Describe the physiological consequences of breathing 100% oxygen at sea level.

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College Answer

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
effects
Syllabus: B2a, c
Recommended sources: Applied Respiratory Physiology, Nunn, Pages 265 to 268,
288 and 491 to 508

Discussion

 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. 

  • Effect on the control of ventilation
    • Hypoxic respiratory drive is essentially abolished at a PaO2 over 250 mmHg
  • Circulatory changes
    • 100% FiOis a weak systemic vasoconstrictor
    • 100% FiOis a weak pulmonary vasodilator
  • Effect on oxygen content of blood
    • Arterial blood content increases trivially, as most of the haemoglobin would have already been oxygenated (the increase will be by about 2ml/L)
    • Any hypoxemia which was due to "V/Q scatter" (lung units with V/Q less than 1.0) will be reversed
    • Any hypoxemia due to "true" shunt will remain unchanged
  • Effect on oxygen stores
    • The following changes will occur in the size of the stores of metabolically available oxygen:
       
    • Form of storage O2 stores on room air (ml) O2 stores after 100% FiO preoxygenation
      As gas in the lungs (FRC)  270 1825
      Bound to haemoglobin 820 910
      Bound to myoglobin 200 200
      Dissolved in tissue fluids 45 50
  • Adverse effects of (isobaric) oxygen include:
    • Drying of mucous membranes and inspissation of secretions
    • Inflammatory tracheobronchitis
    • Decreased central respiratory drive (minimally)
    • Hypecapnia in "CO2 retainers" mainly by virtue of V/Q mismatch and Haldane effect
    • Absorption atelectasis
    • Increased left-to-right shunting in ASDs
    • Increased peripheral vascular resistance
    • Cerebral and coronary vasoconstriction
    • Euphoria
    • Retrolental fibroplasia of the newborn
    • Decreased erythropoiesis

References

References

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.