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Original Research |
1 Department of Radiology, Waikato Hospital and Waikato Clinical School,
University of Auckland, Hamilton, New Zealand.
2 Department of Radiology, Mayo Clinic and Mayo College of Medicine, 200 1st St.
SW, Rochester, MN 55905.
3 Department of Biostatistics and Health Services Research, Mayo Clinic and Mayo
College of Medicine, Rochester, MN.
4 Department of Respiratory Medicine, Waikato Hospital, Hamilton, New
Zealand.
5 Department of Intensive Care, Waikato Hospital and Waikato Clinical School,
University of Auckland, Hamilton, New Zealand.
Received July 11, 2007;
accepted after revision December 17, 2007.
Address correspondence to P. J. Peller
(peller.patrick{at}mayo.edu).
Abstract
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SUBJECTS AND METHODS. A total of 523 consecutive patients who underwent CT pulmonary angiography for a suspected moderate to high pretest probability of pulmonary embolism were recruited from March 2003 to October 2004. There were 105 patients with positive CT pulmonary angiography examinations. Two consultant respiratory physicians and two consultant radiologists independently and prospectively calculated an ECG score and a quantified pulmonary artery clot burden, respectively. Twelve-month follow-up was completed in all patients.
RESULTS. The mean ECG score was 2.36 (SD, 2.84) and the mean clot burden score percentage was 23.74% (16.8%). Poor correlation (r = 0.09) was seen between the average ECG score and the average clot burden score percentage (p = 0.39) at diagnosis. Thirteen patients had died at the 12-month follow-up. The mean ECG score for those patients who were alive was 2.4 (2.91) and for those who had died was 2.03 (2.34) at 12 months (p = 0.65). The mean clot burden score percentage for those patients who were alive was 24% (17%) and for those who had died was 22.1% (15.7%) at 12 months (p < 0.73).
CONCLUSION. No statistically significant association was seen between ECG score and CT pulmonary angiographic clot burden at diagnosis and the 12-month all-cause mortality rate of patients diagnosed with pulmonary embolism.
Keywords: CT angiography • CT pulmonary angiographic clot burden score • ECG score • outcome • pulmonary embolism • thorax
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Multiple retrospective studies have evaluated the accuracy of CT pulmonary angiographic clot burden scores [14-22]. Only a few of these retrospective studies have investigated the relationship between the clot burden score and patient outcome [19-22]. The literature contains discrepancies regarding the potential for association between the severity of pulmonary angiographic clot burden and the immediate outcome [23]. Studies to date are limited by their retrospective nature and the small number of patients investigated. The association between the intermediate and long-term outcomes and the severity of pulmonary angiographic clot burden at diagnosis is not known.
Daniel et al. [24] established an ECG score that increases with pulmonary hypertension due to pulmonary embolism. A score of greater than 10 is suggestive of severe pulmonary hypertension. This ECG score also predicted those with the greatest percentage of perfusion defects on ventilation-perfusion examinations [25]. But whether this ECG score has any correlation with the embolic clot burden at diagnosis or association with outcome (as measured by mortality rate) is not known.
The objectives of this study were to prospectively evaluate whether a correlation exists between the ECG score [24] and the CT pulmonary angiography clot burden score [15] at diagnosis and to evaluate any potential association between the scores and the 12-month all-cause mortality rate among patients diagnosed with pulmonary embolism.
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The 523 patients had a median age of 58 years (range, 16-96 years). There were 284 (54.3%) females and 239 (45.7%) males. Two hundred sixty-seven (51%) patients were referred from the emergency department; 121(23%) from medical specialists, and 135 (26%) from surgical specialists.
The study population included 18 patients with active cancer. Patients were examined by physicians, and baseline investigations (chest radio graph, ECG, and arterial blood gases) were per formed in all patients before CT pulmonary angiography.
CT Pulmonary Angiography
CT pulmonary angiography was performed using a single-detector helical
scanner (HiSpeed CT/i, GE Healthcare). A contrast-enhanced CT evaluation of
the pulmonary arteries was performed in a caudocranial direction from the top
of the diaphragm to the level of the aortic arch. Scans were acquired during a
single breath-hold or shallow breathing, depending on the patient's ability to
hold his or her breath during the acquisition time. Scans were obtained at
3-mm collimation and 1.6-1.8 pitch and were reconstructed at 1.5-mm intervals
using a standard algorithm. Contrast material was injected via an 18- to
20-gauge cannula in an antecubital vein and both arms were placed above the
patient's head. A total volume of 150 mL of nonionic contrast material
([iohexol], Omnipaque 300, GE Healthcare, formerly Nycomed) was injected at a
rate of 4.5 mL/s. Scanning began when the contrast material was first seen in
the pulmonary trunk using a bolus-tracking software (SmartPrep, GE
Healthcare).
