An electrocardiogram (EKG or ECG) strip records the heart’s electrical activity over time. This tracing allows healthcare professionals to visualize the cardiac cycle, which is fundamental for diagnosing various heart conditions. Accurately measuring the strip’s components is the mandatory first step before interpretation. Quantifying the time and voltage represented on the graph paper allows calculation of the heart rate and the duration of specific electrical events.
Decoding the EKG Grid
The foundation of EKG measurement requires understanding the specialized grid paper. The horizontal axis represents time, and the vertical axis represents voltage or amplitude. The standard recording speed is 25 millimeters per second (mm/sec), which defines the time values.
The grid consists of small squares. Horizontally, each small box represents 0.04 seconds of time (1 mm / 25 mm/sec). Vertically, standard calibration sets one small box equal to 0.1 millivolt (mV).
Five small squares form a larger, thick-lined box. One large box on the horizontal axis represents 0.20 seconds. On the vertical axis, a large box represents 0.5 mV. Standard calibration is often confirmed by a standardization mark at the beginning of the strip, typically showing a deflection of 10 small boxes (1 mV).
Calculating Heart Rate
Calculating the heart rate (BPM) is the first measurement performed on an EKG strip. The appropriate calculation method depends on whether the rhythm is regular (consistent R-R intervals) or irregular. For regular rhythms, two primary methods focus on the distance between consecutive R waves, which represent ventricular depolarization.
Regular Rhythm Methods
The “300 Method” is a quick estimation, accomplished by dividing 300 by the number of large boxes between two successive R waves. This method relies on the fact that 300 large boxes represent one minute of time. For example, if there are four large boxes between R waves, the rate is 300 divided by 4, equaling 75 BPM.
The “1500 Method” provides a more precise calculation for regular rhythms. This technique involves dividing the constant 1500 by the total number of small boxes between two consecutive R waves. Since 1500 small boxes make up one minute, this provides a more exact rate. For instance, 1500 divided by 20 small boxes yields a rate of 75 BPM.
Irregular Rhythm Method
For irregular rhythms, where R-R intervals vary significantly, the “6-Second Method” provides the most reliable average rate. Locate a six-second section of the EKG strip, which corresponds to 30 large boxes. Count the number of QRS complexes within this segment, then multiply that count by 10 to estimate the rate for a full minute. This method is essential because the R-R interval is not constant.
Measuring Wave Durations and Intervals
Measuring the duration of specific waves and intervals provides insight into the speed of electrical conduction through the heart. These measurements use the horizontal axis, where each small box represents 0.04 seconds.
The P-R Interval measures the time from the start of the P wave to the start of the QRS complex, representing the time for the electrical signal to travel from the atria to the ventricles. A normal P-R interval typically spans between 0.12 and 0.20 seconds, which correlates to three to five small boxes on the grid.
The QRS Duration quantifies the time of ventricular depolarization, measured as the width of the QRS complex. This measurement is taken from the first deflection of the complex until the final deflection returns to the baseline. A healthy QRS duration falls between 0.06 and 0.10 seconds, or 1.5 to 2.5 small boxes.
The Q-T Interval measures the total time for ventricular depolarization and subsequent repolarization, extending from the start of the Q wave to the end of the T wave. This interval is highly dependent on the heart rate, and its duration is often corrected (QTc) to account for rate variations. While normal uncorrected QT intervals are often cited between 0.36 and 0.44 seconds, the corrected value is generally less than 450 milliseconds for men and less than 460 milliseconds for women.
The vertical axis is primarily used for voltage. Amplitude of waves, such as the height of the R wave, is determined by counting the small boxes and multiplying by 0.1 mV.