If a PR interval is prolonged, the impulse from the sinoatrial node took too long to reach the intra-atrial pathways and the AV node. A prolonged or widened QRS complex may indicate that the impulse took too long to travel through the ventricles. 

A normal PR interval is 120–200 milliseconds long (0.12–0.20 seconds).

• Short PR intervals suggest the presence of Wolff-Parkinson-White syndrome.
• Long PR intervals are commonly seen in first-degree AV block. 

Normal QRS intervals last 60–100 milliseconds (i.e., 1.5–2.5 small squares). 

• Prolonged QRS intervals indicate the presence of a bundle branch block, ventricular preexcitation, ventricular pacing, or ventricular tachycardia.