, 2024). Up to a third of patients aged 18 to 60 who present with new Mobitz II or third-degree block and no other evident structural heart disease are ultimately diagnosed with cardiac sarcoidosis, which makes an unexplained higher-grade block in a younger adult a reason to keep that diagnosis on the list (Merck Manual Professional Edition, 2024).

Interpretation Guide

Key Features:

• Rate: not itself defined by this label — the ventricular rate depends on which grade is actually present, from normal (first-degree, most second-degree) down to a slow escape rate (third-degree, or high-grade second-degree)
• Rhythm: varies by grade — regular in first-degree and in fixed-ratio second-degree block, group-beating (progressively shortening R-R intervals before a dropped beat) in Wenckebach/Mobitz I, and either regular or irregular in third-degree block depending on the escape focus
• P waves: normal morphology and rate in all four grades; the atria continue to depolarize on their own schedule regardless of how many impulses reach the ventricles
• PR interval: the single most useful feature for sorting a record into a specific grade — fixed and prolonged beyond 200 ms with every P wave conducting is first-degree; progressively lengthening before a dropped QRS is Wenckebach/Mobitz I; constant among conducted beats but with an unpredictably dropped QRS is Mobitz II; and showing no consistent relationship to the QRS at all, because the atria and ventricles are dissociated, is third-degree
• QRS complex: normal and narrow unless the block sits at or below the His bundle (Mobitz II) or a slow ventricular escape rhythm is driving the ventricles in third-degree block, either of which can widen it; QRS width helps localize how far down the conduction system the block sits
• ST segment: not a primary diagnostic feature of AV block itself; normal unless a coexisting condition is present
• T waves: not a primary diagnostic feature of AV block itself; normal unless a coexisting condition is present
• QT interval: not a primary diagnostic feature of this finding
• Other findings: the pattern of P waves that fail to produce a following QRS — none missing, progressively-then-suddenly missing, unpredictably missing, or completely unrelated to the QRS — is what actually assigns a record to one of the four grades; measure that pattern before treating a bare “AVB” strip as belonging to any one of them

Key Leads

• Lead II — Typically the clearest single lead for tracking the P-to-QRS relationship across several consecutive beats, since it usually shows the most distinct, upright P wave separated from the QRS complex.
• Lead V1 — A second, independent view of P-wave shape and timing, useful for confirming a pattern seen in Lead II when the P wave there is small or runs into the preceding T wave.

Differential Diagnosis

• First-Degree AV Block (1AVB) — a fixed, prolonged PR interval with every P wave still conducting and no dropped beats at all; the mildest grade this umbrella spans.
• 2 Degree Atrioventricular Block (2AVB) — this dataset’s generic second-degree label: intermittently non-conducted P waves without a stated mechanism, covering second-degree block that is not specifically identified as the Wenckebach/Mobitz I pattern below.
• 2 Degree Atrioventricular Block (Type One) (2AVB1) — the Wenckebach/Mobitz I form specifically: PR interval that lengthens beat to beat until one P wave fails to conduct, then resets.
• 3 Degree Atrioventricular Block (3AVB) — complete AV dissociation: P waves and QRS complexes each march out on their own independent rhythm, with no consistent PR relationship at all.

Treatment Brief

Because this label does not specify which of the four grades is present, the first task is measuring the strip rather than acting on the label alone: check whether every P wave conducts, whether the PR interval is fixed or progressively lengthening, and whether any P waves fail to produce a QRS at all — that pattern, not the bare “AVB” label, determines urgency. Confirm lead placement and obtain a longer strip to verify the pattern is consistent rather than an artifact, and correlate the finding with the patient’s vital signs, symptoms, and current medication list, especially AV-nodal-blocking drugs. For a pattern that turns out to be simple first-degree block or asymptomatic Wenckebach, routine observation is typically sufficient. Notify the provider promptly for any pattern showing unpredictably dropped beats without progressive PR lengthening, a widened escape QRS, or any sign that the P waves and QRS complexes are running independently of one another — these findings point toward Mobitz II, high-grade, or third-degree block, which carry a materially higher risk of progressing to a hemodynamically unstable rhythm and are managed far more urgently than the milder grades on this spectrum.