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Examples

Examples

High altitude; respiratory failure breathing air Normal person breathing air Normal person breathing 50% O2 Normal person breathing 100% O2 Normal person breathing hyperbaric O2

*This table illustrates the carriage of oxygen in the blood under a variety of circumstances. As arterial O2 tension increases, the amount of dissolved O2 increases in direct proportion to the Po2, but the amount of oxygen bound to hemoglobin reaches a maximum of 196 mL O2/L (100% saturation of hemoglobin at 15 g/dL). Further increases in O2 content require increases in dissolved oxygen. At 100% inspired O2, dissolved O2 still provides only a small fraction of total demand. Hyperbaric oxygen therapy is required to increase the amount of dissolved oxygen to supply all or a large part of metabolic requirements. Note that, during hyperbaric oxygen therapy, the hemoglobin in the mixed venous blood remains fully saturated with O2.

The figures in this table are approximate and are based on the assumptions of 15 g/dL hemoglobin, 50 mL O2/L whole-body oxygen extraction, and constant cardiac output. When severe anemia is present, arterial Po2 remains the same, but arterial content is lower; oxygen extraction continues, resulting in lower O2 content and tension in mixed venous blood. Similarly, as cardiac output falls significantly, the same oxygen extraction occurs from a smaller volume of blood and results in lower mixed venous oxygen content and tension.

shunting of cardiac output, oxygen supplementation must be regulated carefully because of the risk of further reducing pulmonary vascular resistance and increasing pulmonary blood flow.

Metabolism Inhalation of 100% oxygen does not produce detectable changes in oxygen consumption, carbon dioxide production, respiratory quotient, or glucose utilization.

Oxygen Administration

For safety, oxygen cylinders and piping are color-coded (green in the U.S.), and some form of mechanical indexing of valve connections is used to prevent the connection of other gases to oxygen systems. Oxygen is delivered by inhalation except during extracorporeal circulation, when it is dissolved directly into the circulating blood. Only a closed delivery system with an airtight seal to the patient's airway and complete separation of inspired from expired gases can precisely control FIo2. In all other systems, the actual delivered Fio2 will depend on the ventilatory pattern (i.e., rate, tidal volume, inspiratory-expiratory time ratio, and inspiratory flow) and delivery system characteristics.

Low-Flow Systems

Low-flow systems, in which the oxygen flow is lower than the inspiratory flow rate, have a limited ability to raise the Fio2 because they depend on entrained room air to make up the balance of the inspired gas. The F1O2 of these systems is extremely sensitive to small changes in the ventilatory pattern. Devices such as face tents are used primarily for delivering humidified gases to patients and cannot be relied on to provide predictable amounts of supplemental oxygen. Nasal cannu-lae—small, flexible prongs that sit just inside each naris—deliver oxygen at 1-6 L/min. The nasopharynx acts as a reservoir for storing the oxygen, and patients may breathe through either the mouth or nose as long as the nasal passages remain patent. These devices typically deliver 24-28% F1O2 at 2-3 L/min. Up to 40% F1O2 is possible at higher flow rates, although this is poorly tolerated for more than brief periods because of mucosal drying. The simple facemask, a clear plastic mask with side holes for clearance of expiratory gas and inspiratory air entrainment, is used when higher concentrations of oxygen delivered without tight control are desired. The maximum F1O2 of a facemask can be increased from around 60% at 6-15 L/min to >85% by adding a 600- to 1000-mL reservoir bag. With this partial rebreathing mask, most of the inspired volume is drawn from the reservoir, avoiding dilution by entrainment of room air.

High-Flow Systems

The most commonly used high-flow oxygen delivery device is the Venturi mask, which uses a specially designed mask insert to entrain room air reliably in a fixed ratio and thus provides a relatively constant Fio2 at relatively high flow rates. Typically, each insert is designed to operate at a specific oxygen flow rate, and different inserts are required to change the Fiot Lower delivered Fio2 values use greater entrainment ratios, resulting in higher total (oxygen plus entrained air) flows to the patient, ranging from 80 L/min for 24% F1O2 to 40 L/min at 50% Fiot These flow rates still may be lower than the peak inspiratory flows for patients in respiratory distress, and thus the actual delivered oxygen concentration may be lower than the nominal value. Oxygen nebulizers, another type of Venturi device, provide patients with humidified oxygen at 35-100% F1O2 at high flow rates. Finally, oxygen blenders provide high inspired oxygen concentrations at very high flow rates. These devices mix high-pressure compressed air and oxygen to achieve any concentration of oxygen from 21-100% at flow rates of up to 100 L/min. Despite the high flows, the delivery of high F1O2 to an individual patient also depends on maintaining a tight-fitting seal to the airway and/or the use of reservoirs to minimize entrainment of diluting room air.

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