Prolonged QT

Measuring the QT can be challenging, and many different methods exist. One simple method is the “half the RR” rule – the QT interval is prolonged if it occupies more than half the R-R interval. The “half the RR” rule is a conservative estimate at normal and tachycardic rates, but perhaps a better option at bradycardic rates is an absolute cut-off of 485 ms. Causes of long QT include congenital cardiac ion channel mutations (i.e., Na or K channelopathies), electrolyte imbalance (i.e., hypokalemia, hypomagnesemia, hypocalcemia), medications (e.g., amiodarone, tricyclic antidepressants, methadone, antibiotics, ondansetron, antipsychotics, and many, many others), hypothermia, and hypothyroidism. Prolonged QT places someone at risk of developing Torsades de Pointes due to early afterdepolarizations/triggered activity during the repolarization phase (R on T phenomenon). Torsades de pointes is characterized by QRS complexes that “twist” around the isoelectric line.

Torsades de Pointes.

Examples

Prolonged QT

Prolonged QT interval likely drug-induced. This patient was on several QT-prolonging medications including amiodarone, metoclopramide, and ondansetron.

Hypokalemia

Hypokalemia tends to prolong the QT interval by way of delaying repolarization, creating a broad-based T wave, U waves, and T-U fusions.

Hypokalemia

While there is quite a bit of baseline artifact, a prolonged QT interval with broad-based T waves is evident. This patient was severely hypokalemic and intermittently went into polymorphic ventricular tachycardia. Overdrive pacing via a temporary transvenous pacer was initiated to avoid further episodes of polymorphic ventricular tachycardia.

Hypokalemia

This patient presented with global weakness and was diagnosed with hypokalemic periodic paralysis. The potassium level was 1.9 mEq/L. Note the prolonged QT interval and the U waves.

Hypocalcemia

This patient presented with fatigue and was found to have newly discovered renal failure. The calcium was 5.6 mg/dL.

References

Al-Khatib SM, Stevenson WG, Ackerman MJ, et al. 2017 AHA/ACC/HRS Guideline for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death. Journal of the American College of Cardiology. 2018;72(14):e91-e220.
Isbister GK. Risk assessment of drug-induced QT prolongation. Australian prescriber. 2015;38(1):20-24.
Rischall ML, Smith SW, Friedman AB. Screening for QT Prolongation in the Emergency Department: Is There a Better “Rule of Thumb?” Western Journal of Emergency Medicine. 2020;226(2).
Danzi S, Klein I. Thyroid disease and the cardiovascular system. Endocrinology and Metabolism Clinics of North America. 2014;43(2):517-528.