A 31-year-old male presents with weakness.
What electrolyte disturbances cause the interval problem seen in this ECG? Which dysrhythmia is this patient at risk for?
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What electrolyte disturbances cause the interval problem seen in this ECG?
Hypokalemia, hypomagnesemia, and hypocalcemia can all cause a prolongation of the QT interval. While hypokalemia and hypomagnesemia both delay the repolarization phase (phase 3) of the of the cardiac action potential (creating wide-based T waves, U waves, or a fusion of both), hypocalcemia prolongs the QT interval by way of extending the plateau phase (phase 2) of the cardiac action potential (lengthening the ST segment but with a normal T wave, Figure 1). This ECG is consistent with hypokalemia and/or hypomagnesemia.1
Figure 1. This schematic demonstrates the various phases of the cardiac action potential as they relate to the ECG waveform. In phase 0, fast acting voltage-gated sodium channels open and a rapid influx of sodium results. Sodium channel blocking agents tend to predominantly affect phase 0 and widen the QRS complex. During phase 2, voltage-gated potassium (efflux) and calcium (influx) channels tend to maintain a relative potential plateau. Since phase 2 represents the ST segment, hypocalcemia tends to produce a long ST segment, but a normal T wave. Finally, in phase 3, potassium channels allow more potassium to leak and “repolarize” the cell. Hypokalemia tends to delay this phase, creating broad-based T waves, U waves, and T-U fusions.
Which dysrhythmia is this patient at risk for?
Torsades de Pointes due to early afterdepolarizations/triggered activity during the repolarization phase (R on T phenomenon).2 It’s worth noting that the QT correction formulas (e.g. Bazett’s formula) are not very good outside a normal heart rate range, and that data supporting the use of a QT nomogram is emerging.3,4 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.4
Figure 2. Torsade de Pointes
Pearls for Urgent Care Management
- Symptomatic patients with prolonged QT should be transferred to a higher level of care.
- Common electrolytes that cause prolonged QT include hypokalemia, hypomagnesemia, and hypocalcemia. If able, commence electrolyte repletion prior to transfer.
- Diercks DB, Shumaik GM, Harrigan RA, Brady WJ, Chan TC. Electrocardiographic manifestations: Electrolyte abnormalities. Journal of Emergency Medicine. 2004;27(2):153-160. doi:10.1016/j.jemermed.2004.04.006
- 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. doi:10.1161/CIR.0000000000000549
- Isbister GK. Risk assessment of drug-induced QT prolongation. Australian prescriber. 2015;38(1):20-24. doi:10.18773/austprescr.2015.003
- 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). doi:10.5811/westjem.2019.10.40381