CT Pulmonary Angiographic Readings
Images were reviewed on a workstation at settings for pulmonary vasculature
(window width, -400 H; window level, 50 H) and lung parenchyma (window width,
1,200 H; window level, 700 H). The presence or absence of occlusive or
nonocclusive thrombus in the main, lobar, segmental, and subsegmental arteries
was recorded. The studies were classified as positive for pulmonary embolism
if thrombus was observed; negative for pulmonary embolism if no thrombus was
observed; or indeterminate if a poor examination, poor contrast enhancement,
or motion artifacts precluded confident interpretation of the study.
Helical CT Criteria for Scoring Vascular Obstruction
All CT pulmonary angiographic examinations were independently read by two
radiologists, and clot burden scores
[15] were calculated
prospectively for all 105 patients with positive CT pulmonary angiographic
examinations for pulmonary embolism. Qanadli et al.
[15] evaluated a specific
index for quantifying arterial obstruction with helical CT in acute pulmonary
embolism and showed that their proposed score was reproducible and correlated
highly to the pulmonary angiography index. In this study, we used the same
scoring system based on the site of obstruction and the degree of occlusion of
the pulmonary arteries.
Site of obstruction—The presence of thrombus in a segmental artery received a point value of 1. Emboli in the most proximal arterial level received a total score equal to the number of segmental arteries arising distally, according to the predetermined anatomic subdivisions described previously (maxi mum score of 3 for the upper lobe arteries, 2 for the middle lobe and the lingual arteries, 5 for the lower lobe arteries, 7 for the intermediate arteries, and 10 for the main pulmonary arteries). A single filling defect extending into more than one anatomic location was scored for each location up to, but not exceeding, the maximum designated for each region. The maxi mum possible score for involvement was 20 points [15].
Degree of obstruction—In addition to assessing the level of obstruction, and to provide additional information about the perfusion distal to the thrombus, we multiplied all scores related to the level of obstruction by a weighting factor (x 1 when the thrombus was partially occlusive; x 2 when the thrombus was totally occlusive), depending on the degree of vascular obstruction caused by embolism [15]. Each obstruction therefore received a score depending on the vessel involved multiplied by the weighting factor. The value of the most proximal thrombus in the pulmonary arterial tree scored a maximum of 6 (3 x 2) for the upper lobe arteries, 4 (2 x 2) for the middle lobe and the lingual arteries, 10 (5 x 2) for the lower lobe arteries, 14 (7 x 2) for the intermediate arteries, and 20 (10 x 2) for the main pulmonary arteries; thus, the maximal CT obstruction score for any patient could not exceed 40. Percentages were calculated from the raw scores.
ECG
ECG was performed in all patients as part of the clinical assessment before
CT pulmonary angiography was performed. Two respiratory physicians scored each
ECG examination independently according to a previously established scoring
system [25]
(Table 1) in those patients who
were diagnosed with pulmonary embolism on CT pulmonary angiography. The ECG
readers were blinded to the clot burden scores calculated by the two
radiologists but were aware that patients were diagnosed with pulmonary
embolism on CT pulmonary angiography.
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Follow-Up and Outcome
Patients were interviewed by two research nurses and a radiologist at the
end of 3 and 12 months, by telephone, to establish whether the patients were
alive or had died during the follow-up. In instances in which patients could
not be contacted, the family medicine practitioner was contacted. Clinical
records and death certificates were reviewed by a radiologist if the patient
had died during the follow-up period. The patient outcome was defined as
"all-cause mortality at 12-month follow-up." This definition
indicates death of study patients within 12 months from the date of CT
pulmonary angiography diagnosis of pulmonary embolism due to any medical cause
rather than specifically pulmonary embolism.
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Patient Outcome and the Average Clot Burden Score Percentages
The mean CT pulmonary angiographic clot burden score percentage was 23.74%
(SD, 16.8%; range, 1.25-67.5%). Excellent intraclass correlation of 0.91 (95%
CI, 0.89-0.92) was seen, as was a good Pearson's correlation of 0.72
(p < 0.0001) between the two radiologists' calculations of clot
burden score percentages (Fig.
1). The mean clot burden score percentage for patients who were
alive was 24% (SD, 17%; range, 1.25-67.5%) and for those who had died was
22.1% (15.7%; range, 1.25-43.8%) at the 12-month (p < 0.73)
follow-up (Fig. 2).
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The pulmonary arterial obstruction index used in this study is based on the description by Qanadli et al. [15], in which the segmental pulmonary arteries are the basic units for scoring, and a weighted factor is considered for occlusive versus nonocclusive emboli. We report a mean percentage of pulmonary arterial obstruction of 23.7% (range, 1.25-67.5%), which is similar to the figure (22%) described by Wu et al. [19] and lower than the figure described by Qanadli et al. (29%). This difference may be due to differences in the demographics of the patients studied.
