Effect of needle puncture on morphine and lidocaine flux through the spinal meninges of the monkey in vitro. Implications for combined spinal-epidural anesthesia.

ABSTRACT

BACKGROUND: Combined spinal-epidural anesthesia is a technique growing in popularity. However, there have been no attempts to investigate the risk of epidural drug reaching the subarachnoid space in high concentration by passing through the meningeal hole left by the spinal needle. This study begins to address this question by quantitating the flux of morphine and lidocaine through the spinal meninges of the monkey in vitro after puncture with three different-sized needles. METHODS: Spinal meningeal tissue from anesthetized monkeys was mounted in a diffusion cell and drug flux was measured through intact tissue and through tissue punctured with a 27-G Whitacre, a 24-G Sprotte, and an 18-G Tuohy needle. RESULTS: The flux of morphine through the meningeal tissue was significantly increased by puncture with each of the study needles. The flux of lidocaine was significantly increased only by puncture with the 24-G Sprotte and 18-G Tuohy needles. The flux of morphine through intact tissue was less than the flux of lidocaine through intact tissue. In contrast, the flux of morphine and lidocaine were the same through tissue punctured with the study needles. The magnitude of the drug flux through the needle puncture was a function of the diameter of the study needle. CONCLUSIONS: Epidural anesthesia after accidental or intentional puncture of the spinal meninges has occasionally resulted in high spinal blocks and total spinal anesthesia. This study suggests that drug movement through the meningeal hole is responsible for this complication and that the risk may be decreased by using the smallest possible needle to puncture the meninges.