critical care medicine
Oxygen Desaturation
Treatment of Oxygen Desaturation
Identified causes are treated as discussed elsewhere in THE MANUAL. If hypoventilation persists, Overview of Mechanical Ventilation via noninvasive positive pressure ventilation or endotracheal intubation is necessary. Persistent hypoxia requires supplemental oxygen. The amount of oxygen given is guided by arterial blood gases (ABG) or pulse oximetry to maintain PaO2 between 60 and 80 mm Hg (ie, 92 to 100% saturation) without causing oxygen toxicity. This level provides satisfactory tissue oxygen delivery; because the oxyhemoglobin dissociation curve is sigmoidal, increasing PaO2 to>80 mm Hg increases oxygen delivery very little and is not necessary. The lowest fractional inspired oxygen (FiO2) that provides an acceptable PaO2 should be provided. Oxygen toxicity is Concentration-dependent Time-dependent Sustained elevations in FiO2>60% result in inflammatory changes, alveolar infiltration, and, eventually, pulmonary fibrosis. An FiO2>60% should be avoided unless necessary for survival. An FiO260% is well tolerated for long periods. An FiO2<40% can be given via nasal cannula or simple face mask. A nasal cannula uses an oxygen flow of 1 to 6 L/minute. Because 6 L/minute is sufficient to fill the nasopharynx, higher flow rates are of no benefit. Simple face masks and nasal cannulas do not deliver a precise FiO2 because of inconsistent admixture of oxygen with room air from leakage and mouth breathing. However, Venturi-type masks can deliver very accurate oxygen concentrations. An FiO2>40% requires use of an oxygen mask with a reservoir that is inflated by oxygen from the supply. In the typical nonrebreather mask, the patient inhales 100% oxygen from the reservoir, but during exhalation, a rubber flap valve diverts exhaled breath to the environment, preventing admixture of carbon dioxide and water vapor with the inspired oxygen. Nonetheless, because of leakage, such masks deliver an FiO2 of at most 80 to 90%. High-flow nasal cannula (HFNC) oxygen therapy, in contrast to traditional nasal oxygen, delivers oxygen at rates from 20 to 60 L/minute; the oxygen is humidified. Humidification helps prevent airway desiccation and inflammation, maintain mucociliary function, and improve mucus clearance. HFNC therapy tends to wash out upper airway dead space and decrease the work of breathing more than non-rebreather masks. This therapy may help patients with hypoxemic respiratory failure not due to heart failure and who are not hypercapneic. Refractory hypoxia may require neuromuscular blockade, recruitment maneuvers, prone ventilation, or venovenous extracorporeal membrane oxygenation (ECMO). 