Introduction: High opening injection pressure (> 15 psi) can detect needle - nerve contact. However, the reliability of injection pressure monitoring to detect needle – nerve or impingement may be affected by syringe size. We hypothesized that monitoring of opening injection pressure (the pressure at which injectate is detected by US) is affected by the size of the syringe used for injection. Methods: After Ethics Commitee approval, 22 gauge 50 mm needles were inserted under US to contact the C5, C6 and C7 nerve roots of fresh human cadavers. Hand-held injections were made using 3 different syringe sizes (5, 10, 20 mL) at a rate commensurate with typical clinical practice. Injections were made bilaterally at each of the above nerve roots. Opening injection pressure data were aquired with an in-line digital pressure recorder using a 60 mL syringe (10ml/min), and injection halted when spread was detected. Results: A total of 48 injection measurements were made. The peak (opening) pressures at which injection commenced in two cadavers were 30.50 psi and 34.07 psi with 5 mL syringe, 29.20 psi and 34.95 psi for 10 mL syringe, 26.03 psi and 29.42 psi for 20 mL syringe, all han-held injection. In automated pump injection 60 mL syringe was used, and maximum achieved pressures were 23.42 psi and 34.03 psi. Opening injection pressures were similar regarless of syringe size (p>0.05). Conclusion: The size of the syringe commonly used in clinical practice of peripheral nerve blockade did not significantly affect the monitoring of the open injection pressure. All injection with the needle – nerve contact resulted in injetion pressure > 20 psi, regardless of syringe size or method of injection. Our findings ate thus consistent with the fluid mechaminc described by Pascal's Law, where pressure exerted anywhere in a confined incompressible fluid system is transmited equally throughout until the opening pressure is reached and injection begins.
