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    6.3
    Home > ECG findings

    6.3

    Minnesota Code 6-3: PR interval ≥ 220 ms (first-degree AV block) — causes, long-term risks and when to worry.

    Code 6.3 — Minnesota Code 6-3: prolonged PR interval (first-degree AV block). The PR interval, the time from the start of atrial activation to the start of ventricular activation, is 0.22 s (220 ms) or longer in most beats in any of the limb leads. Every atrial impulse still reaches the ventricles, but with a delay in the AV node or the His–Purkinje system. Clinical guidelines define first-degree AV block as a PR interval over 200 ms; the Minnesota Code uses the stricter 220 ms threshold.

    Minnesota Code 6PR intervalFirst-degree AV block
    Normal PR120–200 ms
    Code 6-3PR ≥ 220 ms
    Guideline definitionPR > 200 ms
    Conduction1:1, every P conducted
    Normal PR interval of about 160 milliseconds compared with a prolonged PR interval of about 280 milliseconds Normal PR PR 120–200 ms PR 160 ms Code 6-3 PR ≥ 220 ms PR 280 ms
    Figure 1. Left: normal PR interval (about 160 ms). Right: prolonged PR interval (about 280 ms) — every P wave is followed by a QRS complex, but later than normal (Minnesota 6-3).

    Causes

    • High vagal tone: athletes, sleep — usually normalises with exercise
    • Medicines that slow AV conduction: beta-blockers, verapamil, diltiazem, digoxin, amiodarone
    • Age-related fibrosis of the conduction system
    • Inferior myocardial infarction, myocarditis, Lyme disease, rheumatic fever, sarcoidosis
    • Electrolyte disorders (high potassium), hypothyroidism

    Clinical significance

    Isolated first-degree AV block with a moderately prolonged PR usually causes no symptoms and needs no treatment. In the Framingham cohort, however, a PR over 200 ms was associated with a higher long-term risk of atrial fibrillation, pacemaker implantation and mortality. A very long PR (over 300 ms) can cause symptoms similar to pacemaker syndrome, because atrial contraction occurs too early relative to ventricular contraction. A long PR combined with bundle branch block may point to more extensive conduction disease.

    Usually harmless

    Young athlete, no symptoms, PR shortens with exercise, normal QRS.

    Needs evaluation

    Dizziness, fainting, fatigue; PR > 300 ms; new bundle branch block; progression on repeat ECGs; drugs that slow conduction.

    Seek urgent care

    if you have fainting, near-fainting, very slow pulse or breathlessness — these may indicate a higher-degree AV block.

    References

    1. 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(8):e382–e482. doi:10.1161/CIR.0000000000000628
    2. Cheng S, Keyes MJ, Larson MG, et al. Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block. JAMA. 2009;301(24):2571–2577. doi:10.1001/jama.2009.888
    3. Prineas RJ, Crow RS, Zhang ZM. The Minnesota Code Manual of Electrocardiographic Findings. 2nd ed. London: Springer; 2010. doi:10.1007/978-1-84882-778-3

    This article is for general information and does not replace a consultation with a doctor. ECG findings must be interpreted by a physician together with symptoms and other tests.

    Published on 14 May 2019