ECG intervals

Definitions & Key takeaways

ECG intervals are major elements to look at when reading an ECG strip. These include the PR interval, the QRS complex itself, the QT, and the RR intervals. The PR interval is the time from the beginning of the P wave to the beginning of the QRS complex. It represents the time between the beginning of atrial depolarization and the beginning of ventricular depolarization, which normally lasts about 0.12-0.20 seconds. The QRS complex represents ventricular depolarization and typically lasts less than 100 milliseconds. Next, the QT interval is measured from the beginning of the QRS complex to the end of the T wave. It represents the ventricular systole and typically lasts less than half of the individual's cardiac cycle. Finally, the RR interval is measured from one R wave peak to the next. It helps to calculate the heart rate and tell if it's regular.

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Introduction0:00–1:06

An electrocardiogram is also known as an ECG; the Dutch and German version of the word, elektrokardiogram, is shortened to EKG.
It is a tool used to visualize, or “gram,” the electricity, or “electro,” that flows through the heart, or “cardio.” Specifically, an ECG tracing shows how the depolarization wave, which is a wave of positive charge, moves during each heartbeat by providing the perspectives of different sets of electrodes.
This particular set of electrodes is called lead II; one electrode is placed on the right arm and the other on the left leg.
Essentially, when the wave’s moving toward the left leg electrode, you get a positive deflection. This big, positive deflection corresponds to the wave moving down the septum.
When reading an ECG, there are a few key elements to keep in mind; one of them is looking at the intervals.In a typical waveform, there’s a p-wave, QRS complex, and t-wave.
In addition, there are certain intervals, including the PR interval, the QRS complex itself, and the QT interval. The PR interval spans from the beginning of the p-wave to the beginning of the QRS complex, and it represents the time from the beginning of atrial depolarization to the beginning of ventricular depolarization.

PR interval1:06–2:26

It’s normally 0.12-0.20 seconds, which is three to five little boxes, since each little box is 0.04 seconds. Therefore, the PR interval shown is about four boxes or 0.16 seconds.
Any deviation in the normal depolarization pathway from the SA node to the ventricles can change the PR interval. For example, consider if the atria are depolarized by an ectopic atrial focus, such as an irritable atrial cell outside of the SA node.
If it was farther away from the AV node, it’d result in a longer PR interval, because the signal has to travel a greater distance.
Alternatively, if it was really close to the AV node, the PR interval might be super short. Another example is first degree heart block, which is when the electrical signal travels more slowly through the AV node than it normally does, causing the PR interval to lengthen beyond 0.2 seconds.The QRS complex represents depolarization of the ventricles; it’s normally less than 100 milliseconds or two and a half little boxes.

QRS complex2:26–3:16

Just like the PR interval, the QRS duration can differ in its path from the av node to the ventricles. For example, if an ectopic ventricular focus, such as an irritated ventricular cell, fires off, the resulting depolarization wave will move slowly from muscle cell to muscle cell, instead of traveling quickly through the electrical conduction system.
Therefore, it takes a longer time to depolarize the ventricles, and the QRS is wider. It’s considered intermediate if it’s 100 to 120 milliseconds, and prolonged if it’s over 120 milliseconds, or three little boxes.The QT interval spans from the beginning of the QRS complex to the end of the t-wave.

QT interval3:16–6:39

It represents ventricular systole, which is the entire span from depolarization through repolarization. Normally, the QT interval should be less than half of a cardiac cycle.
In fact, for a heart rate of 60 beats per minute, the QT interval is generally considered to be abnormally long when it’s greater than 440 milliseconds in men, or 460 milliseconds in women.
If you measure someone’s QT interval at a different rate, say 90 beats per minute, and it was 400 milliseconds, you might think that that’d be considered normal; however, you can’t really use these values to compare to the normal QT interval at 60 beats per minute, or bpm, because the QT interval changes depending on the rate.
As rate increases, the QT interval decreases; thus, at 60 bpm, the abnormal cutoff might be 440 ms, whereas at 90 bpm, it’s likely something lower.
So, what we have to do is find the corrected QT interval, or QTc, at the different rate. Then, you can compare it to the QT interval at 60 beats per minute.
Even though there are several formulas you can use, Bazett’s formula is probably the simplest, where the corrected QT interval equals the QT interval in milliseconds, divided by the square root of the R to R interval in seconds, divided by one second.
As a side-note, this formula is usually expressed without the “divide by 1 second” part, but the astute observer will notice that the units won’t work out if you leave it out.
Let’s use our 90 bpm and 400 milliseconds QT interval from before, and imagine the person is male. If we plug in 400 for QT and 90 beats per minute, or .66 seconds per beat, we have a QT of 400 milliseconds divided by the square root of 0.66 seconds over 1 second, which is 400 milliseconds divided by 0.81, which is unitless, and we get a corrected QT interval of 493 milliseconds.
Now we can compare this sum to the value at 60 bpm. We see that the corrected QTc is actually greater than 440 ms, which means that this measured QT interval of 400 ms at 90 bpm is indeed abnormally long!Finally, here are some factors that can affect the QT interval.
First up, both hyper- and hypocalcemia, meaning too much or too little calcium in the blood, respectively! Hypercalcemia typically shorterns the QT interval, while hypocalcemia lengthens it.
The QT interval can also be prolonged by medications, like amiodarone, which is an antiarrhythmic that affects cardiomyocyte ion channels, or caused by inherited long-QT syndromes, where there are mutations in genes, including LQT1, LQT2, or LQT3.
One of the most feared complications of having a prolonged QT interval is a type of ventricular tachycardia called torsade de pointes, which can lead to sudden cardiac death; here, the ECG shows a “twisting of the points.”All right, let’s quickly recap.

Review6:39–7:05

An ECG waveform has a PR interval that’s usually 120 to 200 milliseconds, a QRS complex that’s less than 100 milliseconds, and a QT interval that’s less than 440 milliseconds in men and 460 milliseconds in women; these values are for rates at 60 bpm.
Bazett’s formula can be used to find the corrected QT interval, which you can use to