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BACKGROUND

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Hypoxia and hypoxemia are common reasons for hospital admission, usually in the context of a primary respiratory illness1. An understanding of the evaluation, diagnosis, and monitoring of patients with hypoxemia can improve patient care and resource utilization.

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PATHOPHYSIOLOGY

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The Oxygen Cascade
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The process of delivering oxygen (O2) to tissues is complex, but knowledge of this process facilitates an understanding of the clinical aspects of hypoxemia and hypoxia. Providing adequate tissue oxygenation involves the pulmonary, cardiovascular, and hematology systems and is hampered by the relatively slow tissue diffusion of O2. O2 always moves down its partial pressure gradient from dry atmospheric air (partial pressure of O2 [PO2] 160 mmHg at sea level) to the mitochondria in tissues (PO2 3–20 mmHg). The gaseous waste product of aerobic metabolism, carbon dioxide (CO2), is assisted in its transport to atmospheric air like O2, but unlike O2, diffuses readily across tissues.

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Alveolar Oxygen
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The fraction of inspired O2 (Fio2) in atmospheric air is 21%, but the partial pressure of inspired O2 (PIO2) is also dependent on barometric pressure (PB), primarily as a function of altitude. O2 entering the alveolus is “diluted” by humidification (PH2O = 47 mmHg), and CO2 in the alveolus. Arterial partial pressure of CO2 (PaCO2) approximately equals alveolar partial pressure of CO2 (PAco2) due to ready diffusion across capillary and alveolar membranes. PACO2 is a function of energy metabolism (production of CO2) and minute ventilatory volume (elimination of CO2). CO2 production reflects the respiratory quotient (RQ); RQ of a balanced diet is ~0.83. CO2 elimination is reflected by the PACO2. CO2 is also involved in acid–base balance, which can impact its rate of elimination.

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The simplified alveolar gas equation calculates an estimate of alveolar partial pressure of O2 (PAO2): PAO2 ~ PIO2 – (PaCO2/RQ). At sea level in atmospheric air, PB is 760 mmHg. The resulting Pio2 is Pio2 = (FIO2 *(PB–PH2O), or (0.21*(760 – 47)) = 149 mmHg. In the alveolus, CO2 is present in proportion to the RQ and minute ventilation, and the PIO2 is diminished by the factor PaCO2/RQ. With a PACO2 = paco2 of 40 mmHg and an R of 0.83, the ideal PAO2 at sea level is 149 mmHg – (40/0.83) = 101 mmHg. In contrast, in Salt Lake City, Utah, 1280 m above sea level, the PB is 647 mmHG, and the ideal PAO2 is 77 mmHG. Hyperventilation, environment, diet, and disease can influence PAO2 through their ...

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