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EKG Challenge – March 2019

EKG Contributor: Massoud Kazzi, MD
Resident Author: Jennifer Rabjohns, MD
Faculty Mentor: Massoud Kazzi, MD
Date of Presentation: 3/28/19

Introduction:
30 year old female presents with alcohol intoxication and suicidal ideation. You're unable to get much history because she is obtunded, but she's protecting her airway. You order psych labs and screening EKG:

You notice her QTc is long, but as soon as you get her on the cardiac monitor, you see this:

How would you describe this EKG?
Sinus beat followed by tachycardia.

What is the differential diagnosis?
This is a wide tachycardia. It's either supraventricular with a bundle branch block or ventricular. Deciding between the two is a whole other discussion; for this case, we'll assume it's ventricular tachycardia (VT). VT can be monomorphic or polymorphic (1).

In this case, there are different morphologies, so it's polymorphic VT.

Background:
There are two different kinds of polymorphic VT; those associated with long QT (torsades de pointes) and those that aren't.

Polymorphic VT without long QT is most often associated with cardiac ischemia.

In contrast, polymorphic VT with long QT usually is acquired (not congenital), from electrolyte abnormalities like hypokalemia and hypomagnesemia, and/or a long list of medications. Then an R-on-T starts torsades.

A normal QTc in men is <450mS; in women, it's <460mS (2). Courtesy of LITFL (3), once your actual (not corrected) QT goes above this nomogram line (~500mS), you're more likely to go into torsades. Remember that principle, but you can also see Dr. Smith's blog for an explanation of how QTc can throw you off (4).

 

Management:
This patient has polymorphic ventricular tachycardia with a documented long QT; she is in Torsades. Luckily, most Torsades resolves quickly and spontaneously; episodes usually are <90 seconds long, but you may have up to 30 episodes before they go away completely (5)! Here's how you escalate if you need to (1, 5, 6)):

  • If unstable with a pulse, cardiovert. Rosen's recommends not synchronizing, which makes sense; your machine never be able to because of the twisting points.
  • If unstable without a pulse, defibrillate. Torsades can degenerate into ventricular fibrillation anyway, and often VF is mistaken for torsades (2,3).
  • Replete K, even to mildly supranormal levels
  • Give Magnesium sulfate 2-4g, regardless of level
  • Give isoproterenol if assumed acquired torsades; use beta-blocker if congenital since it's usually triggered by catecholamines.
  • Overdrive pace (100-120 bpm).
  • Consider lidocaine.

Congratulations! You've successfully gotten this patient out of torsades, but you may not be out of the woods yet. Magnesium is poorly absorbed by cardiac myocytes and drugs that induce long QTc usually have a longer half-life then the magnesium that you gave; start a magnesium drip. A case series in Circulation showed success (no recurrence) with magnesium 3-20mg/min until QT < 500mS (5, 8).

Take-home points:

  • Polymorphic VT can be Torsades (requires long QT) or not (most often due to cardiac ischemia).
  • Once actual (not corrected) QT >500mS, there's a higher risk of Torsades.
  • Most Torsades is brief and episodic...
  • but give magnesium bolus and drip, and be ready to escalate care if needed: unsynchronized cardioversion/defibrillation, potassium, isoproterenol, overdrive pace, consider lidocaine.

References:

  1. http://hqmeded-ecg.blogspot.com/2013/10/polymorphic-ventricular-tachycardia.html
  2. https://jamanetwork.com/journals/jama/fullarticle/1357296
  3. https://litfl.com/polymorphic-vt-and-torsades-de-pointes-tdp/
  4. http://hqmeded-ecg.blogspot.com/2017/11/qt-correction-formulas-compared-to-rule.html
  5. Rosen's: https://www.clinicalkey.com/#!/content/book/3-s2.0-B9780323354790000696?scrollTo=%23hl0001640
  6. Tintinalli's: https://accessmedicine.mhmedical.com/content.aspx?sectionid=109426644&bookid=1658&jumpsectionid=109426892&Resultclick=2
  7. https://emcrit.org/pulmcrit/tdp-magnesium/
  8. https://www.ahajournals.org/doi/abs/10.1161/01.cir.77.2.392

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