IN-VITRO AND IN-VIVO MICRODIALYSIS CALIBRATION USING RETRODIALYSIS FOR THE STUDY OF THE CEREBROSPINAL DISTRIBUTION OF BUPIVACAINE

Microdialysis coupled to HPLC was used to study the disposition of loc al anesthetics in the cerebrospinal fluid (CSF) because of the difficu lty in sampling CSF. A retrodialysis method for the microdialysis cali bration was investigated in vitro and in vivo. Calibration by retrodia lysis was simultaneously validated through the use of the zero net flu x method. Two local anesthetics (bupivacaine and ropivacaine), which d iffer structurally by only one methyl group, were respectively utilize d as substance of interest and as internal standard. Different paramet ers were tested in vitro to compare the relative recovery (RR) of bupi vacaine and the relative loss (RL) of ropivacaine. Several flow rates were tried to select an optimal in vivo flow rate (1 mu l/min). The RR and RL values were not influenced by the variation of bupivacaine con centration. A significant variability among different probes within a batch was established (RR ranging from 41.1-65.3%; RL ranging from 30. 7-61.0%). The K-factor values, defined as RLropivacaine/RLbupivacaine, were calculated in vitro and in vivo. This ratio decreased in vivo bu t was constant (K-in (vitro) = 1.06 +/- 0.04, K-in (vivo) = 0.87 +/- 0 .03). The extracellular tissue concentration of the compound of intere st was given by C = C-in (dialysate) x (K/RL). Following in vivo impla ntation in rabbit CSF during 4 h, the probes were tested again in vitr o and no deterioration of probe during the in vivo experiment was foun d. After administration of bupivacaine in the epidural space of rabbit s, plasma and microdialysis CSF samples were simultaneously collected. Plasma and CSF disposition of bupivacaine displayed different kinetic s. The maximum CSF concentration of B averaged 394 +/- 170 mu g ml(-1) , with a mean T-max of 3.8 +/- 1.8 min. The maximum plasma concentrati on of B averaged 0.44 +/- 0.09 mu g ml(-1) with a mean T-max occurring at 1 min, Microdialysis, combined with accurate calibration, should b e a reliable technique to gain further insight in the spinal dispositi on of local anesthetics. (C) 1998 Elsevier Science B.V. All rights res erved.