Monitoring and titration are required to meet the therapeutic goal of oxygen therapy and to avoid complications and side effects. Although cyanosis is a physical finding of substantial clinical importance, it is not an early, sensitive, or reliable index of oxygenation. Noninvasive monitoring of arterial oxygen saturation now is widely available from transcutaneous pulse oximetry, in which oxygen saturation is measured from the differential absorption of light by oxyhemoglobin and deoxyhemoglobin and the arterial saturation determined from the pulsatile component of this signal. Pulse oximetry measures hemoglobin saturation and not PO2; thus, it is insensitive to increases in PO2 beyond that required to saturate the blood fully. However, pulse oximetry is very useful for monitoring the adequacy of oxygenation during procedures requiring sedation or anesthesia, rapid evaluation and monitoring of potentially compromised patients, and titrating oxygen therapy in situations where toxicity from oxygen or side effects of excess oxygen are of concern.
CHAPTER 15 Therapeutic Gases: O2, CO2, NO, and He 257 COMPLICATIONS OF OXYGEN THERAPY
Administration of supplemental oxygen is not without potential complications. In addition to the potential to promote absorption atelectasis and depress ventilation, high flows of dry oxygen can dry out and irritate mucosal surfaces of the airway and the eyes, as well as decrease mucociliary transport and clearance of secretions. Humidified oxygen thus should be used when prolonged therapy (>1 hour) is required. Finally, any oxygen-enriched atmosphere constitutes a fire hazard, and appropriate precautions must be taken both in the operating room and for patients on oxygen at home. Hypoxemia still can occur despite the administration of supplemental oxygen. Furthermore, when supplemental oxygen is administered, desaturation occurs at a later time after airway obstruction or hypoventilation, potentially delaying the detection of these critical events. Therefore, it is essential that oxygen saturation and adequacy of ventilation be assessed frequently.
The primary therapeutic use of oxygen is to correct hypoxia. However, hypoxia is generally a manifestation of an underlying disease, and administration of oxygen thus should be viewed as a symptomatic therapy. Efforts must be directed at correcting the cause of the hypoxia. For example, airway obstruction is unlikely to respond to an increase in inspired oxygen tension without relief of the obstruction. More important, while hypoxemia owing to hypoventilation after a narcotic overdose can be improved with supplemental oxygen administration, the patient remains at risk for respiratory failure if ventilation is not increased through stimulation, narcotic reversal, or mechanical ventilation. The hypoxia that results from most pulmonary diseases can be alleviated at least partially by administration of oxygen, thereby allowing time for definitive therapy to reverse the primary process. Thus, administration of oxygen is used in all forms of hypoxia, with the understanding that the response will vary in a way that generally is predictable from knowledge of the underlying pathophysiology.
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