The measurement of injection pressure during the performance of peripheral nerve blocks can be pivotal to detect intraneural placement of the needle tip and thus avoid intrafascicular injection. However, injection pressure can only be measured along the injection line (tubing), which is influenced by several factors. The primary aim of this feasibility study was to describe and validate the principle of a novel nerve-block needle conceived for real-time continuous monitoring of injection pressures at the needle tip. Our secondary aim was to provide measurements and compare injection pressure values at the needle tip and in the injection line. Four porcine lower limb anatomic models were prepared and extraneural injections were performed with fractioned boluses of 2 ml saline at a controlled infusion rate of 10 ml.min (0.16 ml.s ). Injection pressure at the needle tip was monitored and compared with the pressure in the injection line. The system proved to be reliable. Thirty injections were successfully performed without technical failures. The mean (95%CI) difference between pressures at the needle tip and the injection line varied substantially from 14.33 (12.58-16.08) kPa at 0.5 ml injected volume to 41.56 (39.66-43.45) kPa at the end of the injection. This study demonstrates that the described system allows for real-time continuous monitoring of injection pressure at the needle tip. Moreover, this study shows that injection pressure values measured in the injection line cannot be assumed to be a reliable indicator of the injection pressure at the needle tip.

Injection pressure monitoring provides valuable information to prevent intraneural injections and possibly consequent nerve damage during peripheral nerve block. However, the measurement of injection pressure along the injection line is inaccurate as it is influenced by several variables. The aim of this study was to test a new system for precise injection pressure monitoring at the needle tip in a cadaveric model. The system consists of a miniaturised pressure sensor embedded within the needle shaft that is connected via an optical fibre to an external control unit. In order to test the capacity of the system to discriminate between perineural and intraneural injections, a total of 24 ultrasound-guided injections at various locations of the sciatic nerve (12 perineural and 12 intraneural) were performed in fresh cadavers. The injections were delivered at a constant rate by an electronic pump (5 ml saline at 10 ml.min ). Two perineural and two intraneural injections were excluded from analysis, since the operator could not confirm the exact needle-tip location. Mean (SD) peak injection pressure was significantly lower for perineural compared with intraneural injections (14 (6) kPa vs. 131 (56) kPa; p < 0.001). This study shows that this system is a reliable method to accurately monitor injection pressure at the needle tip, allowing for discrimination between perineural and intraneural injections of the sciatic nerve in fresh cadavers.

Alongside ultrasonic visualisation, measurement of injection pressure is an effective tool for reducing the risk of intraneural injection during peripheral nerve block. The aim of this study was to compare injection pressure profiles when measured along the injection line with the pressure measured directly at the needle tip using different rates of injection. A syringe pump delivered a 5-ml injection of saline into silicone gel at three different speeds (5 ml.min , 10 ml.min and 15 ml.min ). Fibreoptic pressure sensors recorded real-time pressure profiles of the injection pressure directly at the needle tip and along the injection line. A total of 15 injections were successfully performed, five for each injection rate, totalling 30 recorded pressure profiles. More rapid rates of injection caused peak pressure measured in-line to increase, whereas pressure measured at the needle tip remained constant (mean (SD) pressure in-line 30.76 (3.45) kPa vs. 72.25 (1.55) kPa and mean (SD) pressure at needle tip 19.92 (1.22) kPa vs. 20.93 (2.66) kPa at 5 ml.min and 15 ml.min , respectively). Injection pressure profiles showed that in-line pressure measurement failed to record precise real-time pressure changes occurring at the needle tip (mean (95%CI) pressure difference 10.8 (6.98-14.70) kPa vs. 51.2 (47.52-54.89) kPa for in-line and needle-tip measures, respectively). We conclude that, in order to accurately monitor the true injection pressure generated, independent from operator and injection parameters, measurement at the needle tip is necessary, as injection pressure measured along the injection line is an unreliable surrogate.

Background and Objectives Injection pressure monitoring can help detecting the needle tip position and avoid intraneural injection. However, it shall be measured at the needle tip in order to be accurate and reproducible with any injection system and non operator‐dependent. With an innovative system monitoring the injection pressure right at the needle tip we show that it is possible to early detect an intraneural and also an intravascular injection. Methods We performed supraclavicular block‐like procedures under real‐time ultrasound guidance on two fresh cadaver torsos using a sensing needle with an optical fiber pressure sensor within the shaft continuously measuring injection pressure at the needle tip. A total of 45 ultrasound‐guided injections were performed (15 perineural, 15 intraneural and 15 intravenous injections). Results Mean (SD) injection pressure after only 1 mL injected volume was already significantly higher for the intraneural compared to the perineural injections: 70.46 kPa (11.72) vs 8.34 (4.68) kPa; P < .001. Mean (SD) injection pressure at 1 mL injected volume was significantly lower for the intravascular compared to the perineural injections: 1.51 (0.48) vs 8.34 (4.68) kPa; P < .001. Conclusions Our results show that injection pressure monitoring at the needle tip has the potential to help identifying an accidental intraneural or intravascular injection at a very early stage.

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