Lidocaine, Serum or Plasma

This drug is used in therapy of ventricular but not supraventricular arrhythmias. Following initial parenteral administration of a bolus, lidocaine is rapidly cleared with a short half-life of approximately 10 minutes (first-pass effect). After approximately 30 minutes, there is a slower elimination phase about 90 minutes long. With continuous intravenous administration, a half-life of about 1.5 to 2 hours may be achieved, hence, prolonged administration by the I.V. route is often necessary to achieve the desired therapeutic result. Time to reach steady-state by I.V. is 6 to 12 hours. In most cases, a relatively constant plasma level may be maintained by slow intravenous infusion administered over a period of 6 to 10 hours. Blood levels are also elevated by impaired cardiac or hepatic function. The drug is metabolized by the liver to two active metabolites, monoethylglycinexylidide (MEGX) and glycinexylidide (GX). Both accumulate and MEGX most likely contributes to toxicity. Toxic symptoms may include confusion, respiratory depression, seizures, dizziness, drowsiness, paresthesias, hypotension, bradycardia, and double vision. Convulsions, cardiac and respiratory arrest may occur. Barbiturates and phenytoin seem to enhance drug metabolism and lower serum levels, whereas propranolol, cimetidine, and norepinephrine increase levels.

Lidocaine is approximately 70% bound to plasma proteins, especially α₁ acid glycoprotein (the concentration of which is variable) and to albumin.