Large block method for heart rate calculation

Some paramedics are not proficient at estimating the heart rate using the large block method for heart rate calculation.

I realize that modern ECG machines calculate the heart rate, but I my attitude toward machines is similar to Ronald Reagan’s attitude toward the Soviet Union. “Trust but verify.”

I created the following image using nothing but PowerPoint.

large block method blog wm
You only have to remember 6 numbers. 50, 60, 75, 100, 150, and 300.

Or, if it helps you to remember it fast-to-slow: 300, 150, 100, 75, 60, and 50.

Why is this important?

A “normal” heart rate for an adult is 60 to 100. In other words, there should be between 3 and 5 large blocks between R waves. If there are less than 3, it’s a tachycardia. If there are more than 5, it’s a bradycardia.

Many tachyarrhythmias present with heart rates around 150 (for example, 2:1 flutter).

If a heart rhythm is so fast that it only shows 1 large block between R waves, then you know the rate is about 300, and you need to suspect an accessory pathway. Sometimes this turns out to be atrial flutter with 1:1 conduction. If the rhythm is atrial fibrillation, and the shortest R-R interval is 1 large block or less (actually 6 small blocks or less) then leave the drugs for rate control in the drug box! You’re probably looking at atrial fibrillation with WPW, and almost all drugs for rate control are contraindicated.

Anytime you’re holding a rhythm strip in your hand, take a good look at the R-R intervals, and get a feel for how many large blocks are between the R waves. Before long, you’ll be able to estimate the heart rate at a glance!

See also

Large Block Method to Calculate Heart Rate at ECG Medical Training

11 Comments

  • ecgblog says:

    That is a method that I have used every single day since I learned it. It’s both fast and precise enough to use in emergency situations.

  • Jim Tarro says:

    Great article and great concept! However, for people like me an equation makes it easier to understand and remember. For this particular method heart rate is equal to 300 divided by the amount of big blocks between R waves. R=300/BR- Heart RateB- the amount of big blocks between R wavesNow go ahead, call me a geek… 🙂

  • Max, Ukraine (max.romanchenko@gmail.com) says:

    Thanks, Tom!I "invented" this method on my own and used it for almost a year of internship practice, and now it appears being well known! Oh, man.. The Nobel prise just slipped away from me! 😉2 Jim:great formula! Even though it takes some time to divide 300 by 3,2 in case of 94 bpm.

  • Ahh, the sweet sweet basics. Very nicely done. How are you generating these great graphics that you use Tom?

  • Tom B says:

    Believe it or not Adam, I just use the drawing tools in PowerPoint. This is something my friend Matt can’t get over since he uses Photoshop and thinks it’s so much easier to use.

    Tom

  • Susan says:

    I recently taught this method during a 12 lead class. 9 out of 10 people never heard of it. I agree it is easy and under used.

  • LuisG Sandoval says:

    Great article! congrats!

  • Dennis Korneff says:

    can someone explain where the numbers come from? 300 150 …

    • Jay says:

      They are dividends of 300 since on an ecg 60 secs (1 min) is represented by 300 large squares. So a QRS complex occurring once every large square gives it a rate of 300bpm. If you have a QRS complex for every 2 large squares it is rate of 150 bpm and so on.

    • Cecilia says:

      In order to find the ventricular heart rate you count the blocks between two R peaks. If they are only one block away it’s gonna be 300 bpm. If for example there are 4 blocks in between it’s gonna be 300 divided by 4, so 75 bpm.

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