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AS5.{1-3,5-6} | Regional Anaesthesia — PBL Case

CLINICAL SETTING

Mr. Rajan Kumar, a 55-year-old man with a 15-year history of type 2 diabetes mellitus and peripheral vascular disease, is listed for a right below-knee amputation secondary to an infected, gangrenous foot that has failed conservative management. His past history includes hypertension (controlled on amlodipine), ischaemic heart disease with a previous NSTEMI two years ago, and chronic kidney disease stage 3 (eGFR 38). He is a non-smoker. On examination, he weighs 75 kg and is afebrile but tachycardic at 98 bpm. Blood pressure is 142/88 mmHg. His ECG shows Q-waves in leads II, III, and aVF with a rate-controlled sinus rhythm. Echocardiography done one month ago showed an EF of 48% with inferior wall hypokinesia. His platelet count is 178 × 10⁹/L and coagulation profile is normal. The anaesthesiologist reviews the patient and, given the compromised cardiac reserve and the need to avoid general anaesthesia if possible, considers a regional technique for the below-knee surgery.

Trigger 1: Technique Selection and Anatomy

The anaesthesiologist decides to use a combined sciatic nerve block (popliteal approach) plus saphenous nerve block (adductor canal) for the below-knee amputation, with light sedation. She plans to use 0.5% bupivacaine for both blocks. Mr. Kumar weighs 75 kg. She asks the registrar to confirm the maximum safe dose of bupivacaine for this patient and to describe the anatomy at the popliteal fossa that will guide the sciatic nerve block.

DISCUSSION POINTS

What is the maximum safe dose of bupivacaine (plain, without adrenaline) for Mr. Kumar? Calculate the actual volume you would use for the sciatic block and confirm it is within safe limits.
Describe the anatomy of the popliteal fossa. Where exactly does the sciatic nerve bifurcate into the common peroneal and tibial nerves, and what structures border the popliteal fossa? Why is this block site preferred over a more proximal sciatic block for below-knee surgery?

Click to reveal Trigger 2: Intraoperative Crisis — Cardiovascular Collapse (discuss previous trigger first!)

Trigger 2: Intraoperative Crisis — Cardiovascular Collapse

The sciatic block is performed under ultrasound guidance. Thirty millilitres of 0.5% bupivacaine are injected around the sciatic nerve at the apex of the popliteal fossa. The patient is also given 10 mL of 0.5% bupivacaine for the saphenous block. Total bupivacaine = 40 mL × 5 mg/mL = 200 mg = 2.67 mg/kg. Two minutes after completing the injection, Mr. Kumar becomes agitated and reports a metallic taste and ringing in his ears. His speech becomes slurred and he has a brief generalised tonic-clonic seizure lasting 45 seconds. His cardiac monitor then shows a wide-complex tachycardia at 140 bpm degenerating to ventricular fibrillation. His blood pressure falls to 50/30 mmHg.

DISCUSSION POINTS

What complication has occurred? Describe the sequence of clinical signs (from earliest to cardiovascular collapse) and explain the mechanism underlying each stage of this progression.
The total bupivacaine dose was 200 mg (2.67 mg/kg), exceeding the 2 mg/kg safe limit. What is the specific antidote? Describe the dose and administration route. Why is bupivacaine particularly cardiotoxic compared to lignocaine?

Click to reveal Trigger 3: Resuscitation and Reflection (discuss previous trigger first!)

Trigger 3: Resuscitation and Reflection

The team initiates ACLS protocols. Defibrillation is performed once for the VF (monophasic 360 J / biphasic 200 J). 20% intralipid emulsion is administered: 1.5 mL/kg IV bolus (112.5 mL) followed by 0.25 mL/kg/min infusion. After 3 cycles of CPR and intralipid, sinus rhythm is restored. Mr. Kumar is transferred to the ICU. He recovers over 48 hours without permanent neurological or cardiac sequelae. A root-cause analysis meeting is convened. It is determined that the dose error (failure to add both block volumes before comparing to the mg/kg limit) was the primary cause, compounded by failure to use a test dose before the full injection.

DISCUSSION POINTS

If you were to redesign the regional anaesthesia protocol for Mr. Kumar, what safety checks would you build in BEFORE injecting any local anaesthetic? What is the role of the 'test dose' in peripheral nerve blocks, and what constitutes a positive test?
Based on the learning from this case, what alternative regional technique could have been chosen for this below-knee amputation that would minimise the total volume of bupivacaine required while still providing surgical anaesthesia? Justify your choice using anatomy.

Group Task Assignments

Group 1: Collaborative Task

As a group, construct a 'Pre-Block Safety Checklist' for peripheral nerve blocks that would prevent the dose error that occurred in this case. Include: patient weight documentation, drug concentration confirmation, volume calculation for each block, cumulative dose calculation, and mandatory test-dose criteria.

Group 2: Collaborative Task

Prepare a brief (5-minute) presentation comparing spinal anaesthesia vs combined peripheral nerve blocks for below-knee amputation in a patient with cardiac compromise, covering: haemodynamic profile of each technique, total LA dose required, reversibility, and suitability for Mr. Kumar's specific risk profile.

Learning Issues

Research these questions and bring your findings to the discussion.

[AS5.1] What are the indications, contraindications, and advantages of peripheral nerve blocks over neuraxial anaesthesia for lower-limb vascular surgery in patients with cardiac compromise?
[AS5.2] Describe the anatomy of the sciatic nerve at the popliteal fossa and the saphenous nerve at the adductor canal. What are the sensory territories of each and why are both required for below-knee anaesthesia?
[AS5.3] How is local anaesthetic systemic toxicity (LAST) diagnosed and managed? What is the mechanism of bupivacaine cardiotoxicity and why is 20% intralipid the specific antidote?
[AS5.6] What is the test dose for peripheral nerve blocks — composition, dose, and interpretation of a positive result? When should it be used?