Cardiac Rhythm Series: First Degree AV Block

In first degree AV block, the sinoatrial (SA) impulse is delayed longer than normal at the AV node before being transmitted to the ventricles causing a prolonged PR interval1, 2. As a reminder, the PR interval represents the time needed for an electrical impulse to travel from the SA node throughout the cardiac conduction system to the Purkinje fibers2.

Characteristics: P waves present and uniform with one per QRS, PR interval greater than 0.2 seconds and consistent, QRS less than 0.12 seconds, regular rate and rhythm.

First degree AV block has a number of possible causes:

  • AV node ischemia: The AV node’s blood supply comes from the right coronary artery in 90% of individuals and the left circumflex coronary artery in the remaining 10%2, 3.
  • Enhanced vagal tone: Well-trained athletes can develop first-degree AV block due to an increased vagal tone3.
  • Acute myocardial infarction: First-degree AV block occurs in just under 15% of patients with acute MI (particularly of the inferior wall) admitted to coronary care units3.
  • Drugs: Digoxin, amiodarone, beta-blockers, and calcium-channel blockers increase the refractory time of the AV node which slows conduction. These drugs are not completely contraindicated, but their use increases the risk of developing a higher degree of AV block3.
  • Infective endocarditis or rheumatic fever2, 3.

“If the R is far from the P, then you have a first degree.”

First degree AV block is typically considered benign and usually does not progress to higher degrees of AV block2. If asymptomatic, no treatment is needed2. The patient should be monitored for progression to higher degrees of AV block2.

  1. Bojar, R. M. (2016). Manual of perioperative care in adult cardiac surgery (5th ed.). West Sussex, UK: Wiley-Blackwell
  2. University of Maryland Medical Center Office of Clinical Practice and Professional Development. (2014). Introduction to cardiac rhythm interpretation (6th ed.).

For the next post of this series, click here: second degree AV block, Mobitz type I (Wenckebach).




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