Study Finds Ways to Improve Detection
of Blood Clots in the Lung
Enhancing Diagnostic Tests Could Reduce Deaths from
Pulmonary Embolism
A new study of a commonly used imaging test of the chest to detect
potentially deadly blood clots in the lung shows that extending
the scan to the legs – where the clots typically originate
– or adding a standard clinical assessment significantly improves
physicians' abilities to accurately diagnose pulmonary embolism.
A sudden and potentially deadly blockage in a lung artery, pulmonary
embolism affects an estimated 600,000 Americans each year, making
it the fourth most commonly occurring cardiovascular problem in
the United States. The multicenter study was funded by the National
Heart, Lung, and Blood Institute (NHLBI) of the National Institutes
of Health (NIH).
The Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED)
II is the largest study ever conducted to assess the role of contrast-enhanced
multidetector computed tomographic (CT) angiography for diagnosing
pulmonary embolism. In the June 1, 2006, issue of the New England
Journal of Medicine, PIOPED researchers from eight clinical
centers report that chest CT angiography alone detects suspected
pulmonary embolism in only 83 percent of patients; in contrast,
combined results of the chest CT angiogram and the leg CT scan detect
clots in 90 percent of patients. The researchers recommend that
physicians consider additional test results before ruling out pulmonary
embolism in patients whose scan does not detect clots but whose
clinical assessment suggests a high likelihood of pulmonary embolism.
"Imaging technologies are one of the most rapidly evolving
areas of medicine, and they greatly expand our ability to diagnose
and treat disease," said Elias A. Zerhouni, MD, NIH Director
and a board-certified radiologist. "In recent years, chest
CT scans have become the most widely used technique for diagnosing
pulmonary embolism. But, until now, we have not had enough scientific
evidence to really understand how accurate they are for detecting
this often-fatal condition."
Pulmonary embolism (PE) leads to death in nearly one-third of untreated
cases, but therapies lower the death rate to between 3 percent and
8 percent. In nine out of 10 cases, PE begins as a clot in the deep
veins of the leg, a condition known as deep vein thrombosis (DVT).
The clot breaks free from the vein and travels to the lung, where
it can block an artery. Commonly used treatments for both PE and
DVT include anti-clotting medications and injections of clot-busting
agents.
"There are many effective ways to prevent and treat blood-clotting
diseases," noted NHLBI Director Elizabeth G. Nabel, MD. "Pulmonary
embolism is underdiagnosed and therefore often untreated. If detected
early, however, blood clots can often be prevented from causing
permanent damage or death."
In PIOPED II, researchers compared the accuracy of three ways to
diagnose blood clots in 824 patients suspected of having pulmonary
embolism: chest CT angiogram alone, chest CT angiogram with venous-phase
imaging (leg CT), and chest CT angiogram with an objective clinical
assessment known as the Wells Score. The Wells Score is a validated
tool to determine the likelihood that a patient has PE based on
characteristics such as signs and symptoms, heart rate, and risk
factors. A high score indicates that a patient has a high probability
of having PE. To determine their accuracy, the tests were compared
with the participants' composite results from other validated diagnostic
tests for PE.
Overall, the sensitivity (the ability to detect clots) of the combined
chest CT and leg CT was 90 percent, compared to 83 percent sensitivity
of the chest CT angiogram alone. The specificity (the ability to
rule out the presence of clots) of the chest CT alone compared to
the chest CT and leg CT combined was similar (about 95 percent).
A high clinical probability combined with positive chest CT correctly
indicated PE in 96 percent of participants. However, in patients
with a high clinical probability, a negative chest CT result did
not confidently rule out a diagnosis of PE, the researchers report.
"This study suggests that chest CT angiogram for detecting
dangerous blood clots in the lung is good, but sometimes it is not
enough," noted Paul D. Stein, MD, director of research education
at St. Joseph Mercy Oakland Hospital in Pontiac, Michigan, professor
of medicine at Wayne State University, and lead author of the paper.
"We can more accurately detect or rule out pulmonary embolism
by taking pictures of the leg veins in addition to pictures of the
lung arteries."
The results of the chest CT combined with the patient's clinical
probability assessment were comparable to the results from the combined
chest CT and leg CT scans.
"Our study spells out the strengths and weaknesses of chest
CTs for diagnosing pulmonary embolism, and will help guide physicians
on when more tests are needed," added Stein, who also chaired
the PIOPED II steering committee.
During a chest CT angiogram, contrast material (dye) to make the
blood vessels in the lungs more visible is injected into a vein
in the patient's arm. The patient lies on a table as a machine with
dozens of detectors rotates around to quickly take X-ray pictures
of the blood vessels -- in the lungs for the chest CT or in the
pelvis or thighs for the leg CT. No additional dye is needed for
the leg CT, which can be performed immediately after the chest CT.
A computer combines the images to make detailed pictures.
CT scans are noninvasive and well tolerated by most patients. Adverse
effects are rare and are primarily related to the iodine-based dye
that is injected; possible complications include kidney damage in
patients with kidney disease and allergic reaction. The tests involve
some exposure to radiation, but the benefits outweigh the risks.
Risk factors for pulmonary embolism include blood clots in the
leg or a history of such problems, and certain inherited conditions
that increase the risk for blood clotting. Individuals who recently
have been treated for cancer, have been bedridden, or have had surgery
or suffered a fracture in the past few weeks are also more likely
to develop PE. Other risk factors for DVT, which can lead to PE,
include sitting for long periods of time, pregnancy and the 6-week
period after pregnancy, and being overweight or obese. Women who
take hormone therapy or birth control pills are also at increased
risk for DVT.
Signs of PE include unexplained shortness of breath, pain with
deep breathing, and coughing up blood. Rapid breathing and a fast
heart rate can also indicate possible PE. In some cases, there are
only signs of DVT, such as swelling of the leg or along the vein
in the leg, pain or tenderness in one leg, feeling of increased
warmth in the area of the leg that is swollen, and red or discolored
skin on the affected leg. Other patients do not experience any symptoms
or signs of PE or DVT.
To interview an NHLBI expert, please contact the NHLBI Communications
Office at (301) 496-4236. To interview Dr. Stein, please contact
Jennifer Munro at (248) 858-6662.
Resources
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It is the primary federal agency for conducting and supporting basic,
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the causes, treatments, and cures for both common and rare diseases.
For more information about NIH and its programs, visit http://www.nih.gov.
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