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October 14, 2013
10:00:00 AM - 11:00:00 AM
Room Room 104-Area E
Regional Anesthesiology Injection Pressures comparing Skilled Assistants With SAFIRA in a Simulated Ultrasound Guided Technique
Robin Heij, M.B.,B.Ch., Emad Eldin, M.B.,Ch.B., Peter Young, M.B.,Ch.B., Joseph Carter, M.B.,Ch.B., John Gibson, M.B.,Ch.B., Amr Ali, M.B.,Ch.B., Monica Liu, M.B.,Ch.B.
Queen Elizabeth Hospital, Kings Lynn, Kings Lynn, United Kingdom

Injection during regional anesthesiology risks permanent nerve injury when pressures in excess of 11 psi (75KPa) are applied1. Previous studies have demonstrated that the perception of appropriate, safe injection pressure varies widely2. There were two phases to this study. In phase one we tested operating department practitioners (who routinely perform the injection in our institution) during a simulated ultrasound guided regional technique recording the pressure of injection. In phase 2 we used a new prototype injection pump called the Safe Injection in Regional Anesthesiology pump [SAFIRA, Kings Lynn, UK] where 20 Anesthesiologists injected against a series of pressures using the pump.

Materials and Methods:

Requirement for formal ethical approval was waived by our institutional research governance committee. Written consent was obtained.

In phase one: whilst an investigator was performing an US guided block on a chicken leg, each participant (n=20) was asked to assess US visualization of 2mL aliquots of local anesthetic. Injection was performed by each participant using a 20mL syringe (Terumo, Egham, UK) as per normal institutional practice. Each was asked to inject and aspirate exactly as they would do clinically and specifically told not to inject if they did not feel happy to. Unknown to the participants, prior to the commencement of each 2mL aliquot injection, a second investigator applied a constant known pressure to the injection system using a calibrated constant pressure generator (Fluke Biomedical, Everett, WA) at 5, 10, 15, 20, 25 and 30 psi in random order. Participants indicated when they were unhappy to inject and the needle was moved. The next pressure in the random sequence was then surreptitiously applied. If injection was performed then the injection pressure was changed during the aspiration period of every 2mL aliquot.

In phase 2 the SAFIRA pump was used. This pump has 2 flat buttons attached with adhesive pads to the Ultrasound probe for injecting and aspirating. It also limits injection pressure to less than 11 psi ( 75KPa ). 20 anesthesiologists injected against 5, 10, 15, 20, 25 and 30 psi, they also confirmed whether the pump injected at a given pressure or not.


Phase one: Fourteen of the 20 participants (70%) injected at 15 psi or above.

Four participants (20%) injected at 20 psi, two (10%) at 25 psi and two (10%) at 30 psi. Interestingly, all participants who injected at 20 psi and above were male. Furthermore, three of the six participants who were not willing to inject at 15 psi or higher had more than 20 years experience.

Phase two: At 5 and 10 psi (34, 69 KPa) the pump injected in all (100%) attempts. At 15 psi (103 KPa) or above the pump did not inject in any attempts.


Avoiding intra-neural injection is a fundamental requirement during the performance of any regional anesthetic technique. An anesthesiologist using a two-handed US guided approach relies on a skilled assistant to inject the local anesthetic solution. Our results confirm that the majority of skilled participants would inject at higher pressures than considered to be safe and a pump like SAFIRA which limits the pressure would not only be safer but also allows the anesthesiologist to control the injection.



Hadzic et al. Combination of Intraneural Injection and High injection Pressure Leads to Fascicular Injury and Neurological Deficit in Dogs Reg Anes Pain Med 2004;29:417-423


Claudio et al. Injection Pressures by Anesthesiologists During Simulated Peripheral Nerve Block Reg Anes Pain Med 2004;29:201-205

Funding disclosure:

We have no funding disclosures.

Conflict of Interest:

Drs Eldin, Carter and Young own provisional patents for safe injection of regional anesthesiology systems (SAFIRA).

Copyright © 2013 American Society of Anesthesiologists