Cisternal cerebrospinal fluid concentrations of morphine following intrathecal and epidural administration in the baboon

Eleven baboons, anaesthetised with ketamine, had catheters introduced into the cisterna magna. Morphine was injected at lumbar level intrathecally in six and epidurally in five. Cisternal CSF was sampled hourly and the morphine concentration measured using a high pressure liquid chromatograph. In two cases following intrathecal injection peaks of 180 ng/ml at 4 hours, and 2,200 ng/ml at 3 hours were detected. In the latter case there was associated error in sampling therefore this baboon had a repeat injection four weeks later. The maximum level of morphine obtained then was 139 ng/ml at 4 hours. In the epidural group peaks of 113 ng/ml and 53 ng/ml at 1 hour were measured in 2 baboons and 27 ng/ml at 6 hours in a third. In all six other baboons following either procedure no morphine was detected in the cisterna. We conclude that morphine injected either intrathecally or epidurally in primates does migrate centrally in varying quantities. This finding would seem to have some bearing on the unpredictability of reported episodes of respiratory depression following intraspinal morphine.