Ventilation perfusion lung scan in acute pulmonary embolism, a good negative but a bad positive diagnostic test

ABSTRACT

Provisional diagnosis of acute pulmonary embolism [P.E.] is based on clinical evaluation Definitive diagnosis however, used to be non invasive with Ventilation Perfusion lung scan [V/Q] considered the test of choice pointing to high, intermediate and low probabilities of PE. Diagnostic pulmonary angiography is obviously the golden standard for establishing the final diagnosis of a highly probable PE. The purpose of the present study is to validate the technique of Ventilation Perfusion lung scan through comparison with the directly proven diagnosis via pulmonary angiography. This study was conducted on a total of 20 pts, all having DVT [proven by duplex], with 10 having symptoms suggestive of pulmonary embolism and 10 asymptomatic. Nine were males and 11 were females with a mean age of 41 +/- 16 [range from 16 to 71y.] Following clinical evaluation, including 12 lead ECG and X-ray chest, all pts were subjected to routine lab. tests, specific lab, tests including arterial blood gases, plasma D-dimer level, FDPs and fibrinogen, transthoracic echocardiography[M-mode and 2D],V/Q lung scan using Tc99m labeled micro aggregated albumin and pulmonary angiography. The latter was performed under hemodynamic monitoring and O2 supplementation, using the trans- venous approach [Sildenger technique] through the right or the left common femoral vein[Internal jugular veins were an alternative in case of iliac or caval thrombosis]. A 6 F pulmonary catheter [angled pigtail] was used for injection of low osmolar iodinated contrast medium. Two views of each lung were performed [The frontal and 45° ipsilateral posterior oblique projections]. Out of the 10 asymptomatic pts only one [10%] had angiographically proven PE compared to 5 out of 10 symptomatic pts [50%]. Corresponding figures for +ve V/Q were 50% and 90% respectively. Compared to the 14 cases without angiographic evidence of PE, the 6 pts with proven PE exhibited significantly higher incidence of dyspnea and chest pain, lower BP, faster respiratory rate, lower PaO2, significantly higher PAP and higher incidence of cyanosis, pulmonary hypertension, TR, RV dilatation. Considering pulmonary angiography as the standard diagnostic tool, all 6 proven cases had +ve V/Q lung scan, while of the remaining 14 unproven cases, V/Q was +ve in 8 i.e sensitivity 100%, specificity 43%, negative predictive value 100%, positive predictive value 43%. In conclusion, compared with the standard pulmonary angiography, ventilation perfusion lung scan proved to be an excellent negative but a bad positive test [sensitivity 100%, specificity 43%, negative predictive value 100%, positive predictive value 43%]. Because of the non invasive nature, V/Q is to be recommended at least as an initial screening test prior to subjecting pts to the invasive technique of pulmonary angiography. Clinical evaluation and bed side objective assessment should not be under estimated as shown by the 10% true incidence of PE in asymptomatic vs 50% in symptomatic cases