## Outline

Calculating the rate of the ECG is an essential skill.

Wondering whether to skip ahead? Here are some of the questions you should be able to answer by the end of this topic:

- What is the difference between the small and large grid squares on ECG paper?
- Is bradycardia a rate that is too fast or too slow?
- If there are 3 large grid squares between each beat, what is the rate?
- How do you find the rate if the beats are all different distances apart?
- Why might you want to calculate a separate rate for the atria and the ventricles of the heart?

## ECG grids

Standard ECGs are printed on large and small grids. These grids allow you to calculate the rate, and measure segments, intervals and waves more easily.

### Large grid squares

The large grid squares are 5 mm wide and tall. At standard ECG settings, each large grid square is 200 msec long (0.2 seconds), and 0.5 mV high.

### Small grid squares

The small grid squares are 1 mm wide and tall. At standard ECG settings, each small grid square is 40 msec long, and 0.1mV high.

## FAQ

How do you know if the ECG was recorded at standard settings?

If the ECG was recorded at standard settings, it should have a paper speed of 25 mm/sec and a calibration of 10 mm/mV. It may have these settings printed on the edge, or there may be a calibration signal at the start of the trace that is a rectangle 5mm wide and 10mm tall. Some ECG machines do half of each: they record a very narrow calibration signal that is 10mm tall and write "25 mm/sec" separately on the edge of the page.

## Rate

The ECG rate describes the speed of the electrical beats. It can be described as normal, tachycardia (fast) or bradycardia (slow).

### Normal Heart Rate

In adults, a **normal heart rate** is usually 60-100 beats per minute. At standard settings, this means there will be 3-5 large grid squares between each beat.

### Tachycardia

Tachycardia is a rate that is faster than normal. In adults, a fast heart rate is usually over 100 beats per minute. At standard settings, this means there will be 3 or fewer large grid squares between each beat.

### Bradycardia

Bradycardia is a rate that is slower than normal. In adults, a slow heart rate is usually under 60 beats per minute. At standard settings, this means there will be more than 5 large grid squares between each beat.

## Large squares method

One of the fastest methods of calculating the rate is to count the number of large grid squares between two beats.

If the ECG beats are regularly spaced apart, you can find the rate by measuring the distance between two beats.

### Steps:

- Choose two beats
- Count the number of large grid squares between the R waves
- Divide 300 by this number to find the rate.

Each method of finding the rate has its own advantages and disadvantages.

### Advantages

- Quick and easy
- You don't need a long recording, as you only use two beats.
- You can calculate a new rate if the rhythm suddenly changes.

### Disadvantages

- This method can be misleading if the beats you choose are not representative of the overall rhythm, or if the beats are not regularly spaced apart.
- This method only calculates an approximate rate

## FAQ

What if there are no R waves?

You can use any part of the QRS complex to measure the RR interval, as long as you measure between the same part of each beat. For example, you could use the Q waves or S waves.

What if the R waves don't line up with the large grid squares?

If the R waves don't line up with the large grid squares, you can either choose two different beats or just measure the closest grid squares. For example, you could use the next grid to the right of each R wave. You could also measure the RR interval using parts of grid squares, e.g. 3.5 large grid squares.

What if the RR interval is not a whole number of grid squares?

You can measure the RR interval as a fraction if you like, e.g. if there are 4.5 large grid squares between R waves, the rate is 300/4.5 = 67 beats per minute.

## Average rate method

If the heart rate is irregular or changing across the ECG, you may need to calculate the average rate.

### Steps:

- Find a 10 second length of recording
- Count the number of beats
- Multiply this number by 6 to find the rate.

Each method of finding the rate has its own advantages and disadvantages.

### Advantages

- This method is more accurate with irregular rhythms such as Atrial Fibrillation or ectopic beats

### Disadvantages

- You need a longer rhythm strip to use this method, but the formula can be adjusted if you have a different length of recording.
- This method only calculates an average rate

## FAQ

What if the recording is not 10 seconds long?

The formula can be changed for different lengths of recording. You could modify the formula like this:

Rate = beats x 60 / length of recording (seconds)

For example, if you have a 6 second recording, Rate = beats x 10.

In general, the rate will be more accurate with a longer strip of recording - unless there is a sudden change in rate during the recording.

How do I know the length of my ECG recording?

Assuming it was recorded at standard settings, there will be 5 large grid squares per second. Some ECG machines also mark each second as a small vertical dash above the trace.

