Breathing Systems, Carbon Dioxide Absorption, and Ventilator Mechanics — WACS Viva & Clinical Scenarios (Physics in Anaesthesia)

Exam-style breathing systems, carbon dioxide absorption, and ventilator mechanics viva scenarios with examiner probes and model answers for ANAESTHESIA…

Scenarios covered

• SCENARIO 1: A 4-year-old child weighing 16 kg is scheduled for repair of an umbilical hernia. You select a Mapleson F (Jackson-Rees modification of Ayre's T-piece) breathing circuit for spontaneous breathing during induction followed by controlled ventilation. Using physical principles, explain why the Mapleson A system is the most efficient for spontaneous breathing while the Mapleson D/F system is preferred for controlled ventilation, and calculate the exact fresh gas flow required for this child under both conditions.
• SCENARIO 2: You are preparing to anaesthetize a patient using a coaxial Bain system (Mapleson D). Describe step-by-step how you perform Pethick's test, explain the physical principles (Bernoulli effect and pressure differential) that make this test work, and explain what happens to inspired gases if the inner tube is detached or fractured.
• SCENARIO 3: During a prolonged laparoscopic colectomy under low-flow anaesthesia (FGF 0.7 L/min) using a circle system, the end-tidal carbon dioxide trace fails to return to baseline during inspiration (inspired CO₂ reads 8 mmHg), and the soda lime canister feels hot to the touch with a deep violet colouration. Describe the chemical reactions of CO₂ absorption, explain the physical reason for heat generation, outline why ethyl violet changes colour, and detail your immediate management.
• SCENARIO 4: A newly installed operating theatre ventilator has descending (hanging) bellows instead of ascending (standing) bellows. Explain the physical differences between ascending and descending bellows, explain why descending bellows represent a critical hazard during accidental circuit disconnection, and outline the mechanical safeguards required.