Thread by @AvrahamCooperMD

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Ever wonder why magnesium (Mg) is used to treat Torsades de Pointes?

The answer is fascinating, elegant, and also explains why Mg treats another clinical entity - eclampsia

#medthread #tweetorial #medtwitter

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First, let's find how torsades was originally described (and named!):

💡Francois Dessertenne noted "ventricular tachycardia with two variable foci" in a patient in 1966
💡He coined the arrythmia torsades de dointes, connoting "twisting peaks"

bit.ly/2XN9Hp5

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To understand how Mg treats torsades we need to understand the pathophys of the arrythmia itself.

▶️Torsades is a ventricular arrythmia of repolarization, occurring in patients with prolonged QT intervals (usually acquired from drugs, ischemia, or⬇️electrolytes)

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There are 4 phases of repolarization of cardiac myocytes

1: K efflux
2: Ca influx
3 and 4: K efflux

Calcium influx in phase 2 "stalls" repolarization as in/out charge flow is balanced (more on this later)

bit.ly/34khj4T

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Torsades arises from a specific sequence of events that disrupts repolarization.

✅Sinus beat
❌Prolonged QT
❌Ventricular ectopic beat during depol (aka an "early after depolarization" or EAD, thought to be calcium-mediated)
❌Re-entrant arrythmia

bit.ly/2ruAj1R

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It turns out that seeing torsades on the monitor is literally watching a re-entry circuit circulate

💥Terrifying but also striking to see

bit.ly/2OKJdjL

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Now that we understand torsades, how did Mg come to be preferred therapy?

The original description was in 1984 when Mg infusion treated 3 patients w/ torsades who had acquired long QT.

All had normal serum Mg levels, the QT intervals didn't shorten

bit.ly/2OhCm2q

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It is not totally clear how they figured out that this might work, but they cite a paper from 1968 where 2 patients w/ Vfib from low Mg (which was actually torsades) were treated with overdrive pacing.

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So how does Mg actually work?

💡It turns that Mg, as a cation, is an effective Ca channel blocker.

bit.ly/2OkFt9H

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💡Blocking Ca channels suppresses the EADs, allowing the re-entrant arrythmia to terminate. This was first described in dogs who had torsades-like arrythmias induced by cesium infusions with magnesium rescue.

bit.ly/2XKz7DK

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This was described by @PulmCrit as naloxone for torsades (I really like that description)

He also advocates for protocol-driven, continuous Mg infusions to make sure the serum levels stay up (goal 3.5-5 mg/dl) and those Ca channels stay blocked.

bit.ly/2OLEJcK

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Calcium channel blockade also explains why Mg treats eclamptic seizures:

💡Eclamptic seizures are driven by dysregulated cerebral vasoconstriction
💡Mg blocks Ca channels and induced vasodilation
✅Seizures stop

bit.ly/2KUupxX

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To sum up:

💡Torsades is a re-entrant ventricular arrythmia in the setting of a prolonged QT interval
💡Early after depolarizations, triggered by Ca influx, lead to torsades
💡Mg blocks Ca channels and ❌the arrythmia
💡Mg treats eclamptic seizures by a similar mechanism

What do you think #medtwitter?

CC: @laxswamy @ChrisWorsham @virenkaul @grecoa3 @dina_khateebDO @AdamRodmanMD @tony_breu @ETSshow @cjchiu @gbosslet @aoglasser

This is the correct source for tweet #8

ahajournals.org/doi/pdf/10.116…