Our study showed no statistically significant correlation between the pulmonary angiographic clot burden score calculated according to Qanadli et al. [15] and the 12-month all-cause mortality rate. This result may be due to a selection bias, in that patients with a large clot burden may have died before CT pulmonary angiography was performed, so that patients with pulmonary embolism who survive to undergo CT pulmonary angiography have less severe clot burden. Although other studies have reported pulmonary angiographic clot burden scores to be a poor predictor of short-term mortality in patients with acute pulmonary embolism [18, 21], to our knowledge, no studies in the literature examine the potential association between the CT pulmonary angiographic clot burden at diagnosis and the intermediate outcome (such as 12-month mortality rate).
Two small retrospective studies have suggested a correlation between the CT pulmonary angiographic clot burden and patient short-term mortality [19, 20]. Wu et al. [19] reported that patients with a pulmonary angiographic clot score of more than 60% tended to die. Those authors [19] and van der Meer et al. [20] found the clot burden score proposed by Qanadli et al. [15] to be a significant predictor of death, with positive results at CT pulmonary angiography (p < 0.002). The median clot burden score for our cohort is 21%, which is similar to that obtained by Wu et al. and van der Meer et al. (10% and 32%, respectively). This supports that selection bias is unlikely to be an important confounder in our cohort. In addition, our study showed all patients who had died at 3 or 12 months had a CT pulmonary angiographic clot burden of less than 45%.
Two dedicated CT pulmonary angiographic scoring systems are available, as proposed by Qanadli et al. [15] and Mastora et al. [16]. We used the system developed by Qanadli et al. because it was the scoring system available at the time our study was planned in 2002. A recent retrospective study involving 89 patients diagnosed with pulmonary embolism concluded that the Mastora obstruction score showed a significant correlation with early death within 30 days, whereas the Qanadli pulmonary embolism obstruction score showed no significant correlation with 30-day mortality [22]. Although both scores are simple to use and reproducible, the Mastora obstruction score is significantly varied (analysis of variance, p < 0.0001) and lower than the Qanadli obstruction score used in this study. This is most likely due to differences in calculation of clot burden in these scoring models.
A number of ECG changes have been associated with severity of pulmonary embolism and outcome [30-32]. The ECG scoring system developed by Daniel et al. [24] increases with severity of pulmonary hypertension due to pulmonary embolism and also predicts those with the greatest percentage of perfusion defect on ventilation-perfusion examinations [25]. Iles et al. [25] retrospectively recruited 229 patients who had pulmonary embolism diagnosed on ventilation-perfusion scans and ECG performed within 48 hours of the lung scans. The mean ECG scores were 2.6, 3.2, and 5.3 in patients with < 30%, 30-50%, and > 50% perfusion defects on the ventilation-perfusion scans, respectively. In comparison, our patients were prospectively recruited, ECG was performed before CT pulmonary angiography on the same day, and the diagnosis of pulmonary embolism was made on CT pulmonary angiograms. Our study showed a poor correlation between the ECG scores and the CT pulmonary angiographic clot burden at diagnosis. Our study also suggests that no statistically significant association exists between the ECG score and the 12-month mortality rate. To our knowledge, ours is the first prospective study examining the correlation between ECG score and CT pulmonary angiographic clot burden at diagnosis and the 12-month mortality rate.
This prospective study refutes the hypothesis that ECG changes and pulmonary artery clot burden at diagnosis may be useful in risk-stratifying and prognosis of patients diagnosed with pulmonary embolism. This is in contrast to previous promising findings of using ECG scoring and pulmonary angiographic clot burden as tools for risk stratification or prognosis in patients with pulmonary embolism. Our results are likely due to patient outcome or because death is determined by a combination of other comorbidities such as malignancy or surgery, rather than the diagnosis of pulmonary embolism itself.
Our study had some limitations. First, we analyzed the ECG studies only of patients who were diagnosed with pulmonary embolism on CT pulmonary angiography. Second, the ECG scoring system we used may have its own limitation because it is largely based on T wave inversion, which may vary as a result of right bundle branch block; these inversions are frequently found in leads V1 and V3 of healthy individuals. Third, we used single-detector helical CT pulmonary angiograms for calculation of clot burden scores. With advances in technology, MDCT pulmonary angiographic clot burden scores may more accurately estimate the subsegmental pulmonary artery clot burden. That we did not use the current CT technologic advances limits the value of this study, although to our knowledge it is the largest prospective study published on this topic. Finally, no CT parameters for right heart failure or echocardiographic parameters for pulmonary hypertension were estimated in our study.
In conclusion, we found a poor correlation between the ECG score and the CT pulmonary angiographic clot burden in our cohort of patients with pulmonary embolism. No statistically significant association was seen between the ECG score, CT pulmonary angiographic clot burden at diagnosis, and the 12-month all-cause mortality rate.
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