Radioactive tracers in the differential diagnosis of pulmonary embolism

A Clinically Meaningful Interpretation of the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) Scintigraphic Data

RATIONALE AND OBJECTIVES

Pulmonary embolism (PE) is a common condition associated with significant morbidity and mortality. Diagnostic test characteristics reported in terms of sensitivity and specificity are difficult to translate at the clinical level. More relevant measures are likelihood ratios (LRs), which can convert a pretest into a posttest probability. The aim of our study was to calculate the LRs and posttest probabilities for multiple-level test result for ventilation/perfusion (V/Q) lung scintigraphy and for perfusion scintigraphy combined with chest radiography using modified Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) II and the Prospective Investigative Study of Pulmonary Embolism Diagnosis (PISAPED) criteria for each clinical probability level for the most commonly used clinical prediction rules (CPR) using the PIOPED data.

MATERIALS AND METHODS

PE pretest probability was estimated for the most commonly used CPRs (Wells, Geneva, Miniati, and Charlotte) at each clinical probability level (two-, three-, and four-level). Multiple-level LRs (high, indeterminate, low, very low probability, and normal) and the positive, indeterminate, and negative results for V/Q scintigraphy, and the positive, indeterminate, and negative results for perfusion scintigraphy were generated and used to calculate posttest probabilities based on the sensitivity and specificity data from PIOPED for each clinical probability level (low, intermediate, and high) for each CPR. Nomograms were also created.

RESULTS

The LRs for a positive V/Q and perfusion scintigraphy test using modified PIOPED II and PISAPED criteria were 20.6, 11, and 23.7, and for a negative test result were 0.15, 0.16, and 0.2, respectively. In the three-level Wells score, the posttest probability for an initial low clinical probability PE for a positive, indeterminate, and negative test result, respectively, for V/Q scintigraphy is 56, 5, and 0.9; for perfusion scintigraphy with modified PIOPED 40, 7, and 0.9, and with PISAPED 59, not available (N/A), and 1.2; for an initial moderate clinical probability PE for V/Q scintigraphy 86, .22, and 4; for perfusion scintigraphy with modified PIOPED 77, 26, and 5, and with PISAPED 88, N/A, and 6; for an initial high clinical probability of PE for V/Q scintigraphy 95, 48, and 13; and for perfusion scintigraphy with modified PIOPED 92, 53, and 13, and with PISAPED 96, N/A, and 16.

CONCLUSIONS

With LRs >10, a positive test result for V/Q and perfusion scintigraphy can confirm the presence of PE. Only a normal test result has low enough LR to exclude PE.

The role of multidetector computed tomography angiography for the diagnosis of pulmonary embolism

From a radiological point of view, computed tomography pulmonary angiography (CTPA) has effectively become the de-facto first-line imaging test for the evaluation of pulmonary embolism (PE), as patients with a high-quality negative CTPA do not require further examination or treatment for suspected PE. We are likely to see further technical developments in CT technology in the near future. These advances will most likely further improve image quality. Several questions or issues remain, including strategies for further imaging when CT is inconclusive or contraindicated, issues regarding radiation exposure, the prevalence of PE in specific populations, best tests and pathways in specific patient groups, including patients with specific comorbidities such as oncology patients or patients with chronic obstructive pulmonary disease. Also, the question whether all PE patients need anticoagulation, the clinical effect of follow-up imaging, and the accuracy of different clinical prediction rules, remains.

