Respiratory Physiology, Mechanics of Ventilation and Gas Exchange — WACS Viva & Clinical Scenarios (Physiology and Applied Physiology)

Exam-style respiratory physiology, mechanics of ventilation and gas exchange viva scenarios with examiner probes and model answers for ANAESTHESIA candidates.

Scenarios covered

• SCENARIO 1: A 45-year-old female with morbid obesity (BMI 42 kg/m²) undergoes laparoscopic cholecystectomy under general anaesthesia with endotracheal intubation. Immediately following induction and peritoneal insufflation in the Trendelenburg position, her SpO₂ drops from 99% on FiO₂ 0.5 to 89%. Explain the physiological mechanisms governing the changes in Functional Residual Capacity, Closing Capacity, respiratory system compliance, and ventilation-perfusion matching that contribute to this acute desaturation.
• SCENARIO 2: Draw, label, and explain the normal static pressure-volume loop of the total respiratory system. Contrast this with the dynamic pressure-volume loop during mechanical positive pressure ventilation, identifying the lower and upper inflection points, hysteresis, and the physical determinants of both resistive and elastic work of breathing.
• SCENARIO 3: A 60-year-old male with chronic obstructive pulmonary disease is scheduled for emergency exploratory laparotomy. Describe the central and peripheral control of ventilation. Discuss the physiological mechanisms of hypoxic and hypercapnic ventilatory drive, the cellular transduction at the carotid body, and the physiological consequences of administering uncontrolled high-flow oxygen to this patient.
• SCENARIO 4: Write comprehensive notes on the transport of carbon dioxide in blood. Graph the carbon dioxide dissociation curve, contrast it with the oxygen-haemoglobin dissociation curve, and detail the physiological basis and clinical significance of the Haldane effect and the chloride shift (Hamburger phenomenon).
• SCENARIO 5: Define the anatomical, alveolar, and physiological dead space. State the Bohr equation and its Enghoff modification, including the underlying assumptions, mathematical derivation, and physiological factors that increase dead space under general anaesthesia.