The PR interval on an electrocardiogram (ECG) represents the time an electrical impulse takes to travel from the atria to the ventricles. This measurement is fundamental for assessing the heart’s electrical conduction system.
What the PR Interval Represents
The PR interval reflects the journey of an electrical impulse as it originates in the sinoatrial (SA) node, the heart’s natural pacemaker, located in the right atrium. This impulse then spreads across both atria, causing them to contract. As the electrical signal moves from the atria, it converges at the atrioventricular (AV) node.
A brief delay occurs at the AV node. This delay allows the atria to fully empty blood into the ventricles before they contract. After passing through the AV node, the impulse rapidly travels down the bundle of His and the Purkinje fibers, distributing the electrical signal throughout the ventricles. The PR interval specifically measures the time from the beginning of atrial depolarization to the start of ventricular depolarization.
Reading the ECG Grid
An ECG is typically printed on specialized graph paper, featuring a standardized grid. This grid consists of small squares, each measuring 1 millimeter (mm) by 1 mm. These small squares are grouped into larger 5 mm by 5 mm squares.
On the horizontal axis, the grid measures time. At a standard paper speed of 25 mm per second, each small 1 mm square represents 0.04 seconds (40 milliseconds). Each large 5 mm square, containing five small squares, represents 0.20 seconds (200 milliseconds). The vertical axis measures voltage, with each small square typically representing 0.1 millivolt (mV).
Step-by-Step Measurement
Measuring the PR interval begins by precisely identifying its start and end points on the ECG tracing. The interval commences at the very beginning of the P wave, which signifies atrial depolarization. This is the point where the tracing first deviates from the baseline, indicating the start of atrial electrical activity.
The PR interval concludes at the beginning of the QRS complex, which represents ventricular depolarization. This is the point where the Q wave (initial downward deflection) or the R wave (initial upward deflection) first departs from the baseline. Identifying these precise points requires careful observation to ensure accuracy.
Once the start of the P wave and the beginning of the QRS complex are identified, count the number of small squares between these two points. For instance, if the P wave begins at one vertical line and the QRS complex starts five small squares later, the PR interval spans five small squares. Using calipers can enhance precision in marking these points and counting the squares.
To convert the counted squares into a time measurement, multiply the number of small squares by 0.04 seconds (the time value of one small square). For example, if the PR interval covers five small squares, the calculation is 5 0.04 seconds, resulting in a PR interval of 0.20 seconds. Many modern ECG machines and software also offer digital measurement tools that automatically calculate this interval.
Interpreting PR Interval Values
In adult individuals, a typical PR interval generally falls within a range of 0.12 to 0.20 seconds. This corresponds to three to five small squares on the ECG grid when the paper speed is set at 25 mm per second. This duration indicates that the electrical impulse is traveling from the atria to the ventricles within an expected timeframe, allowing for proper heart chamber coordination.
A PR interval shorter than 0.12 seconds suggests the electrical impulse reaches the ventricles more quickly than usual. This can indicate an accelerated conduction pathway. Conversely, a PR interval longer than 0.20 seconds indicates a delay in impulse conduction from the atria to the ventricles. This prolongation suggests the signal takes more time than expected to pass through the AV node or the His-Purkinje system.