An electrocardiogram (ECG) is a graphic recording of the heart’s electrical activity over a period of time, serving as a non-invasive tool to assess cardiac function. This test uses electrodes placed on the skin to detect the small electrical changes that occur as the heart muscle depolarizes and repolarizes during each heartbeat. The resulting ECG strip is essentially a graph, with the horizontal axis representing time and the vertical axis representing the voltage, or amplitude, of the electrical signal. Analyzing the patterns and measurements on this gridded paper allows medical professionals to identify various cardiac conditions, including rhythm disturbances and signs of inadequate blood flow.
The Time Value of a Large Square
One large square, often delineated by darker lines on the ECG paper, represents 0.20 seconds, or 200 milliseconds, of time. This specific time duration is directly determined by the standard paper speed at which the ECG machine prints the tracing. In most clinical settings, the paper moves at a speed of 25 millimeters per second (mm/s).
Since each large box measures 5 millimeters horizontally, the relationship between distance and time is fixed at this standard speed. This means that one millimeter of distance equals 0.04 seconds of time, and five millimeters equals one-fifth of a second. This fixed calibration is necessary to ensure that measurements like the duration of the P wave or the QRS complex are consistent. Altering the paper speed requires a corresponding adjustment in the interpretation of the time values.
Understanding the Smaller Grid Divisions
Within each large square on the ECG paper, there is a finer grid composed of smaller squares, which allows for more granular time measurement. A large square is divided into 25 of these smaller squares, arranged in a 5-by-5 configuration. Since one large square represents 0.20 seconds, each small square must represent one-fifth of that time.
The time value of a single small square is 0.04 seconds, or 40 milliseconds. This measurement is used to determine the precise duration of the various components of the cardiac cycle, such as the PR interval and the QRS complex. For example, a normal QRS complex, which represents the depolarization of the ventricles, typically measures between 0.06 and 0.10 seconds, corresponding to 1.5 to 2.5 small squares.
Measuring Voltage and Amplitude
While the horizontal axis of the ECG grid measures time, the vertical axis measures the amplitude, or voltage, of the heart’s electrical signal. This vertical measurement indicates the strength of the electrical current generated by the heart muscle. Standardization is applied to this axis, typically set so that 10 millimeters of deflection corresponds to 1 millivolt (mV) of electrical potential.
Under this standard calibration, one large square vertically represents 0.5 mV, while one small square vertically represents 0.1 mV. Clinicians often check a calibration marker at the beginning of the tracing, confirming the 10 mm/mV and 25 mm/s settings. The amplitude of the waves is significant because it can reveal issues like ventricular hypertrophy, where a larger muscle mass generates greater voltage, or low voltage, which may suggest conditions such as fluid around the heart.
Applying Grid Measurements to Heart Rate Calculation
The time values represented by the large and small squares are used practically to determine the patient’s heart rate in beats per minute (bpm).
The 300 Method
For rhythms that are regular, a quick estimation can be made using the “300 Method.” This involves counting the number of large squares between two consecutive R waves (the peak of the QRS complex). The rate is then calculated by dividing 300 by that number, based on the fact that 300 large boxes pass in one minute. For example, if two R waves are separated by four large squares, the heart rate is estimated to be 75 bpm (300/4).
The 1500 Method
A more precise method for regular rhythms is the “1500 Method,” which uses the small squares for a finer calculation. This method divides 1500 by the number of small boxes between two R waves.
The 6-Second Strip Method
If the rhythm is irregular, the “6-Second Strip Method” is the preferred technique. This involves counting the number of R waves that appear within a 6-second strip (30 large boxes), and then multiplying that count by 10 to get the rate in beats per minute.