Reply to: importance of half-the-air pressure test in Shamrock lumbar plexus block

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Editor,We wish to thank Dr J-A Lin1 for commenting on our publication regarding the Shamrock method for lumbar plexus blocks and suggesting methods for refining our technique for safety reasons.When performing the Shamrock lumbar plexus block,2 we wish to avoid the following clinical scenarios: intravascular spread of the injectate, damaging the lumbar arteries, intraneural injection and, ultimately, epidural spread of the injectate.Risk of intravascular injection can be minimised with the use of ultrasound visualising of real-time spread of the injectate along with negative aspiration test and intermittent aspirations during injection. Risk of puncturing any of the lumbar arteries running beneath the tendinous arches of the psoas major muscle and behind the lumbar plexus can potentially be avoided with the use of colour Doppler, although this technology is not always efficient with deep blocks. The use of electrical nerve stimulation is still viewed by many physicians as a reliable method for avoiding intraneural injection, as motor responses at current intensities below 0.2 mA is thought to indicate intraneural placement. However, some doubts regarding this technology have been demonstrated in one study wherein motor response was not elicited in exposed sciatic bovine nerves at intensities below 1.8 mA despite intraneural placement.3 Furthermore, the stimulus of 2.4 mA was required to elicit muscle twitch/dysaesthesia in diabetic patients with neuropathy even though the tip of the stimulating needle was placed perineurally to the sciatic nerve guided by ultrasound.4 Similar findings with the sciatic nerve have also been shown very recently with intraneural needle placement in the same patient category.5 Finally, the question of whether high injection pressure leads to unwanted spread of the injectate is important to take into consideration. The results from the study by Gadsden et al.6 clearly indicated that high pressure injection (>20 psi) with needle guidance by electrical nerve stimulation could be correlated with contralateral and epidural spread from the lumbar plexus blocks, although the study had to be prematurely terminated. Thus, we agree with Drs Lin and Lu7 that triple monitoring might be considered for lumbar plexus block. However, very recent results regarding high opening injection pressure-monitored blocks revealed that even with needle tip position indenting the epineurium of the femoral nerve, injection pressures less than 15 psi were observed in 10% of cases.In conclusion, no single monitoring technique seems to exclude the risk of adverse effects with lumbar plexus block. However, the combination of visualisation of the spread of the injectate with real-time ultrasound, intermittent negative aspirations, the use of colour Doppler, electrical nerve stimulation and injection pressure monitoring (e.g. Bsmart or half-the-air technique)6,7 together might reduce occurrence of epidural spread, potentially nerve damaging intraneural injections, local anaesthetic systemic toxicity and vascular puncture.Acknowledgements related to this articleAssistance with the reply: none.Financial support and sponsorship: none.Conflicts of interest: none.

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