The big box method for HR determination on the ECG (for nurses & nursing students)!

The big box method is the third common way to evaluate heart rate (the 6 second and small box methods being the others).

Using the big box method for calculating heart rate involves some degree of memorization.

Before you begin, memorize the following 6 numbers: “300, 150, 100… 75, 60, 50.” Next, find an R wave that resides on a thick (0.20 second) solid line as shown above. With each “big box” line that comes, count off the numbers, “300, 150, 100… 75, 60, 50.”

In the example above, the second R wave from the “start”-ing line falls between 100 and 150 beats per minute. Now, you’ve probably already guessed that the disadvantage to this method is its lack of precision relative to the small box method, pictured below.

The overwhelming advantage, however, is the rapid determination of heart rate the big box method allows.

As with the small box method, the big box method works using time as a constant variable:

✓ Each big box on our ECG paper measures 0.20 seconds

✓ We’re interested in heart rate as a function of beats per minute

✓ There are 60 seconds in each minute

✓ This means there are 300 big boxes in each minute (because 60 seconds divided by 0.20 equals 300 beats per minute if an R wave were to fall exactly on each big box line)

✓ If an R wave were to fall on every other big box line, this equals a beat occurring every 0.40 seconds (0.20 x 2)

Now…….. 60 seconds divided by 0.40 equals 150 beats per minute if an R wave were to fall exactly every OTHER big box line. See the pattern? Again, the great advantage to the big box method is that it allows for very rapid approximation of heart rates, an important feature when decisions need to be made quickly and a heart rate approximation is all that’s needed.

Clearly the advantage the small box method as well as the 6 second method for rate calculation have over the big box is a more acute rate determination for the reasons mentioned. Hope this helps!

TAKE HOME POINTS:

✓ The big box method allows for rapid approximation of heart rate when quick decisions are needed

✓ Commit the following numbers to memory: 300, 150, 100 … 75, 60, 50