An electrocardiogram (ECG) is a graphic representation of the electrical activity that powers the heartbeat. Interpreting these recordings requires precise measurement of the time intervals between specific electrical events. Accurately measuring these durations allows clinicians to assess the sequence and timing of the heart’s electrical conduction system. Abnormalities in the timing of these events can signal potential issues with the heart’s rhythm or internal circuitry.
Understanding the ECG Grid and Paper Speed
The standard ECG is recorded on specialized graph paper designed to simplify the conversion of distance into time and voltage. This grid is composed of small squares, each measuring 1 millimeter (mm) by 1 millimeter. These small squares are grouped into larger squares, which are delineated by darker lines and measure 5 mm by 5 mm. The vertical axis represents the amplitude, or voltage, of the electrical signal, while the horizontal axis consistently measures time.
The universal standard speed for recording an ECG is 25 millimeters per second (mm/s). This specific speed dictates the time value assigned to each square on the horizontal axis. Since the paper moves at 25 mm every second, each 1 mm small square represents a duration of 0.04 seconds (40 milliseconds).
This calculation means that every large square, which spans five small squares, represents a time duration of 0.20 seconds (200 milliseconds). Before beginning any measurement, one should always confirm the paper speed, as some circumstances may require a faster or slower recording speed.
Step-by-Step Measurement of Key Intervals
The measurement process begins by identifying the precise start and end points of the key waves and complexes on the ECG tracing. It is best practice to select the clearest complexes, often in lead II or V5, and measure the interval across several consecutive beats.
PR Interval
The PR interval represents the time it takes for the electrical impulse to travel from the atria through the atrioventricular node and into the ventricles. Measurement starts at the very first deflection, which is the beginning of the P wave. The measurement continues horizontally to the point where the QRS complex begins, whether that is a Q wave or the initial R wave.
QRS Duration
The QRS duration measures the time required for the electrical impulse to spread through the ventricles, resulting in their depolarization. The measurement starts at the beginning of the Q wave, or if no Q wave is present, the start of the R wave. The interval ends at the precise point where the S wave returns to the isoelectric baseline, which is the flat line between complexes.
QT Interval
The QT interval encompasses the total time of ventricular electrical activity, including both depolarization (QRS) and repolarization (T wave). This measurement starts at the beginning of the QRS complex, just like the QRS duration. The interval concludes at the very end of the T wave, which is the point where the T wave returns to the isoelectric baseline. Identifying the exact end of the T wave can be challenging, so care must be taken to distinguish it from a subsequent U wave.
Calculating Duration and Normal Ranges
After counting the number of small squares for any given interval, the final step involves converting this count into a measurable time duration. This is accomplished by multiplying the total number of small squares by the time value of a single small square, which is 0.04 seconds. For instance, an interval that spans four small squares has a duration of 4 x 0.04 seconds, equaling 0.16 seconds.
The calculated duration is then compared to established normal ranges to determine if the heart’s conduction time is appropriate. A normal PR interval typically falls between 0.12 and 0.20 seconds, representing three to five small squares. The QRS duration should be narrow, usually measuring between 0.06 and 0.10 seconds, which is less than three small squares.
The QT interval is unique because its duration changes naturally with the patient’s heart rate; a faster heart rate shortens the QT interval, while a slower rate lengthens it. Therefore, the raw QT measurement is often mathematically adjusted to calculate the corrected QT interval, or QTc. This correction standardizes the measurement to a heart rate of 60 beats per minute. A QTc duration is generally considered normal if it is less than 0.44 seconds for men and less than 0.46 seconds for women.