The blocked limb, the contested question, and the late toxicity

Why it matters

Two things arrive in the PACU under a working regional block, and both are easy to misread as the block. The first is compartment syndrome in a limb that cannot feel it, a question on which the strongest society statement and everyday clinical practice pull in different directions ivani-2015. The second is local anaesthetic systemic toxicity, which the current ASRA advisory describes as presenting in changed patterns, including delayed and atypical presentations, so that it looks unlike the block regression it is mistaken for neal-2018.

The consequence of getting this wrong is a limb, or a patient, whose deterioration is filed under the block.

Mechanism

A peripheral nerve block interrupts conduction in the nerves that would carry the pain of a rising compartment pressure, which is the whole basis of the worry that it masks compartment syndrome. Against that, ischaemic pain is intense and often breaks through a sensory block as the compartment pressure rises, which is the basis of the clinical practice that treats breakthrough pain as the signal. Both mechanisms are real; the disagreement is about which dominates, and the evidence cannot yet say.

Local anaesthetic toxicity is a function of plasma concentration at the heart and the brain. After a block, that concentration depends on the dose, the site’s vascularity and the rate of systemic absorption, which can be slow. Absorption from a large-volume infiltration or from a depot preparation can produce a peak long after the injection, which is why toxicity can present in the PACU rather than in the block room, and why it can present with cardiovascular rather than neurological signs first neal-2018.

Evidence

The joint ESRA/ASRA committee practice advisory states that there are no current evidence-based data that the use of regional anaesthesia increases the risk of acute compartment syndrome or delays its diagnosis, and that high-level evidence is not available for the topics it evaluated, with most recommendations based on Evidence B studies ivani-2015. Its scope is paediatric, which is a real limitation for an adult curriculum and is stated here rather than buried.

The 2015 ASRA advisory on neurological complications supplies the framework for distinguishing prolonged block, block failure and neurological injury on examination neal-2015.

The 2018 ASRA interim update on local anaesthetic systemic toxicity covers the mechanisms of lipid resuscitation, updated frequency estimates for toxicity, the preventive role of ultrasound guidance, changes in case presentation patterns, and the limited evidence around local infiltration anaesthesia and liposomal bupivacaine; it is the source for the atypical and delayed presentations that make toxicity look unlike block regression neal-2018. The 2021 ASRA checklist is the current cognitive aid, updated in response to user feedback and simulation studies; it carries a correction notice, because its Figure 1 was replaced after online-first publication, so any reproduced checklist must come from the corrected version or from the society’s own site neal-2021-checklist.

What this does not show

The compartment syndrome question is not resolved on this page and the page does not pretend otherwise. The advisory finds no evidence of delayed diagnosis; it does not find evidence against delay, and it says so by recording that high-level evidence is not available ivani-2015. The opposing position, that the case literature and medicolegal practice treat breakthrough pain and a rising analgesic requirement as the signal to unblind the limb, has no ledger row of its own: [TODO_VERIFY: a citable source for the case-literature and medicolegal position on breakthrough pain in a blocked limb as the trigger to examine for compartment syndrome.] Until one exists, that half of the claim rests on clinical practice, and the status chip says uncertain.

Every source here is an advisory or a checklist. None is a trial, and the frequency estimates in the 2018 update have not been extracted, so no incidence figure appears neal-2018.

Items on the director’s content list for this day that the held sources do not carry are placeholders: [NUMBER NEEDED: lipid emulsion bolus and infusion dosing for local anaesthetic systemic toxicity] [NUMBER NEEDED: interscalene block and phrenic nerve palsy, incidence and Horner syndrome] [NUMBER NEEDED: pneumothorax after supraclavicular block, incidence and presentation].

At the bedside

Pain that escalates through a block that was working is a finding to explain, not a top-up to give. Examine the limb, measure what your institution measures, and involve the surgeon early; the advisory’s absence of evidence for delay is not a reason to wait ivani-2015. [PRACTICE VARIES: whether a block is used at all in limbs at risk of compartment syndrome, and who is called when breakthrough pain occurs, differ between institutions and surgical services.]

Know the current ASRA checklist and where the corrected version lives, because toxicity in the PACU may present hours after the block and with the heart rather than the brain neal-2021-checklist neal-2018. Have lipid emulsion where you can reach it and know the local dose.

When a block seems prolonged or a new deficit appears, use the advisory’s framework to separate prolonged block from failure from injury before you reassure anyone neal-2015.

Check yourself

Three items. Every option carries an explanation. Progress is not saved.