An electrocardiogram (EKG) strip is a graphical recording that captures the electrical activity of the heart over time. This tracing translates the heart’s electrical signals into a series of waves and deflections printed on specialized paper. Analyzing these measurements is fundamental for medical professionals to assess the heart’s function, determine its rate and rhythm, and identify potential cardiac conditions.
Understanding the EKG Grid Paper
EKG measurements rely on standardized grid paper marked with a fine grid of small and large squares, representing specific units of time and voltage. The horizontal axis measures time, while the vertical axis tracks the amplitude or voltage of the electrical signal.
Each small square on the grid measures 1 millimeter by 1 millimeter (1 mm x 1 mm). Because the paper typically moves at a standard speed of 25 millimeters per second, one small square horizontally represents a duration of 0.04 seconds. This precise unit of time is the basic building block for calculating all intervals and heart rates on the strip.
Large squares are formed by groups of five small squares, creating a 5 mm x 5 mm box. Consequently, one large square represents 0.20 seconds in time, which is five times the duration of a small square.
Calculating the Heart Rate
Determining the heart rate requires different calculation methods based on whether the rhythm is regular or irregular. For regular rhythms, where the distance between successive QRS complexes (the R-R interval) is consistent, interval methods convert the distance between beats into beats per minute (bpm) using the standardized time values of the EKG grid.
The 300 method is a quick estimation for regular rhythms that uses the large boxes. To apply this, locate an R wave that falls directly on a heavy vertical line on the grid. Count the number of large squares until the next R wave and divide 300 by that number to get the approximate heart rate. For instance, if there are four large squares between R waves, the rate is calculated as 300 divided by 4, resulting in 75 bpm.
A more precise calculation for regular rhythms is the 1500 method, which uses the smaller units of time. This technique involves counting the number of small boxes between two consecutive R waves. The constant 1500 is then divided by the total count of small squares to yield an accurate heart rate. This method is often used when greater precision is required.
When the rhythm is irregular, such as in certain types of atrial fibrillation, the R-R intervals vary significantly, making the 300 and 1500 methods unreliable. For these rhythms, the 6-second method provides an average heart rate over a longer period. A 6-second strip is equivalent to 30 large boxes, based on the calculation that five large boxes equal one second.
To use the 6-second method, count the number of QRS complexes that appear within the 30 large boxes marked on the strip. Since 30 boxes represent 6 seconds, multiplying the number of QRS complexes counted by 10 estimates the average heart rate for a full minute. This approach determines the heart rate despite inconsistent R-R intervals.
Measuring Intervals and Segments
Beyond rate calculation, the EKG grid is used to measure the duration of specific electrical events, which are known as intervals and segments. These measurements provide insight into the efficiency of the heart’s conduction system, and they are calculated by counting the number of small boxes and multiplying by the time constant of 0.04 seconds.
The PR interval measures the time it takes for an electrical impulse to travel from the atria to the ventricles. This measurement begins at the start of the P wave and ends at the beginning of the QRS complex. A normal PR interval typically falls between 0.12 and 0.20 seconds, corresponding to three to five small squares on the grid.
The QRS complex represents the time required for the ventricles to depolarize and contract. The duration is measured from the point where the first wave of the complex leaves the baseline to the point where the last wave returns to the baseline. This complex is normally narrow, lasting between 0.06 and 0.10 seconds, which translates to a width of one and a half to two and a half small squares.
The QT interval reflects the total time for the ventricles to depolarize and then fully repolarize. Measurement starts at the beginning of the QRS complex and continues to the end of the T wave. While the duration is dependent on the heart rate, a general normal range is between 0.35 and 0.44 seconds. Clinicians measure the raw duration and often correct it to account for the patient’s specific heart rate.