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Normal Aa gradient, decreased surface area for diffusion, normal diffusion distance
5%
15/312
Decreased Aa gradient, decreased surface area for diffusion, normal diffusion distance
11%
34/312
Decreased Aa gradient, increased surface area for diffusion, decreased diffusion distance
3%
10/312
Increased Aa gradient, normal surface area for diffusion, increased diffusion distance
18%
56/312
Increased Aa gradient, decreased surface area for diffusion, increased diffusion distance
60%
188/312
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This patient's clinical presentation is most consistent with pulmonary edema, likely due to congestive heart failure. Pulmonary edema is characterized by an increased Aa gradient, decreased surface area for diffusion, increased diffusion distance. There are five major causes of hypoxemia, which should first be differentiated by evaluation of the alveolar-arterial oxygen gradient (Aa gradient). Mechanisms of hypoxemia that are characterized by an increased Aa gradient include ventilation/perfusion (V/Q) mismatch, shunt, and diffusion limitation. Pulmonary edema causes both V/Q mismatch and diffusion limitation. Resting hypoxemia is primarily due to V/Q mismatch, since oxygen exchange in the alveolar capillary is not diffusion limited at rest (even if there is a diffusion limitation present), as there is ample time for the red blood cell in alveolar capillary. However, when a diffusion limitation is present, it does contribute to hypoxemia on exertion, as the increased cardiac output with exertion decreases time for diffusion in the alveolar capillary. Vital et al. perform a meta-analysis of the use of non-invasive positive pressure ventilation (CPAP or BiPAP) for treatment of pulmonary edema. They find that this intervention reduced in-hospital mortality and rates of intubation in the patient populations under study. However, there was no change in hospital length of stay or rates of myocardial infarction. Ware et al. discuss a patient presenting with acute pulmonary edema. They note that cardiogenic pulmonary edema is caused by back-up of fluid from a poorly functioning left heart, leading to increased hydrostatic pressure in the pulmonary veins and arteries. In contrast, non-cardiogenic pulmonary edema results from increased permeability of pulmonary vasculature. The two mechanisms can be differentiated by the protein content of pulmonary fluid, which should have a high protein content in cases of non-cardiogenic pulmonary edema. Figure A shows a chest X-ray showing pulmonary edema. Illustration A shows different causes of hypoxia. Incorrect Answer: Answer 1: Causes of hypoxemia with a normal Aa gradient include decreased partial pressure of oxygen (high altitude) and hypoventilation. Answers 2, 3: No known causes of hypoxemia cause a decreased Aa gradient. Answer 4: The surface area for diffusion is decreased in pulmonary edema.
4.2
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