First-degree atrioventricular (AV) block


  • A delay in conduction from the atria to the ventricles resulting in a prolonged PR interval (>0,20 s) with normal morphology and duration of the QRS complex and maintenance of 1 : 1 conduction

  • First-degree AV block does not cause bradycardia unless it progresses intermittently to a higher degree block or is associated with sinus node dysfunction.

ECG features:

  • PR interval longer than 200 ms
  • 1:1 conduction – every P wave is followed by QRS complex
  • normal morphology and duration of QRS complex (unless there is another aberrancy in conduction)

Prolonged PR interval - 300 ms

ECG 1  First degree AV block / AV block type 1

  • prolonged PR interval 300 ms
  • 1:1 conduction – every P wave is followed by a QRS complex
  • sinus rhythm at a rate 78 bpm


  • First-degree AV block is generally self-limited and not associated with progression to complete heart block (CHB), but it may be associated with poorer prognosis in those with heart disease
  • An increase in vagal tone – causes a reduction in the rate of conduction
  • Drugs that slow AV node conduction – beta blockers, digoxin or non-dihydropyridine calcium channel blockers
  • Structural abnormalities of the AV node – including dual AV nodal pathways (two sets of PR intervals)
  • Myocardial infarction – anterior or inferior wall
  • Infiltrative cardiomyopathies (sarcoidosis, etc.)


  • Diagnosis is usually easy, based on the surface electrocardiogram (ECG) showing a prolonged PR interval
  • First degree AV block is usually asymptomatic and does not require a specific treatment
  • Markedly prolonged PR interval (>300 ms) can lead to symptoms of heart failure:
    - atrial systole occurs too early in diastole, resulting in an ineffective or decreased contribution of the atrial systole to cardiac output
    - As a consequence, the increase of pulmonary capillary wedge pressure may cause dyspnoea and retrograde blood flow in the jugular veins
    pacemaker syndrome-like symptoms: fatigue, near syncope, shortness of breath, sensation of fullness in the neck and palpitations described as ‘pauses’ or ‘strong beats’

Picture 1 Indications for pacemaker implantation as in ESC guidelines (2013)

Picture 2  First-degree AV block management

Olshansky, B. (2017). First-degree atrioventricular block management [Graph]. In Arrhythmia Essentials (2nd ed., p. 36).


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