Vaporizers, Saturated Vapour Pressure, Heat, and Humidification Physics — WACS Viva & Clinical Scenarios (Physics in Anaesthesia)

Exam-style vaporizers, saturated vapour pressure, heat, and humidification physics viva scenarios with examiner probes and model answers for ANAESTHESIA…

Scenarios covered

• SCENARIO 1: An anaesthesia resident accidentally fills an empty Isoflurane-calibrated variable-bypass vaporizer with Halothane, and another resident fills an empty Sevoflurane vaporizer with Isoflurane. Based on the saturated vapour pressures and boiling points of these agents, explain using physical principles whether the delivered concentration will be higher or lower than the dialled setting in each case, and detail the mathematical relationship involved.
• SCENARIO 2: In a tropical rural operating theatre where the air conditioner has broken down, the ambient room temperature rises to 34°C. You are using a standard variable-bypass Isoflurane vaporizer (Tec 5). Explain the physical mechanism by which the vaporizer compensates for this elevated temperature, the role of specific heat capacity of the vaporizer body, and the failure threshold where temperature compensation breaks down.
• SCENARIO 3: A 68-year-old male is undergoing an open radical cystectomy under general anaesthesia with Desflurane. Explain why Desflurane cannot be delivered safely using a conventional unheated variable-bypass vaporizer at room temperature, describe the inner physical mechanisms of the Tec 6 vaporizer, and explain what happens to the delivered partial pressure of Desflurane if you operate this vaporizer at high altitude.
• SCENARIO 4: A 2-year-old child undergoing a 4-hour craniotomy under mechanical ventilation is fitted with an adult-sized Heat and Moisture Exchanger (HME) filter placed at the tracheal tube connector. Explain the physical mechanism of hygroscopic vs hydrophobic HMEs, evaluate the physical impact of this filter on dead space, airway resistance, and humidity, and justify the correct humidification strategy for this child.