The Low-Probability Lung Scan

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Abstract
Background: The prognosis in patients with suspected pulmonary embolism who have a low-probability lung scan has been the focus of much clinical debate. This is particularly so in patients with underlying cardiac and pulmonary disease, because these disorders frequently cause low-probability lung scans in the absence of pulmonary embolism. Historically, the clinical response has been to exclude pulmonary embolism and withhold treatment on the basis of a low-probability lung scan, which has been regarded as synonymous with a good prognosis. Methods: A prospective cohort-analytic study to evaluate prognosis, using long-term follow-up, in patients with inadequate cardiorespiratory reserve who have presented with suspected pulmonary embolism (inadequate cardiorespiratory reserve, ie, pulmonary edema, right-ventricular failure, hypotension, syncope, acute tachyarrhythmia, abnormal spirometry [forced expiratory volume in 1 second, < 1.0, or vital capacity, < 1.5 L], or abnormal arterial blood gases [PO2, < 50 mm Hg, or PCO2, > 45 mm Hg]). Results: The outcomes of the 77 consecutive patients with low-probability lung scans, suspected pulmonary embolism, and inadequate cardiorespiratory reserve were compared with those in 711 consecutive patients with good cardiorespiratory reserve and nondiagnostic lung scans who were entered into the study over the same period of time. Six (7.8%) of the 77 patients died within days of entry with autopsy-proven pulmonary embolism compared with one (0.14%) of the 711 patients with nondiagnostic lung scans (P<.0001). Conclusions: Our findings indicate that the term low-probability lung scan should be abandoned in reference to patients with inadequate cardiorespiratory reserve, because it is not synonymous with a good prognosis and is, indeed, misleading. (Arch Intern Med. 1995;155:1845-1854)