Respiratory Monitoring, Capnography, Pulse Oximetry, and Blood Gas Instrumentation — WACS Viva & Clinical Scenarios (Clinical Measurement and Equipment)

Exam-style respiratory monitoring, capnography, pulse oximetry, and blood gas instrumentation viva scenarios with examiner probes and model answers for…

Scenarios covered

• SCENARIO 1: A 24-year-old male with severe extensive burn wounds is undergoing surgical debridement under general anaesthesia. Following induction, the dual-wavelength pulse oximeter shows a saturation (SpO₂) reading of 85% despite ventilation with 100% oxygen, with clear bilateral air entry and a normal capnograph trace. Arterial blood gas shows a PaO₂ of 450 mmHg. The co-oximeter reveals a methaemoglobin fraction of 32%. Explain the physical optical principles of dual-wavelength pulse oximetry, explain why SpO₂ plateaus at 85% in methaemoglobinaemia, and outline your pharmacological and ventilatory mana
• SCENARIO 2: A 42-year-old female is undergoing an emergency laparoscopic appendectomy. Five minutes following carbon dioxide pneumoperitoneum insufflation, the sidestream capnograph displays a classic 'shark-fin' waveform with an increasing Phase III slope and loss of the horizontal plateau. Simultaneously, peak airway pressure rises from 18 to 38 cmH₂O, and wheezing is audible on chest auscultation. Describe the phases and angles of a normal capnogram, explain the mechanics behind this pathological trace, and outline your immediate anaesthetic intervention.
• SCENARIO 3: During a 6-hour open thoracic aneurysm repair under moderate hypothermia (core temperature 30°C), an arterial blood gas (ABG) sample is drawn and analyzed at the standard blood gas machine temperature of 37°C. The uncorrected ABG reads: pH 7.40, PaCO₂ 40 mmHg, PaO₂ 150 mmHg. Explain the electrochemical physics of the Sanz, Severinghaus, and Clark electrodes, discuss the physical changes in gas solubility at 30°C vs. 37°C, and justify your choice between Alpha-stat and pH-stat acid-base management strategies.
• SCENARIO 4: A 30-year-old trauma victim is intubated in the emergency department and connected to an automated transport ventilator with a mainstream capnometer. Immediately after transfer to the theatre table, the capnograph trace suddenly drops to zero, and the exhaled tidal volume alarm sounds. Describe the physical differences between mainstream and sidestream capnometers, list your rapid differential diagnosis for an immediate flat-line capnogram, and describe your systematic clinical response.
• SCENARIO 5: During a prolonged spine surgery under sevoflurane anaesthesia, the inspired oxygen monitor reads 21% despite the rotameter showing a delivery setting of 50% O₂ and 50% Medical Air. You perform calibration checks on the oxygen sensor. Explain the physical operating principles of galvanic (fuel cell) versus paramagnetic oxygen analyzers, identify potential sensor failure mechanisms, and formulate your intraoperative action plan.