Abstract ID: A50
Abstract Title: In Vitro Study of a Compressed Air Injection Technique to Monitor Safe Injection Pressures During Peripheral Nerve Blocks
Authors: Tsui B1, Li L2
University of Alberta Hospitals Edmonton Alberta Canada1, University of Alberta Hospitals Edmonton Alberta Canada2
Poster Type: Either
ABSTRACT BODY
Introduction: Presently, no standardized technique exists to monitor injection pressures during peripheral nerve blocks, despite the association of high pressures ( >25psi) with intraneural injection of local anesthetic and nerve injury risk. (1) Objective and real-time measurement is beneficial as anesthesiologists vary widely in their perception of an appropriate pressure and rate of injection (2) and many hold the needle in place while an assistant performs the actual injection. Using Boyle's Law (PV = constant), pressure in a system can be approximated by the changes in air volume. Since below 25 psi is the reported safe range (1), our objective was to develop a compressed air injection technique that can generate and monitor safe pressures much below 25 psi using an in vitro model (Figure 1) based on Boyle's Law.
Materials & Methods: A model using two infusion pumps with built-in manometers and plastic extension tubing was created. Injection pressures for 20 and 30 mL syringes with various needle sizes (18 G, 21 G, 22G, 24G) were measured at a rate of 600 mL/h in two systems: 1) a closed system consisting of tubing capped off at the distal end to create a static environment and, 2) an open system with some resistance created by opening the clamp slightly on the extension tubing to allow for fluid flow. A set volume of air (10 ml) was drawn into the syringe prior injection and then compressed and maintained at various percentages during injections while pressure was measured.
Results: At 50% air compression, the pressure was 14.7 psi (760 mmHg) in the closed system, and 13.6 psi (703 mmHg) in the open system (Figure 2). There were no statistically significant differences between any of the syringe/needle combinations tested.
Discussion: Using a closed (ideal) system confirmed that Boyle's Law applies to the in vitro model. The static environment allowed pressures to be measured with good accuracy; each syringe and needle combination was essentially the same with very little variability. The results showed what was expected with 50% volume compression doubling the atmospheric pressure (760mmHg). The open system verified the clinical usefulness of the technique as pressures change in a dynamic environment. The open system showed that injection pressures in a physiological setting would likely lie in a safe range when using this technique. By maintaining air compression < 50% within syringes, pressures created will be appropriately 15 psi (well below 25 psi) and the risk of neurological injury may be reduced. This compressed air technique can offer anesthesiologists a simple and objective alternative injection method to monitor injection pressure to ensure safe peripheral nerve blocks.
References
1. Hadzic A, Dilberovic F, Shah S, Kulenovic A, Kapur E, Zaciragic A, Cosovic E, Vuckovic I, Divanovic KA, Mornjakovic Z, Thys DM, Santos AC: Combination of intraneural injection and high injection pressure leads to fascicular injury and neurologic deficits in dogs. Reg Anesth Pain Med 2004; 29: 417-23
2. Claudio R, Hadzic A, Shih H, Vloka JD, Castro J, Koscielniak-Nielsen Z, Thys DM, Santos AC: Injection pressures by anesthesiologists during simulated peripheral nerve block. Reg Anesth Pain Med 2004; 29: 201-5
ATTACHED FILES
Reg Anesth Pain Med 2005; 30(3):A50