Radionuclide imaging of acute pulmonary embolism

Pulmonary embolism

Pulmonary embolism (PE) remains one of the most challenging medical diseases in the emergency department. PE is a potentially life threatening diagnosis that is seen in patients with chest pain and/or dyspnea but can span the clinical spectrum of medical presentations. In addition, it does not have any particular clinical feature, laboratory test, or diagnostic modality that can independently and confidently exclude its possibility. This article offers a review of PE in the emergency department. It emphasizes the appropriate determination of pretest probability, the approach to diagnosis and management, and special considerations related to pregnancy and radiation exposure.

Diagnosis of suspected venous thromboembolic disease in pregnancy

Venous thromboembolic disease is a leading cause of maternal mortality during pregnancy. Early and accurate radiological diagnosis is essential as anticoagulation is not without risk and clinical diagnosis is unreliable. Although the disorder is potentially treatable, unnecessary treatment should be avoided. Most of the diagnostic imaging techniques involve ionizing radiation which exposes both the mother and fetus to finite radiation risks. There is a relative lack of evidence in the literature to guide clinicians and radiologists on the most appropriate method of assessing this group of patients. This article will review the role of imaging of suspected venous thromboembolic disease in pregnant patients, highlight contentious issues such as radiation risk, intravenous contrast use in pregnancy and discuss the published guidelines, as well as suggesting an appropriate imaging algorithm based on the available evidence.

Predictors of an intermediate ventilation/perfusion lung scan in patients with suspected acute pulmonary embolism

BACKGROUND

An intermediate ventilation/perfusion (V/Q) lung scan lacks diagnostic utility in excluding acute pulmonary embolism (PE).

OBJECTIVE

To identify the potential predictors of an intermediate V/Q lung scan in patients with suspected acute PE.

METHODS

A review of the medical records of 306 consecutive patients with suspected acute PE, in whom a V/Q lung scan was performed (interpretation using modified Prospective Investigation of Pulmonary Embolism Diagnosis criteria).

RESULTS

A total of 274 patients (89.5%) had a non-intermediate V/Q lung scan: 88 (28.8%) high probability, 149 (48.7%) low probability, and 37 (12.1%) near normal/normal. Thirty-two patients (10.5%) had an intermediate V/Q lung scan. Patients with an intermediate V/Q lung scan presented a significantly greater frequency of age greater than 70 years (81.2 versus 50.4%, P=0.01), previous cardiopulmonary disease (75 versus 46.3%, P=0.003), chronic obstructive pulmonary disease (34.4 versus 15.7%, P=0.01), and chest radiograph evidence of emphysema (18.7 versus 4.7%, P=0.008). We found no other significant differences in the rest of the characteristics studied between patients with an intermediate and a non-intermediate V/Q lung scan.

CONCLUSION

The possibility of an intermediate V/Q lung scan is higher in elderly patients and in patients with previous cardiopulmonary disease (especially with chronic obstructive pulmonary disease). Emphysema is the only chest radiograph abnormality associated with a greater possibility of an intermediate V/Q lung scan.

[Helical CT and perfusion scintigraphy: diagnosis of pulmonary embolus in the clinical practice]

The purpose of this study was to compare helical CT with lung perfusion scintigraphy (LPS) as the initial investigations of patients with suspected pulmonary embolism (PE).

PATIENTS AND METHOD

A total of 54 patients with clinically suspected acute PE were studied retrospectively. Each patient was assigned a pretest clinical probability of acute PE (very likely, 90%; possible, 50%; unlikely, 10%). Within 72 hours of presentation, patients underwent LPS and contrast material-enhanced helical CT. Perfusion LS was classified following the PISAPED criteria as normal or near normal; abnormal consistent with PE or abnormal not consistent with PE. Helical CT studies were categorized as positive for PE, negative for PE or non-diagnostic. The standard reference was a consensus based on LS, helical CT and clinical outcome.

RESULTS

In 38 of the 54 patients, the results of LS a hCT were concordant, 13 with PE and 25 without. There were 4 indeterminate hCT. In 12 patients LS and hCT were discordant. There were 4 LS false negative; 2 with parenchyma damage and 2 chronic PE. There were 5 LS false negative; 3 extrinsic vascular compressions and one low clinical probability. There was 1 hCT false positive because of breathing artifact and 2 false negatives because of subsegmental emboli.

