ECG Heart Rate Interpretation

Heart rate interpretation on ECG is one of the most important ability that must be mastered by any medical professionals. Fortunately, it is very easy to calculate this based on ECG.

The horizontal axis of ECG paper represent time and the vertical axis of ECG represent voltage. When we see a blank ECG paper, we will notice there are a lot of squares of varying size. The standard speed of ECG is 25 mm/second. By definition, this means in one second, the ECG will have moved 25 mm. One small boxes equals 0.04 second. This makes 5 small boxes 0.2 second. So in order to complete 1 second, we require 25 small boxes.

There are 3 ways to calculate heart rate based on ECG

For regular rhythm                                              

• using large boxes                                                   
• using small boxes

For irregular rhythm

• Using total QRS in 6 seconds 

Using Medium Boxes

The method for using small boxes:

1. Find R wave that is located exactly or near one of the thick vertical line
2. Calculate the total of large boxes until the next R wave
3. Determine the rate using the formula below 

The formula for using medium boxes :                                      300                                          

                                                               total number of small boxes between 2 R waves

If there is only one large box between two R waves, the duration is 0.2 second (0.04 x 5 small boxes). It means that in one second there is 5 heart cycle and in 1 minute there is 300 heart cycle (5 x 60 seconds).

This is one of the easiest and fastest method but could be the most inaccurate. See those tall R wave that are aligned to the vertical line? Between them are 3 medium boxes.

Using the formula: 300/3 = 100 beats/minute

Using Small Boxes

The formula for using small boxes:                                     1500                                       
                                                           total number of small boxes between 2 R waves

It is very important to utilize this method most of the time since most ECG will not show QRS complexes aligned to thick vertical line. Unless of course you are in emergency room, then feel free to use the first method.

From the example above: 1500/22 = 68 beats/minute

One can also use large boxes method. Between the R wave, there are 4 large boxes + 2 small boxes, making 300/4 = around 75 beats.minutes. A similar result with the first method, is it not? Of course for the math mania can use 300/4.2 from 4 large boxes + 2 small boxes.

On Irregular Rhythm

Count the total of QRS complexes within 6 seconds and multiply it by 10

Irregular rhythm is tricky but still easy. If the QRS complex is irregular just like above from premature atrial contraction, it is not recommended to use small and large box method to determine the heart rate. Instead, we need to take at least 6 seconds of ECG and count the total of the QRS complexes in that 6 seconds. For simplicity, I have marked the ECG above. One must always remember the value of the boxes. 1 small box is 0.04 second, 1 large box equals 5 small boxes. Therefor 1 large box is 0.2 second and 5 large box is 1 second. To make 6 seconds, we need exactly 30 large boxes.

8 QRS x 10 = 80 beats/minute