Anaesthetic Breathing Systems, Carbon Dioxide Absorption, and Waste Gas Scavenging — WACS Viva & Clinical Scenarios (Clinical Measurement and Equipment)

Exam-style anaesthetic breathing systems, carbon dioxide absorption, and waste gas scavenging viva scenarios with examiner probes and model answers for…

Scenarios covered

• SCENARIO 1: A 4-year-old child weighing 16 kg is scheduled for repair of a congenital umbilical hernia. You choose to use a Mapleson F (Jackson-Rees modification of Ayre's T-piece) breathing system for spontaneous and subsequent controlled ventilation. Describe the physical classification of this circuit, calculate the required fresh gas flow rates for both phases, and discuss how you minimize apparatus dead space and airway resistance.
• SCENARIO 2: During a 5-hour pelvic exenteration under circle system general anaesthesia with sevoflurane at a fresh gas flow of 1.0 L/min, you observe that the end-tidal carbon dioxide trace does not return to zero during inspiration (inspired CO₂ reads 8 mmHg). The soda lime in the transparent canister shows a purple discoloration covering two-thirds of the volume. Detail your immediate diagnostic differentiation and intraoperative management steps.
• SCENARIO 3: A 32-year-old male is undergoing an open reduction and internal fixation of a fractured femur under general anaesthesia with controlled mechanical ventilation using a Bain (Mapleson D) co-axial circuit. The capnograph demonstrates marked elevation of baseline CO₂. You suspect an inner tube disconnection. Outline your systematic testing of the Bain circuit (including the Pethick test) and management of this intraoperative failure.
• SCENARIO 4: A 50-year-old female undergoes laparoscopic cholecystectomy under low-flow (0.5 L/min) anaesthesia using an automated circle system. After three hours, the pulse oximeter reads a declining oxygen saturation (SpO₂ 91%), and arterial blood gas reveals an elevated carboxyhaemoglobin level (COHb 18%). Detail the chemical and physical mechanism responsible for this event, the role of absorbent hydration, and your therapeutic interventions.
• SCENARIO 5: During an audit of occupational safety in a tertiary hospital theatre block, you observe that the active waste anaesthetic gas scavenging system (WAGS) reservoir bag is fully distended and taut, and several patients experience unexplained positive end-expiratory pressure (auto-PEEP) and hemodynamic compromise during expiration. Explain the pneumatic mechanics of WAGS, identify the point of failure, and outline remediation steps.