Why Does Vasopressin cause QT prolongation?

  1. And is it concerning? Meaning, should be avoided with amiodarone or levquin?
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    1. The QT interval should be measured manually, preferably by using one of the limb leads that best shows the end of the T wave on a 12-lead ECG.
    2. The QT interval should be measured from the beginning of the QRS complex to the end of the T wave and averaged over 3 to 5 beats. U waves possibly corresponding to the late repolarization of cells in the mid myocardium should be included in the measurement only if they are large enough to seem to merge with the T wave.
    3. The QT interval should be measured during peak plasma concentration of a QT-prolonging medication.
    4. The QT interval should be adjusted for heart rate.

      Above is the problem with worrying about drugs that prolong the Q-T interval. Actually putting that consideration into real practice is uber impractical. Usually the warnings come with no mention at all of dosage and if they do, those doses are not the routinely utilized amounts. A case in point would be Zofran, which had a black box warning about prolonged QT. The FDA had to clarify that it involved doses of 32 mg or more. The usual dose for routine nausea treatment or prophylaxis is 4 to 8 mg.

      Another one is droperidol, but the uses for this drug in nausea and vomiting are not large enough for a concern about QT interval.

      Vasopressin, as used for vasoplegia in critical care, does not present a risk greater than the condition that it is being used to treat when used appropriately. IME, I've never once, nor has anyone I know, avoided it's use because of QT prolongation concerns.