## Small squares method

Sometimes when the ECG is very fast and the beats are very close together we might want to count the small grid squares to calculate the rate.

### Steps:

- Choose two beats
- Count the number of small grid squares between the R waves
- Divide 1500 by this number to find the rate.

Each method of finding the rate has its own advantages and disadvantages.

### Advantages

- More accurate than using the large squares, especially if the rate is very fast
- You don't need a long recording, as you only use two beats.
- You can calculate a new rate if the rhythm suddenly changes.

### Disadvantages

- This method can be misleading if the beats you choose are not representative of the overall rhythm, or if the beats are not regularly spaced apart.
- You often need a calculator!

## Choosing a rate method

With so many options for calculating the rate, it is important to know when to use each method.

### Regular rhythms

If the rhythm is regular and not too fast, you can use any method to calculate the rate. Counting the large grid squares is probably the most commonly used method.

### Irregular rhythms

If the rhythm is very irregular and there is no clear pattern, you should calculate an average rate. You could also describe a rate range by using the two closest and two farthest apart beats, e.g. Atrial Fibrillation could have a rate of 120-160 bpm.

### Very fast rhythms

If the rhythm is very fast, the large squares may not be very accurate, so use the small squares or an average rate.

### Other methods

There are various other ways of finding the rate that are derived from these methods:

#### ECG rulers

ECG rulers are printed with a scale that is calibrated to the standard grid squares. If you line up a beat with the start of the ruler, you can read off the rate at the next beat. This is the same as calculating a beat-to-beat rate using the large squares method.

#### Memorise the pattern

Once you've done a few examples of calculating the rate using the large squares method, you might start to memorise the sequence 300, 150, 100, 75, 60, 50... You can quickly approximate the rate by counting this sequence out across the large squares between two beats. This is the same as using the large squares method.

## FAQ

Can I just trust the computer's calculation for the rate?

Computer interpretation of ECGs is notoriously unreliable. Even though the computed rate is not as unreliable as some of the other ECG features, it can still be tricked by artefact or very abnormal waves such as tall T waves. If you are going to use the computer interpretation for the rate, it is worthwhile knowing how to quickly estimate the rate (e.g using the large grid squares or average rate method) so you can check the computer is in the right ballpark.

## Atrial and ventricular rates

We can calculate the atrial and ventricular rates separately if needed.

### Atrial rate

The atrial rate can be calculated by using the P waves. Normally the atrial rate is the same as the ventricular rate. In some arrhythmias the atrial rate may be faster than the ventricular rate. For example in Atrial Flutter, the atrial rate can be 300 beats per min while the ventricular rate is often 150 beats per min. This occurs because the AV node protects the ventricles from beating too fast. In other arrhythmias such as AV blocks, the atrial rate might be slower than the ventricular rate. This can occur when not all of the P waves are conducted through to the ventricles.

### Ventricular rate

The main rate we usually calculate is the ventricular rate. It is measured using the QRS complexes because these represent ventricular depolarisation. It is important to remember that this is an electrical rate and it may not be the same as the palpable pulse. For example in Atrial Fibrillation, when the ventricular rate is fast there are often more electrical beats visible on the ECG than palpable pulse beats. In a cardiac arrest, if the rhythm is Pulseless Electrical Activity there may be visible electrical beats on the ECG but no palpable pulse at all!

### Example

Consider the above rhythm strip where the P waves (the atria) are running at a completely separate rate to the QRS complexes (the ventricles).

- The P waves are 3 large squares apart, so the atrial rate is 300 / 3 = 100 beats per minute.
- The QRS complexes are 6 large squares apart, so the ventricular rate is 300 / 6 = 50 beats per minute.

This is a rhythm where every second P wave is blocked at the AV node, so the ventricular rate is only half of the atrial rate. Patients with blocks like this might be light headed, tired or fainting because the ventricular rate is too slow for them.

## Summary

- ECGs are usually printed onto large grids (200msec wide) and small grids (40msec wide).
- A normal heart rate is 60-100 bpm in adults
- If the beats are regularly spaced apart, rate = 300/RR (large grid squares).
- If the rhythm is irregular, an average rate may be more reliable.
- If the rate is very fast, counting the small grid squares may give a more accurate rate.
- The atrial and ventricular rates are usually the same, but they can be calculated separately if there is an arrhythmia such as Atrial Flutter where the atria may have a very different rate to the ventricles.