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AS11.1-6 | Oxygen Therapy and Airway Management Foundations — PBL Case

CLINICAL SETTING

Mrs Kamala Devi, a 72-year-old retired schoolteacher with a background of moderate COPD (on home salbutamol inhaler, never required hospitalisation), presents to the emergency department at 11:30 PM accompanied by her daughter. Her daughter reports that she has had a productive cough with rusty-coloured sputum for five days, progressive breathlessness over two days, and has been increasingly confused for the past six hours. She was unable to speak in full sentences during the ambulance journey. On arrival: temperature 38.9°C, heart rate 122/min, blood pressure 94/58 mmHg, respiratory rate 32/min, SpO₂ 79% on room air, GCS 12 (E3V4M5). She is visibly using accessory muscles and has intercostal recession. Chest auscultation reveals coarse crepitations at the right base with dullness to percussion. She is able to open her eyes on calling her name and obeys simple commands but is clearly disoriented. Her weight is 68 kg; her daughter estimates her usual height at approximately 5 feet 4 inches.

Trigger 1: Initial Stabilisation: Choosing the Right Oxygen Device

The emergency nurse immediately places a non-rebreather mask (NRM) at 15 L/min. Within 10 minutes, Mrs Devi's SpO₂ rises to 98%. However, the emergency registrar, Dr Arjun, is concerned and orders an urgent ABG. The result returns: pH 7.30, PaCO₂ 62 mmHg, PaO₂ 118 mmHg, HCO₃⁻ 30 mmol/L, SaO₂ 98%. Dr Arjun immediately changes the oxygen delivery device. The registrar also asks the medical student to calculate Mrs Devi's CURB-65 score.

DISCUSSION POINTS

  • Why was Dr Arjun concerned despite the SpO₂ of 98%? What does the ABG result tell you about the risk of over-oxygenation in a COPD patient?
  • Which oxygen delivery device should replace the NRM, and at what initial setting? Justify your answer using the principle of fixed versus variable performance devices and the appropriate SpO₂ target for known COPD.
Click to reveal Trigger 2: Airway Compromise: Adjuncts and Escalation Decision (discuss previous trigger first!)

Trigger 2: Airway Compromise: Adjuncts and Escalation Decision

Mrs Devi is placed on a 28% Venturi mask with the flow rate set correctly. Over the next 90 minutes, her SpO₂ stabilises at 90% and her RR decreases to 24/min. However, at 1:15 AM the nurse calls urgently — Mrs Devi's GCS has fallen to 9 (E2V3M4), she is no longer following commands, and there is an audible gurgling sound with each breath. Her SpO₂ has dropped to 83%. The registrar asks you to assist with airway management while he prepares for possible intubation.

DISCUSSION POINTS

  • The registrar asks you to insert an airway adjunct immediately. Given Mrs Devi's current GCS of 9, which adjunct — oropharyngeal airway (OPA) or nasopharyngeal airway (NPA) — is more appropriate, and why? What anatomical and physiological principle governs this choice?
  • The registrar asks you to list the criteria that, if met, would mandate immediate endotracheal intubation. Which of these criteria does Mrs Devi currently meet? What are the risks of delaying intubation at this point?
Click to reveal Trigger 3: Intubation and Mechanical Ventilation Setup (discuss previous trigger first!)

Trigger 3: Intubation and Mechanical Ventilation Setup

The decision is made to intubate. Rapid sequence induction is performed and Mrs Devi is successfully intubated on the first attempt using direct laryngoscopy (Cormack-Lehane grade II view). Tube position is confirmed with continuous waveform capnography (ETCO₂ 38 mmHg, normal waveform) and bilateral chest rise. She is now connected to the ICU ventilator. The anaesthesiologist asks the team to propose initial ventilator settings, reminding the team that her IBW must be used — not her actual weight of 68 kg — and that she has COPD in addition to her acute pneumonia.

DISCUSSION POINTS

  • Calculate Mrs Devi's IBW and use it to determine a safe lung-protective tidal volume. What mode of ventilation would you choose initially, and what PEEP and FiO₂ would you set? Explain the physiological rationale for each setting, including why IBW must be used rather than actual body weight.
  • One hour after starting ventilation, the ABG shows: pH 7.25, PaCO₂ 70 mmHg, PaO₂ 95 mmHg, HCO₃⁻ 30 mmol/L, with plateau pressure 28 cmH₂O. The respiratory rate is currently 14/min. Which ventilator parameter would you adjust and why? What constraint prevents you from simply increasing tidal volume to clear CO₂?

Group Task Assignments

Group 1: Collaborative Task

  • Construct a structured escalation algorithm for airway and oxygen management in a deteriorating patient — from initial oxygen device selection through adjuncts to advanced airway. Include the decision criteria at each escalation step (e.g., SpO₂ threshold, GCS threshold, ABG values). Present as a flow diagram or structured table.

Group 2: Collaborative Task

  • Using the ventilator settings you proposed for Mrs Devi, calculate and present the minute ventilation. If the plateau pressure were to rise to 34 cmH₂O, outline the sequence of ventilator adjustments you would make, and explain the physiological rationale for each change.

Learning Issues

Research these questions and bring your findings to the discussion.

  1. [AS11.1] How does the mechanism of the Venturi mask differ from the non-rebreather mask? Why is the Venturi mask preferred for patients with COPD when precise FiO₂ delivery is required?
  2. [AS11.2] What is the recommended SpO₂ target range for known COPD patients receiving supplemental oxygen? What are the mechanisms by which excessive oxygen can worsen CO₂ retention in COPD?
  3. [AS11.3] When is jaw thrust preferred over head-tilt chin-lift as the initial airway opening manoeuvre? Describe the correct jaw thrust technique step by step.
  4. [AS11.4] What is the physiological basis for choosing NPA over OPA in a semi-conscious patient? What is the absolute contraindication to NPA insertion and why?
  5. [AS11.5] List the clinical criteria that mandate advanced airway management. How does waveform capnography confirm correct endotracheal tube placement, and why is it superior to other methods?
  6. [AS11.6] Why must tidal volume for mechanical ventilation be calculated using ideal body weight rather than actual body weight? What is the target plateau pressure for lung-protective ventilation, and what does an elevated plateau pressure indicate?