An electrocardiogram (EKG) is a graphic recording of the electrical activity that powers the heart’s rhythmic contractions. The machine captures these electrical changes using electrodes placed on the skin and prints them onto specialized paper with a precise grid. This standardized grid translates the heart’s electrical events into quantifiable units of time and voltage. By accurately counting the small and large boxes, medical professionals can determine heart rate, assess the duration of key electrical phases, and diagnose various cardiac conditions.
Decoding the EKG Grid: Time and Voltage Values
The EKG paper is a graph with a consistent pattern of small and large squares, which are standardized measurements used worldwide. The horizontal axis measures time, while the vertical axis represents voltage, or the amplitude of the electrical signal. The standard speed at which the paper moves through the machine is 25 millimeters per second, which establishes the time value for each box. Horizontally, a small box represents 0.04 seconds of time, and vertically, it represents 0.1 millivolts (mV) of voltage. A large box encompasses five small boxes, representing 0.20 seconds of time and 0.5 mV of voltage.
Calculating Heart Rate Using Box Counting Methods
Determining the heart rate in beats per minute (bpm) is a primary use of the EKG grid. The method chosen depends on whether the heart rhythm is regular or irregular. For a regular heart rhythm, where the distance between successive R waves is consistent, two accurate methods are available.
The 300/Large Box Method
The 300/Large Box Method offers a rapid estimation of the heart rate. This involves locating an R wave that falls exactly on a thick line and then using a simple sequence for the subsequent thick lines: 300, 150, 100, 75, 60, and 50. The heart rate is approximated by the number associated with the thick line closest to the next R wave. For instance, if the second R wave falls on the third thick line after the starting R wave, the rate is 100 bpm.
The 1500/Small Box Method
The 1500/Small Box Method provides a more precise calculation for regular rhythms. Since there are 1,500 small boxes in one minute, the heart rate is found by dividing 1500 by the total number of small boxes between two consecutive R waves. Counting the small boxes between the peaks of two R waves and using this division yields a highly accurate heart rate.
The 6-Second Strip Method
When the heart rhythm is irregular, such as in atrial fibrillation, the distance between R waves constantly changes, making the previous methods unreliable. The 6-Second Strip Method is the preferred technique for irregular rhythms because it averages the activity over a set period. This involves first identifying a 6-second segment on the EKG strip, which is equivalent to 30 large boxes. Next, count the total number of QRS complexes, or heartbeats, that occur within that timeframe. This count is then multiplied by 10 to estimate the number of beats that would occur in a full minute.
Measuring Key EKG Intervals and Amplitude
Beyond calculating the rate, the boxes are used to measure the duration of specific electrical phases, called intervals, by counting horizontally. The PR interval measures the time it takes for the electrical signal to travel from the atria to the ventricles, measured from the start of the P wave to the start of the QRS complex. Its duration is determined by counting the small boxes between these two points and multiplying that number by 0.04 seconds. The QRS duration represents the time it takes for the ventricles to depolarize, measured from the beginning of the Q wave to the end of the S wave. The QT interval measures the total time for ventricular depolarization and repolarization, calculated by counting small boxes from the start of the Q wave to the end of the T wave.
The grid’s vertical axis is used to measure the amplitude, or height, of the heart’s electrical waves, which is a measure of voltage. The height of a wave, such as the P wave, R wave, or T wave, is determined by counting the number of small boxes vertically from the baseline to the wave’s peak. Since each small box is 0.1 mV, the count is multiplied by 0.1 to get the voltage in millivolts. Voltage measurements are used to check for abnormalities, such as increased height in the QRS complex, which can suggest conditions like ventricular hypertrophy.