Why the QT interval matters

QT interval

When you begin taking care of patients who are on cardiac monitoring (AKA “tele monitoring”) you’ll need to chart a few key ECG measurements once per shift and any time there is a change in the rhythm. These are the PR interval, the QRS, the QT and the QTc. You probably are already very familiar with the PR interval and QRS measurements…but what is a QT and why do you need to measure it?

First, let’s review ECG components

Take a look at your ECG…you’ve got your P-wave, then a QRS, then a T-wave. There are also U-waves, but you can’t always see them, so we’re not going to bother ourselves with those at this point.

QT interval

  • The P-wave represents atrial depolarization and, in turn, atrial contraction.
  • The QRS, on the other hand, represents ventricular depolarization (and thus, ventricular contraction)
  • The T-wave represents the repolarization of the ventricles.

So then, what is the QT interval? Glad you asked!

The QT interval is the measure of time between the onset of the Q wave and the end of the T wave. This interval represents the time that depolarization and repolarization of the ventricle occur. Why is this important? To understand why this is important, we need to talk for just a moment about the refractory period.

After the cardiac cells depolarize, they must “recharge” or repolarize. This “recharging” phase is what we refer to as the refractory period. Generally speaking, a refractory period is one in which the cell is unable to have another action potential. More specifically, there’s an “absolute refractory period” where no new action potentials can take place…which is followed by a “relative refractory period” where an action potential COULD occur, if the stars are aligned, Venus is in Scorpio and everything falls into perfect place. But here’s the thing…you don’t want an action potential to occur during this time. If an electrical stimulus hits during this relative refractory period, premature depolarization can occur before the cardiac cells are completely repolarized…the result is that the heart can go into dangerous and deadly ventricular arrhythmias and ain’t nobody got time for that!

And this, ladies and gentlemen, is why we keep such a close eye on the QT interval.

How do we measure the QT interval?

Looking at your ECG strip in Lead II, V5 or V6…measure from the beginning of the Q wave to the end of the T wave. Sounds simple enough, right? Well, the thing is…we need to account for variations in heart rate in order for the QT interval to be accurate. This is called a “corrected QT interval” or a “QTc” and it represents what the QT interval would be if the heart were plugging along at a steady rate of 60 bpm. By correcting the QT to represent a rate of 60, we are able to compare QT intervals of varying heart rates with a pretty set degree of consistency.

So, how do we correct the QT interval?

There are several formulas for correcting the QT interval…the most commonly used (and most simple) is Bazett’s formula, and it’s the one to go to for rates 60-100. If your rate is outside this range, then the next best formula to use is Fredericia’s formula or Framingham formula. However, before you start having a panic attack, the easiest thing to do is use the QT calculator at MDCalc.com or download an app such as MedCalc and let technology figure it out for you! I know the Philip’s monitors we use in my ICU has an e-caliper function that does the calculation for you…so never fear…you won’t have to walk around memorizing a bunch of complicated maths…would I do that to you?

So now that you’ve got your QT corrected for the heart rate…what do you want it to look like? A normal QTc is 350-440ms for men and 350-450ms for women. Once it gets above 500ms, you’re at risk for bad things to happen (we’ll talk about that in a bit!)

What causes a prolonged QT interval?

Lots of things can cause a prolonged QTc…here are just a few:

  • electrolyte imbalances (K, Mg, Ca)
  • hypothermia
  • increased ICP
  • ischemic cardiac tissue or s/p cardiac arrest
  • congenital long QT syndrome (an inherited disorder)
  • drugs…lots and lots of drugs can prolong the QT. A few of the most common you’ll run across are amiodarone, Haldol, procainamide, citlalopram, tricyclic antidepressants, Benadryl, erythromycin and levaquin

The QT is long…what’s the worst that could happen?

The answer is a short one.  Your patient could die.

When the action potential hits on that relative refractory period (which is prolonged when the QT is prolonged), then it can send the heart into a deadly rhythm…typically you’ll see either torsades de points (explained beautifully here) or v-fib…both of which are lethal. If your patient goes into either of these rhythms you’re going to:

  1. start chest compressions while your friend gets the crash cart/defibrillator
  2. if they’re on a ventilator, take ’em off the vent and manually breathe for them using a BVM (Don’t extubate them! Just attach the BVM to the ETT!)
  3. go through your ACLS protocol for V-fib
  4. if they’re in torsades de pointes, give magnesium!

What are you going to DO about it?

If your QTc is prolonged but not scarily prolonged, you’d want to make sure the MD Is aware and then keep a close eye on it. Identify any culprits such as those listed above and assess the QTc more frequently than once per shift. If it gets into the scary zone (>500), you’re probably going to STOP all culprit medications and work with the MD to identify any other potential causes….and then fix them! If their electrolytes are out of whack, replace ’em! If their ICP is up…reduce it (see how to do that here!). If they’re on an amiodarone gtt for atrial fibrillation with RVR, then consider stopping it and going to another therapy. Document the measurement, document that it was communicated to the MD, document what you did about it, document the patient’s response to your intervention and document the new QTc. Document, document, document!

So there you have it! A quick introduction to the QT interval and how you’re going to monitor it and treat it in the clinical setting. This is one ECG measurement you don’t want to ignore…so get in there and measure with confidence!

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