Pentobarbital elimination in patients with poor renal function.

Drug oxidation is slowed in experimental uremia in animals but has been reported to be normal or accelerated in uremic patients. Eleven normal subjects and 9 uremic patients were each given 100 mg pantobarbital orally. Several blood samples were drawn over a 36-hr period starting on the morning after the dose. Plasma pentobarbital concentration was measured by GLC. The log concentration values were graphed against time elapsed after dose for each patient, and the plasma T/2 and extrapolated value of concentration at the time of drug administration (Co) were determined. The T/2 values in normals ranged from 18 to 48 hr; mean, 26.5 +/- 9.2 (SD). In uremic patients, T/2 values are 10 to 38 hr, mean, 21.3 +/- 8.7. Four of the uremic patients had T/2 values below 18 hr (which was the lowest in the normals). The apparent volumes of distribution of pentobarbital (aVD = dose divided by Co) were 62 +/- 25 L in the normal subjects and 58 +/- 24 L in the uremic patients. The four uremic patients with the short T/2 values tended to have small apparent volumes of distribution so that the metabolic clearnce rates (aVd X 0.693 divided by half-life) were normal in 3 of them. We conclude that pentobarbital elimination is normal in renal failure. Some uremic patients have short plasma T/2 values for pentobarbital, and these more likely result from low apparent volumes of distribution with normal metabolic clearance rates than from accelerated metabolism of pentobarbital.