Heart Rate

What is the heart rate? Even if you forget the rest of this chapter, please, remember the following: The heart rate is the number of ventricular beats per minute.

When determining the heart rate, depolarization in the atria are ignored. If you prefer, you can, of course, calculate the rate of the atrium as well. However, if you do this, you must explicitly state that you have calculated the atrial rate. At the same time, remember that calculation of the atrial rate is only really helpful when considering regular supraventricular tachycardias.

The heart rate is shown at the upper right or lower left corner of the ECG strip. However, as we all know, machinery can be unreliable. We therefore recommend that you determine the heart rate for yourself. This is very easy, and involves simply counting the R waves and checking the paper feed. Paper feed is the speed at which the paper is printed by the ECG device. In most countries, the standard is 25mm per second. In others, the printing rate is 50mm per second. Please make sure to find out which paper feed is used at your institution!

If you don’t have an ECG ruler handy, you can calculate the heart rate with the help of the small boxes on the squared ECG paper.

How does this work?

Suppose that you want to determine the ventricular rate: that is, the number of QRS complexes per minute.

A typical ECG strip represents a period of several seconds. The following example shows how you get from the number of QRS complexes to the heart rate:

Given a feed rate of 25 mm per second on a paper sheet with a longitudinal edge length of 30 cm or 300 mm, one paper sheet equals exactly 300 divided by 25, that is, 12 seconds.

If you then count all of the QRS complexes that you find in those 300mm (or during those 12 seconds) you only need to multiply them by a factor of 5 to get the number of beats per 60 seconds or per minute. That’s it! If you only have 15 cm (or 150 mm) then, at a paper feed of 25 mm per second, 6 seconds are recorded and you must multiply by a factor of 10 to reach 60 seconds. You can, of course, adapt this calculation to any millimeter section you like.

However, you should avoid distances that are too short, so that you maintain a valid value even with fluctuating rates. When in doubt, take a longer strip.

And, of course, it’s always best to choose a seconds interval that is a divider of 60, and thus simplify the mental arithmetic involved.

Let’s look now at another tool for assessing the length of ECG waves, namely the small boxes on the ECG-paper.

The small box of an ECG strip is standardized to 1mm, and a large one is standardized to 5 mm. Again, the time unit that the box represents is dependent on the paper feed.

At 50 mm per second, a large box, which is 5 mm long, has a duration of one tenth of a second which equals 100 milliseconds (ms). One large box is divided into five little boxes. One little box represents 100 divided by 5, equaling 20 ms. One second equals 1000 ms. A large box equals 100 ms. Thus, at a paper feed of 50 mm per second, one second on the ECG paper equals ten large boxes or 50 little boxes, or 50 mm.

At a paper feed of 25 mm per second, the ECG printer prints twice as fast. This means that the waves of the ECG are half as wide, and twice as much time fits in the the same length or number of boxes. A small box, which equals 1 mm, corresponds to 40 milliseconds. One large box corresponds to five little boxes, which is five times 40 or 200 milliseconds.

Now, why do we need this whole box business in the first place?

The main reason why we need it is to determine the PR interval, the QRS duration, and the QT interval. More information on these will be presented in the later chapter on "Waves“.

Bradycardia is defined as a heart rate of less than 60 beats per minute irrespective of the underlying heart rhythm.

Common symptoms of bradycardia include a sensation of weakness or dizziness, and syncope.

However, bradycardia doesn’t necessarily mean that someone is sick. Athletes often have heart rates of less than 30 bpm due to a distinctly high vagal tone without being adversely affected at all, and many people easily get by with resting heart rates of 50 bpm or even less.

For the correct assessment of bradycardia, there are always two criteria:

First, you have to determine the underlying rhythm. If a high-degree heart block is present, then bradycardia is, for example, more dangerous than in sinus rhythm.

Second, you should determine whether your patient has any symptoms of bradycardia.

A whole chapter on bradycardia is presented later in the course.

Tachycardia is defined as a heart rate of more than 100 bpm.

Tachycardia extends from 100 bpm to as high as 500 bpm. Many patients with supraventricular tachycardias have heart rates of between 100 and 200 bpm. Heart rates in patients with ventricular tachycardia can be significantly higher. Ventricular fibrillation starts at about 300 bpm. Heart rates above 200 bpm are generally considered an emergency, and so must be treated immediately. When assessing the relevance of heart rates of between 100 and 200 bpm, the underlying rhythm is very important. You will learn a lot more about tachycardia later in the course.