Oxygen toxicity

Background

  • The harmful effects of breathing oxygen at higher partial pressures than normal
    • Partial pressure of O2 at sea level = 0.21 ATM
  • Toxicity based on both time of exposure and partial pressure of oxygen
    • Generally, FiO2 of 40% or less (0.40 ATM at sea level, breathing unpressurized air) can be tolerated indefinitely[1]
    • Most common in hyperbaric oxygen therapy, scuba divers and prolonged administration of normobaric supplemental oxygen
    • Pulmonary toxicity occurs sooner and at lower partial pressures than CNS toxicity[1], however there is no predictable pattern or sequence of symptoms for CNS toxicity (initial symptom may be seizure/coma)[2]

Diving Physiology

  • Pascals Law applies to the diving body (without air filled areas such as lungs) states that the pressure applied to any part of the enclosed liquid will be transmitted equally in all directions through the liquid.
  • Boyles Law applies to the diving body's air filled areas such as lungs, sinuses, middle ear, and states that the volume and pressure of a gas at a given temperature are inversely related.
    • At 2 ATA (10m/33ft) a given gas would be 1/2 it's volume, at 3 ATA (20m/66ft) it would be 1/3 it's volume and so on.
Boyle's Law
  • Dalton's Law applies to the total pressure of an ideal gas mixture being the sum of the partial pressures of each individual gas.
    • Divers may used Enriched Air NITROX mixtures to proportionally increase partial pressures of oxygen and reduce partial pressures of nitrogen while diving.
    • At extremes of depth, additional inert gasses such as helium in TRIMIX are used to further reduce partial pressures of both oxygen and nitrogen below toxic levels.
Dalton's Law
  • Henry's Law applies to the dissolvability of gasses into fluids, including body tissues, being proportional to the partial pressure of the gas.
    • The increased pressure at depth causes divers to breath their gas mix at increased pressure to defeat the external water pressure.
      • Increased inhaled partial pressures of nitrogen increase risk of nitrogen narcosis, and dissolved nitrogen in tissues re-expanding in micro-bubbles on ascent is the essential cause of decompression sickness. This can affect divers at any depth, including commonly-seen recreational diving depths of 20m/60ft or less.
      • Increased inhaled partial pressures of oxygen, generally beyond 1.4-1.6atm, increases risk of oxygen toxicity. This is typically not a substantial risk in common depths of recreational divers at 20m/60ft of depth or less, but can be for more advanced divers at deeper depths.

Clinical Features

Pulmonary

Central nervous system

Ocular

  • Retinopathy of prematurity (retrolental fibroplasia) - seen in premature infants
  • Hyperoxic myopia - seen in adults exposed to repeated toxic levels of oxygen[4]
    • Resolves spontaneously over several weeks

Differential Diagnosis

Diving Emergencies

Evaluation

  • Generally clinical

Management

  • Lower inhaled partial pressure of oxygen to as low as tolerated while maintaining tissue perfusion[5]

Disposition

  • Admit

See Also

External Links

References

  1. ↑ 1.0 1.1 Hedley-Whyte J. Pulmonary Oxygen Toxicity: Investigation and Mentoring. The Ulster Medical Journal. 2008;77(1):39-42.
  2. ↑ 2.0 2.1 Bitterman H. Bench-to-bedside review: Oxygen as a drug. Critical Care. 2009;13(1):205. doi:10.1186/cc7151.
  3. ↑ Thomson L, Paton J. Oxygen toxicity. Paediatr Respir Rev. 2014 Jun;15(2):120-3.
  4. ↑ Anderson B, Farmer JC. Hyperoxic myopia. Transactions of the American Ophthalmological Society. 1978;76:116-124.
  5. ↑ Deutschman, C. S., & Neligan, P. J. (2010). Evidence-based practice of critical care. Philadelphia, PA: Saunders/Elsevier.
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