Imaging evaluation of suspected pulmonary embolism

Venous thromboembolism (VTE) is a common disorder that is difficult to diagnose clinically but carries significant morbidity and mortality if untreated. Additionally, although demonstrated to be of benefit in cases of proven deep vein thrombosis (DVT) and pulmonary embolism (PE), anticoagulation therapy is not without risk. Because the clinical exam is known to be unreliable for the detection of both DVT and PE, many imaging modalities have been used in the diagnostic imaging algorithm for the detection of VTE, including chest radiography, ventilation/perfusion (V/Q) scintigraphy, pulmonary angiography, and recently, spiral computed tomography (CT) and magnetic resonance imaging (MRI). Chest radiographic findings in acute PE include focal oligemia, vascular enlargement, atelectasis, pleural effusions, and air space opacities representing pulmonary hemorrhage or infarction. The chest radiograph can occasionally be suggestive of PE but is more often nonspecifically abnormal. The main use of the chest radiograph in the evaluation of suspected PE is to exclude entities that may simulate PE and to assist in the interpretation of V/Q scintigraphy. Lower extremity venous compression ultrasonography (CU) is both sensitive and specific for the diagnosis of femoropopliteal DVT, and the value of negative CU results has been established in outcomes studies. However, the reliability of CU for the detection of isolated calf vein thrombosis is not well established, and the clinical significance of such thrombi is debatable. Additional methods such as color and spectral Doppler analysis are also useful in the diagnostic evaluation of DVT but are best considered as adjuncts to the conventional CU examination rather than as primary diagnostic modalities themselves. Compression ultrasonography and Doppler techniques are useful in the evaluation of suspected upper extremity DVT; spectral Doppler waveform analysis is particularly useful to assess for the patency of veins that cannot be directly visualized and compressed with conventional gray-scale sonography. V/Q scintigraphy has been the initial modality obtained in patients suspected of PE for a number of years. Although many studies have investigated the role of V/Q scintigraphy in the evaluation of VTE, the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study has provided the most useful information regarding the utility of V/Q scintigraphy in this setting. A high probability scan interpretation is sufficient justification to institute anticoagulation, and a normal perfusion scan effectively excludes the diagnosis of PE. A normal/near normal scan interpretation also carries a sufficiently low prevalence of angiographically proven PE to withhold anticoagulation. Although the prevalence of PE in the setting of low probability scan interpretations is low and several outcomes studies have demonstrated a benign course in untreated patients with low probability scan results, patients with inadequate cardiopulmonary reserve do not necessarily have good outcomes. Such patients deserve more aggressive evaluation. Patients with intermediate probability scan results have a 20% to 40% prevalence of angiographically proven PE and thus require further investigation. The radionuclide investigation of DVT includes such techniques as radionuclide venography and thrombus-avid scintigraphy. Although these methods have not been as thoroughly evaluated as CU, studies thus far have indicated encouraging results, and further investigations are warranted. Pulmonary angiography has been the gold standard for the diagnosis of PE for decades. Studies have indicated that angiography has probably been underutilized by referring physicians for the evaluation of suspected PE, likely because of the perception of significant morbidity and mortality associated with the procedure. (ABSTRACT TRUNCATED)

Angiographic evaluation of scintigraphic abnormalities in screening for pulmonary embolism after total hip replacement

Among 240 consecutive patients subjected to total hip replacement, 206 underwent perfusion scintigraphy in screening for pulmonary embolism 10 to 14 days after surgery. All patients with perfusion abnormalities (n = 77) also underwent ventilation scintigraphy with a dry 99Tcm microaerosol. The scintigraphic abnormalities were classified according to a detailed coding system and in 50 patients pulmonary angiography was performed within 24 hours for correlation. Emboli were diagnosed in 19 patients at angiography. All patients with ventilated segmental (n = 6) or larger (n = 5) perfusion defects or multiple subsegmental defects larger than half a segment (n = 2) had emboli. Two of 8 patients with abnormally ventilated subsegmental perfusion defects larger than half a segment had emboli. Four of 21 patients with wedge-shaped subsegmental perfusion defects smaller than half a segment or defects with a rounded border towards the hilum and a peripheral extension equal to or less than that of a segment had emboli. In one of them the site of the emboli correlated to the site of the perfusion defect but occurred in normally perfused areas in the remaining 3. Ventilation scintigraphy was of no value in this group of patients. No emboli were found in patients who had areas with decreased but not totally lost perfusion (n = 8). No emboli were diagnosed in 27 lungs with a normal perfusion.

An analysis of the diagnostic methods for acute pulmonary embolism

In spite of numerous sophisticated investigative procedures, acute pulmonary embolism (PE) is very frequently misdiagnosed. In order to improve the diagnostic approach to PE, the sensitivity and specificity of the commonly used methods were reviewed in a group of 421 patients with angiographically proved PE without associated cardio-pulmonary disease. The specificity of diagnostic procedures was, by decreasing order: positive pulmonary angiography (to affirm) = negative perfusion lung scan (to eliminate) greater than chest X-ray much greater than clinical symptoms, positive perfusion lung scan, ECG, blood gas, serum enzymes.

