Overview
- Minor atrioventricular (AV) conduction defect: partial block at/below AV node
- P wave always precedes each QRS complex, but there is fixed, prolongation of the PR interval that is >200 ms
- In other words, there is a constant conduction delay of the atrial impulse to the ventricles
- Considered “marked” first-degree AV block is PR interval >300 ms
ECG Features
- Constant, prolonged PR interval >200 ms
- No dropped beats — one P wave for each QRS complex (i.e. each P wave is followed appropriately by a QRS complex)
- Note: prolonged PR interval represents delay from the onset of atrial conduction to ventricular conduction; does not evaluate the time for the sinoatrial node to atrial tissue; hence, a prolonged PR interval with a narrow QRS complex localized the site of the block to the AV node
Causes
- Cardiac: Inferior MI, mitral valve surgery, myocarditis (e.g. Lyme disease)
- Medications: AV nodal blocking agents (e.g. beta-blockers, calcium channel blockers, digoxin, amiodarone)
- Other: Increased vagal tone (e.g. athletes), hyperkalemia, normal variant
Clinical Significance
- Does not cause hemodynamic instability
- No specific treatment is typically required
ECG Example
Patient: 84-year-old male with hypertension on lisinopril and diltiazem
Interpretation: Sinus bradycardia with first-degree AV block
– Ventricular rate: 54 BPM
– PR interval: 262 ms
– QRS duration: 84 ms
– QT/QTc interval: 412/390 ms
– P-R-T axes: +68 +45 +45
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