Acute pulmonary embolism: Correlation of CT pulmonary artery obstruction index with blood gas values

OBJECTIVE. The purpose of our study was to investigate the relation between the pulmonary artery obstruction index assessed with helical CT and impairment in blood gases in patients with acute pulmonary embolism. SUBJECTS AND METHODS. Helical CT pulmonary angiography was performed in 78 patients who were suspected of having acute pulmonary embolism and selected as being free of underlying cardiopulmonary disease. Findings consistent with acute pulmonary embolism were observed in 34 patients. The severity was assessed by the pulmonary artery obstruction index, defined as Sigma (n x d), where n is the number of segmental arteries occluded and d is the degree of obstruction. Spearman's rank correlation coefficients were used to assess the correlation between the index of arterial obstruction and arterial partial pressure of oxygen (PaO2); alveolar-arterial difference in partial pressure of oxygen (PAO(2) - PaO2); arterial partial pressure of carbon dioxide (PaCO2); and arterial oxygen saturation (SaO(2)). The statistical difference of the arterial blood gas values between the two groups of patients (those with and those without pulmonary embolism) was evaluated using the Mann-Whitney U test. Blood gases were comparatively evaluated below and above different index values (from 40% to 70%) and different PaCO2 values (25, 30, and 35 mm Hg) as possible indexes of embolism severity using the same test. The level of significance was set at 95% (p = 0.05). RESULTS. The values of PaO2, SaO(2), and PaCO2 were significantly lower (p = 0.024, p 0.0062, and p = 0.000075, respectively) and the values of PAO(2) - PaO2 were significantly higher (p = 0.0169) in the pulmonary embolism group than in the no-pulmonary-embolism group. A significant correlation was observed between the obstruction index and PaO2 (r = -0.33, p = 0.05), PaCO2 (r = -0.34, p = 0.05), PAO(2) - PaO2 (r = 0.39, p = 0.02), and SaO(2) (r = -0.35, p = 0.04). Using cutoff values for the pulmonary artery obstruction index of 40%, 50%, 60%, and 70%, we observed that PaCO2 and PAO(2) - PaO2 differed significantly between above and below the 40% (p = 0.018 and p = 0.03), 50% (p = 0.0087 and p = 0.029), and 60% (p = 0.005 and p 0.003) cutoffs. PaO2 differed significantly for the cutoff values of 60% (p = 0.03) and 70% (p = 0.004). The same was observed for SaO(2) at 60% (p = 0.05) and 70% (p = 0.03). Comparisons for PaCO2 showed that a value of 30 turn Hg significantly separates levels of the pulmonary artery obstruction index (p = 0.002), with 78% sensitivity and 82% specificity indicating a pulmonary artery obstruction index greater than 50%. CONCLUSION. Inpatients with acute pulmonary embolism but no other underlying cardiopulmonary disease, the severity of the pulmonary arterial tree obstruction assessed using the CT obstruction index is significantly correlated to the blood gas values. The strongest correlation was observed between the index and the PAO(2) - PaO2. Furthermore, a PaCO2 value of 30 mm Hg or less is highly suggestive of an obstruction index of more than 50% of the arterial bed.