A patient with paroxysmal atrial fibrillation is administered sotalol to maintain sinus rhythm. Which of the following is most likely to cause toxicity of this drug?
A)Oxygen saturation 88% (normal >95%)
B)Hemoglobin 8g/dl (normal >13g/dl)
C)Heart rate 44/min (normal >60)
D)AST 500 U/l (normal <40)
E)Corrected QT interval 430ms (normal <450ms)
Answer C (correct answer): Sotalol is a class III antiarrhythmic drug that blocks potassium channels. Like all potassium channel blockers, it can prolong the QT interval on ECG. If QT prolongation is excessive, Torsades de Pointes may occur which can be life-threatening. For this reason, QT interval monitoring is often done in the hospital when initiating therapy with sotalol (also with dofetilide, another class III drug).
Heart rate can alter the risk of toxicity with sotalol. This drug displays “reverse use dependence” meaning the drug exhibits enhanced potassium channel binding at low heart rates. The risk of QT prolongation is greater at slower heart rates.
Answers A and B (incorrect answers): Hypoxemia and anemia do not increase the risk of toxicity from sotalol.
Answer D (incorrect answer): Sotalol is not metabolized by the liver and modestly increased liver function tests will not alter the risk of toxicity.
Answer E (incorrect answer): Sotalol is contraindicated with a baseline corrected QT interval of greater than 450ms. The risk of toxicity is not increased at normal QT intervals.