Describe the physiological consequences of breathing 100% oxygen at sea level.
The question related to physiological changes occurring when FiO2=1 Many
candidates focused on the toxic effects of oxygen, which were often incorrect (CNS
symptoms will not occur at one atmosphere). Candidates simply lacked knowledge,
those that did have some understanding failed to provide adequate detail (ie. it was
occasionally mentioned oxygen stores are increased but not the mechanism by
which or extent to which stores are increased).
In addition it was expected that candidates would outline and describe the
mechanism behind the changes in PaO2 in arterial and mixed venous blood, shift in
CO2 ventilation, hypoxic pulmonary vasoconstriction as well as pulmonary toxic
Syllabus: B2a, c
Recommended sources: Applied Respiratory Physiology, Nunn, Pages 265 to 268,
288 and 491 to 508
Judging from their comments, the examiners were looking for an answer to the question, "what are the physiological consequences of increasing a patient's FiO2 from room air to 100%, at sea level atmospheric pressure?"), and so it is puzzling why they decided to word it differently. The pass rate of 0% suggests that this added level of complexity did nothing to discriminate good trainees from bad. The fact that this question has never appeared again in future papers suggests that the examiners performed some sort of item analysis and came to the same conclusion.
|Form of storage||O2 stores on room air (ml)||O2 stores after 100% FiO2 preoxygenation|
|As gas in the lungs (FRC)||270||1825|
|Stored in blood||
Hb-bound: 805 ml
Dissolved: 15 ml
Total: 820 ml
Hb-bound: 810 ml
Dissolved: 100 ml
Total: 910 ml
|Bound to myoglobin||200||200|
|Dissolved in non-blood tissue fluids||45||50|
The numbers for the oxygen stores are from Kerry Brandis' The Physiology Viva, because that's probably where the examiners will naturally go to fish for ideas when they are writing their CICM First Part questions.
O'Driscoll, B. R., et al. "BTS guideline for oxygen use in adults in healthcare and emergency settings." Thorax 72.Suppl 1 (2017): ii1-ii90.
Samaja, Michele. "Prediction of the oxygenation of human organs at varying blood oxygen carrying properties." Respiration physiology 72.2 (1988): 211-217.
Cherniack, NEIL S., and G. S. Longobardo. "Oxygen and carbon dioxide gas stores of the body." Physiological reviews50.2 (1970): 196-243.
ROUGHTON, Francis John Worsley, and John Cowdery KENDREW. "Haemoglobin." Haemoglobin. (1949).
RAHN, HERMANN. "Oxygen stores of man." Oxygen in the Animal Organism. 1964. 609-619.
Farhi, L. E., and H. Rahn. "Gas stores of the body and the unsteady state." Journal of Applied Physiology 7.5 (1955): 472-484.
Campbell, I. T., and P. C. W. Beatty. "EDITORIAL II: Monitoring preoxygenation." British journal of anaesthesia 72.1 (1994): 3.