Pulmonary embolism (PE) can be a fatal condition. It occurs when there is a blockage in one of the pulmonary arteries in the patient’s lungs, which most often occurs when a blood clot in the legs (deep venous thrombosis or DVT) or sometimes other parts of the body dislodges and travels to the lungs.
Blood clots can occur in patients of all ages and conditions. As the National Blood Clot Alliance states, “Blood clots affect people from all walks of life…blood clots don’t discriminate.” (NBA all-star, Chris Bosh, will miss the remainder of 2014-15 season due to blood clot found in his lung.)
More than 200,000 people in the U.S. die of acute PE each year, making it a leading cause of cardiovascular death. However, when a patient is accurately diagnosed and then treated, researchers have found that hospital mortality rates can fall from 30% to 8%. Consequently, when a patient exhibits symptoms of having PE, making a safe, non-invasive, and inexpensive assessment that can be rapidly done at the bedside is an important first step in improving health outcomes.
The Physician-Patient Alliance for Health & Safety has worked with panels of experts to develop recommendations that may assist in preventing venous thromboembolism (VTE), a source of PE, in maternity and stroke patients. One of our experts, Anna Hemnes, MD, Assistant Director Pulmonary Vascular Center at Vanderbilt University Medical Center, and her colleagues completed research that investigates making a safe, non-invasive, and inexpensive assessment to exclude PE.
Dr. Hemmes and I recently discussed why this research is so important. Here is a synopsis of what we talked about:
Wong: Why is there is a need for safer, more accurate and readily available diagnostic testing for PE?
Hemnes: Current diagnostic algorithms typically combine D dimer testing and CT angiography. The use of both of these has shortcomings. D dimer testing requires venipuncture and time for test performance. CT angiography, although used quite extensively, exposes the patient to contrast and radiation risk; these risks need to be weighed against its low percentage of actually demonstrating a PE. Therefore, there is a need for a safer, more accurate readily available diagnostic testing for PE.
Wong: You and your colleagues researched the use of capnography in the diagnosis of PE. This technology is usually used to monitor breathing, how can it help exclude PE?
Hemnes: End-tidal partial pressure of CO2 (ETCO2) is a physiological surrogate for vascular obstruction from PE. Pulmonary thromboembolism results in dead space ventilation and therefore prevents meaningful gas exchange in the subtended lung unit, yielding an alveolar CO2 content as low as zero mmHg. As a result, carbon dioxide content measured at end expiration, which represents admixture of all alveolar gas, drops in proportion to dead space ventilation. While there are many potential etiologies of increased dead space ventilation—for example, advanced chronic obstructive pulmonary disease—these diseases are usually easily identified. Increased dead space ventilation is not associated with common clinical conditions that can present similarly to PE—for example, unstable angina or gastroesophageal reflux. So, in an emergency department or outpatient setting, a low ETCO2 value may indicate that a PE is present and a normal ETCO2 value should suggest that PE is very unlikely, since there is no increased dead space ventilation.
Wong: What did you conclude from your research?
Hemnes: We found that ETCO2 is highly predictive in excluding PE. Moreover, we also found that using capnography in combination with Wells’ Score improved the negative predictive value to a higher level of accuracy. We looked at more than 300 patients who were 18 years or older who were admitted to the ER at Vanderbilt University Medical Center from October 2007 to April 2008. These patients were screened electronically for a computer order for contrasted chest helical CT, ventilation-perfusion lung scan, pulmonary angiogram or lower extremity Duplex evaluation. Patients meeting screening criteria were approached for consent to undergo end-tidal CO2 determination within 24 hours of study order placement.
We found that the group with PE had a significantly lower ETCO2 (30.5 ± 5.5 mmHg vs. healthy volunteers, p<0.001), which was also significant compared with the no PE group (P<0.001). Mean ETCO2 was not different in the two D dimer groups (35.3 ± 5.9 mmHg D dimer positive vs. 36.1 ± 5.2 in D dimer negative groups, p=0.35). As I mentioned, these results improved when used in combination with Wells’ Score. Moreover, there were no adverse events related to ETCO2 measurement.
Wong: Could capnography be used to exclude PE in all patients?
Hemnes: Unfortunately, no. In our research, patients who were pregnant had known hypercarbic respiratory failure, were on mechanical ventilation, using a face mask oxygen or more than 5L/minute nasal cannula oxygen, or had a known neuromuscular disease were excluded.
Wong: What benefits do you see in using capnography to exclude PE?
Hemnes: The importance of not using CT scanning cannot be ignored. In our cohort, 226 patients (76%) underwent diagnostic CT scanning. The long-term risks of exposure to radiation from chest CT scanning are a concern. The monetary savings from preventing unnecessary CT studies is also potentially substantial. At a cost per study of $1,739, patients in our study underwent a total of 226 contrast enhanced helical chest CTs, 120 of which could potentially be spared saving $208,680.