Several technical modifications of the various perivascular techniques of brachial plexus block have been advocated but their efficacy has never been documented. The present study used injections of radiopaque anesthetic solutions to illustrate the effect of various technical maneuvers on the flow of the solutions injected into the sheath at several levels. Axillary block studies indicated that solutions injected through a needle placed high in the axilla and centrally directed achieve a higher level than similar volumes injected through a needle that is perpendicular to the neurovascular bundle at the more traditional lower level. Furthermore, the studies demonstrate that a rubber tourniquet applied around the arm distal to the needle is unable to prevent retrograde flow of the injected solution, especially when the usual volumes utilized clinically are injected. Firm digital pressure applied immediately distal to the needle, on the other hand, is effective in preventing retrograde flow and, in so doing, enhances central flow. Bringing the arm to the side while maintaining digital pressure results in even further central spread by removing the obstacle provided by the head of the abducted humerus.
Studies carried out using the infraclavicular technique indicate that because the direction of the properly placed needle is lateral, most of the injected solution passes distally out along the sheath. Again, firm digital pressure high in the axilla occludes the sheath, preventing distal flow and causing the solution to move centrally.
Studies carried out using the interscalene technique show that with the usual technique the injected solution moves cephalad into the cervical portion of the sheath as well as caudad into the brachial portion. Firm digital pressure applied superior to the needle prevents cephalad flow and thus forces all of the injected solution to move caudally as far as the dome of the lung and laterally into the axillary sheath.