pulmonary embolism is a common and potentially lethal disorder that frequently recurs and is associated with long - term impairment and suffering . the overall mortality rate of patients with untreated pulmonary embolism is approximately 30% ; however , prompt diagnosis and appropriate therapy can reduce the mortality rate to < 10% . pulmonary embolism is the most commonly overlooked disease in patients with pleural effusion ( pe ) . it has been reported that the numbers of patients with pulmonary embolism from 20% of the acute hospital beds were 229,637 in 2005 in usa . if these numbers are extrapolated to all acute hospital beds , the estimated annual occurrence of pulmonary embolism is 1.15 million . given that 3050% of patients with pulmonary embolism have a pe , the annual prevalence of pe due to pulmonary embolism in usa is 300,000500,000 . pulmonary embolism has been established to be the fourth main cause of pe in usa after congestive heart failure , parapneumonic effusion , and malignant effusion . in china , the annual incidence of pulmonary embolism is 0.1% ( 95% confidence interval , 0.10.2% ) in overall hospitalized patients , and an increasing incidence gradient for pulmonary embolism is noticed from southern to northern china . however , no data about the incidence of pe in chinese patients with pulmonary embolism are available to date . we therefore performed the current study to investigate the incidence and computed tomography ( ct ) characteristics of pe in a chinese population of patients with pulmonary embolism . our data showed that pes , usually small and unilateral effusions , are present in about one fifth of a chinese population of patients with pulmonary embolism . the study protocol was approved by the institutional review boards for human studies of beijing chao - yang hospital , beijing , china and no ethical concerns were raised . we retrospectively reviewed the consecutive medical records of ct pulmonary angiography ( ctpa ) scan data on all patients with clinically suspected pulmonary embolism from january 2008 until december 2013 at beijing chao - yang hospital , capital medical university , beijing , china . in our hospital , ctpa is ordered for all for patients with suspected pulmonary embolism . using the electronic database deposited in the department of radiology , we reviewed the study population 's clinical records to ascertain demographic details of patients . all ctpa scans were analyzed independently by two thoracic radiologists , who were blinded to the original ct report and patients diagnoses . the images were viewed on a picture archiving and communications system workstation with these settings : width 400 and level 60 hounsfield units . the picture archiving and communications system allowed the radiologists to scroll through the images . next , the radiologists independently determined if pulmonary embolism and pe were present in each patient : any discrepancies were resolved by reanalyzing the images until consensus was reached . in patients with pulmonary embolism , the clots were classified as central , peripheral or both as described by de mony et al . a central clot was defined as the presence of filling defects within the main to lobar pulmonary arteries , whereas a peripheral clot was defined as the presence of filling defects within the segmental and subsegmental arteries . the pulmonary ct obstruction index was calculated as the method described by qanadli et al . to quantify arterial obstruction with ct in acute pulmonary embolism . briefly , the ct obstruction index was defined as (n  d ) ( n , value of the proximal clot site , equal to the number of segmental branches arising distally ; d , degree of obstruction scored as partial obstruction [ value of 1 ] or total obstruction [ value of 2 ] ) . if a pe was present , the size was semi - quantified as small , moderate , or large according to the ct imaging features with anteroposterior quartile and maximum anteroposterior depth measured at the midclavicular line as described by moy et al . : first anteroposterior - quartile effusion was small , second quartile effusion was moderate , and third or fourth quartile effusion was large . in borderline cases , anteroposterior depth was measured with 3-cm and 10-cm thresholds for the upper limits of small and moderate , respectively . descriptive statistics was used , to summarize , patient characteristics , as well as ctpa and pleural fluid findings . chi - square test was used to compare the incidence of pe in patients with or without embolism , and to determine the statistical significance of the association between the clot score , clot location , and pe . chicago , il , usa ) , and a p < 0.05 was considered to indicate statistical significance . the study protocol was approved by the institutional review boards for human studies of beijing chao - yang hospital , beijing , china and no ethical concerns were raised . we retrospectively reviewed the consecutive medical records of ct pulmonary angiography ( ctpa ) scan data on all patients with clinically suspected pulmonary embolism from january 2008 until december 2013 at beijing chao - yang hospital , capital medical university , beijing , china . in our hospital , ctpa is ordered for all for patients with suspected pulmonary embolism . using the electronic database deposited in the department of radiology , we reviewed the study population 's clinical records to ascertain demographic details of patients . all ctpa scans were analyzed independently by two thoracic radiologists , who were blinded to the original ct report and patients diagnoses . the images were viewed on a picture archiving and communications system workstation with these settings : width 400 and level 60 hounsfield units . the picture archiving and communications system allowed the radiologists to scroll through the images . next , the radiologists independently determined if pulmonary embolism and pe were present in each patient : any discrepancies were resolved by reanalyzing the images until consensus was reached . in patients with pulmonary embolism , the clots were classified as central , peripheral or both as described by de mony et al . a central clot was defined as the presence of filling defects within the main to lobar pulmonary arteries , whereas a peripheral clot was defined as the presence of filling defects within the segmental and subsegmental arteries . the pulmonary ct obstruction index was calculated as the method described by qanadli et al . to quantify arterial obstruction with ct in acute pulmonary embolism . briefly , the ct obstruction index was defined as (n  d ) ( n , value of the proximal clot site , equal to the number of segmental branches arising distally ; d , degree of obstruction scored as partial obstruction [ value of 1 ] or total obstruction [ value of 2 ] ) . if a pe was present , the size was semi - quantified as small , moderate , or large according to the ct imaging features with anteroposterior quartile and maximum anteroposterior depth measured at the midclavicular line as described by moy et al . : first anteroposterior - quartile effusion was small , second quartile effusion was moderate , and third or fourth quartile effusion was large . in borderline cases , anteroposterior depth was measured with 3-cm and 10-cm thresholds for the upper limits of small and moderate , respectively . descriptive statistics was used , to summarize , patient characteristics , as well as ctpa and pleural fluid findings . chi - square test was used to compare the incidence of pe in patients with or without embolism , and to determine the statistical significance of the association between the clot score , clot location , and pe . chicago , il , usa ) , and a p < 0.05 was considered to indicate statistical significance . during the 6-year study period from january 2008 through december 2013 , there was a population of 3196 patients with clinically suspected pulmonary embolism underwent ctpa at the department of radiology in our hospital . the final study population consisted of 3141 patients ( 1504 male ) with a mean age of 60.2  15.1 years ( range 1593 years ) . a diagnosis of pulmonary embolism on ctpa was made in 1220 of the total 3141 patients ( 38.8% ) . as shown in table 1 , patients with pulmonary embolism had more dyspnea and chest pain syndromes and less chest tightness than those without pulmonary embolism did . the other syndromes , such as hemoptysis , cough , and fever , between two groups were not different . concomitant central and peripheral emboli were seen in the majority ( n = 735 , 60.2% ) of patients , peripheral emboli were seen in the remaining patients ( n = 485 , 39.8% ) , and no central clots alone were seen . the percentages of concomitant central and peripheral emboli or peripheral emboli alone in patients with pe were similar to those in patients without pe [ table 2 ] . types of pulmonary emboli in patients with and without pes  = 0.935 , p = 0.342 . a total of 423 patients had pe identified on ct in 3141 patients ( 13.5% ) who underwent ctpa . the incidence of pes was significantly higher in patient with pulmonary embolism ( 243/1220 , 19.9% ) than that in those without embolism ( 180/1921 , 9.4% ) (  = 71.236 , p < 0.001 ) . as shown in table 3 , the presence of pe drove more patients with pulmonary embolism suffering from chest pain and hemoptysis , but not other related syndromes , including chest tightness , dyspnea , cough , and fever , etc . comparisons of symptoms in pulmonary embolism patients and without pe * student s t - test . pe : pleural effusion ; sd : standard deviation . among the 243 pulmonary embolism patients with a pe , 88 patients ( 36.2% ) had unilateral left - sided effusions , 74 ( 30.5% ) had unilateral right - sided effusions and 81 ( 33.3% ) had bilateral effusions . in either left or right side , the size of most pes was small or moderate , very few patients showed large effusions [ table 4 ] . size of pes in patients with pulmonary embolism  = 44.918 , p < 0.001 . the incidence of pe in patients with peripheral pulmonary embolism ( 18.6% ) and in those with concomitant central and peripheral embolism ( 20.8% ) was similar (  = 0.935 , p = 0.342 ) [ table 2 ] . we noted that pulmonary embolism was bilateral in 922 patients ( 75.6% ) , on the right side in 254 ( 20.8% ) , and on the left side in 44 ( 3.6% ) . in 243 pulmonary embolism patients with pe , pulmonary embolism was bilateral in 166 patients ( 68.3% ) , on right side in 67 ( 27.6% ) , and on left side in 10 ( 4.1% ) . we further noted that unilateral pulmonary embolism and pe were on the ipsilateral side in 30 patients , on the contralateral side in 24 patients . we calculated ct pulmonary obstruction index according to the method described by qanadli et al . , and found that the obstruction index in the total population studied was 15.7  10.8 ( range 1  40 ) . the obstruction index in pulmonary embolism patients without pe ( 15.6  10.9 ) was not different from that in the patients with pe ( 16.6  10.9 ) ( t = 0.921 , p = 0.357 ) . as expected , ct abnormalities , including wedge - shaped opacity , linear opacity , and ground - glass attenuation , were more frequently seen in patients with pulmonary embolism than in those without embolism [ table 5 ] . parenchymal ct findings in patients with and without pulmonary embolism or : odds ratio ; ci : confidence interval ; ct : computed tomography . of 243 patients with pulmonary embolism and pe , 89 had wedge - shaped opacity , 51 had atelectasis , 98 had consolidation , 15 had pulmonary masses , 20 had nodule , 96 had mosaic sign , 77 had emphysematous bullae . all the above abnormalities on ct , excluding lung nodule , were much more in pulmonary embolism patients with pe than in those without effusion [ table 6 ] . in a bivariate analysis of these parameters , parenchymal ct findings in pulmonary embolism patients with and without pes or : odds ratio ; ci : confidence interval ; ct : computed tomography ; pe : pleural effusion . during the 6-year study period from january 2008 through december 2013 , there was a population of 3196 patients with clinically suspected pulmonary embolism underwent ctpa at the department of radiology in our hospital . the final study population consisted of 3141 patients ( 1504 male ) with a mean age of 60.2  15.1 years ( range 1593 years ) . a diagnosis of pulmonary embolism on ctpa was made in 1220 of the total 3141 patients ( 38.8% ) . as shown in table 1 , patients with pulmonary embolism had more dyspnea and chest pain syndromes and less chest tightness than those without pulmonary embolism did . the other syndromes , such as hemoptysis , cough , and fever , between two groups were not different . concomitant central and peripheral emboli were seen in the majority ( n = 735 , 60.2% ) of patients , peripheral emboli were seen in the remaining patients ( n = 485 , 39.8% ) , and no central clots alone were seen . the percentages of concomitant central and peripheral emboli or peripheral emboli alone in patients with pe were similar to those in patients without pe [ table 2 ] . types of pulmonary emboli in patients with and without pes  = 0.935 , p = 0.342 . a total of 423 patients had pe identified on ct in 3141 patients ( 13.5% ) who underwent ctpa . the incidence of pes was significantly higher in patient with pulmonary embolism ( 243/1220 , 19.9% ) than that in those without embolism ( 180/1921 , 9.4% ) (  = 71.236 , p < 0.001 ) . as shown in table 3 , the presence of pe drove more patients with pulmonary embolism suffering from chest pain and hemoptysis , but not other related syndromes , including chest tightness , dyspnea , cough , and fever , etc . comparisons of symptoms in pulmonary embolism patients and without pe * student s t - test . pe : pleural effusion ; sd : standard deviation . among the 243 pulmonary embolism patients with a pe , 88 patients ( 36.2% ) had unilateral left - sided effusions , 74 ( 30.5% ) had unilateral right - sided effusions and 81 ( 33.3% ) had bilateral effusions . in either left or right side , the size of most pes was small or moderate , very few patients showed large effusions [ table 4 ] . size of pes in patients with pulmonary embolism  = 44.918 , p < 0.001 . the incidence of pe in patients with peripheral pulmonary embolism ( 18.6% ) and in those with concomitant central and peripheral embolism ( 20.8% ) was similar (  = 0.935 , p = 0.342 ) [ table 2 ] . we noted that pulmonary embolism was bilateral in 922 patients ( 75.6% ) , on the right side in 254 ( 20.8% ) , and on the left side in 44 ( 3.6% ) . in 