The progression of fluid accumulation in CPE can be identified as three distinct physiologic stages.
Stage 1
In stage 1, elevated LA pressure causes distention and opening of small pulmonary vessels. At this stage, blood gas exchange does not deteriorate, or it may even be slightly improved.
Stage 2
In stage 2, fluid and colloid shift into the lung interstitium from the pulmonary capillaries, but an initial increase in lymphatic outflow efficiently removes the fluid. The continuing filtration of liquid and solutes may overpower the drainage capacity of the lymphatics. In this case, the fluid initially collects in the relatively compliant interstitial compartment, which is generally the perivascular tissue of the large vessels, especially in the dependent zones.
The accumulation of liquid in the interstitium may compromise the small airways, leading to mild hypoxemia. Hypoxemia at this stage is rarely of sufficient magnitude to stimulate tachypnea. Tachypnea at this stage is mainly the result of the stimulation of juxtapulmonary capillary (J-type) receptors, which are nonmyelinated nerve endings located near the alveoli. J-type receptors are involved in reflexes modulating respiration and heart rates.
Stage 3
In stage 3, as fluid filtration continues to increase and the filling of loose interstitial space occurs, fluid accumulates in the relatively noncompliant interstitial space. The interstitial space can contain up to 500mL of fluid. With further accumulations, the fluid crosses the alveolar epithelium in to the alveoli, leading to alveolar flooding. At this stage, abnormalities in gas exchange are noticeable, vital capacity and other respiratory volumes are substantially reduced, and hypoxemia becomes more severe.
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Radiograph shows acute pulmonary edema in a patient who was admitted with acute anterior myocardial infarction. Findings are vascular redistribution, indistinct hila, and alveolar infiltrates.
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Radiograph shows acute pulmonary edema in a patient known to have ischemic cardiomyopathy. Findings are Kerley B lines (1mm thick and 1cm long) in the lower lobes and Kerley A lines in the upper lobes.
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Radiograph demonstrates cardiomegaly, bilateral pleural effusions, and alveolar opacities in a patient with pulmonary edema.
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Radiograph shows interstitial pulmonary edema, cardiomegaly, and left pleural effusion presenting at an earlier stage of pulmonary edema.
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Lateral chest radiograph shows prominent interstitial edema and pleural effusions.
