Medical Gas Supply, Cylinders, Regulators, and Oxygen Concentrators — WACS Viva & Clinical Scenarios (Physics in Anaesthesia)

Exam-style medical gas supply, cylinders, regulators, and oxygen concentrators viva scenarios with examiner probes and model answers for ANAESTHESIA candidates.

Scenarios covered

• SCENARIO 1: You are preparing to transfer an intubated, mechanically ventilated 32-year-old trauma patient with acute respiratory distress syndrome from the theatre to an intensive care unit 45 minutes away. You have a size E oxygen cylinder with a gauge pressure reading of 100 bar. The patient requires a minute ventilation of 8 L/min and an FiO₂ of 1.0 via a transport ventilator driven by oxygen. Using physical principles, calculate the volume of oxygen available, determine if this cylinder is adequate for the journey, and justify your safety buffer.
• SCENARIO 2: In a regional district hospital in West Africa, the main oxygen pipeline is supplied by an on-site Pressure Swing Adsorption (PSA) oxygen concentrator plant. During a prolonged craniotomy, the inspired oxygen fraction (FiO₂) measured by the anaesthetic machine cannot exceed 0.93 despite setting the rotameter to 100% oxygen. Explain the physical mechanism of zeolite molecular sieves, explain why the FiO₂ plateaus at 93% to 95%, and assess whether this poses any physiological risk to the patient.
• SCENARIO 3: During a night shift, an emergency caesarean section is underway under general anaesthesia. The anaesthetic machine oxygen supply failure alarm sounds, the pipeline pressure gauge drops to zero, and the nitrous oxide supply is automatically cut off. Outline the step-by-step physical and clinical actions you will take to maintain oxygenation, depth of anaesthesia, and patient safety.
• SCENARIO 4: An Entonox cylinder (50% oxygen, 50% nitrous oxide) stored outdoors in harmattan night conditions where temperature dropped to 0°C is brought into the labour ward for an active labouring parturient. Explain the physical concept of the pseudocritical temperature, the Poynting effect, what physical changes occur within the cylinder, and the lethal hazard if used immediately.