Interpretation of aspiration tests in local anesthetic injections.

Three young adult laboratory rabbits were anesthetized with a barbiturate and their abdominal cavities were surgically opened to expose a variety of blood vessels. A number of veins and arteries were penetrated with 25- and 27-gauge needles attached to standard dental aspirating cartridge-type syringes. Aspiration tests were performed and the results were recorded. No differences in the performance of aspiration were detected with the two gauges of needles. Where the aperture of the needle lay unobstructed and wholly within a vein greater than 1.5 mm in diameter or within any artery, it was impossible not to aspirate blood even with a peer technique. In certain circumstances, false-negative and misleading positive readings are possible. Recommendations for a clinically reliable aspirating technique have been given and four grades of positive result described. Absolutely infallible interpretation of results is impossible but, provided the aspirating technique itself is sound, intelligent evaluation of observed effects should result in very few errors of interpretation.