Effects of tocainide, an oral analogue of lidocaine, on thromboembolism after total hip replacement

In an investigation of deep venous thrombosis and pulmonary embolism, where neither dextran nor other antithrombotic drug prophylaxis was employed, 30 patients subjected to total hip replacement under general anaesthesia were randomly allotted to one of two groups. One group (n=15) received tocainide, an oral analogue of lidocaine, as a means of preventing thromboembolism; the other group (n=15) served as a control. In patients given tocainide the frequency of deep venous thrombosis involving the femoral veins, as observed at phlebography, was 60% (9 of 15), and in the control group 73% (11 of 15). The frequency of pulmonary embolism, as determined by pulmonary perfusion lung scanning, was 20% (3 of 15) in the the tocainide group and 33% (5 of 15) in the control group. It was concluded that tocainide administration had no effect as an antithromboembolic agent. Phlebography revealed that the pattern of deep venous thrombosis after total hip replacement was characterized by a high frequency of isolated thigh vein thrombi in the operated leg, probably related to the surgical procedure. A finding of possible clinical significance was that patients given tocainide had a significantly lower intraoperative blood loss than control patients.

Preoperative normovolaemic haemodilution with dextran 70 as a thromboembolic prophylaxis in total in hip replacement

The effect of preoperative normovolaemic haemodilution with dextran 70 in 57 patients undergoing total hip replacement for osteoarthritis, was evaluated with respect to the production of thrombosis and pulmonary embolism. The patients were randomly allocated to one group undergoing preoperative haemodilution and one control group receiving 500 ml dextran 70 during operation and again on the second postoperative day. Before operation blood was replaced by dextran 70 to lower the hematocrit to between 25 and 30. The blood drained from the patients was used to replace blood lost at operation. Between 10 and 14 days after operation the patients were examined by perfusion lung scan, chest radiography and bilateral phlebography. Twelve patients also had 133Xe ventilation scans. The incidence of deep vein thrombosis in the two groups did not differ significantly. Pulmonary embolism was significantly decreased in the preoperative haemodilution group. One patient in the control group had clinical symptoms of, and died from, pulmonary embolism.

Lung ventilation-perfusion scintigraphy in pulmonary embolism. Diagnostic specificity compared to pulmonary angiography

In 53 patients with possible pulmonary embolism, pulmonary abnormalities of 133Xe ventilation and 99Tcm albumin microsphere perfusion scintigraphy were compared with absence or presence of pulmonary emboli documented by concurrent pulmonary angiography. It was found that patients with combined scintigraphy considered as unlikely for pulmonary embolism (ventilation defect larger than perfusion defect) or indicative of pulmonary embolism (ventilation defect smaller than perfusion defect) provide high diagnostic specificity. Patients with equal ventilation-perfusion abnormalities (possible pulmonary embolism) require further evaluation by pulmonary angiography to ascertain diagnosis. Importantly, diagnostic accuracy, using ventilation-perfusion scintigraphy and the quantified method of evaluation delineated, is preserved in patients with severe congestive heart failure.

Detection of experimental pulmonary emboli in dogs by sequential positron imaging after inhalation of 15O-carbon dioxide

After inhalation, C15O2 (T1/2 = 2 minutes) rapidly diffuses into pulmonary blood and is cleared from the lungs within 10 seconds. The purpose of this study was to determine whether impaired clearance of inhaled C15O2 from oligemic zones, distal to areas of obstructed pulmonary blood flow, could be detected by serial pulmonary imaging with a positron camera. Experimental obstruction of branches of the pulmonary artery was induced in 19 anesthetized dogs by inflation of balloon-tipped catheters (8-12 mm in diameter), injection of radiopaque silicone spheres (0.5-4.0 mm), and embolization with barium-impregnated autologous blood clots (1-5 mm) via the right external jugular vein. After a single bolus injection of 2 mCi of C15O2 into the endotracheal tube, serial lung images of 15O activity were obtained over 60-180 seconds. Obstruction of pulmonary arterial branches resulted in visualization of discrete zones of impaired 15O clearance which varied in area with catheter diameter. Location and size of these zones were confirmed by repeat imaging after direct injection of 15O-labeled blood through the distal catheter lumen. In dogs receiving autologous clots (n = 8), similar zones of impaired 15O clearance were consistently imaged, and single emboli as small as 2 mm in diameter produced regions of retained 15O activity. Zones of retained 15O activity corresponded to the location of radiopaque emboli on chest radiographs. This study introduces a new technique of radionuclide imaging for detection of pulmonary emboli that is noninvasive, safe, sensitive, and repeatable at short intervals.