Normal PR-interval

ECG: Sinus rhythm with a rate of 56 bpm; fixed PR-interval of normal duration (130 ms); isoelectric PR segment;

 Comments: A step by step systematic analysis of an electrocardiogram includes measuring the duration and the polarity of the PR-interval. The PR-interval (or PQ-interval) corresponds to the time frame between the depolarization of the first atrial cell and the depolarization of the first ventricular cell. The PR-interval can be divided into a P-wave, the width of which is the reflection of the depolarization time of the two atria, and the PR segment, measured from the end of the P-wave to the beginning of the R-wave, which reflects the conduction delay in the atrioventricular node and the His-Purkinje system.

•  the PR-interval is measured from the beginning of the P-wave to the first deflection of the QRS-complex (Q wave if initial negative deflection or R wave if initial positive deflection). The term PR is frequently used because Q waves are often absent. It is common practice to measure the PR-interval in lead II (good visualization in this lead of the onset of the P-wave which displays good voltage and the onset of the QRS-complex) or to use the longest value measured in limb leads. For a same patient, the PR-interval may not be fixed throughout the day and can present physiological variations (shortening on exertion, prolongation in a vagal context). Indeed, the conduction velocities and refractory periods in the atrioventricular node are dependent on calcium channels and vary according to various parameters (influence of the autonomic nervous system, certain medications and certain metabolic abnormalities). Despite these physiological variations, it is not usual to index and adjust the PR-interval value based on heart rate (unlike the QT-interval). The normal PR-interval is between 120 and 200 ms. These thresholds vary very little according to gender and age even if the standard is between 100 and 180 ms in children.
• The PR segment corresponds to 1) atrial repolarization which generally has a polarity opposite to that of the P-wave but is most often invisible due to its very low amplitude and its coincident occurrence with the QRS-complex; it can sometimes be unmasked in the event of atrioventricular block and atrioventricular dissociation. 2) the propagation of impulses in the atrioventricular node, the bundle branches and the Purkinje network. The PR-interval is thus usually isoelectric given that the transited structures do not generate signals of sufficient amplitude to be detected on the surface ECG.

Take-home message: The PR-interval is considered normal between 120 and 200 ms; a PR segment elevation is significant if greater than 0.5 mm, and a depression if greater than 0.8 mm.