Adjuvants such as opioids or epinephrine are commonly added in small volumes to multicomponent spinal anesthetic solutions. In this study, we tested the hypothesis that final adjuvant concentrations vary depending on the devices and techniques used to prepare the anesthetic solution. We compared two aspiration devices, the filter needle and the filter straw, in a laboratory study. Two techniques for drawing up and estimating adjuvant volumes were assessed, as was variation in the composition of a model spinal anesthetic solution resulting from intra- and interindividual variability. A model hyperbaric anesthetic solution consisting of tetracaine, dextrose, and methylene blue (MB) as a small-volume tracer solution was studied. The components were drawn up into a syringe through one of two commercially supplied aspiration devices, a filter straw or a filter needle. The effect of the order of aspiration of the components into the syringe was measured by determining the MB concentration in the final solution by optical absorbance. Ten experienced anesthesiologists then prepared samples of the test solution using one of two different techniques to estimate tracer volume in the aspiration syringe. In comparison studies, the MB tracer was added to the hyperbaric solution with a tuberculin syringe. The order of aspiration of the solution components had a large effect on the final concentration of the MB tracer in the ultimate mixture. Variation in the MB concentration was on the order of four- to fivefold. Effects were larger for the filter straw compared with the filter needle. A comparison of 10 anesthesiologists revealed large intra- and interindividual variations in the final composition of the model anesthetic solution. The concentration of tracer added to the mixture with a tuberculin syringe approximated the planned yield. We conclude that the devices and techniques used to prepare mixtures of drugs for delivery to the cerebrospinal fluid may influence the concentrations of drugs in the anesthetic and, thus, the dose supplied to the patient receiving spinal anesthesia. Variation in clinical effects of spinal anesthetics may be attributable, in part, to variation in the composition of the anesthetic.
Implications: This laboratory study demonstrates the potential for large variation in the composition of spinal anesthetic mixtures.