243 pulmonary embolism patients with pe , pulmonary embolism was bilateral in 166 patients ( 68.3% ) , on right side in 67 ( 27.6% ) , and on left side in 10 ( 4.1% ) . we further noted that unilateral pulmonary embolism and pe were on the ipsilateral side in 30 patients , on the contralateral side in 24 patients . we calculated ct pulmonary obstruction index according to the method described by qanadli et al . , and found that the obstruction index in the total population studied was 15.7  10.8 ( range 1  40 ) . the obstruction index in pulmonary embolism patients without pe ( 15.6  10.9 ) was not different from that in the patients with pe ( 16.6  10.9 ) ( t = 0.921 , p = 0.357 ) . as expected , ct abnormalities , including wedge - shaped opacity , linear opacity , and ground - glass attenuation , were more frequently seen in patients with pulmonary embolism than in those without embolism [ table 5 ] . parenchymal ct findings in patients with and without pulmonary embolism or : odds ratio ; ci : confidence interval ; ct : computed tomography . of 243 patients with pulmonary embolism and pe , 89 had wedge - shaped opacity , 51 had atelectasis , 98 had consolidation , 15 had pulmonary masses , 20 had nodule , 96 had mosaic sign , 77 had emphysematous bullae . all the above abnormalities on ct , excluding lung nodule , were much more in pulmonary embolism patients with pe than in those without effusion [ table 6 ] . in a bivariate analysis of these parameters , parenchymal ct findings in pulmonary embolism patients with and without pes or : odds ratio ; ci : confidence interval ; ct : computed tomography ; pe : pleural effusion . there were several mechanisms might be responsible for the development of pe secondary to pulmonary embolism : ( 1 ) pulmonary hypertension and increases in the right ventricular pressure can result in an increase of the systemic venous pressure at the parietal pleural surface . ( 2 ) the embolus occludes the artery and leads to ischemia distal to the embolus , which leads to an increase in the amount of interstitial fluid in the lung . the interstitial fluid resulting from this increased permeability traverses the visceral pleura , enters the pleural space and leads to pe . ( 3 ) when the embolus lodges in the pulmonary artery , cytokine are released , which also increase the permeability of the vessels . a pe occurs when the amount of fluid formed overwhelms the capacity of the lymphatic vessels to remove the fluid from the pleural space . retrospectively analyzed the medical records of a total of 230 consecutive patients with pulmonary embolism over an 8-year period , and found that pe was observed in 32% and 47% of patients by chest x - ray and ct , respectively . by reviewing all ctpa data performed over the past 6 years on patients with clinically suspected pulmonary embolism , we noted in the present study that a chinese population with pulmonary embolism was more likely to have a pe than those without pulmonary embolism , and the incidence of pe in chinese patients with pulmonary embolism was 19.9% , which was significantly higher than that in those without embolism ( 9.4% ) . the incidence of pe in chinese pulmonary embolism patients was lower than the finding reported by porcel et al . and was similar to the finding reported by yap et al . that pe was diagnosed in 21% of 285 patients . the incidence of pe in patients with pulmonary embolism varied dependent on different methods used in different populations . using thorax ultrasound , mathis et al . reported that a small pe is found in 49% of patients with pulmonary embolism . however , in the most series , pulmonary embolism accounts for < 5% of pes in patients who have undergone a thoracentesis . more recently , hooper et al . performed a prospective study to evaluate the incidence of pulmonary embolism in patients with unilateral pe , and found that pulmonary embolism is detected in 6.4% patients , indicating that pulmonary embolism is not a common primary cause for unilateral pe . there may be three explanations for this contradiction : ( 1 ) the majority of pes secondary to pulmonary embolism are small , which preclude a diagnostic and therapeutic thoracentesis ; ( 2 ) most patients with clinically suspected pulmonary embolism are immediately anticoagulated while awaiting a confirmatory test , and pe gradually resolve , a thoracentesis is not necessary ; ( 3 ) pulmonary embolism is frequently not considered in patients with undiagnosed pe . the study by porcel et al . showed that most pes are small and unilateral , but occasionally they reach more than a half of the hemithorax . a study of yap et al . also showed that pes are generally very small . our current data confirmed that most pes presented in patients with pulmonary embolism were unilateral and small , and the frequency was similar to the findings in patients without