Torsades de pointes most often accompanies prolonged QT intervals. Which factor increases risk when the QTc exceeds 500 ms?

• A. Hyperkalemia
• B. QTc greater than 500 milliseconds
• C. A normal QT interval
• D. Shortened QT

Answer: (B)

Prolonged repolarization increases the heart’s vulnerability to early afterdepolarizations, which can trigger torsades de pointes. When the corrected QT interval is pushed beyond about 500 milliseconds, this risk rises markedly because the longer repolarization phase allows these abnormal impulses to initiate a twisting, polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation. So the factor that raises risk as QTc goes past 500 ms is the QTc itself—its extension beyond this threshold is a warning sign for torsades. In contrast, hyperkalemia tends to shorten the QT interval and carries different arrhythmia risks, a normal QT interval does not carry the same heightened risk, and a shortened QT reduces risk rather than increases it. If QTc approaches or exceeds 500 ms, address potential QT-prolonging drugs and correct electrolytes, with treatments like magnesium sulfate and, if needed, pacing or rate control to prevent bradycardia and further episodes.