Cabin Heat, a Headache and a Pulse Oximeter Over the Lindis Pass
Mid-winter, 10,500 ft over the Lindis Pass in a Cessna T182T, two on board, cabin heat fully on. The rear-seat passenger complains of a throbbing headache, nausea and dizziness, and looks pale. You have a mild headache that you put down to turbulence and a long day. The passenger's pulse oximeter reads 90% and yours reads 88%, both after several minutes of quiet breathing. You are 25 minutes from Queenstown. The aircraft's heater muff was inspected at the last 100-hour inspection and no exhaust cracks were found, but the cabin has smelt faintly 'exhausty' since run-up. Conditions at altitude are smooth, so turbulence is not an explanation for the symptoms.
1. What is the most likely diagnosis, and how would you distinguish it from altitude hypoxia or hyperventilation in this aircraft?
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Carbon monoxide poisoning is the leading diagnosis. The combination is characteristic: two occupants with the same headache, nausea, dizziness and pallor; a combustion heater in use; a faint exhaust smell; and no mechanical explanation such as turbulence. CO binds haemoglobin with an affinity hundreds of times that of oxygen and shifts the dissociation curve left, so oxygen delivery falls even when saturation looks acceptable. At 10,500 ft there is also mild altitude hypoxia, but it would not normally produce a throbbing headache and nausea in a resting passenger while the pilot remains functional.
Hyperventilation is less likely: it produces perioral tingling, carpopedal spasm and light-headedness, usually in one anxious person, and is not shared by both occupants. Altitude hypoxia alone at 10,500 ft might give saturations around 88–92%, but the symptoms are disproportionate and both people are affected simultaneously. The presence of an exhaust smell plus a heater in use tips the balance decisively to CO. Treat the aircraft as contaminated until proven otherwise.
2. What immediate actions would you take, and why does the pulse oximeter reading not reassure you?
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Immediate actions: shut off the cabin heat and any other combustion source, open fresh-air vents, descend to the lowest safe altitude (well below 10,000 ft, ideally 8,000 ft or lower), use 100% oxygen if carried, and land at the nearest suitable aerodrome. Declare a PAN with ATC, state suspected CO poisoning and the number of people affected, and request medical support on arrival. Do not continue to the planned destination while symptoms persist.
The oximeter cannot distinguish carboxyhaemoglobin from oxyhaemoglobin; it reports both as 'saturated' haemoglobin. A reading of 88–90% may therefore be falsely reassuring and does not exclude significant CO levels. The clinical picture and the shared symptoms matter more than the number. Treat the patient, not the display: if symptoms and circumstances fit CO, act as though it is CO.
3. Describe the post-flight medical, maintenance and reporting actions required.
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Both occupants should be assessed by a medical practitioner, with a blood carboxyhaemoglobin level if CO is suspected; 100% oxygen is continued until symptoms resolve or until advised otherwise, and hyperbaric therapy may be considered for significant exposure. Neither pilot should fly again that day; cognitive effects can persist after the headache resolves.
The aircraft must not be flown until the heating and exhaust system has been inspected by an engineer. The heater muff, exhaust stacks, shroud and cabin ducting should be checked for cracks, staining, loose clamps and sealant failure; the presence of an exhaust smell is itself a defect. The operator and the CAA should be notified under the applicable occurrence-reporting requirements, and the event recorded in the maintenance log and safety management system. Post-flight reporting is what stops the next crew having the same event.