High or low current threshold for nerve stimulation for regional anaesthesia

A Swiss nationwide survey shows that dual guidance is the preferred approach for peripheral nerve blocks

Ultrasound has significantly increased safety and effectiveness in regional anesthesia. However, little is known about its clinical use. We studied clinical approaches currently used by anesthesiologists, conducted a nationwide survey, and analyzed data collected in ordered logistic regression models. All active members of the Swiss Society for Anaesthesiology and Resuscitation (SSAR/SGAR) were asked to participate. Reported practice in nerve localization, safety, and techniques used for peripheral nerve blocks (PNB) were main outcome measures. Experience ranged from 3 to >30 years. The mean number of block techniques mastered was 11.5 ± 5.9. Standard monitoring was regularly used, whereas sterile coats were less frequently used by anesthesiologists who self-estimated a higher level of expertise in PNB (ordered logit coefficient −0.05, 95% CI −0.07 to −0.02, P < 0.001; pseudo r2 = 0.019; probability > Chi2 = 0.02). The more self-estimated expertise anesthesiologists had, the less likely they were to use nerve stimulation in combination with ultrasound (dual guidance) (ordered logit coefficient −0.31; 95% CI −0.85 to −0.03: P = 0.03; pseudo r2 = 0.007; probability > Chi2 = 0.05). The high share of reported standard monitoring meets the recommendations of the Helsinki Patient Safety Declaration. Dual guidance appears to be the preferred approach for safely localizing nerves for PNB in Switzerland.

[Localization of peripheral nerves. Success and safety with electrical nerve stimulation]

Peripheral electrical nerve stimulation is one of the standard applications in peripheral regional anesthesia in addition to the ultrasound technique. Among other findings, the visualization of needle and nerve during ultrasound-guided blockade caused a change in clinical practice of peripheral nerve stimulation in the last decade. In the present article old and new aspects of principles and clinical practice of the nerve stimulation technique are presented and summarized in a total clinical concept in order to achieve safe and successful peripheral regional anesthesia using electrical peripheral nerve stimulation.

Dual guidance improves needle tip placement for peripheral nerve blocks in a porcine model

BACKGROUND

The objective of the study was to evaluate whether the use of ultrasound (US) together with nerve stimulation (USNST) provides a better needle tip position for performing peripheral regional anaesthesia than the use of US or nerve stimulation (NST) alone.

METHODS

Needle placements were applied at the brachial plexus and sciatic nerves in 32 anaesthetised pigs. Following needle placement near the target nerve, using either the USNST or the US or NST, a volume of 0.3 ml synthetic resin was injected mimicking a 'test-dose' injection. The primary outcome was the incidence of close needle-to-nerve placement assessed by injectate localisation in direct contact with the nerve epineurium. Secondary endpoints were the incidences of intraneural injection and haematoma formation in direct contact with the target nerve.

RESULTS

A total of 611 punctures were performed. The evaluation for the criterion 'close needle placement' revealed significant differences in favour of the USNST group (98.5%) compared with the NST (90.1%) and the US group (81.6%) (P = 0.001). Significant differences were observed regarding 'intraneural needle placement' between the groups as well (USNST, 0.5%; US, 4%; NST, 2.5%; P = 0.034). The incidence of haematoma formation was significantly higher in the NST group (10.8%) than in the US group (2.5%) and in the USNST group (1.5%) (P = 0.001).

CONCLUSION

These findings suggest that the USNST approach combines the benefits of the US and the NST techniques in terms of a higher rate of close needle tip placements and a lower incidence of haematoma formation.

Simultaneous percutaneous implantation of a microdialysis probe for monitoring perineural concentrations of local anaesthetics during peripheral nerve block in rabbits

OBJECTIVE

To develop a technique that allows simultaneous percutaneous implantation of both a microdialysis probe and injection catheter in order to monitor the perineural pharmacokinetics of local anaesthetics (LA) after a femoral block.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Five anaesthetized male New Zealand rabbits with a mean ± SD weight of 3.2 ± 0.2 kg.

METHODS

After femoral nerve localization by electrostimulation, an injection catheter and a microdialysis probe were slowly and simultaneously inserted into a cannula left into place in the perineural region. Both were then secured into place, after removal of the cannula. At the end of the experiment, methylene blue was injected to confirm the distance from the femoral nerve during subsequent postmortem anatomical dissection of the injection site.

