First degree AV block

Background

First-degree atrioventricular (AV) block, or first-degree heart block, is defined as prolongation of the PR interval on an electrocardiogram (ECG) to more than 200 msec(5 small squares on the EKG tracing). First-degree AV block is considered “marked” when the PR interval exceeds 300 msec(more than 7 small squares on the EKG tracing).

Clinical Features

Patients with first-degree atrioventricular (AV) block mostly are asymptomatic at rest and it is diagnosed by chance. Exercise may reveal decreased tolerance especially when there is markedly prolonged PR interval and with left ventricular systolic dysfunction. It may also be the cause of syncope with transient high-degree AV block.

Differential Diagnosis

AV blocks

• First degree AV block
• Second degree
  • Second degree AV block type I (Wenkebach)
  • Second degree AV block type II
• Third degree AV block
• AV dissociation without complete heart block

See Also

• AV blocks