Oxygen Toxicity

The most common complications are a consequence of the delivery systems. Plastic systems, oxygen masks and nasal cannulas are used, and all of these devices are skin irritants which can cause significant skin irritation and breakdown when used long term. Patients who are on long-term oxygen systems often try to prevent skin irritation by padding their delivery systems, such as by padding their nasal cannula behind the ears with nasal tissues. Other common areas of skin breakdown are across the bridge of the nose and beneath the nares.

Typically oxygen systems deliver oxygen that has nearly zero moisture content. When this oxygen passes through the mucous membranes in the mouth and nose, it is humidified by pulling moisture from the mucous membranes so it is humid by the time it reaches the alveoli. While this protects the alveoli and bronchioles, the nasal and oral mucous membranes quickly dry out. Dry mucous membranes lose their ability to humidify the air we breathe and also become uncomfortable. Applying oxygen via a humidifier can help prevent this from occurring.

Oxygen Toxicity

Recall from earlier in this article that under high oxygen environments, cells metabolize oxygen more quickly. This is because there is an increased pressure from the dissolved oxygen, the PaO₂, forcing oxygen into the cell, thereby increasing oxygen consumption and the production of the toxic oxygen molecule byproduct O₂^-. Since production of the enzyme to eliminate O₂^- is fixed, the toxic molecules build up over time.⁴ After roughly 24 hours of this oxygen-rich environment, enough toxic molecules accumulate to clinically see evidence of cellular damage.¹

An oxygen-rich environment is determined by looking at how much oxygen a patient receives. Delivering less than 60% oxygen to otherwise healthy lungs is generally considered a low oxygen delivery rate and typically is not associated with the development of clinical oxygen toxicity. However, diseased or injured lungs have been shown to develop symptoms of oxygen toxicity when receiving 50% oxygen or more.⁴

An early result of oxygen toxicity is capillary leakage, which leads to edema throughout the body, particularly pulmonary edema. Pulmonary edema generally appears first and when untreated can lead to acute lung injury and acute respiratory distress syndrome (ARDS).¹ Central nervous system symptoms include altered mental status, respiratory depression and seizures. When awake, some patients also experience visual and auditory disturbances.

Oxygen toxicity has been well documented since the early 1900s and still today remains clinically significant for patients on ventilator support, premature infants and patients receiving hyperbaric oxygen treatment.⁴ A detailed discussion of ventilator management is beyond the scope of this article. However, EMS is seeing a rise in patients being managed with hyperbaric oxygen and newborns are regularly born outside of the hospital setting.

Toxicity in Hyperbaric Medicine

Hyperbaric oxygen therapy is an important tool in modern medicine for management in a variety of situations including diving emergencies, wound management and carbon monoxide toxicity. Regardless of what hyperbaric medicine is being used to manage, its goal is to increase oxygen availability to organ tissues by increasing oxygen dissolved in the plasma through an increase in the atmospheric pressure. To illustrate this, administering 100% oxygen at sea level, or 1 atmospheric pressure, can produce a maximum PaO₂ of 510 mm Hg. By increasing the environment to 3 atmospheric pressures, PaO₂ can be increased to 1,530 mm Hg.⁴ This increase speeds healing by allowing tissues to have increased oxygen available for metabolism. Specifically in diving-related emergencies, hyperbaric medicine compresses nitrogen bubbles that may have formed in the patient’s body tissues to allow the body to more easily eliminate nitrogen that may cause pain (i.e., the bends) and emboli.

While hyperbaric oxygen has true benefits, there are legitimate dangers to its utilization as well. As stated above, hyperbaric oxygen increases oxygen available at the tissue level. Also recall from earlier that the more oxygen available, the faster the cell will metabolize oxygen, and over time this can lead to an accumulation of free oxygen radicals. At normal atmospheric pressures (1 atmosphere) this takes 12 to 16 hours of constant 100% oxygen exposure; this timeframe is reduced to 3 to 6 hours at 2 atmospheres.⁴ This is significant because the same valuable treatments can become dangerous; thus the utilization of hyperbaric oxygen must be closely monitored and controlled.