Hypoxemia occurs in about 90 percent of patients with PE, but the Pao2 may be normal. While a Pao2 of 80 to 90 mmHg is 90 to 95 percent sensitive in identifying patients with PE, it is less than 50 percent specific.5 Further, the degree of hypoxemia fails to accurately predict the size of the PE.5 However, a Pao2 of less than 70 mmHg not explained by findings on the CXR strongly suggests PE. The calculation of the alveolar-arterial (A-a) oxygen gradient may be useful in reducing the suspicion of PE if it and the PaCo2 are both normal.14
a-a grudiirtl 5 kf.oj) 11 (tartmetrk: pressure - 47)]
However, the A-a gradient has been shown to be normal in nearly 25 percent of patients with PE15 and should never be used alone to exclude the diagnosis. The A-a gradient has been shown to be even less useful in the elderly.16 These screening tests (ECG, CXR, and ABG) are most useful in defining the patient's complaints to be caused by diseases other than PE. A normal ECG, presenting CXR, Pao2, PaCo2, and A-a gradient cannot be used to exclude the diagnosis of PE in patients at risk for thromboembolic events. In this subset of patients, a number of additional tests are used to increase or decrease the clinical suspicion of PE.
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