Oxygen Supply Calculation and a COPD Patient at Altitude
You are transporting a 72-year-old man with a COPD exacerbation and pneumonia. On the ground he is on a non-rebreather at 10 L/min with an SpO2 of 94%, a respiratory rate of 28, and a baseline PaCO2 of 58 mmHg. The flight is 55 minutes in a pressurized aircraft with a cabin altitude of 6,000 ft, plus about 15 minutes of ground and taxi time, plus a required 15-minute reserve. The aircraft has no built-in oxygen system. You have three D-cylinders, each rated at 350 L when full at 2,016 psi. The gauge readings are 1,900 psi, 1,150 psi, and 700 psi.
1. Calculate whether the oxygen supply is adequate. Show your work.
Read the worked answer
Do the arithmetic before you load.
Available liters = 350 L × (psi ÷ 2,016)
- Cylinder 1: 350 × (1,900 ÷ 2,016) ≈ 330 L
- Cylinder 2: 350 × (1,150 ÷ 2,016) ≈ 200 L
- Cylinder 3: 350 × (700 ÷ 2,016) ≈ 122 L
- Total ≈ 652 L
Time to cover = 55 min flight + 15 min ground/taxi + 15 min reserve = 85 min
At 10 L/min: 85 × 10 = 850 L required
652 L available against 850 L required - inadequate, and that is before accounting for altitude. At a 6,000 ft cabin altitude his alveolar oxygen partial pressure falls, so 10 L/min that produces 94% at sea level will probably not hold 94% at altitude. The true requirement is higher than the arithmetic, not lower.
A supply plan that only works if nothing changes is not a plan.
2. What are your options, and in what order do you apply them?
Read the worked answer
Solve it in this order: patient target, then device, then supply, then aircraft.
- Reassess the target. He is a chronic CO2 retainer with a baseline PaCO2 of 58. An SpO2 target of 88–92% is acceptable and arguably safer for him than 94%. A non-rebreather at 10 L/min is not a titration device.
- Change the device, not just the flow. A controlled Venturi mask at 28–35% uses roughly 4–8 L/min. At 6 L/min the requirement becomes 85 × 6 = 510 L, which fits inside 652 L with margin for a small increase at altitude. Recheck saturation after every change and document the target.
- Add supply if you can. A full D-cylinder from the referring hospital, verified in test date, converts a marginal plan into a comfortable one.
- Change the aircraft or the altitude. A lower cabin altitude reduces physiologic demand and makes the lower flow defensible.
Do not launch on 652 L at 10 L/min because it will 'probably be fine.'
3. Thirty minutes into the flight his SpO2 is 97%, he is drowsy and difficult to rouse, and his respiratory rate has fallen to 14 with shallow effort. What is your interpretation and what do you do?
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A falling respiratory rate with a good saturation is more alarming than a low saturation.
The picture - 97% on oxygen, drowsy, shallow at 14 - is most consistent with hypercapnia from over-oxygenation in a chronic retainer, possibly compounded by fatigue and altitude. Hypoxia is not the explanation; his saturation is the best it has been all day.
What to do:
- Titrate down, not up. Bring the saturation toward 88–92% with a controlled device.
- Exclude the mimics: glucose, a silent aspiration, and the ETCO2 if you carry capnography. A rising ETCO2 supports the diagnosis.
- Support ventilation. Coach him, sit him up, have the BVM ready, and have a plan for advanced airway support if his effort continues to fall. Do not assume he will simply wake up.
- Tell the receiving center so that a blood gas and NIV capability are waiting at the door.
The reflex to increase oxygen when a patient looks worse is exactly wrong here. The reasoning to defend is target range, device, and trend.