Long QT syndrome and Torsade de pointes

On a normal ECG, you’ve got the P, Q, R, S, and T waves.

The QT interval spans from the start of the Q to the end of the T wave.

Long QT syndrome, or LQTS, is when somebody’s QT interval is longer than normal, which should typically be less than half of a cardiac cycle.

In fact, for a heart rate of 60 beats per minute, the QT interval’s generally considered to be abnormally long when it’s greater than 440 milliseconds in males or 460 milliseconds in females.

If you measure someone’s QT interval at a different rate though, say 90 beats per minute and it was 400 milliseconds, you can’t really use that to compare that to these value at 60 beats per minute, since the QT interval changes depending on the rate.

As rate increases, the QT interval decreases.

So what we have to do is find the corrected QT interval, or QTc, at the different rate so that you can compare it to the QT interval at 60 beats per minute.

Even though there are several formulas you can use, the 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 1 second.

As a bit of a side-note, usually this formula is expressed without the “divide by 1 second” bit, but the astute observer will notice that the units won’t work out if you do that.

Interestingly, the original formula did include dividing by 1 second to get the units to work out, but for some reason in a paper way back when that step wasn’t included, and basically the version without the 1 second, the sort of unit-incorrect version, has been used ever since!

Anyways, let’s do a quick example of a male with a 400 milliseconds QT interval at a rate of 90 beats per minute.

Comparing to the values at 60 beats per minute, 400 milliseconds wouldn’t be considered a long QT, right?

If we use our handy formula, though, we’ll plug in 400 for QT and 90 beats per minute or .66 seconds per beat.

So 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, which is greater than 440, so actually, a 400 milliseconds QT interval at 90 beats per minute is considered long.

Alright so the QT interval’s a little long, what’s wrong with that?

Well, the QRS complex corresponds to the ventricles depolarizing and contracting.

After they depolarize, they have to repolarize, and that’s captured by the T wave.

When someone has a long QT interval, it means that they have an abnormally long repolarization of some of their heart cells, but not all of their heart cells - which is an important point to remember.

Specifically some of the heart cells are taking longer than normal to repolarize compared to their neighboring heart cells.