An electrocardiogram (ECG or EKG) is a non-invasive test that records the heart’s electrical activity. It provides a visual representation of the heart’s rhythm and rate, showing how electrical signals travel with each beat. Calculating heart rate from an ECG tracing is a key step in interpreting heart health, offering a basic assessment of heart speed. This article outlines methods for determining heart rate from an ECG.
Understanding ECG Paper
ECG recordings are printed on specialized grid paper, marked with small and large squares. Each small square measures 1 millimeter.
The horizontal axis represents time, with each small square equaling 0.04 seconds. Five small squares form a large square, representing 0.20 seconds. The vertical axis measures voltage or amplitude, with 10 millimeters equaling 1 millivolt (mV).
Calculating Rate for Regular Rhythms
For regular heart rhythms, where R-R intervals (the distance between consecutive R-waves, prominent upward deflections on the ECG) are consistent, two methods calculate heart rate. These methods rely on the standardized measurements of the ECG paper to determine beats per minute (bpm).
The 300 Method, also known as the “Large Squares” method, offers a quick estimation. Locate an R-wave on a thick vertical line. Count the large squares between this R-wave and the next. Divide 300 by this number to obtain the approximate heart rate in beats per minute. For instance, if there are four large squares between two R-waves, the heart rate would be 75 bpm (300/4).
For a more precise calculation, the 1500 Method uses the smaller squares. Count the total number of small squares between two consecutive R-waves. Divide 1500 by that number. For example, if there are 20 small squares between two R-waves, the heart rate calculates to 75 bpm (1500/20). This method is considered more accurate because it accounts for smaller time increments.
Calculating Rate for Irregular Rhythms
For irregular heart rhythms, where R-R intervals vary significantly, the previously mentioned methods are not suitable for accurate rate calculation. The 6-Second Method provides a reliable way to determine an average heart rate, particularly useful for rhythms like atrial fibrillation where beats are not evenly spaced.
To apply this method, identify a 6-second segment on the ECG strip, which typically corresponds to 30 large squares. Count the R-waves within this segment. Multiply this count by 10 to obtain the heart rate in beats per minute. For example, if you count 7 R-waves within a 6-second strip, the heart rate would be 70 bpm (7 x 10). This multiplication extrapolates the 6-second count to a full minute, providing an average rate for the irregular rhythm.
What Your Calculated Rate Means
After calculating heart rate from an ECG, understanding what the number signifies is important. For adults, a normal resting heart rate typically falls within the range of 60 to 100 beats per minute. A heart rate consistently below 60 beats per minute is generally referred to as bradycardia. Conversely, a heart rate consistently above 100 beats per minute is known as tachycardia.
While these classifications provide a basic understanding, a slow heart rate is not always a concern, especially in trained athletes who may have resting rates as low as 40 bpm. Similarly, a fast heart rate can also be a normal response to exercise or stress. These calculations serve as educational tools; professional medical interpretation is always necessary to determine the implications of any heart rate findings.