Common causes of QT prolongation
Drug effects and electrolyte disturbance dominate. Low potassium, low magnesium, and low calcium all prolong repolarization, as do many antiarrhythmics, antipsychotics, macrolide and fluoroquinolone antibiotics, and some antiemetics.
- • Correct potassium and magnesium before attributing prolongation to a drug.
- • Congenital long QT syndrome is a distinct inherited cause requiring specialist care.
- • Manual QT measurement on lead II or V5 remains more reliable than machine reads.
FAQ
Which QTc formula should I use?
Fridericia or Framingham when the heart rate is far from 60 bpm. Bazett is the historical default but distorts at rate extremes.
What QTc is dangerous?
Values of 500 ms or above are associated with meaningfully increased torsades de pointes risk, as is an increase of more than 60 ms from a patient's baseline.
Why is the threshold different for men and women?
Women have a slightly longer normal QTc on average, so sex-specific cutoffs of 450 ms and 460 ms are used.
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