ECG tracing paper is a standardized grid system used to record the heart’s electrical activity. Medical professionals use this specialized paper to measure and interpret the heart’s rhythm and function. The grid consists of small squares delineated by faint lines, which are grouped by darker, thicker lines to form larger squares. One large square contains 25 small squares, arranged in a five-by-five configuration.
Defining the Small Square Measurements
The smallest box on the grid, measuring 1 millimeter (mm) by 1 mm, represents the fundamental unit of measurement for both time and electrical amplitude. The horizontal axis measures time, based on a standard paper speed of 25 mm per second. Each small square horizontally represents a time duration of 0.04 seconds.
The vertical axis quantifies the amplitude, or voltage, of the heart’s electrical impulse. By convention, 10 mm on the grid equals 1 millivolt (mV). A single small square vertically measures 0.1 mV. These standardized measurements allow for the analysis of waveforms like the P wave, QRS complex, and T wave.
Defining the Large Square Measurements
The large square is formed by heavier grid lines and encompasses five small squares horizontally and five small squares vertically. Since the large square spans five small squares on the horizontal time axis, its total temporal measurement is 0.20 seconds (5 small squares × 0.04 seconds/small square). This interval is used for assessing the duration of various segments and intervals of the cardiac cycle.
The vertical dimension covers five small squares, resulting in a total electrical amplitude of 0.5 mV (5 small squares × 0.1 mV/small square). The large square acts as a reference point for estimating heart rate and assessing whether electrical activity falls within normal parameters. The multiplication factor of five simplifies the interpretation of the heart’s rhythm.
Calculating Heart Rate Using the Grid
The ECG grid is a practical tool for determining the patient’s heart rate. A common method for regular rhythms is the “300 Method,” which utilizes the large squares. This involves counting the number of large squares between two consecutive R waves (the tall spike in a heartbeat complex) and dividing 300 by that number. For instance, if there are four large squares between R waves, the heart rate is 75 beats per minute (300/4 = 75).
For irregular rhythms, the “Six-Second Method” offers an alternative. This technique uses the knowledge that a 6-second strip is equivalent to 30 large squares. The clinician counts the number of QRS complexes within that segment and then multiplies that count by 10 to approximate the heart rate in beats per minute. This method provides an average rate over a specific time period.