First-Degree Heart Block 

1st Degree AV Heart Block

Defined as prolongation of the PR interval on an electrocardiogram (ECG) to more than 200 msec. The PR interval of the surface ECG is measured from the onset of atrial depolarization (P wave) to the beginning of ventricular depolarization (QRS complex). Normally, this interval should be between 120 and 200 msec in the adult population. Whereas conduction is slowed, there are no missed beats. In first-degree AV block, every atrial impulse is transmitted to the ventricles, resulting in a regular ventricular rate. 


Rate Usually within normal range, but depends on underlying rhythm

Rhythm Regular

P-Waves Normal in size and shape; 1 positive (upright) P wave before each QRS in leads 2, 3 and aVF

PR Interval Prolonged. More than 0.20 seconds (5 boxes) but constant

QRS Duration 0.11 seconds (almost 2 full boxes)


Atropine 0.5-1mg SIVP over 1-2 min (Repeat every 3-5 min. 3.0mg max)


Monitor Diagnostic 12 lead

Usually requires none,
 but if Bradycardia is present, treat Bradycardia.

Non-Symptomatic Brady

No Treatment

Symptomatic Brady

Monitor Diagnostic 12 lead

Oxygen 15 LPM NR Mask

IV 18g

Atropine 0.5-1mg SIVP over 1-2 min (Repeat every 3-5 min. 3.0mg max)

Noticeable by: 

Prolonged PR Interval (Greater than 0.20 sec or Greater than 5 Boxes)

© Matt Dillard 2012