Syringe sizes and injection techniques in regional anaesthesia - Does it really matter? 
Tam Al-Ani, United Kingdom, E-Mail: tam.al-ani@nhs.net

Al-Ani T., Staber M.
Inverclyde Royal Hospital, Dept of Anaesthesiology & Intensive Care, Greenock, United Kingdom

Background and Goal of Study: Forceful needle-nerve contact and injection may result in nerve damage. We tested the hypothesis that a larger diameter syringe combined with a low-pressure injection technique can more readily detect high opening injection pressures (OIPs) ≥15 PSI.1
Materials and Methods: In a prospective observational study thirty anaesthetic staff were randomly allocated to 3 syringe sizes: 20ml, 30ml and 50ml. Each participant was asked to inject local anaesthetic during manikin-based simulated nerve block. The distal end of a non-distensible injection line was hidden and connected to a 3-way tap and manometer (Fluke DPM4). First, participants were asked to inject 3 ml while the 3-way tap was open to atmosphere, then to simulate high OIPs the 3-way tap was closed and they were asked to inject further 5 ml. Using their allocated syringe size, peak injection pressures were recorded for three injection techniques: 1.''Single hand, thumb on the plunger'' 2.''Single hand, palm on the plunger'' 3.''Two hands, thumbs on the plunger''. The participant's own injection technique was recorded first, then randomly allocated to the other two techniques. Injection technique 1 and the 20 ml syringe were used as references for data analysis using the mixed effects model.
Results and Discussions: Regardless of syringe size, injection techniques 2 and 3 recorded higher pressures ≥15 PSI compared to technique 1 (p< 0.001). No significant pressure differences were recorded for syringe sizes 30ml and 50ml when compared to 20ml [(p< 0.2261,95% CI -9.69, 2.46) and (p< 0.1330,95% CI -10.90, 1.52)], respectively. In figure (1) below the first three boxes are for technique 1, the next for 2 and the final for 3.

[Figure 1]
Conclusions: Within the measured parameters, syringe size does not appear to significantly affect OIPs.
The ''one hand, thumb on the plunger'' injection technique is superior to the other techniques in limiting high injection pressures, however this does not completely eliminate the risk of high OIPs and forceful injections. 
References: 1- Gadsden J et al. Reg. Anesth. Pain. Med. 2016; 41:50-5.