Epidural and intrathecal opiates in obstetrics.

The use of epidural and intrathecal opiates in obstetrics is reviewed. Opiate receptors in the substantia gelatinosa of the spinal cord appear to be the main site of drug action after both epidural and intrathecal modes of drug administration. However, an additional systemic effect for this selective spinal analgesia cannot be excluded, especially after epidural drug administration. Contrary to that of general surgery patients, the response of pregnant women at term to epidural opiates is unpredictable. The action of the opiates is of short duration. There are at least three anatomical reasons for this phenomenon: an extensive epidural venous blood flow during pregnancy, visceral fibers (uterine contraction pain) located deeper in the substantia gelatinosa than somatic fibers (skin and peritoneal pain), and A delta fibers (uterine pain) which bypass opiate receptors in the substantia gelatinosa. After intrathecal injection of opiates, there was a strong analgesic action during delivery, but an unacceptable amount of side effects prevents their routine use. In post-cesarean patients, epidurally administered opiates are quite effective analgesics, but they still have one serious unwanted effect: respiratory depression of delayed onset. Thus, in routine obstetric practice, epidural or intrathecal opiates play only a limited role.