PR interval

PR interval is a term used in electrocardiography to describe the period of time from the onset of the P wave (atrial depolarization) to the start of the QRS complex (ventricular depolarization). It is an important measure in the diagnosis of several heart conditions, including heart blocks and pre-excitation syndromes.

Long PR interval (ECG)

Measurement

The PR interval is measured in milliseconds and is usually between 120 and 200 ms in duration. This interval represents the time the electrical impulse takes to travel from the sinoatrial (SA) node through the atrioventricular (AV) node and into the ventricles. The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex on the ECG.

Clinical significance

The length of the PR interval is important because it can indicate a number of different cardiac pathologies. A short PR interval may indicate a pre-excitation syndrome such as Wolff-Parkinson-White syndrome, while a long PR interval may indicate a heart block.

PR interval buildup

Short PR interval

A short PR interval, less than 120 ms, may indicate a pre-excitation syndrome. In these conditions, the electrical signals bypass the AV node via an accessory pathway, leading to premature ventricular depolarization. The most common of these is Wolff-Parkinson-White syndrome.

Long PR interval

A long PR interval, more than 200 ms, is known as a First-degree atrioventricular block. This condition is often asymptomatic, but can be a sign of increased risk for developing a more serious heart block.

See also

• Electrocardiography
• P wave
• QRS complex
• Heart block
• Pre-excitation syndrome