Accuracy of injection pressure measurement at peripheral nerves using high-resolution 40 MHz ultrasound in an anesthetized porcine model

Abstract

BackgroundFluid injection pressure measurement is promoted as a marker of needle tip position that discriminates between tissue layers. However, clinical ultrasound has insufficient resolution to identify the exact position of the needle tip. Our primary objective was to use 40 MHz ultrasound in anesthetized pigs in order to precisely locate the tip of the needle and measure opening injection pressure in muscle, at epineurium and in subepineurium.MethodsWe surgically exposed the axillae of four anesthetized pigs. Two operators placed a 40 MHz ultrasound transducer over the pectoral muscle and imaged axillary, median and radial nerves. Injections (0.5 mL) were randomized to in-plane and out-of-plane needle trajectories and flow rates of 1, 6 and 12 mL/min.ResultsWe identified 541 fascicles in 23 nerves. The ratio of fascicle area to nerve area remained constant at ~0.30 for all nerves. Axillary nerves were smaller than median and radial nerves, difference in diameter (95% CI) 1.61 (0.87 to 2.36) mm, p<0.001 and 1.59 (0.82 to 2.36) mm, p=0.001, respectively. Axillary nerves had less fascicles per nerve than median nerves, difference 7.63 (2.43 to 12.83) and radial nerves, difference 9.02 (3.64 to 14.40). We visualized the circumneurium and injection within the subcircumneural compartment. Intraneural injection increased nerve area (SD) from 5.7 (2.2) mm2to 13.7 (5.5) mm2, difference 8.0 (5.4–10.6) mm2, p<0.001. Mean injection pressure was greater in subepineurium compared with muscle, geometric ratio 2.29 (1.30 to 4.10), p<0.001; and greater on epineurium compared with muscle, geometric ratio 1.73 (1.03 to 3.00), p=0.01. Twenty-two out of 23 injections in muscle, 14 out of 23 injections at epineurium and 11 out of 22 injections in subepineurium were <138 kPa (20 psi).ConclusionNeedle tip position was not discernible using pressure monitoring. The circumneurium and subcircumneural injection compartment were observed but not intrafascicular injection.

The Forces Associated with Bolus Injection and Continuous Infusion Techniques during Ultrasound-Targeted Nerve Contact: An Ex Vivo Study

“…This research has identified a wide range of forces and pressures that are measured in muscles when subjected to a gentle touch and the indentation of the epineurium. This variation in force can be attributed to the needle tip position and is consistent with the previous studies conducted on pigs [6] and soft embalmed cadavers [10,21]. However, the standard ultrasound technology used in medical procedures does not provide a sufficient resolution to accurately determine the needle tip's precise location.…”

“…However, this approach fails to account for the myriad of circumneural collagen fibres and fat cells [4] that reduce local anaesthetic delivery to the axons, according to Fick's law of diffusion [5]. Fat-filled circumneural fascial tissue is only visible via histology [4] or experimental 40 MHz microultrasound [6]. Accordingly, different authors [2,5,6] have recommended that the nerve is touched gently [6] so that the aperture of the block needle partially covers the epineurium and the injectate takes the path of least resistance away from the stiff epineurium to the elastic circumneural fat compartments [7].…”