title: Compressed air injection technique to standardize block injection pressures.
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Anesthesiology and Pain Medicine, 8-120 Clinical Sciences Building, Edmonton, Alberta T6G 2G3, Canada. btsui@ualberta.ca
authors.forename: Ban C H
authors.initials: BC
authors.lastname: Tsui
authors.forename: Lisa X Y
authors.initials: LX
authors.lastname: Li
authors.forename: Jennifer J
authors.initials: JJ
authors.lastname: Pillay
mesh_terms: D000399:Air Pressure; D000776:Anesthesiology / Q000295:instrumentation* / Q000706:statistics & numerical data; D003627:Data Interpretation, Statistical; D016014:Linear Models; D009339:Needles; D009407:Nerve Block / Q000295:instrumentation*; D010525:Peripheral Nerves; D013594:Syringes