Clinical Overview

First-degree atrioventricular (AV) block is a delay in conduction through the AV node, defined on the surface ECG by a PR interval greater than 200 ms (0.20 s) that stays constant from beat to beat, with every P wave still followed by a QRS complex. It is a commonly cited “misnomer”: no impulse is actually blocked — every atrial impulse reaches the ventricles, just more slowly than normal — so it is more accurately described as an AV conduction delay than a true block.

The delay is most often located in the AV node itself. In younger patients and trained athletes it is typically a benign, vagally-mediated finding; in older patients it more often reflects fibrotic degeneration of the conduction system. Because the delay is usually nodal rather than in the His-Purkinje system, first-degree block on its own carries a comparatively low risk of progressing to a higher-degree block or sudden cardiac arrest.

First-degree AV block is generally well tolerated and, in isolation, does not cause hemodynamic disturbance — the great majority of patients are asymptomatic and the finding is picked up incidentally on a routine strip. That said, it is no longer viewed as uniformly benign: longer PR intervals are increasingly linked with higher rates of atrial fibrillation and other adverse outcomes, so the finding is worth noting even in an otherwise asymptomatic patient. When the PR interval becomes markedly prolonged (>300 ms, sometimes called “marked” first-degree block), the next atrial contraction can land against a mitral/tricuspid valve that hasn’t finished closing from the previous beat, producing pacemaker-syndrome-like symptoms even without any higher-degree block.

Symptoms are usually absent. When the block is marked (PR > 300 ms), some patients report dyspnea, fatigue, lightheadedness, chest discomfort, or near-syncope from the loss of normal AV synchrony described above.

Common causes and risk factors include increased vagal tone (common in young, healthy athletes; reported prevalence in this group is roughly 7-12% across studies), age-related fibrotic degeneration of the AV node, ischemic heart disease including acute inferior myocardial infarction, myocarditis (including Lyme disease), infiltrative disease such as sarcoidosis, collagen vascular disease, electrolyte disturbances (particularly hyperkalemia), and AV-nodal-blocking medications (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, amiodarone). Prevalence in the general population is roughly 1-1.5% in adults under 60 and rises to roughly 6% beyond that age; the finding is about twice as common in men as in women.

Interpretation Guide

Key Features:

• Rate: not itself altered by the block — reflects the underlying rhythm (most often normal sinus rhythm)
• Rhythm: regular, with 1:1 conduction preserved between P waves and QRS complexes
• P waves: normal morphology, one preceding every QRS complex
• PR interval: prolonged beyond 200 ms (0.20 s) and fixed/constant from beat to beat; “marked” when it exceeds 300 ms (0.30 s)
• QRS complex: normal and narrow (<0.12 s) unless a separate, coexisting bundle branch block is present
• ST segment: not a primary diagnostic feature; normal unless a coexisting condition is present
• T waves: not a primary diagnostic feature; normal unless a coexisting condition is present
• QT interval: not a primary diagnostic feature of this block
• Other findings: every P wave conducts — there are no dropped beats; in markedly prolonged cases the P wave can sit close to, or even encroach on, the preceding T wave

The single most important recognition point is that first-degree AV block is a delay, not a dropped beat: if any P wave fails to produce a following QRS, the rhythm is a second- or third-degree block instead, not first-degree.

Key Leads

• Lead II — Typically the clearest single lead for measuring the PR interval, since it usually shows the most distinct, upright P wave separated from the QRS complex.
• Lead V1 — A useful second view of P-wave morphology when the P wave is subtle or partially obscured in the inferior leads.

Differential Diagnosis

• 2 Degree Atrioventricular Block (Type One) — Wenckebach/Mobitz I also starts from a prolonged PR interval, but the PR interval progressively lengthens beat to beat until one P wave fails to conduct and a QRS is dropped; first-degree block never drops a beat and its PR interval stays fixed.
• 3 Degree Atrioventricular Block — complete heart block shows P waves and QRS complexes marching out completely independently of one another (AV dissociation), with no consistent PR relationship at all, unlike first-degree block’s constant, only-prolonged PR interval with 1:1 conduction fully preserved.
• Shortened PR Interval — the opposite-direction finding: a PR interval abnormally short (<120 ms), often associated with a pre-excitation pathway, rather than the prolongation seen in first-degree block.
• PR Interval Extension — this dataset label describes the same underlying ECG finding (a PR interval beyond normal limits) but as a standalone finding rather than the formal AV-block diagnosis; the two labels can co-occur on the same record, so confirm which label the specific record is filed under before treating them as identical.

Treatment Brief

Confirm lead placement and obtain a longer strip (or a full 12-lead) to verify the PR interval is truly and consistently prolonged rather than an artifact, and correlate the finding with the patient’s vital signs, symptoms, and current medication list — especially AV-nodal-blocking drugs, since a new or worsening prolongation after a dose change is worth flagging to the provider. For the large majority of asymptomatic patients with a modestly prolonged, stable PR interval, no specific treatment is required beyond routine observation and outpatient follow-up; major society guidance does not support routine pacemaker placement for isolated first-degree block. Watch and notify the provider for markedly prolonged PR intervals (>300 ms) with pacemaker-syndrome-like symptoms, a new or widening QRS suggesting infranodal (His-Purkinje) involvement, or any progression toward a higher-degree block.