pulmonary embolism . these findings indicated that the presence of pulmonary embolism increases the possibility of pe formation ; however , it does not affect the size of pe . in most of our patients , the emboli were located in both central and peripheral pulmonary arterials , and the percentages of concomitant central and peripheral emboli in patients with or without pe were similar . we also found that in pulmonary embolism patients with or without pe , more emboli were bilateral than unilateral , and the frequencies of bilateral pes in two groups were quite similar . in addition , the ct pulmonary obstruction index in pulmonary embolism patients with or without pe was not different . the above data indicated that the locations of pulmonary emboli in central and peripheral pulmonary arterials , sidedness of pulmonary embolism , as well as the ct pulmonary obstruction indexes are not related to the development of pe . parenchymal abnormalities at ct , especially peripheral wedge - shaped opacity , have gained attention to be associated with having pulmonary embolism . our current data showed that ct abnormalities , including wedge - shaped opacity , atelectasis , consolidation , masses , mosaic sign , and emphysematous bullae , were more frequently seen in patients with pulmonary embolism than in those without embolism . although wedge - shaped opacity , atelectasis , consolidation , pulmonary masses , mosaic sign , and emphysematous bullae was much more in pulmonary embolism patients with pe than in those without pe , a bivariate analysis did not show a relationship between any one of these ct abnormalities and the presence of pe . a significant strength of the present study was that a quite big study population ( more than 3000 patients with clinically suspected pulmonary embolism ) was included , and 243 pulmonary embolism patients with pe were finally identified . to the best of our knowledge , our series was the biggest one of this kind of studies . as a matter of fact , the numbers of patients with pulmonary embolism diagnosed and treated in our hospital have been being more than those in any one hospital around over the country . it has been documented that pe due to pulmonary embolism is always exudates , frequently hemorrhagic , and with a marked mesothelial hyperplasia . one limitation of our current study was that no data concerning on specimens were available for analyzing biochemical and cytological characteristics of pes , because very few pulmonary embolism patients with pe in our study underwent diagnostic or therapeutic thoracentesis . we also noted that all patients pes gradually resolved soon after the treatment with anticoagulants . another limitation was that patients undergoing ctpa came from multiple departments of our hospital , including emergency , outpatient departments , and inpatient departments , it therefore was not possible for us to analyze the relationship between the appearance of pe and the prognosis of patients with pulmonary embolism . in summary , we have demonstrated that pes , usually small and unilateral effusions , are present in about one fifth of a chinese population of patients with pulmonary embolism . therefore , when the etiology of an exudative pe is uncertain , the diagnosis of pulmonary embolism should be considered , and confirmatory tests for pulmonary embolism should be added to the routine evaluations . our data also suggested that ctpa is a suitable way to identify the presence of pe in patients with pulmonary embolism and to evaluate the possibility of pulmonary embolism in patients with pe .
background : no data on the incidence of pleural effusion ( pe ) in chinese patients with pulmonary embolism are available to date . the aim of the current study was to investigate the frequency of pe in a chinese population of patients with pulmonary embolism.methods:this was a retrospective observational single - center study . 
 all data of computed tomography pulmonary angiography ( ctpa ) performed over 6-year period on adult patients with clinically suspected pulmonary embolism were analyzed.results:from january 2008 until december 2013 , pe was identified in 423 of 3141 patients ( 13.5% ) with clinically suspected pulmonary embolism who underwent ctpa . 
 the incidence of pe in patients with pulmonary embolism ( 19.9% ) was significantly higher than in those without embolism ( 9.4% ) ( p < 0.001 ) . 
 majority of pes in pulmonary embolism patients were small to moderate and were unilateral . 
 the locations of emboli and the numbers of arteries involved , ct pulmonary obstruction index , and parenchymal abnormalities at ct were not associated with the development of pe.conclusions:pes are present in about one fifth of a chinese population of patients with pulmonary embolism , which are usually small , unilateral , and unsuitable for diagnostic thoracentesis .