In 2008, Tsui and Colleagues  published data on the effect on intraneural placement of a needle on electrical impedance (EI) as measured with a commercially available nerve stimulator (HNS 12 B. Braun). They used a porcine model and found that when their needle penetrated the nerve the EI increased by about 50%. More recently a study carried out in human subjects  looking at EI was performed to determine how it was effected with accidental interneuronal placement of the block needle as seen on US guidance. Their data suggested that puncture of the nerve will increase EI, but in their study the increase was only about 4.3%. Clinically this number is not useful. There was a problem with the most recent study, and that was that they used "suspected puncture" as the criteria instead of verified puncture. One of the criteria that counted as a suspected puncture was a motor response with a current level less than 0.4 mA. This is not a reliable method to determine intraneuronal placement of a needle.
Recently I performed an ultrasound guided femoral nerve block with EI measurement in real time. My baseline reading with the needle near the nerve was 14 Kohms. With penetration into the nerve tissue by direct visualization, the EI increased to 22 Kohms. The needle was withdrawn from the nerve fiber and once again the EI returned to a baseline value of 14 to 15 Kohms. This increase was just over the 50% increase threshold seen in the Tsui study in the porcine model.
Avoidance of intraneural injection is critical in the practice of anesthesia. Temporary or permanent nerve damage is a catastrophic outcome since patients see the nerve block as an add on procedure done for their convenience rather as therapeutic. This mind set typically leads to a much lower tolerance from our patients for negative outcomes, particular ones which are as debilitating as permanent nerve injury. Currently, standard methods such as injection pressure, pain on injection, low current threshold, and paresthesias during block have a low positive and negative predictive value. US guidance likely makes it safer, but this margin of safety is really user dependent and there are case reports of permanent nerve damage after nerve blocks placed with US guidance in experienced hands. At this time, the only objective test to rule out intraneural placement of the stimulating needle seems to be constant measurement of EI. Further studies are awaited and as my case count increases, I will report on my experience.
1. Tsui BC, Pillay JJ, Chu KT, Dillane D. Electrical impedance to distinguish intraneural from extraneural needle placement in porcine nerves during direct exposure and ultrasound guidance. Anesthesiol 2008; 109: 479-83.
2. Bardou P, Merle JC, Woillard JB, Nathan-Denizot N, Beaulieu P. electrical impedance to detect accidental nerve puncture during ultrasound-guided peripheral nerve blocks. Can J Anaesth 2013; 60:253-8.
Blog with interesting cases and/or problems related to anesthesia with discussion based on best evidence in the literature.