Flying with a pneumothorax: a model of altitude limitations due to gas expansion.
BACKGROUND
Pneumothorax(PTX) is considered an absolute contraindication to flying. Guidelines for recovery time are arbitrary and fail to acknowledge that some passengers with PTX have flown without incident. One concern is pleural air expansion, causing extrinsic lung compression, increased intrathoracic pressure, and the subsequent risk of tension pneumothorax. We used a model to investigate critical endpoints resulting from PTX expansion at altitude.


METHODS
Pneumothorax expansion was investigated using physiological simulation in the form of a mathematical model comprising elastic lungs, rib cage, hemidiaphragms, mediastinum, and abdomen. Compliance curves were assigned to each compartment based on published data. Cyclical muscle pressures drive normal ventilation. Initial sea-level pleural air volumes were set in the range from 10 to 60% pneumothorax. Pressures, volumes, and mediastinal shift were tracked during ascent to cruising altitude at 8000 ft (2438 m) and during cabin depressurization to 30,000 ft (9144 m).


RESULTS
Pleural pressure oscillations during normal breathing became less negative during ascent. Positive pleural pressure was encountered at cabin altitude only if sea-level PTX exceeded 45%. Corresponding peak pressure gradient across the mediastinum did not exceed 5 cm H2O.


CONCLUSIONS
Our results provide insight into the mechanics of pneumothorax expansion during flight. Sea-level PTX up to 45% would be tolerable in otherwise healthy persons if positive intrathoracic pressure is the dominant mechanism causing respiratory discomfort. Critical limitation in our model is more likely due to hypoxemia caused by altitude and pulmonary shunt from lung collapse. Studies of PTX tolerance to altitude should be conducted with caution.
Department of Medicine, McMaster University, Hamilton, Ontario, Canada.
John
J
Fitz-Clarke
David
D
Quinlan
Rahim
R
Valani
D000337:Aerospace Medicine; D000531:Altitude; D006801:Humans; D008170:Lung Compliance / Q000502:physiology; D055641:Mathematical Concepts; D008954:Models, Biological; D011030:Pneumothorax / Q000503:physiopathology*; D015656:Respiratory Mechanics / Q000502:physiology*