Calculating The Heart Rate & Intervals

Calculation

ECG calculation requires that you have a working knowledge of the graph paper that is used for ECG analysis. The paper has small boxes that help us calculate the heart rate and other things on an ECG. The first thing to note is that the horizontal movement (left to right) is used to calculate time. The vertical movement (up and down) is used to calculate voltage. Let's look at the graph paper up close.

The Big Box

On the ECG paper, there is often a darker line outlining a series of smaller boxes. This is done to help save time when reading the time measurements. As mentioned in the picture above, the big box outlined represents 0.20 seconds when we look horizontally. While the entire box is being outlined here, the actual boxes we are looking at when dealing with time are the ones that are displayed horizontally.

The Small Box

The small box on the ECG paper represents 0.04 milliseconds. If you took 5 of these small boxes, moving horizontally, you would reach 0.20 seconds. The boxes vertically and horizontally help us make sense of the waves that we see on an ECG scan.

IMPORTANT

We are focusing only on calculating the heart rate based on the boxes in this presentation. When we combine the small boxes together, at any given location, we can count how much they represent. Whether that number is 0.20 or 0.58 will be determined by the waves we see on the ECG paper. Let's dive into actually reading ECGs.

Measuring The Intervals

PRI or PR Interval

The PR Interval measures the time it takes the atrium to depolarize. By way of review, depolarization is when the electricity passes over the cell and causes the muscle to contract. So the P wave represents the atrium contracting.



QRS Width

The QRS measures the time it takes the electricity to pass through the ventricles. It represents ventricular depolarization. For the measurements above, we can see that the first line is at the beginning of the Q wave when it descends below the isolectric line. Then we terminate the measurement at the end of the S wave, when it returns to the isoelectric line. It is important to note that te

Calculating The Heart Rate

Now that we understand how to measure the amount of time (horizontal movement/small boxes or big box) we can focus on how to measure or calculate the heart rate for a patient. To calculate the heart rate we first need to understand that an ECG can be long or short. The amount of beats that we see (QRS Complexes) is dependent on how long the recording is. For most ECG analysis, we use 6 second strips to both interpret and analyse the electrical signals.

The method we will be using to calculate the heart rate is very simple. To calculate the heart rate for a patient, simply count how many QRS complexes you see in a 6 second strip and multiply by 10.

It's that easy. Let's try a couple of examples.


Figure 1

What is the heart rate for the rhythm strip to the right? Remember, all you need to do is count the QRS complexes and multiply by 10. This gives us a heart rate of 90bpm or 90 beats per minute.

Figure 2

What is the heart rate for Figure 2?

Figure 3

What is the heart rate for Figure 3?

Notes:

  • Horizontal movement on the graph paper represents time.
  • 1 small box (the tiny ones) equal 0.04 milliseconds.
  • 5 small boxes (big box) equal 0.20 milliseconds.
  • The PR interval is the distance from the start of the P wave until the QRS complex.
  • The QRS Width is the distance from the start of the Q, R, or S wave until the termination of the S Wave.
  • To calculate the heart rate, simply count how many QRS complexes you see in a 6 second strip and multiply by 10.