Reading EKGs

Reading EKGs > Rate

1.  Rate

The first and easiest step to take when approaching EKGs is to calculate the heart rate.  This is a two step process, first calculating the ventricular rate and then calculating the atrial rate.  Please do not forget to calculate the atrial rate.  While it is not often referenced in texts, skipping the atrial rate is an easy way to miss arrhythmias.

Calculating the Heart Rate

There are a variety of different methods used to calculate the heart rate, all of which rely on knowing the time scale on the EKG paper.  Recall that each large box represents 0.2 seconds.  With that in mind, we can calculate the time elapsed between waves. 

For the first method, all you need to remember is the following series of numbers:

300, 150, 100, 75, 60, 50.

Find an R wave that falls on a thick vertical line and count the number of boxes until the next R-wave.  The duration between two consecutive R-waves is referred to as the RR interval.    Then use the number range above, counting down for each large box.

The following paragraph is strictly the calculation behind this series of numbers.  If you are satisfied with memorizing the list, feel free to skip the next paragraph. 

We know that each large box is 0.2 seconds.  If there is a large box between each R-wave, we can calculate how many R-waves would occur over a minute span (assuming the rate remains constant, but we will get to that later).  In this particular example, each large box correlates with a new heart beat.  To calculate the heart rate we need to use a little bit of algebra:

(1 beat / box) x (1 box / 0.2s) x (60s / minute) = 300 beats / minute

If the heart rate was to slow down to every other large box, the math would change to:

(1 beat / 2 boxes) x (1 box / 0.2s) x (60s / minute) = 150 beats / minute

Taking this pattern one step forward, you can also count the number of large boxes between progressive R-waves and divide that number by 300 to get the heart rate, effectively reducing the amount of memorization necessary.  As an example:

4 Boxes between successive R-waves:  300 / 4 boxes = 75 beats per minute

The next method is more effective with slow heart rates (<60 beats per minute) and irregular heart rates.  Many EKG strips mark off 3 second intervals with a little tic mark at the top of the page.  You can use these marks if available, or count off 15 large boxes to determine a 3 second time period.  Double this period, using 2 consecutive 3 second sections or 30 large boxes.  Count the number of R-waves within the section and multiply by 10.  As a practical example, if you had 5 R-waves in 6-second period, the math would look like this:

(5 beats / 6s) x (60s / minute) = 5  x 10 = 50 beats per minute

Interpreting the Calculated Heart Rate

Calculating the atrial and ventricular rates can provide a wealth of information.  If the atrial and ventricular rate are the same, you can be reasonably certain that each beat is conducted to both the upper and lower chambers.  If there is a discrepancy between the atrial rate and the ventricular rate, this is an early sign of possible heart block or other pathology which will be reviewed under the Rhythm analysis.  In addition, the heart rate can help you predict the location of the pacemaker and narrow the differential diagnosis for arrhythmias. 

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