Aa gradient 4 age4

P>Blood gas analysis may be less reliable in patients with chronic obstructive pulmonary disease.

Plain film radiographic studies are required in any patient presenting with the symptoms of pulmonary embolus. Recent posteroanterior chest films (within 6 h) are also required when further work-up determines that ventilation-perfusion scanning is necessary. The chest radiographic findings most commonly seen include pulmonary consolidation and hemidiaphragmatic elevation, with each being seen in 40 per cent of patients. As a screening device, this quickly obtainable study can suggest pneumonia, atelectasis, pleural effusion, or pneumothorax as alternative diagnoses. The most interesting finding in patients with pulmonary embolus may be a normal chest radiograph, which can be seen in nearly 40 per cent of cases.

Historically, several radiographic findings were believed to be specific for pulmonary embolus, although more recently these features have been determined to be neither sensitive nor specific. Hampton's hump (a round density at the pleural margin pointing towards the hilum) and Westermark's sign (pulmonary vascular congestion together with regional oligemia) have both been shown to be poor predictors of pulmonary embolus.

CT scanning can be a useful addition to the screening armamentarium. Contrast-enhanced CT scanning, particularly spiral CT with its fast acquisition times, which was initially found to discover mural thrombi in the pulmonary vasculature when scanning for other diagnoses, can delineate acute thrombi in second- to fourth-order pulmonary vessels. It can also discern oligemic lung parenchyma, a decrease in the number and caliber of vessels, and a decreased attenuation of the signal in this region. It is also sensitive for diagnosing chronic thromboembolic disease, as it shows both the central mural thrombi and occlusion of smaller arteries. Lung parencyhmal infarction can also be determined, although only 10 per cent of pulmonary emboli will actually result in infarction. A thorough knowledge of bronchovascular anatomy is necessary, as a diagnosis of pulmonary embolism may be misinterpreted by intersegmental lymph nodes or from partially opacified pulmonary veins and thrombosed arteries.

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