AV block, first degree, on ECG

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The PQ interval is measured from the onset of the P wave to the onset of the QRS complex. Normally it is 120–200 ms (3–5 mm). In first degree AV block, PQ is prolonged beyond 200 ms, but every recognizable P wave is conducted to the ventricles (1:1 conduction).

Measure PR in the lead with the most clearly defined onset of P and onset of QRS; this is often II or V1. Important: the PQ interval is in most cases stable from complex to complex (unlike the progressive prolongation in Wenckebach periodicity in second degree AV block, Mobitz type I), although small PR fluctuations are possible with changes in autonomic tone or heart rate. With marked PQ prolongation, the P wave may overlap the preceding T wave, creating a pseudo-notched T wave.

مخططات حقيقية: 8 أمثلة

مخطط قلب من قاعدة البيانات المفتوحة PTB-XL (Wagner P. et al., Scientific Data, 2020; PhysioNet)، ترخيص CC BY 4.0. الرسم - منصة WhiteCoat Pro.

الآلية

The impulse from the sinus node spreads through the atrial myocardium to the AV node, where the main physiological delay occurs (normally 50–120 ms, which constitutes a significant part of the PQ interval), then it is conducted through the His–Purkinje system to the ventricular myocardium. The PR interval itself reflects the total time of intra-atrial, AV nodal, and His–Purkinje conduction. On a standard ECG the level of block is usually not determined; more often the delay is located in the AV node, but an infrahisian level or combined slowing is possible. Electrophysiological studies show that in most patients the delay is located in the compact portion of the AV node (AH interval prolonged), and such block responds well to atropine. Less commonly, prolongation occurs in the His–Purkinje system (HV interval), which indicates a more distal level of involvement and may be a harbinger of higher degrees of block.

الأسباب

الفسيولوجية

• High vagal tone (athletes, healthy young people, sleep)
• Use of parasympathomimetics, beta-blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), class III antiarrhythmics (amiodarone, sotalol), digitalis

Pathological

• Degenerative changes of the conduction system — Lev–Lenègre disease (sclerodegenerative involvement of the conduction system), age-related fibrosis
• Coronary artery disease, especially inferior wall infarction (occlusion of the right coronary artery supplying the AV node)
• Myocarditis and inflammatory heart diseases; in rheumatic fever, carditis with AV conduction disturbances is possible
• Infiltrative processes (sarcoidosis, amyloidosis, hemochromatosis)
• Hyperkalemia (in combination with QRS widening)
• After cardiac surgery (aortic valve replacement, correction of congenital defects) or in certain congenital defects

التشخيص التفريقي

Second degree AV block, Mobitz type I (Wenckebach): the PQ interval is not constant but gradually lengthens from cycle to cycle, followed by a dropped QRS (the P wave is not conducted). In first degree AV block, PQ is stably prolonged, but every P is conducted.

P wave partially overlapping T: if the P wave is inconspicuous and hidden at the end of the preceding T, PQ may be erroneously measured from the "hump" on T to QRS. Always look for the onset of P — it may be on the descending limb of T.

Bundle branch block: a QRS width > 120 ms results from intraventricular delay, but PQ may be normal. A prolonged PQ with bundle branch block means a combination of two conduction disturbances.

Normal upper limit of PQ in the young: in children and adolescents the upper limit of normal may be 180 ms. In the Russian Ministry of Health clinical guidelines on bradyarrhythmias (2020) and AHA/ACCF/HRS (2009), the 200 ms threshold applies to adults; in children, age-based nomograms are used.

الأهمية السريرية والتكتيكات

First degree AV block is usually benign and does not cause symptoms. Prevalence depends on age, population, and selection criteria and varies considerably between studies; it is more common in older people and in structural heart disease than in young healthy individuals. With normal or moderately prolonged PR, hemodynamic consequences are usually absent; with marked PR > 300 ms, AV dyssynchrony and symptoms are possible. Patients do not require treatment if there are no symptoms and no risk of progression.

When to be vigilant:

• The combination of PR > 200 ms with bifascicular block requires assessment of the clinical context and the risk of distal conduction disturbance; the term "trifascicular block" should not be used solely on the basis of PR prolongation.
• Marked PQ prolongation (> 300 ms) may be accompanied by complaints of palpitations or reduced exercise tolerance due to shortened diastolic filling and AV dyssynchrony (pseudopacemaker syndrome).
• Acute myocardial infarction: first degree AV block in inferior MI is most often reversible; in anterior MI it may indicate extensive involvement of the interventricular septum and a risk of complete AV block.
• Progressive PQ prolongation or progression to second–third degree AV block.

Management:

• Assess use of drugs that slow AV conduction (beta-blockers, digitalis, verapamil, amiodarone); consider dose adjustment or discontinuation.
• Workup is determined by the clinical context. Assess medications and electrolytes; evaluate for ischemia, myocarditis, or Lyme disease if there are symptoms, acute onset, relevant history, signs of inflammation/ischemia, or unexplained progressive block.
• If intermittent higher-degree block is suspected or if symptoms are present — ECG monitoring, the type and duration of which are chosen according to the clinical picture and the expected event rate.
• With PR usually more than 300 ms and symptoms clearly related to AV dyssynchrony, pacemaker implantation may be considered individually. When combined with bundle branch block, management is determined by symptoms, documented high-grade or infrahisian block, and/or electrophysiological study results; PR prolongation alone is not an indication for pacing.

Most patients with isolated first degree AV block are observed without treatment.

الأخطاء الشائعة أثناء قراءة تخطيط القلب

1. Measuring PQ not from the onset of P: the onset of P may be gradual, especially in low-amplitude leads. Measure in the lead with a clear onset of the P wave (usually II or V1).

1. Missing a P wave overlapping T: in tachycardia or with a long PQ, the P wave may "hide" in the preceding T wave. If suspected, analyze a long recording, the P morphology in other leads, and the P–QRS relationship; alternating T shape is not a reliable sign of P overlap and should not be used as a diagnostic criterion.

1. Diagnosing "first degree AV block" in the absence of atrial activity: if the rhythm originates from the AV node (junctional rhythm), P may be absent or retrograde, and the term "first degree AV block" does not apply.

1. Forgetting to check ECG calibration and paper speed: at 50 mm/s, the same 10 mm corresponds to 200 ms, not 400 ms. Always verify the recording speed.

1. Overlooking functional block: PQ prolongation during beta-blocker use or in an athlete is not a disease but an adaptation or drug effect. After discontinuation of the drug, PQ may normalize.

الأسئلة الشائعة

المصطلحات ذات الصلة

المزيد في قسم «إحصارات AV»

القسم بأكمله

المصادر

• AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
• ESC Guidelines on cardiac pacing and cardiac resynchronization therapy, 2021
• Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice, 6th ed., 2008
• التوصيات السريرية «اضطرابات النظم البطيئة»، وزارة الصحة الروسية، 2020

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