ECG Underwriting Puzzler September 2018 ECG Underwriting Puzzler Dr. James Kadouch Vice President & Medical Director

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Clinical context A 43-year-old woman underwent thyroidectomy for a follicular cancer of thyroid. A few days later, she complained of muscle cramps in the back and sharp chest pain. She had no cardiovascular risk factors and no family history of sudden death. Her physical, BMI and vitals were within normal limits. An ECG was obtained.

What is the major abnormality on this ECG ?

At first look, the QT interval is prolonged. A tip to assess a prolonged QT is that a normal QT interval should be less than half the RR interval. Additionally, the poor R wave progression In V1, V2 is most likely a normal variant. But let’s have a closer look at this QT interval…

QT/QTc intervals are definitely prolonged at 500/570 ms. But the T wave duration is normal. Actually, this prolonged QT is essentially a prolonged ST segment ( ) usually seen with hypomagnesemia or hypocalcemia, which is most likely the case here as a result of hypoparathyroidism following thyroid surgery.

Conclusion QT interval is prolonged either congenitally or through acquired causes. Congenital causes – also called Long QT Syndrome (LQTS) – are usually a result of mutations in ion channels. In contrast, acquired causes may be a result of electrolytes abnormalities and/or drugs that affect those ion channels. There are 2 different ECG patterns for QT prolongation : Long ST segment and normal T wave duration (delayed repolarization). Widening of T wave (prolonged repolarization) usually seen in LQTS. This case was due to hypocalcemia related to the thyroidectomy, and the ECG returned to normal after metabolic correction.