An electrocardiogram (ECG) is a graphic recording of the heart’s electrical activity. The PR interval is a fundamental measurement on the ECG tracing, representing the time the electrical impulse takes to travel from the atria (upper chambers) to the ventricles (lower chambers). This interval captures the brief delay that occurs as the signal passes through the atrioventricular (AV) node. This pause allows the atria to fully contract and empty blood into the ventricles before the ventricles contract.
Identifying the PR Interval Landmarks
The measurement process begins by visually pinpointing the precise start and end points of the interval on the ECG paper. The PR interval starts with the very first deflection of the P wave, which signifies the electrical activation (depolarization) of the atria. This point marks the beginning of the signal’s journey from the sinoatrial node.
The interval concludes at the start of the QRS complex, which represents the rapid depolarization of the ventricles. This terminal point is the first deflection, whether it is a small downward Q wave or the larger, initial upward R wave. The flat line between the end of the P wave and the start of the QRS complex is the PR segment, reflecting the time the impulse spends traveling through the AV node. The PR interval encompasses both the P wave and the PR segment, capturing the entire atrial-to-ventricular conduction time.
Accurate measurement depends on identifying the exact moment the tracing deviates from the flat baseline for the P wave and the exact moment it deviates again to form the QRS complex. In a standard 12-lead ECG, it is best practice to measure the PR interval in the lead that provides the clearest P wave and the longest QRS duration, often lead II. Selecting the optimal lead helps to avoid misinterpretations caused by subtle deflections in other leads.
Calculating the Interval Duration
Once the landmarks are identified, the next step is to convert the distance spanned on the graph paper into a unit of time. Standard ECG machines are calibrated to run the paper at a speed of 25 millimeters per second (mm/s). This standardized speed allows for a simple conversion from distance on the paper to time.
The ECG paper is marked with a grid of small and large squares used for time conversion. Each small square measures 1 millimeter and represents 0.04 seconds at the standard paper speed. Five small squares form one large square, which represents a duration of 0.20 seconds.
To calculate the PR interval, count the number of small squares spanned between the start of the P wave and the start of the QRS complex. For instance, if the interval covers four small squares, the duration is 0.16 seconds (4 x 0.04 seconds). If the measurement falls between the lines, estimate the fraction of the small box to ensure precision. This mathematical conversion provides the precise time the electrical signal took to complete the atrial-to-ventricular journey.
What the PR Measurement Reveals
The calculated PR interval duration provides direct information about the speed of impulse conduction through the atria and the AV node. A normal PR interval in adults ranges between 0.12 seconds and 0.20 seconds, or three to five small squares on the ECG grid. Maintaining this range indicates that the AV node is functioning correctly as a gatekeeper, appropriately delaying the signal to optimize ventricular filling.
A prolonged PR interval, defined as greater than 0.20 seconds, indicates a slowing of the electrical impulse’s travel time. This finding is classified as a first-degree atrioventricular block, suggesting a conduction delay usually occurring within the AV node itself. While this delay can sometimes be benign, it may also be associated with high vagal tone, certain medications, or underlying heart disease.
Conversely, a shortened PR interval (less than 0.12 seconds) suggests the electrical impulse bypassed the normal delay pathway. This occurs in pre-excitation syndromes, such as Wolff-Parkinson-White syndrome, where an accessory pathway connects the atria and ventricles. The shortened time means the ventricles begin to depolarize too early, which can lead to rapid, abnormal heart rhythms. The PR interval serves as a measure to assess the health and timing of the heart’s internal electrical wiring.