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Compressed air injection technique to standardize block injection pressures

[La technique d’injection d’air comprimé pour normaliser les pressions d’injection d’un blocage nerveux]

From the Department of Anesthesiology and Pain Medicine, University of Alberta, Edmonton, Alberta, Canada.

Address correspondence to: Dr. Ban C.H. Tsui, Department of Anesthesiology and Pain Medicine, 8-120 Clinical Sciences Building, Edmonton, Alberta T6G 2G3, Canada. Phone: 780-407-8861, Fax: 780-407-3200; E-mail: btsui{at}ualberta.ca

Purpose: Presently, no standardized technique exists to monitor injection pressures during peripheral nerve blocks. Our objective was to determine if a compressed air injection technique, using an in vitro model based on Boyle’s law and typical regional anesthesia equipment, could consistently maintain injection pressures below a 1293 mmHg level associated with clinically significant nerve injury.

Methods: Injection pressures for 20 and 30 mL syringes with various needle sizes (18G, 20G, 21G, 22G, and 24G) were measured in a closed system. A set volume of air was aspirated into a saline-filled syringe and then compressed and maintained at various percentages while pressure was measured. The needle was inserted into the injection port of a pressure sensor, which had attached extension tubing with an injection plug clamped "off". Using linear regression with all data points, the pressure value and 99% confidence interval (CI) at 50% air compression was estimated.

Results: The linearity of Boyle’s law was demonstrated with a high correlation, r = 0.99, and a slope of 0.984 (99% CI: 0.967–1.001). The net pressure generated at 50% compression was estimated as 744.8 mmHg, with the 99% CI between 729.6 and 760.0 mmHg. The various syringe/needle combinations had similar results.

Conclusion: By creating and maintaining syringe air compression at 50% or less, injection pressures will be substantially below the 1293 mmHg threshold considered to be an associated risk factor for clinically significant nerve injury. This technique may allow simple, real-time and objective monitoring during local anesthetic injections while inherently reducing injection speed.