An electrocardiogram (ECG or EKG) is a non-invasive test that records the electrical activity of the heart over a period of time. This recording appears as a series of waves and straight lines on a graph, which correspond to the sequential phases of the cardiac cycle. By analyzing the shape, timing, and relationship of these components, medical professionals can evaluate the heart’s rhythm and conduction system. The ECG trace is composed of distinct waves, complexes, segments, and intervals, each representing a specific electrical event within the heart muscle.
Defining the PR Segment on an ECG
The PR segment is the flat, horizontal line that appears on the ECG trace after the P wave and before the QRS complex. It is specifically defined as the portion of the tracing that begins at the end of the P wave, which signifies the completion of atrial depolarization, or electrical activation. The segment ends precisely at the onset of the QRS complex, which is the beginning of ventricular depolarization. Because the tracing is flat during this period, the PR segment is considered an isoelectric line, meaning no significant electrical potential is being measured at the electrodes. It effectively serves as the baseline against which the amplitude of other waves and deflections on the ECG are measured.
The Physiological Purpose of the Segment
The presence of this flat line on the ECG represents a physiological delay in the heart’s electrical conduction system. After the electrical impulse activates the atria, the signal reaches the atrioventricular (AV) node, which is a specialized group of cells that slows down the impulse transmission. The AV nodal pause is necessary to ensure that the atria have sufficient time to fully contract and empty their blood into the ventricles before the ventricles begin their own contraction cycle. This short delay optimizes cardiac output by coordinating the mechanical action of the heart chambers. The PR segment itself is flat because the electrical activity during the AV nodal delay is too small to be picked up by the surface electrodes of the ECG.
Standard Measurement and Duration
While the PR segment is a distinct component, the clinically measured conduction time is usually the PR interval, which includes the P wave and the PR segment together. The PR interval is measured from the very beginning of the P wave to the start of the QRS complex. This interval reflects the total time required for the electrical impulse to travel from the atria, through the AV node, and into the ventricular conduction system. In healthy adults, the normal duration for the PR interval is between 0.12 and 0.20 seconds, or 120 to 200 milliseconds. On standard ECG paper, this corresponds to three to five small squares when the paper speed is set to 25 millimeters per second.
What Abnormal PR Segment Readings Indicate
Deviations from the standard PR interval duration are highly indicative of problems with the heart’s electrical conduction.
Prolonged PR Interval
A prolonged PR interval, measuring greater than 0.20 seconds, suggests a slowing of conduction, usually within the AV node. This finding is typically classified as a first-degree atrioventricular block, indicating the impulse is taking an abnormally long time to pass from the atria to the ventricles. This prolongation can be caused by various factors, including certain medications, high vagal tone, or underlying conditions like fibrosis of the AV node.
Shortened PR Interval
Conversely, a shortened PR interval, defined as less than 0.12 seconds, suggests that the electrical signal has bypassed the necessary delay at the AV node. This condition is often associated with pre-excitation syndromes, such as Wolff-Parkinson-White (WPW) syndrome. In WPW, an abnormal accessory pathway acts as an electrical shortcut between the atria and ventricles.
Vertical Abnormalities
The PR segment itself, separate from the interval’s duration, can also show abnormalities in its vertical position relative to the isoelectric baseline. Segment depression or elevation can signal other specific cardiac issues. For instance, PR segment depression is a common finding in pericarditis, which is inflammation of the sac surrounding the heart. Vertical abnormalities may also indicate atrial ischemia, a condition where the atrial muscle tissue is not receiving enough blood flow.