Application of a new method for measurement of plasma methadone levels to the use of epidural methadone for relief of postoperative pain.

A method is described for measurement of plasma methadone concentrations using a gas chromatic technique that is rapid and specific and enables the detection of concentrations of less than 1 ng/ml in plasma samples of only 1 ml. The method, when used to determine the plasma methadone levels in patients given continuous epidural infusions of methadone plus on-demand supplementation for relief of postoperative pain, showed plasma methadone plateaus of around 10 ng/ml (starting at 15 min and lasting over 2 hr) and 20 ng/ml (starting at 3 hr and lasting over the period of treatment) (P less than 0.001) after an initial 2-mg methadone epidural bolus and with continuous epidural methadone. Mean plasma methadone levels of around 10 ng/ml were associated with pain relief. Analgesia could be safely maintained for 24 hr.