Local Anaesthetic Pharmacology, Ultrasound Physics, and Regional Safety Mechanics — WACS Viva & Clinical Scenarios (Anatomy, Applied Anatomy and Regional Anaesthesia)

Exam-style local anaesthetic pharmacology, ultrasound physics, and regional safety mechanics viva scenarios with examiner probes and model answers for…

Scenarios covered

• SCENARIO 1: A 70-kg female receives 30 mL of 0.5% plain bupivacaine for an ultrasound-guided supraclavicular block. Towards the end of the injection, she becomes agitated, reports a metallic taste, tinnitus, and perioral tingling, followed rapidly by loss of consciousness, generalized tonic-clonic convulsions, and ventricular fibrillation on the monitor. Provide a comprehensive physiological analysis of Local Anaesthetic Systemic Toxicity (LAST) and detail your immediate step-by-step pharmacological and electrical resuscitation pathway, including specific lipid emulsion dosing.
• SCENARIO 2: You are asked to select and configure an ultrasound machine to perform a superficial interscalene brachial plexus block in a thin patient versus a deep lumbar plexus block in an obese patient. Explain the fundamental physics of ultrasound generation (piezoelectric effect), beam attenuation, and the physical trade-off between transducer frequency, axial resolution, and tissue penetration. Detail how you would manipulate gain, depth, focal zone, and dynamic range for both cases.
• SCENARIO 3: A 45-year-old male undergoing circumcision receives 15 mL of 2% prilocaine for a penile nerve block. Two hours later in the recovery room, his SpO2 reads 85% on 100% oxygen via a non-rebreather mask, but he has no dyspnoea and his arterial blood gas reveals a PaO2 of 250 mmHg with brown-coloured blood. Explain the pharmacological mechanism of this condition, why pulse oximetry is unreliable, and your definitive treatment protocol.
• SCENARIO 4: Describe the molecular mechanism of action of local anaesthetics at the voltage-gated sodium channel (Nav1.5, Nav1.7). Explain how chemical structure (aromatic ring, intermediate chain, tertiary amine), pKa, lipid solubility, and protein binding determine the clinical onset time, potency, duration of action, and differential sensory versus motor blockade of lignocaine, bupivacaine, and ropivacaine.
• SCENARIO 5: During an ultrasound-guided femoral nerve block, the anaesthetist uses a combination of ultrasound imaging, a peripheral nerve stimulator, and an in-line injection pressure monitor. Explain the biophysical rationale for triple monitoring. Why is an opening injection pressure greater than 15 psi dangerous, what does a muscle twitch at <0.2 mA signify, and how do you differentiate an intraneural intrafascicular from an extrafascicular injection?