dioxygen

Adverse Effects of Oxygen. Although it is true that all living organisms require oxygen to maintain life, an environment of 100 per cent oxygen inhibits growth of living tissue cultures, and laboratory experiments have shown that hyperoxygenation of body tissues can cause irreversible damage. It is known that high concentrations of inhaled oxygen can result in collapse of alveoli because of displacement of nitrogen by oxygen. retinopathy of prematurity in premature infants was found to be caused in part by excessively high levels of oxygen in the blood.

Another serious complication of high-oxygen concentration therapy is the development of a hyaline membrane because of a deficiency of pulmonary surfactant; surfactant is vitally important to normal expansion and deflation of the alveoli. Prolonged exposure to inspired oxygen concentrations in excess of 50 per cent can impair the production of this surfactant in a patient of any age. The result is a loss of lung compliance and reduction of the transport of oxygen across the alveolar membrane.

The danger of oxygen toxicity can be minimized by careful assessment of each patient's need for oxygen therapy and systematic blood gas analysis to determine patient response and effectiveness of treatment. Symptoms of oxygen toxicity are substernal distress, nausea and vomiting, malaise, fatigue, and numbness and tingling of the extremities.

Indications for Oxygen Therapy. In general, the clinical situations in which the administration of supplemental oxygen is indicated are: (1) Profound but potentially reversible hypoxia that appears amenable to the short-term administration of high concentrations of oxygen. Examples would include the patient who is apneic, is suffering from cardiovascular collapse, or is a victim of carbon monoxide poisoning. (2) Conditions in which there is a need to reduce the work load of the cardiovascular and pulmonary systems and at the same time assure an adequate supply of oxygen to the tissues. Congestive heart failure, myocardial infarction, and such acute pulmonary diseases as pulmonary embolism and pneumonia are examples of the types of clinical situations that are best treated by the administration of moderate levels of oxygen concentration. (3) Evidence of hypoventilation, whether from anesthesia and sedation, chronic obstructive pulmonary disease, or other conditions. The patient who is hypoventilating is in danger of suffering from an adverse effect of oxygen therapy because increased oxygenation can lead to decreased respiratory effort. In other words, the oxygen acts as a respiratory depressant and may produce an increase in partial pressure of carbon dioxide in the arterial blood, thus contributing to rather than overcoming the problem of hypoxia. If there is evidence that the patient is hypoventilating, it may be necessary to administer the oxygen by assisted or controlled ventilation.

The delivery of appropriate and effective oxygen therapy requires frequent monitoring of arterial blood gases. An initial blood gas analysis at the time the therapy is started provides baseline data with which to evaluate changes in the patient's status.

In addition to monitoring blood gases to assess the patient's need for and response to supplemental oxygen, it is helpful to observe the patient closely for signs of hypoxemia. However, these signs are not as reliable as blood gas analysis because the clinical manifestations of hypoxemia vary widely in individual patients. The typical clinical manifestations of hypoxemia are confusion, impaired judgment, restlessness, tachycardia, central cyanosis, and loss of consciousness.

Dosage and Method of Administration. It must be kept in mind that oxygen is considered a drug and should be prescribed and administered as such; thus it is apparent that vague orders about its administration are never acceptable. There must be specific written orders for flow rate and mode of administration. Decisions about the initial dosage, as well as any changes in mode of administration and dosage, including the discontinuance of oxygen therapy, should be based on evaluation of the P_O2, the P_CO2, and the blood pH. (See also transcutaneous oxygen monitoring and pulse oximeter.)

The clinical signs and symptoms of hypoxemia may vary from patient to patient, and they should not be depended upon as valid indications of oxygen insufficiency. This is especially true of cyanosis, a symptom that depends on local circulation to the area, the red cell count, and hemoglobin level. In addition to the data obtained from blood gas analyses, an oxygen analyzer should be used occasionally to check inspired oxygen concentration.

In general, the dosage and mode of administration fall into the following categories. High concentrations above 50 per cent usually are prescribed when there is a need for the delivery of high levels of oxygen for a short period of time to overcome acute hypoxemia, as in cardiovascular failure and pulmonary edema. The flow rate may be as high as 12 liters per minute, administered through a close-fitting face mask with or without a rebreathing bag, or via an endotracheal tube.

Moderate concentrations of oxygen are indicated when the patient is suffering from impaired circulation of oxygen, as in congestive heart failure and pulmonary embolism, or from increased need for oxygen, as in thyrotoxicosis, in which the increased metabolic rate creates a need for more oxygen. The rate of flow should be 4 to 8 liters per minute, administered through an air entrainment mask that delivers concentrations above 23 per cent, or in a dosage of 3 to 5 liters per minute through a nasal cannula.

Low concentrations of oxygen are indicated when the patient is receiving oxygen therapy over an extended period of time, as in chronic obstructive pulmonary disease, and there is the possibility of hypoventilation and the danger of increased CO₂ retention. The rate of flow should be 1 to 2 liters per minute, administered through a nasal cannula, or via an air entrainment mask that delivers 24 to 35 per cent oxygen.

Other methods of oxygen administration include the nasal catheter and the oxygen tent. The nasal catheter can cause some discomfort to the patient, and since it is no more and no less effective than the cannula, most therapists and patients prefer not to use it. The oxygen tent is considered by many to be obsolete, its use being limited to the administration of oxygen to children who cannot or will not tolerate other modes of delivery, and to children in whom the objective is to provide oxygen and humidity or humidity alone.

Patient Care. No matter what mode of administration is used, it is essential that the inspired air be moisturized. This is necessary to prevent drying of the respiratory mucosa and thickening of secretions that can further inhibit the flow of air through the air passages. Humidity may be provided by humidifying the oxygen with water, or by aerosoling the water into fine particles and adding it to the oxygen. Most patients need 60 to 65 per cent relative humidity at room temperature. Patients with endotracheal tubes require as close to 100 per cent humidity as possible.

Oxygen is not an explosive gas, but it does support combustion and presents a serious fire hazard. All electrical equipment should be checked for defects that could produce sparks. All appliances that transmit house current must be kept outside an oxygen tent, and all equipment with exposed switches and meters must be considered potential sources of fire. Static electricity is a minimal risk which can be further reduced by maintaining a relatively high humidity in the oxygen tent. Smoking in the immediate area of oxygen administration is prohibited and there should be signs informing visitors and others of this restriction.

When the patient is wearing a mask for an extended period of time, discomfort can be minimized by removing the mask and washing and drying the face at least every eight hours. To be effective the mask must fit snugly and follow the contour of the face. This means that reddened areas will appear where the mask has pressed against the skin. These areas should be gently massaged and the skin lightly powdered to reduce friction.

A program of infection control is especially important in the prevention of cross-infection from the equipment that is used to administer oxygen. Humidifiers and nebulizers may serve as sources of infection because they provide a medium for the growth of bacteria and molds. There is less danger of this happening when disposable equipment is used, but this does not preclude the need for a systematic development of policies and procedures to prevent and control the spread of infection. Every person involved in the care of the patient must be aware of this program and cooperate in its implementation.