Gas Laws, Fluid Dynamics, Pressure Mechanics, and Flowmeters — WACS Viva & Clinical Scenarios (Physics in Anaesthesia)

Exam-style gas laws, fluid dynamics, pressure mechanics, and flowmeters viva scenarios with examiner probes and model answers for ANAESTHESIA candidates.

Scenarios covered

• SCENARIO 1: A 45-year-old male with severe chronic obstructive pulmonary disease is undergoing laparotomy under general anaesthesia. Following tracheal intubation with a 7.0 mm internal diameter tube, peak inspiratory airway pressure reaches 42 cmH₂O with an expiratory wheeze. The ventilator delivers gas with high peak flow rates. How do the Hagen-Poiseuille equation and Reynolds number explain this clinical picture, and how will you physically adjust the ventilation and delivery apparatus to reduce airway resistance?
• SCENARIO 2: During an elective hernia repair under general anaesthesia in a hot, non-air-conditioned district hospital theatre, the anaesthesia resident notes that the rotameter bobbin for oxygen is stuck halfway up the tube, reading 3 L/min despite the control needle valve being fully closed. How does the physics of a variable-orifice variable-height flowmeter explain this malfunction, and what are the immediate anaesthetic safety actions?
• SCENARIO 3: A 28-year-old female in septic shock secondary to ruptured appendicitis requires rapid volume resuscitation. Peripheral intravenous access consists of a single 20-gauge cannula in the dorsum of the hand. How do fluid dynamic principles dictate flow rates through cannulae of varying lengths and internal diameters, and what physical modifications will maximise crystalloid infusion velocity?
• SCENARIO 4: A 60-year-old male with severe subglottic stenosis is scheduled for micro-laryngoscopy. The surgical team requests high-frequency jet ventilation. What physical principles (Bernoulli, Venturi, and Coanda effects) govern gas entrainment and tissue injury during jet ventilation, and how do you configure your anaesthetic technique to avoid barotrauma?