Large Block Method to Calculate Heart Rate

Objective

Understand and learn to use the large block method to estimate heart rate on the electrocardiogram

Graphic

large block method heart rate

Large Block Method

The fastest and easiest method to calculate the heart rate on the ECG is the large block method. It can tell you at a glance whether or not you’re dealing with a bradycardia or a tachycardia but only a minority of clinicians have incorporated this skill into their daily practice!

Granted, it’s more useful for rhythm strips, but when it comes to computers you should “trust but verify!”

This method can be used when:

  • The heart rhythm is regular
  • The paper speed is 25 mm/s (standard in most countries)

All you do is find an R-wave that is lined up with a large block on the ECG paper and count the number of large blocks between that cardiac cycle and the next. In other words, you measure the R-R interval in large blocks.

  • 6 large blocks: 50
  • 5 large blocks: 60
  • 4 large blocks: 75
  • 3 large blocks: 100
  • 2 large blocks: 150
  • 1 large block: 300

We know the “normal” heart rate is 60-100 (although some would argue that 50-90 is more accurate). Using 60-100 the normal heart rate should have 3-5 large blocks between R-waves. More than 5 large blocks is a bradycardia and fewer than 3 blocks is a tachycardia.

Let’s say it doesn’t work out perfect. For example, let’s say there are 3 1/2 large blocks between R-waves. Well, then you know the heart rate is somewhere between 75 and 100!

Let’s look at some examples!

Heart rate: 50

heart rate 50 wm

6 large blocks: 50

Here there are 6 large blocks in between R-waves for an estimated heart rate of 50 BPM.

Heart rate: 60

heart rate 60 wm

5 large blocks: 60

Here there are 5 large blocks in between R-waves for an estimated heart rate of 60 BPM.

Heart rate: 75

heart rate 75 wm

4 large blocks: 75

Here there are 4 large blocks in between R-waves for an estimated heart rate of 75 BPM.

Heart rate: 100

heart rate 100 wm

3 large blocks: 100

Here there are 3 large blocks in between R-waves for an estimated heart rate of 100 BPM.

Heart rate: 150

heart rate 150 wm

2 large blocks: 150

Here there are 2 large blocks in between R-waves for an estimated heart rate of 150 BPM.

Heart rate: 300

1_to_1_atrial_flutter

1 large block: 300

Here there is 1 large block in between R-waves for an estimated heart rate of 300 BPM.

It’s unusual to see a heart rate of 300 (or close to 300) and that’s a good thing! It usually requires the presence of an accessory pathway. This was a patient with Wolff-Parkinson-White Syndrome.

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Tom Bouthillet
Tom Bouthillet - 25 posts

Tom Bouthillet (@tbouthillet) is Editor-in-Chief of ECGMedicalTraining.com (@ECGTraining) and Fire Captain/Paramedic in South Carolina where he is the Emergency Cardiac Care Program Manager and the STEMI and CARES Site Coordinator of his fire department.

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