A 45-year-old woman is evaluated in the surgical ward on the first postoperative day following a total thyroidectomy for a large multinodular goiter. She reports a new onset of tingling and numbness around her mouth and a pins and needles sensation in her fingertips. She also mentions occasional painful muscle cramps in her calves. Her past medical history is significant for hypertension and gastroesophageal reflux disease. She takes amlodipine and omeprazole. She has a 10-pack-year smoking history but quit 5 years ago. She consumes one to two alcoholic beverages per week. Her temperature is 37.1 C (98.6°F), blood pressure is 132/84 mmHg, pulse is 76/min, and respirations are 16/min. On physical examination, she has a healing transverse neck incision without evidence of hematoma or drainage. Inflation of a blood pressure cuff above the systolic pressure on the upper arm for three minutes results in adduction of the thumb and flexion of the metacarpophalangeal joints. An electrocardiogram is performed.
Which of the following is most likely to be seen on this patient's electrocardiogram?
The correct answer is:
D) Prolonged QT interval
This patient is presenting with symptomatic hypocalcemia following a total thyroidectomy. This is a well-known complication caused by the accidental removal of or devascularization of the parathyroid glands during surgery, leading to transient or permanent hypoparathyroidism. The lack of parathyroid hormone results in decreased bone resorption of calcium and decreased renal calcium reabsorption, leading to low serum calcium levels. Clinically, this manifests as neuromuscular irritability, which includes perioral numbness, paresthesias, and muscle cramps. The physical finding described, with carpal spasm induced by blood pressure cuff inflation, is known as the Trousseau sign, which is more sensitive and specific for hypocalcemia than the Chvostek sign.
On electrocardiography, the most characteristic finding of hypocalcemia is a prolonged QT interval. This occurs because low extracellular calcium concentrations delay the plateau phase of the myocardial action potential (phase 2), thereby increasing the duration of ventricular repolarization. While usually asymptomatic, significant prolongation of the QT interval can predispose patients to life-threatening ventricular arrhythmias such as Torsades de Pointes.
Answer choice A: Depressed ST segments, is incorrect. ST segment depression is typically associated with myocardial ischemia or hypokalemia. While electrolyte imbalances can coexist, the primary finding expected in the setting of acute hypocalcemia is related to the duration of the QT interval rather than ST segment morphology.
Answer choice B: Peaked T waves, is incorrect. Narrow, peaked T waves are the hallmark electrocardiographic finding of hyperkalemia. In the context of thyroid surgery, potassium levels are typically not affected unless there is concurrent renal failure or significant tissue trauma, neither of which is suggested here.
Answer choice C: Prolonged PR interval, is incorrect. A prolonged PR interval, representing a first-degree atrioventricular block, can be seen in various conditions including increased vagal tone, medications like beta-blockers, or other electrolyte abnormalities such as hyperkalemia or hypermagnesemia. It is not a classic finding of hypocalcemia.
Answer choice E: Shortened QT interval, is incorrect. A shortened QT interval is the classic finding associated with hypercalcemia, which is the opposite of this patient's clinical state. Hypercalcemia accelerates the plateau phase of the action potential, leading to faster repolarization.
Key Learning Point
Hypocalcemia is a common postoperative complication of total thyroidectomy due to hypoparathyroidism. The most common finding on electrocardiography is prolongation of the QT interval due to a delayed plateau phase of the cardiac action potential.