RESULTS

Staining was adequate and the catheter found to be located within 4 mm of the femoral nerve in three out of five rabbits.

CONCLUSIONS AND CLINICAL RELEVANCE

This procedure allows direct implantation of a microdialysis probe near the injection site of LA during a femoral nerve block without loosing nerve localization accuracy. This procedure has been used successfully to monitor the regional pharmacokinetics of LA after a peripheral nerve block.

Histological Consequences of Needle-Nerve Contact following Nerve Stimulation in a Pig Model

Background. Nerve stimulation can facilitate correct needle placement in peripheral regional anesthesia. The aim of this study was to determine whether the high threshold current is associated with reduced nerve injury due to fewer needle-nerve contacts compared with low current. Methods. In anaesthetized pigs, thirty-two nerves of the brachial plexus underwent needle placement at low (0.2 mA) or high current (1.0 mA). The occurrence of needle-nerve contact was recorded. After 48 hours, the nerves were analyzed for occurrence of histological changes. Nerve injury was scored ranging from 0 (no injury) to 4 (severe injury). Results. The frequency of needle-nerve contact was 94% at low compared to 6% at high current. The score was significantly higher at low (median [interquartile range] 2.0 [1.0-2.0]) compared to high current (0.0 [0.0-1.0] P = .001). Conclusions. Inflammatory responses were directly related to needle-nerve contacts. Hence, posttraumatic inflammation may be diminished using higher current for nerve localization.

Nerve perforation with pencil point or short bevelled needles: histological outcome

BACKGROUND

In the case of needle nerve contact during peripheral blocks, pencil point needles are considered less traumatic compared with bevelled needles. However, there are not enough data to prove this notion. Therefore, the aim of this study was to challenge the hypothesis that nerve perforation with short bevelled needles is associated with major nerve damage compared with pencil point needles.

METHODS

In five anaesthetised pigs, the brachial plexus was exposed bilaterally. Up to eight nerves underwent needle nerve perforation using a pencil point needles cannula or an short bevelled needle. After 48 h, the nerves were resected. The specimens were processed for visual examination and the detection of inflammatory cells (haematoxylin-eosin, i.e. CD68-immunohistochemistry to detect macrophages), myelin damage (Kluver-Barrera staining) and intraneural haematoma. The grade of nerve injury was characterised by an objective score ranging from 0 (no injury) to 4 (severe injury).

RESULTS

Fifty nerves were examined. According to the injury score applied, there was no significant difference between the pencil point needles [median (inter-quartile range) 2.0 (2.0-2.0)] and the short bevelled-needle group [median 2.0 (2.0-2.0) P=0.23]. No myelin damage was observed. Signs of post-traumatic inflammation were equally distributed among both groups.

CONCLUSIONS

In the present study, the magnitude of nerve injury after needle nerve perforation was not related to one of the applied needle types. Post-traumatic inflammation rather than structural damage of nerve tissue is the only notable sign of nerve injury after needle nerve perforation with either needle type. However, neither the pencil point- nor the short bevelled needle can be designated a less traumatic device.

Systematic evaluation of the highest current threshold for regional anaesthesia in a porcine model

BACKGROUND

The purpose of this study was to determine systematically the highest minimal stimulation current threshold for regional anaesthesia in pigs.

METHODS

In an established pig model for regional anaesthesia, needle placements applying electric nerve stimulation were performed. The primary outcome was the frequency of close needle to nerve placements as assessed by resin injects and subsequent anatomical evaluation. Following a statistical model (continual reassessment method), the applied output currents were selected to limit the necessary number of punctures, while providing guidance towards the highest output current range.

RESULTS

Altogether 186 punctures were performed in 11 pigs. Within the range of 0.3-1.4 mA, no distant needle to nerve placement was found. In the range of 1.5-4.1 mA, 43 distant needle to nerve placements occurred. The range of 1.2-1.4 mA was the highest interval that resulted in a close needle to nerve placement rate of > or =95%.

CONCLUSIONS

In the range of 0.3-1.4 mA, all resin deposition was found to be adjacent to nerve epineurium. The application of minimal current intensities up to 1.4 mA does not obviously lead to a reduction of epineural injectate contacts in pigs. These findings suggest that stimulation current thresholds up to 1.4 mA result in equivalent needle tip localisation in pigs.