VARIATIONS IN THE COMPOSITION OF SPINAL ANESTHETIC SOLUTIONS - THE EFFECTS OF DRUG ADDITION ORDER AND PREPARATION METHODS

Adjuvants such as opioids or epinephrine are commonly added in small v olumes to multicomponent spinal anesthetic solutions. In this study, w e tested the hypothesis that final adjuvant concentrations vary depend ing on the devices and techniques used to prepare the anesthetic solut ion. We compared two aspiration devices, the filter needle and the fil ter straw, in a laboratory study. Two techniques for drawing up and es timating adjuvant volumes were assessed, as was variation in the compo sition of a model spinal anesthetic solution resulting from intra- and interindividual variability. A model hyperbaric anesthetic solution c onsisting 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 aspirat ion of the components into the syringe was measured by determining the MB concentration in the final solution by optical absorbance. Ten exp erienced 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 aspir ation of the solution components had a large effect on the final conce ntration of the MB tracer in the ultimate mixture. Variation in the MB concentration was on the order of four- to fivefold. Effects were lar ger for the filter straw compared with the filter needle. A comparison of 10 anesthesiologists revealed large intra- and interindividual var iations in the final composition of the model anesthetic solution. The concentration of tracer added to the mixture with a tuberculin syring e approximated the planned yield. We conclude that the devices and tec hniques used to prepare mixtures of drugs for delivery to the cerebros pinal fluid may influence the concentrations of drugs in the anestheti c and, thus, the dose supplied to the patient receiving spinal anesthe sia. Variation in clinical effects of spinal anesthetics may be attrib utable, in part, to variation in the composition of the anesthetic. Im plications: This laboratory study demonstrates the potential for large variation in the composition of spinal anesthetic mixtures.