Tissue hypoxia occurs in states of low cardiac output, low hemoglobin concentration, or low Sa o2. The percent oxygen saturation of arterial hemoglobin is, in turn, dependent on the Pao2, as described by the oxygen-hemoglobin dissociation curve. Hypoxemia is an abnormally low arterial oxygen tension. Under most situations, cardiac output is within a normal range, and hypoxemia is the most common cause of hypoxia. Although the terms hypoxia and hypoxemia are generally used interchangeably, one can occur without the other. For example, in states of low Pa o2 (hypoxemia) with concomitant polycythemia, the patient may have no tissue hypoxia. Alternatively, very anemic patients may suffer tissue hypoxia despite a normal Pao2. Hypoxemia is arbitrarily defined as a Pao2 less than 60 mmHg. As noted above, patients with hypoxemia may not necessarily have dyspnea, and patients with dyspnea may not have hypoxemia.
Relative hypoxemia is the term used when the arterial oxygen tension is lower than expected for a given level of inhaled oxygen. The degree of relative hypoxemia can be assessed by calculating the alveolar-arterial (A-a) oxygen partial pressure difference. This A-a gradient measures how well alveolar oxygen is transferred from the lungs to the circulation. Alveolar oxygen partial pressure is determined by the inhaled oxygen concentration (21% for room air), atmospheric pressure (760 mmHg at sea level), and displacement by water vapor (47 mmHg for full saturation) and carbon dioxide. Gas in the alveolus is fully saturated with water vapor, and the amount of alveolar oxygen is further reduced by carbon dioxide that freely diffuses from the pulmonary capillaries in an amount determined by the ratio between oxygen consumption and carbon dioxide production, or the respiratory quotient (R). on a typical diet, the respiratory quotient is 0.8. Thus, alveolar oxygen from breathing room air at sea level has a PAo2 = 0.21 * (760 - 47) - PAco2/0.8. The A-a gradient at sea level for room air is P(A-a) o2 = 149 - PaCo2/0.8 - Pao2. A simplified formula is often used (Ch. 22): PAo2 = 145 - Paco2. A normal P(A-a)o2 is under 10 in young healthy patients and increases with age. The predicted A-a gradient is approximately P(A-a)o2 = 2.5 + 0.21 (age) mmHg, with age in years and a standard deviation of 11 mmHg. This value for the normal A-a gradient is for healthy, asymptomatic individuals measured in an upright or sitting position. The supine position alone, as well as many chronic cardiac or pulmonary diseases, may raise the A-a gradient. Thus, many patients seen in the emergency department already have an elevated A-a gradient due to position or underlying chronic diseases, making it difficult to evaluate increases in the gradient that may be due to an acute pathologic condition.
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