Physiological effects of hyperoxia

This chapter is related to the aims of Section F10(ii) from the 2023 CICM Primary Syllabus, which expects the exam candidate to "explain the physiological effects of hyperoxia".  The college have asked about this in a very round-about way (Question 1 from the first paper of 2011) which was actually more about oxygen stores and preoxygenation. Though the chapter on the pharmacology of oxygen does cover this material to some substantial degree, a whole separate page dedicated to this topic seemed appropriate because it has its own syllabus entry (shared with hypoxia hypocapnia and hypercapnia), and also because the chapter on the pharmacology of oxygen takes the form of a disorganised rant, and for an exam answer some sort of an organised summary of hyperoxia needs to exist.

In terms of the aforementioned organisation, it's hard to structure information like this because the effects of increasing oxygen exposure are often dose-dependent and specific in each organ. Therefore, the best one can do is to organise the discussion by organ systems, and then by oxygen concentration. The latter can be separated into normobaric and hyperbaric, which seems like a useful distinction: some ill effects of oxygen can be seen when FiO2 is increased to 100% at normal atmospheric pressure, whereas others can only be seen in patients subjected to supranormal pressures. Anyway, in summary a table can be produced to neatly categorise these issues, which satisfies the author's unnatural lust for tables:

Physiological Effects of Hyperoxia
Organ system  or  tissue Effects seen with normobaric hyperoxia Effects seen only with hyperbaric hyperoxia 
  • Tracheobronchitis
  • Mucositis
  • Alveolar toxicity (Lorrain Smith effect)
  • Absorption atelectasis
  • Decreased respiratory drive
  • Pulmonary vasodilation
  • Accelerated alveolar toxocity, transitioning to frank ARDS
Gas carriage
  • Reverse Haldane effect: release of CO2 from haemoglobin
  • Increased clearance of CO
  • Increased denitrogenation of gas cavities (eg. pneumothorax)
  • With hyperbaric oxygen, dissolved O2 contributes significantly to the total gas transport (i.e. you may not even need haemoglobin)
Cardiovascular system
  • Vasoconstriction due to accelerated rate of oxidative degradation of nitric oxide in the endothelium.
  • Hypertension
  • Bradycardia (reflex)
  • Decreased cardiac output
Central nervous system
  • Mild euphoria
Cerebral circulation
  • Decreased cerebral blood flow
  • Decreased intracranial pressure (30%) and decreased cerebral blood flow (19%) - Miller et al, 1970
  • Minimal effect
  • Blood glucose decreases
  • Serum sodium decreases
  • Potassium increases
  • Increased free radical production
  • Inhibition of enzymes with SH1 groups 
Bone marrow
  • Impaired erythropoiesis
  • Decreased reticulocyte count
Immune system
  • Immunosuppressant effect
  • Increased immunosuppressant effect: decreased circulating lymphocyte count and decreased spleen weight (Hansbrough et al, 1980)
Infectious disease
  • Impaired reproduction of anaerobes
  • Toxicity to anaerobes
  • Decreased toxin synthesis by clostridial species

In terms of references, nobody covers normobaric hyperoxia better than Bruginaux et al (2018). Their article is comprehensive, free online and easy to read. There is also Jain (2017) for hyperbaric oxygen, which seems ideally suited for this topic, and is free from 


Mach, William J., et al. "Consequences of hyperoxia and the toxicity of oxygen in the lung." Nursing research and practice 2011 (2011).

Brugniaux, Julien Vincent, et al. "Highs and lows of hyperoxia: physiological, performance, and clinical aspects." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 315.1 (2018): R1-R27.

Yorgancilar, Ediz, et al. "Effects of hyperbaric oxygen therapy on rat nasal mucosa." Biotechnology & Biotechnological Equipment 26.6 (2012): 3394-3396.

Tibbles, Patrick M., and John S. Edelsberg. "Hyperbaric-oxygen therapy." New England Journal of Medicine 334.25 (1996): 1642-1648.

Miller, J. D., I. McA Ledingham, and W. B. Jennett. "Effects of hyperbaric oxygen on intracranial pressure and cerebral blood flow in experimental cerebral oedema." Journal of Neurology, Neurosurgery & Psychiatry 33.6 (1970): 745-755.

Hansbrough, John F., Joseph G. Piacentine, and Ben Eiseman. "Immunosuppression by hyperbaric oxygen." Surgery 87.6 (1980): 662-667.

Leach, R. M., P. J. Rees, and P. Wilmshurst. "Hyperbaric oxygen therapy.Bmj 317.7166 (1998): 1140-1143.

Jain, Kewal K. "Physical, physiological, and biochemical aspects of hyperbaric oxygenation." Textbook of hyperbaric medicine. Springer, Cham, 2017. 11-22.