Torsades de pointes is very bad. It easily degenerates into Ventricular Fibrillation (VF) and that’s sudden cardiac death. How many cases of torsades have you seen in your career? Are your ready for it when you see it? Let’s see, there’s magnesium…uh, and then I seem to recall that drug isoproterenol (that I’ve never used) and then..oh yeah…I’m supposed to do ‘OVERDRIVE pacing’. Got it. Ummm, how do you do overdrive pacing? How does it work? DOES it work?
First. Let’s set the record straight: emergency doctors don’t do overdrive pacing. How do I know that? Because there is only one indication for “overdrive pacing” in the ED: torsades de pointes. But the rate of torsades is 150-250 BPM. And the most commonly used pacemaker in US ED’s is the Medtronic single-chamber Pacer, which has a maximum RATE of 180 BPM. So how were you planning to OVERDRIVE Torsades, which is running at 250 BPM, when your max pacer rate is 180? See? You can’t even DO overdrive pacing. What are you really supposed to do?
Ventricular Tachycardia: 2 flavors
- Monomorphic VT
- Polymorphic VT
- Normal QT
- Long QT —– BEWARE! This is Torsades de pointes
Torsades de Pointes = Polymorphic VT with long QT
- Torsades occurs when a PVC occurs during the T Wave (R on T)
- THIS explains WHY ‘overdrive pacing’ works – see Treatment below
The QT conundrum:
The QT is heart rate-dependent. When the heart rate is slower, the QT is longer, and vice versa. In order to determine whether your patient truly has a long QT, the QTc was developed as a calculation that that attempts to normalize the QT interval to a rate of 60 BPM.
The bottom line: the QTc is number you should look at. There are several calculations available- even MDCalc has one. But just look at the top of the EKG. It turns out the EKG computer does a fine job of calculating the QTc. Use the QTc.
What is a long QTc
The dirty definition: Long QT = QT > 500 ms
Beware the patient with syncope or near-syncope with a QTc longer than 500. You might be staring straight at a patient who just had polymorphic VT with long QTc. You might be staring at torsades. Torsades likes to degenerate into VF, and VF is a less than desirable rhythm.
Treatment of Torsades
- Stabilize the cardiac myocyte and treat hypomagnesemia: Magnesium 2gm IV q15m
- Increase the resting heart rate. Increasing the HR narrows the QT. Narrowing the QT narrows the T wave (ventricular repolarization). A narrow T wave means your ventricle is spending less time repolarizing. If your T wave is narrow, the chances of throwing a PVC on top of a T (R on T) is much less likely. So….just increase the heart rate. Oh wait…increase the heart rate AFTER you’ve cardioverted them from polymorphic VT.
Drugs to use after sinus rhythm achieved and you want to speed the heart rate
- nonselective beta agonist
- Inotropic, chronotropic, dromotropic
- dose: 10-20 mcg IVP or drip
**Transvenous Pacing: RATE = 110 BPM