The Electrocardiogram - Part V

Rate & Rhythm

Calculating the heart rate:

One thing I always teach is that there is a difference between pulse and heart rate. These terms are used interchangeably, but if you think of them as two different values, you may understand things better. Remember, there is a difference between an electrical pulse and a mechanical pulse. We are going to label the electrical pulse as the heart rate and the mechanical pulse as just plain, pulse. You can see a heart rate, but you have to feel a pulse. They are usually the same value but may be different. The heart rate is the rate at which your heart is sending an electrical impulse. This can be determined with an ECG. A pulse is the beats per minute that are actually sending blood throughout the body. This can be determined by feeling one of many different pulse locations. As you read on in this tutorial, I will eventually explain instances when these two values may be different.

The first method in determining the heart rate using an ECG, 'The Box Method', is probably the most accurate. This method requires you to remember a set of numbers. These numbers are: 300, 150, 100, 75, 60, 50... I only remember to the 50. Anything less than 60 is slow, right?

Now, the first thing you must do is find your R waves, or QRS complexes. Remember those?

Next, count how many big boxes (the 5 x 5 box) are between the two R waves (the RR interval). . This is easiest if you find the R wave closest to a solid line.

Now using this method, how ever many big boxes you have will determine the heart rate. Starting with 300, divide by the number of big boxes. Only one big box would give me a rate of 300 bpm (300 divided by 1 = 300), two big boxes would equal 150 bpm (300 divided by 2 = 150), and so on.

The ECG strip above shows a RR interval of just over three boxes. This means that the heart rate is between 75 - 100 bpm. Since the R wave is just right of center I will stay on the low side and call it a rate of about 80 bpm.

When we talk about rhythm, we are talking about the RR interval. Is the rhythm regularly regular, regularly irregular, or irregularly irregular?

Regularly regular - The RR interval does not vary. This is not hard-fast. RR intervals vary on everyone, when you are having a normal change in heart rate. This just means that there is no pattern of variation or complete irregularity.

Regularly irregular - This is a pattern of irregularity. The RR interval will change but there will be a method to the madness. It may be 600 ms then 200ms, then 600 and then 200 again. Each RR interval is not the same, but a group of RR intervals may mimic the next group.

Irregularly irregular - This is a chaotic irregular rhythm. The RR intervals will be constantly changing without a set pattern.

Below is an example of an irregularly irregular rhythm. To use the rule mentioned above to determine heart rate, you must measure more than one RR interval.

Measuring the shortest RR interval and the longest RR interval will give you the range of the heart rate.

If we measure this strip we see that the smallest RR interval is just over two big boxes wide and the longest RR interval is just over 6 boxes wide. I'd say the range for the heart rate of this ECG is between 45 - 110 bpm.

An ECG ruler, similar to the one below, may be another useful measuring device. These usually use a similar method to the one above, but require less memorization.

The other method that I use to determine the heart rate is by counting the RR intervals on a six second strip. You count each RR interval and then multiply by 10. This gives you a rough estimate of the heart rate. This is much less accurate than the first method.

Depending on the machine you use for ECGs, you may or may not have your second, three second, and six second points marked. Above is a sample ECG strip from my monitor. The hash marks on top, some circled in red, denote one second intervals. One second equals five big boxes. The yellow circles are around the six second hash marks, which are shown on the bottom of the strip.

Give one a try. Tell me about the rhythm of this strip and give the heart rate:

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