ECG rate and rhythm

Definitions & Key takeaways

The heart is a muscle that regularly beats to pump blood throughout the body. An electrocardiogram (ECG) is a test that records the electrical activity of the heart, which shows how fast and strong the heartbeats are.

The heart rate refers to the number of contractions of the heart per minute. This one can vary according to an individual's age, but usually, the normal heart rate for adults falls between 60 and 100 beats per minute. The rhythm refers to the pattern of electrical impulses that cause the heartbeat. The normal cardiac rhythm is a regular sinus rhythm, which means that each heartbeat follows a predictable pattern. There may be abnormal rhythms, such as atrial fibrillation and ventricular fibrillation, both serious and life-threatening conditions.

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Introduction0:00–0:54

An electrocardiogram - an ECG - or the dutch and german version of the word - elektrokardiogram or EKG, is a tool used to visualize “gram” the electricity “electro” that flows through the heart “cardio”.
An ECG tracing specifically shows how the depolarization wave moves during each heartbeat - which is a wave of positive charge - looks from the perspective of different sets of electrodes.This particular set of electrodes is called lead II, with one electrode on the right arm and the other on the left leg, so essentially when the wave’s moving toward the left leg electrode, you get a positive deflection, like this big positive deflection correspond to the wave moving down the septum.
To read an ECG there are a few key elements to keep in mind, one of them includes figuring out the rate and rhythm. There are a couple ways of figuring out the heart rate on an ECG.
The first one is called the “box method” because you count the number of boxes between heartbeats. Each small box represents 0.04 seconds, and each big box is five small boxes, so each big box is 0.2 seconds.

Heart Rate0:54–5:43

To do that, you can count the number of small boxes between R waves since R waves are tall and pointy and easy to see in lead 2 of an ECG strip.
You can find an R wave that has a peak that falls at the beginning of a box, and then count up how many boxes until the same point on the next R wave.Let’s say that there are 4 big boxes and 1.5 small boxes between two R waves, meaning there are 4 x 5 + 1.5 = 21.5 small boxes, and that means there’s 0.04 seconds x 21.5 or .86 seconds between heartbeats.
Now, to get something a little more meaningful we can take the inverse which is 1 over 0.86 beats per second, or 1.16 beats per second.
Now there’re 60 seconds in a minute, so multiplying that by 60 we end up with 70 beats per minute—the heart rate! Now, if the distance between two R waves is exactly 1 big box, then the heart rate would be 300 beats per minute—really fast.
If R waves are two big boxes apart or 0.4 seconds apart, then the heart rate is 150 beats per minute. Three big boxes apart is 100 beats per minute, four, five, and six big boxes apart is 75, 60, and 50 beats per minute respectively.
Remembering these numbers makes it easier to make a rough estimate. For example, if there are three to four large boxes between R waves, then the heart rate must be between 75 and 100 beats per minute.Another method to determine the heart rate is to count the number of beats in ten seconds, which is the standard length of time on the rhythm strip portion of a 12-lead ECG.
So in this case we’ve got 15. All you’ve gotta do is multiply this by 6 to get the heart rate, which would be 90 beats per minute.
The reason this little trick works is that you’ve got 15 beats per 10 seconds, and again to convert to per minute you multiply by 60 seconds in a minute.
Looking at this we see that 60 / 10 equals 6 per minute. So 6 times 15 is 90 beats per minute.If the heart rate is too slow or too fast, it could be because something other than the SA node is pacing the heart rate.
For example, there could be atrial flutter which is when an ectopic focus in the atria - like an irritated atrial cell - starts to spontaneously fire between 250-350 depolarizations per minute, with only one out of every few atrial depolarizations passes through to the ventricles ***B***.
To calculate the atrial rate, you can use the same methods as before, except you look at P waves instead of R waves. If one P wave begins on a heavy line, and the next P wave begins on the next heavy line, or 0.2 seconds later, then again, you’ve got 1 beat for every 0.2 seconds, and multiplying by 60 seconds in a minute you get 300 beats per minute, you could also remember that 1 big box is equal to 300 bpm..
Another situation is atrial fibrillation which is when there are multiple ectopic foci in the atria that start firing all at once, the atrial rate can increase to 350-450 beats per minute ***C***.
In this situation, only the occasional firing of an ectopic focus that happens to be near the AV node is able to make it through and down to the ventricles.
The ectopic foci fire too quickly for the atria to be fully depolarized by any one of them, so with atrial fibrillation, there aren’t any P waves on an ECG, but rather small waves that reflect waves of atrial depolarization that couldn’t make it through the AV node.
Of note, the 350 - 450 beats per minute reflects all the times the atrial foci fire in a minute; and not the rate of ventricular depolarization.
So with atrial fibrillation, the ventricular rate is much lower than the atrial rate, but still over 120 beats per minute, and highly irregular - meaning that the distance between the QRS complexes is not equal.
Normally on an ECG, one waveform with its p-wave, QRS complex, and t-wave looks just like the next one - almost like they were copy and pasted one right after another - that’s how a regular rhythm looks - the heart moves like a smooth dancer on rhythm.
An irregular rhythm, on the other hand, is when there’s any change in the timing of the QRS complexes. To help identify ECG abnormalities you can look to see if every part of the waveform looks exactly the same, which would include the p-wave, the QRS complex, and the t-wave.

Rhythm5:43–7:59

If not, it could mean that there’s an ectopic beat - meaning that it may have originated from an abnormal spot in the atria or ventricles.
For example, this first p wave is deflected upwards and the second is deflected downward, which would indicate that it’s an ectopic beat ***D***.
Alternatively, an odd looking waveform may have originated from the normal spot - the sinoatrial node - but then gotten thrown off course, which is what happens when there’s a block someplace.
In this example, there’s a bundle branch block which is where the signal can’t go down one or both of the bundle branches, usually resulting in these wide QRS complexes ***E***.
Next, check for changes in the sequence of the waves - most often involving the two depolarization waves - the p wave and then the Q RS complex.
You can make sure that there is a p-wave before every QRS complex, and a QRS complex after every p-wave. For example, if there’s a premature ventricular contraction like this, then there may be a QRS complex without a preceding p-wave ***F***.
On the flip side, if there’s a third degree AV block, the atria and ventricles have completely separate electrical activity; kind of like two people dancing to different songs, at the same time.
So in this case, there can be P waves that are not followed by QRS complexes, and the ventricular rate is usually low, around 40 beats per minute.
***G***. Also don’t forget the most common irregular rhythm, which is atrial fibrillation; in which case there are no P waves, and irregular QRS complexes.All right, as a quick recap - one quick way to estimate the heart rate on an ECG, is to remember that the heart rate is 300, 150, 100, 75, 60, 50 depending on whether there’s 1, 2, 3, 4, 5, or 6 boxes between QRS complexes.

Review7:59–8:27

To help identify an irregular rhythm you can look at the waveforms and see if there is any variation in the timing of the QRS complexes.
Beats per minute by big boxes. Is 60 beats per minute and six big boxes is 50 beats per minute.
Help identify an irregular Rhythm, you can. Look at the waveforms and see if there's