Pulmonary Embolism (cont.)
Pulmonary Embolism Diagnosis
Diagnosis of pulmonary embolisms has been difficult for many clinicians over the years because making the diagnosis definitively often required placing a catheter in the heart and injecting dye into the pulmonary vessels. As imaging technology has improved, making the diagnosis has become easier especially with computerized tomographic angiography, a.k.a. CT angiography. Patients with chronic pulmonary embolisms may have nonspecific, insidious symptoms so the diagnosis may be delayed, missed or found at autopsy. Currently, the medical literature has urged doctors to place this diagnosis high in their differential diagnosis because of the potential for lethality. Unfortunately the clinical exam is notoriously inaccurate with regards to pulmonary embolism or DVT. Therefore, frequently other tests need to be done. Many of the tests are not specific but yield clues that either point to or point away from the diagnosis of pulmonary embolism. These tests are as follows:
Chest X-ray (may show other causes for shortness of breath like heart failure or a pneumothorax)
Electrocardiogram (ECC, EKG - tachycardia and a right strain pattern may occur with pulmonary embolism especially with large central clots)
CBC (complete blood count; helps to exclude infections)
D–dimer test (measures breakdown products of blood clots; if negative, suggests there is less chance that the person has a pulmonary embolism; if elevated, it is less useful since many things cause an elevation of this test including many things that may be associated with pulmonary embolism, like pregnancy, cancer, recent surgery, and infection )
Venous Doppler study (legs or occasionally arms) can confirm the presence or absence of a DVT. In fact approximately 50% of lower extremity DVTs will have asymptomatic pulmonary embolisms.
Usually these tests are done first, if the patient's history and preliminary tests suggest pulmonary embolism, then it is likely that at least one or more tests will be done as follows:
Pulmonary angiography is the gold standard for diagnosing pulmonary embolism. In this case, a catheter is placed in a large vein in the groin and moved through the right side of the heart in to the main pulmonary artery. Dye is injected and X-rays obtained of the pulmonary vessels. This test is done less frequently these days because of increased sophistication of CTs.
CT scan of the lungs using a newer generation CT, pulmonary embolism protocol, where dye is injected to visualize the pulmonary arteries; this is not 100% diagnostic for pulmonary embolism but as the newer CTs increase resolution, they are approaching the gold standard angiogram.
VQ scan (Ventilation – Perfusion scan) uses radiolabeled chemicals that identify the location of inhaled air and match it to the blood flow. If there is good air flow in the lungs but segments of the lung have poor or no blood flow, then this suggests that blood clot may be present. This test is often read as normal suggesting no pulmonary embolus is present. A low probability reading depending on the clinical situation can still have a 30% chance of pulmonary embolism. A high probability reading can have upwards of 90% chance of pulmonary embolism. An intermediate or indeterminate reading falls somewhere in between. The key issue related to this test is referred to as the pretest probability. This means the clinical situation (history, physical, and other supporting tests) may determine to some degree the likelihood of pulmonary embolism. If the possibility for pulmonary embolism is high than the VQ scan is more accurate and vice versa.
Medically Reviewed by a Doctor on 11/3/2014
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