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Polymorphic VT

critical
Lead II
25 mm/s10 mm/mV
HR72bpm
RR833ms
QT273ms
Type

VT whose QRS axis alternates on every beat, classically from digoxin toxicity or catecholaminergic polymorphic VT (CPVT).

Updated

On the trace

Read the strip in the order it happens.

Try each step, then check it.
  1. Look at the sinus beats.
    Show A sinus tachycardia around 120 bpm with a normal QT: the exercise-level rate at which CPVT arrhythmias appear.
  2. Watch the run start.
    Show The first ventricular beat is marked by the arrow.
  3. Follow the alternation.
    Show Upright, inverted, upright, inverted: two fixed shapes swapping on every beat, never a third.
  4. Check the rhythm.
    Show The R-R is regular. Two different complexes at a steady rate is the signature.
  5. See it end.
    Show The run stops and sinus rhythm returns.

How to recognise it

FeatureValueNotes
MorphologyTwo alternating QRS shapesThe axis swings on every beat, and the same two shapes hold for the whole run.
RhythmRegularA steady R-R despite the alternating axis. Polymorphic VT is irregular.
Rate140–200 bpmUsually slower than polymorphic VT or torsades.
Baseline QTNormal (QTc < 460 ms)Measured on the sinus beats. A long QT points to torsades instead.
CauseDigoxin toxicity or CPVTRare enough that its presence should prompt a medication review straight away.

The strip

Thirty seconds of lead II with runs of bidirectional VT over a sinus tachycardia, drawn by the simulator. Work through the steps above on it, in the order it happens.

Management

1

Digoxin toxicity: stop the drug, give antibodies

Treat anyone on digoxin as toxic until proven otherwise. Stop the drug and correct potassium and magnesium. Digoxin-specific antibody fragments (Fab) are the antidote for significant toxicity or instability. Avoid cardioversion if possible: the digoxin-toxic myocardium can be tipped into refractory VF.

2

CPVT: beta-blocker, consider ICD

Exercise- or stress-triggered bidirectional VT with no digoxin exposure suggests CPVT. A beta-blocker is first-line, flecainide is added for breakthrough, and an ICD follows cardiac arrest or syncope despite treatment.

Differential

Polymorphic Ventricular Tachycardia

Also changes shape beat to beat, but at random, from many foci, at an irregular rate. Bidirectional VT has exactly two shapes in a strict alternation at a regular rate.

Monomorphic Ventricular Tachycardia

Regular like bidirectional VT, but every complex is identical. Two clearly different shapes alternating on every beat make it bidirectional, and point to a far more specific cause.

Torsades de Pointes

Changes gradually, in a spindle, on a long QT. Bidirectional VT flips on every beat and the QT is normal.

The full picturePolymorphic Ventricular TachycardiaMechanism, causes and management are covered in the base rhythm.

References

  1. 2022 ESC Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death — European Heart Journal, 2022
  2. Catecholaminergic polymorphic ventricular tachycardia: An exciting new era — Annals of Pediatric Cardiology, 2016