The PR Interval & PR segment
Video lecture: The PR interval & PR segment
This video lecture is accompanied by the detailed article The normal electrocardiogram (ECG). A brief summary of the video lecture and the article is provided below.
PR interval and PR segment
The PR interval stretches from the onset of the P-wave to the onset of the QRS complex. It reflects the time interval from start of atrial activation (depolarization) to start of ventricular activation (depolarization). The PR interval reflects whether impulse conduction from the atria to the ventricles is normal. The PR interval must not be shorter than 120 msec or longer than 220 msec.
The PR segment is the flat line between the end of the P-wave and the start of the QRS complex. The PR segment reflects the time delay between atrial and ventricular activation. The PR segment also serves as the baseline (reference line or isoelectric line) of the ECG curve. The amplitude of any deflection/wave is measured by using the PR segment as the baseline.
Several conditions can reduce the ability of the atrioventricular (AV) node to conduct the atrial impulse to the ventricles. As the conductivity diminishes, the PR interval becomes longer. The upper normal limit is 220 msec (0.22 sec). If the PR interval is greater than 220 msec, first degree AV block is manifest and it is considered a pathological finding (Figure 4, second panel).
Under normal circumstances the electrical impulse travels from the atria to the ventricle via the atrioventricular (AV) node. The electrical impulse cannot travel directly from atrial myocardium to ventricular myocardium because the two are electrically isolated from each other by the fibrous rings (anulus fibrosus). However, some people have an additional (also called accessory) connection between the atria and the ventricles. Such accessory connections (pathways) are embryological remnants and they may be located anywhere between the atria and the ventricles. Accessory pathways enable the atrial impulse to bypass the AV node and thereby the impulse delay in the AV node. Therefore, the ventricles will be activated (depolarized) earlier than expected, which is reflected on the ECG by a short PR interval (PR interval <0.12 seconds). This condition is referred to as pre-excitation (the ventricles are excited prematurely; refer to Figure 4, third panel). As illustrated in Figure 4 the initial depolarization of the ventricles is slow because the impulse initially spreads outside of the His-Purkinje system. The slow initial depolarization generates a delta wave on the ECG. Pre-excitation will also cause a prolongation of the entire QRS duration, which will exceed 120 msec (0.12 sec).
PR interval checklist
- Normal PR interval: 0,12–0,22 seconds. Upper reference limit is 0,20 seconds in young adults.
- A prolonged PR interval (>0.22 s) is consistent with first-degree AV-block.
- A shortened PR interval (<0,12 s) indicates pre-excitation (presence of an accessory pathway). This is associated with a delta wave.
For further details on the PR interval and the PR segment, refer to The normal electrocardiogram (ECG).