CONCLUSION

Accurate diagnosis of acute PE is possible combining perfusion scanning and clinical probability. Helical CT has added information in patients with discordant clinical probability and perfusion lung scan results. Helical CT demonstrated lesions other than PE considered responsible for the patient's symptoms, but it was insensitive to embolism of subsegmental branches.

Pulmonary embolism

PE is one of the great challenges in medicine. It is a disease that carries with it a high mortality rate, yet no historical piece of information, physical examination finding, or diagnostic modality is perfect at excluding its possibility. Emergency physicians must be vigilant about considering PE in the differential diagnosis of a variety of presenting complaints and must use a variety of diagnostic and therapeutic options as they manage patients with suspected or confirmed PE. The diagnostic options range from bedside diagnostic tests to highly specialized imaging available at only specialized institutions. Knowing the advantages and disadvantages of each of the diagnostic modalities assists the physician in employing the best test. Therapeutic options also vary widely and include anticoagulation, vena caval interruption, systemic thrombolysis, embolectomy, and other therapeutic adjuncts, such as ECMO and inhaled nitric oxide. Similarly, awareness of the indications and contraindications to the varied therapeutic agents ensures appropriate therapy when the diagnosis is made.

Evaluation of DNA aptamers directed to thrombin as potential thrombus imaging agents

Two DNA aptamers directed against two separate exosites on human alpha-thrombin were evaluated for thrombus-imaging potential. Aptamer ODN 1 is directed to the thrombin substrate binding site (exosite 1). Our finding that ODN 1 competes with fibrin for binding to exosite 1 on thrombin suggests that ODN 1 will not be useful for thrombus imaging. Aptamer ODN 2 is directed against the thrombin heparin binding site (exosite 2). ODN 2 bound to model thrombi that were formed either by clotting purified fibrinogen with thrombin, or by recalcifying citrated plasma. As the thrombin content of thrombi was increased the rate of ODN 2 uptake into preformed thrombi increased, whereas the rate of release of ODN 2 out of preformed thrombi decreased. This in vitro data suggested that ODN 2 might be useful for thrombus imaging because it can bind to exosite 2 on fibrin-bound thrombin. However, in a rabbit jugular vein model using thrombus supplemented with human thrombin, ODN 2 uptake was equal to the ovalbumin control, and did not reflect thrombin content. While the in vitro results with ODN 2 were consistent with thrombus imaging, the rapid clearance of ODN 2 from circulation, combined with slow mass transfer in the clot, seem to work against in vivo thrombin-dependent imaging or washout analysis.

What happens after a lung scan? Management and outcome of patients in a regional hospital

Pulmonary embolism (PE) remains a common preventable cause of death in hospitalized patients. The purpose of this study is to examine the in-hospital management, complications of treatment and clinical outcomes of inpatients undergoing lung scintigraphy for the diagnosis of PE in a regional hospital. Two hundred consecutive inpatients with suspected PE were enrolled. The results of lung scans, stratified according to the probability of pulmonary embolism, were correlated with anticoagulation status, discharge diagnosis, haemorrhagic complications and clinical outcome at 6 months. The use of complementary imaging investigations was also determined. Other imaging was performed infrequently (Doppler ultrasound in 18% of patients, CT pulmonary angiography (CT-PA) in 0.5% and conventional pulmonary angiography in 4% of patients). Long-term anticoagulation was initiated in 66 patients (33%), including 10 with intermediate probability lung scans (IPLS) who had no further investigations. Major haemorrhage occurred in 14% of all long-term anticoagulated patients followed up. The recognized recurrence rate was very low (3%) and there was no documented mortality from PE. Most patients with suspected PE are treated on the basis of the lung scan result without further tests. However, other imaging (especially CT-PA and conventional pulmonary angiography) should be performed prior to anticoagulation in patients with IPLS in whom the diagnosis is in doubt. Standard anticoagulation for 6 months appears to be effective for PE, and the recurrence rate is low. However, it has a significant risk of major haemorrhagic complications.