OXYGEN

Why is Oxygen considered toxic?

• High inspired oxygen concentrations cause toxicity by causing formation of oxygen free radicals (which damage tissues), and by causing absorption atelectasis and V/Q mismatch.

The issue of oxygen toxicity has been topical for a generation, following the discovery that therapeutic oxygen causes blindness in premature babies (retrolental fibroplasias) with respiratory distress syndrome. In addition, it has been established that high inspired concentrations of oxygen may cause acute lung injury, probably due to oxygen free radical production – superoxide, hydroxyl, hydrogen peroxide and singlet O₂ molecules. These agents damage biomolecules such as membrane lipids, enzymes and nucleic acids. The extent of injury appears to depend on 1. The FiO₂, 2. The duration of exposure, 3. The barometric pressure under which exposure occurred. It appears that the critical FiO2 for toxicity is around 50% (1), above which lung recruitment maneuvers should be condidered (CPAP).

High concentrations of inspired oxygen may cause absorption atelectasis. In addition high FiO₂ may cause increased peripheral vascular resistance in congestive heart failure leading to reduced cardiac output.

How much oxygen is safe is a moot point. It is more important that you do not withhold life saving oxygen therapy than to be concerned about oxygen toxicity. It is, nonetheless, important that FiO₂ is minimized to normalization of blood gas in intensive care patients: i.e. there is little to be gained in having a PaO₂ of greater than 100mmHg. Often elevated oxygen requirements can be compensated for by appropriate patient positioning and increasing mean airway pressures to improve matching of ventilation and perfusion.

References

   (1)    Register SD, Downs JB, Stock MC, Kirby RR. Is 50% oxygen harmful? Crit Care Med 1987; 15(6):598-601.

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