In electrocardiography, the PR interval is measured from the beginning of the P wave

P wave (electrocardiography)

In electrocardiography, during normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium...

 to the beginning of the QRS complex

QRS complex

The QRS complex is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram . It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the human heart...

. It is usually 120 to 200 ms long. On the usual 25 mm/s ECG tracing, this corresponds to 3 to 5 small boxes. The PR interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node and entering the ventricles. The PR interval is therefore a good estimate of AV node function.

In case a Q wave was measured with an ECG, the PR interval is also commonly named "PQ interval" instead.

Interpretation

Variations in the PR interval can be associated with certain medical conditions:

• Duration
  • A long PR interval (of over 200 ms) may indicate a first degree heart block
    First degree heart block
    First-degree AV block, or PR prolongation, is a disease of the electrical conduction system of the heart in which the PR interval is lengthened beyond 0.20 seconds....
    
    . Prolongation can be associated with hypokalemia
    Hypokalemia
    Hypokalemia or hypokalaemia , also hypopotassemia or hypopotassaemia , refers to the condition in which the concentration of potassium in the blood is low...
    
     or acute rheumatic fever. or Lyme disease
    Lyme disease
    Lyme disease, or Lyme borreliosis, is an emerging infectious disease caused by at least three species of bacteria belonging to the genus Borrelia. Borrelia burgdorferi sensu stricto is the main cause of Lyme disease in the United States, whereas Borrelia afzelii and Borrelia garinii cause most...
    
    .

• • A short PR interval may indicate a pre-excitation syndrome
    Pre-excitation syndrome
    Pre-excitation syndrome is a condition where the ventricles of the heart become depolarized too early, which leads to their partial premature contraction.-Pathophysiology:...
    
     via an accessory pathway that leads to early activation of the ventricles, such as seen in Wolff-Parkinson-White syndrome
    Wolff-Parkinson-White syndrome
    Wolff–Parkinson–White syndrome is a disorder of the heart in which the ventricles of the heart contract prematurely due to an accessory pathway known as the bundle of Kent. This accessory pathway is an abnormal electrical communication from the atria to the ventricles...
    
    .
  • A variable PR interval may indicate other types of heart block
    Heart block
    A heart block can be a blockage at any level of the electrical conduction system of the heart .* Blocks that occur within the sinoatrial node are described as SA nodal blocks....
    
    .
• PR segment depression may indicate atrial injury or pericarditis
  Pericarditis
  Pericarditis is an inflammation of the pericardium . A characteristic chest pain is often present.The causes of pericarditis are varied, including viral infections of the pericardium, idiopathic causes, uremic pericarditis, bacterial infections of the precardium Pericarditis is an inflammation of...
  
  .
• Variable morphologies of P waves in a single ECG lead is suggestive of an ectopic pacemaker rhythm such as wandering pacemaker
  Wandering pacemaker
  In cardiology, a wandering pacemaker is an atrial arrhythmia that occurs when the natural cardiac pacemaker site shifts between the SA node, the atria, and/or the AV node...
  
   or multifocal atrial tachycardia
  Multifocal atrial tachycardia
  Multifocal atrial tachycardia is a cardiac arrhythmia, specifically a type of supraventricular tachycardia."Multifocal atrial rhythm" is the condition in the absence of tachycardia.-Causes and epidemiology:It is more common in the elderly....