Large block method for heart rate calculation

I've noticed that some paramedics are not proficient at estimating the heart rate using the large block method for heart rate calculation.

This is unfortunate.

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.

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? Simple. 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 (or tachydysrhythmias for you purists out there) present at rate of about 150. Atrial flutter with 2:1 conduction, for example (since flutter waves often occur at a rate of 300 or 1 large block). I would actually like to flog the person who started the rumor than if a heart rate exceeds 150, then it can't be sinus tachycardia.

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!