The better to comprehend isobaric spinal anaesthesia, we constructed a thermoregulated model of the subarachnoid space. With it we investigated the six not exactly isobaric local anaesthetics bupivacaine, carticaine, lidocaine, mepivacaine, prilocaine and tetracaine, in order to determine the effect of the following factors on their spread: injected volume, position, density, temperature of the injected solution, rate of injection, barbotage, direction of the spinal needle and its opening. The injected volume was most important. The small differences in density were of consequence in the vertically mounted model. Higher rates of injection increased the spread. The remaining factors were not relevant. The investigation corroborates the clinical observation that spinal anaesthesia with approximately isobaric solutions is a well controllable technique.

The densities of cerebrospinal fluid and of local anesthetics, applicable to isobaric spinal anesthesia, were determined by using the Digital Density Meter DMA 02. The density of CSF showed little variation and at 37 degrees C was 1.00021 +/- 0.00024 g/cm3 (mean +/- SD, n = 22). The density of the local anesthetics bupivacaine 0.5%, carticaine 2%, lidocaine 2%, mepivacaine 2% and prilocaine 2% varied at 25 degrees C between 1.001 and 1.005 g/cm3, at 37 degrees C between 0.997 and 1.001 g/cm3. Tetracaine 0.5% in CBF increased its density by 0.00046 g/cm3. The addition of the vasoconstrictors adrenaline and ornipressine (POR 8) increased the density of the local anesthetic solutions insignificantly. On the basis of the narrow range of variation of CSF density, reliable statements may be made on the density dependent spread of local anesthetics in spinal anesthesia.