Diagnosis of pulmonary embolism by measurement of alveolar dead space

Olsson K, Jonson B, Olsson CG, Wollmer P (Malmö University Hospital and Lund University Hospital, Sweden). Diagnosis of pulmonary embolism by measurement of alveolar dead space. J Intern Med 1998; 244 : 199–207. Objective Pulmonary embolism (PE) gives rise to alveolar dead space, which can be measured with a single breath test for CO 2 (SBT‐CO 2 ). The characteristics of the SBT‐CO 2 are different in PE and other common conditions giving rise to alveolar dead space, notably airways disease. An analysis of alveolar dead space focusing on the late part of the breath (f D late) has been suggested as a method for diagnosis of PE. Our aim was to evaluate this technique by comparison with lung scintigraphy. Methods We randomly selected patients with clinical suspicion of PE. SBT‐CO 2 and lung scintigraphy were performed on the same day. The scintigraphies were reviewed and classified as high, intermediate and low probability of PE. Results Out of 223 patients able to be evaluated, there were 20 of the high, 29 of the intermediate and 174 of the low probability category. There were large differences between the means of f D late in the high and the intermediate and in the high and the low categories. We obtained a sensitivity of 85% and a specificity of 93% for diagnosis of PE, based on high and low probability categories. If a patient with previous PE, but no scintigraphic evidence of current PE, is excluded the sensitivity increases to 90%. Conclusions This study provides further support for the measurement of f D late by the SBT‐CO 2 as a diagnostic test in patients with suspicion of PE. The test should be especially useful in small hospitals without access to pulmonary scintigraphy or pulmonary angiography.