Rate and Rhythm
Regularity
A regular rhythm means that the interval between consecutive R waves [the R-R interval] is the same (+/- 2 small squares). An irregular rhythm means that the R-R interval is not consistent across all waveforms.
The first rhythm above is regular since all R-R intervals are the same, while the second rhythm is irregular since the R-R intervals are not consistent. If you can't tell, find the longest R-R interval and the shortest R-R interval, and see if they differ by more than 2 small squares. If they do, then the rhythm is irregular.
Heart Rate
Conveniently, most cardiac monitors will automatically determine the heart rate for you, but there are times when you will need to do it manually. While the most accurate way is to count the number of R waves in one minute, you can also estimate the heart rate - which is often sufficient (and necessarily faster) in the emergency setting.
Six-Second method
A method that works for both regular and irregular rhythms is to simply count the number of R waves in a 6-second strip and multiply by 10. This is often the most practical method in the emergency setting.
- In the first example, there are 7 R waves in 6-seconds, so the estimated HR is 7 x 10 = 70 bpm.
- In the second example, there are 9 R waves in 6-seconds, so the estimated HR is 9 x 10 = 90 bpm.
300 method
An (arguably) faster method is to divide the number of large squares between consecutive R waves by 300. While faster, it only works on regular rhythms.
- From left-to-right, the first example has 1 large square between R waves, so the estimated HR is 300 / 1 = 300 bpm.
- The second example has 2 large squares between Rs, so the estimated HR is 300 / 2 = 150 bpm.
- The third example has 3 large squares between Rs, so the estimated HR is 300 / 3 = 100 bpm.
- The fourth example has 4 large squares between Rs, so the estimated HR is 300 / 4 = 75 bpm.
Instead of doing the division each time, some people memorize the above values to make this process even faster.