NASHVILLE, USA – When a pulmonary embolism is suspected, a new bedside test may complement clinical assessment and facilitate diagnosis, according to a new study published in the European Respiratory Journal. 

Pulmonary embolism is associated with a high mortality when not correctly diagnosed and promptly treated. The diagnosis must be considered in many different clinical settings and pulmonary embolism may present with unspecific symptoms such as dyspnoea or chest pain. 

The diagnostic work-up classically includes a ventilation/perfusion scan, which is not only expensive but also often inconclusive. 

Physicians of the Vanderbilt University School of Medicine sought to show how a pulmonary embolism may be easily excluded, thus making further investigations unnecessary.

They included in their analysis 298 patients presenting at the Emergency Department of their university hospital and for whom a contrasted chest helical CT, a ventilation/perfusion lung scan, pulmonary angiogram or a lower extremity duplex evaluation had been requested by the emergency physician, thus indicating a clinical suspicion of pulmonary embolism.

Within 24 hours of their work-up, patients were asked to also undergo measurement of end-tidal carbon dioxide tension (PET,CO2), a surrogate marker of vascular obstruction from pulmonary embolism. The test was performed by blowing into a device – a handheld capnograph with a modified sensor for detection of end-tidal carbon dioxide tension – at the bedside, which generated instant values. 

When evaluating patients for suspected pulmonary embolism, the new test may provide important additional information. “Diagnostic algorithms to simplify testing procedures in the diagnosis of pulmonary embolism have been explored, most combining D-dimer testing (a measure of a specific fibrin in plasma) and CT angiography. PET,CO2 should be prospectively compared to D-dimer in accuracy and simplicity to exclude pulmonary embolism,” write the researchers.

Of the patients included, 39 (13%) were diagnosed with pulmonary embolism. The group with pulmonary embolism had a significantly lower PET,CO2 (30.5±5.5 mmHg) versus healthy volunteers or patients without pulmonary embolism. 

According to study results, a PET,CO2 level of 36 mmHg is an optimal threshold to exclude pulmonary embolism in patients evaluated for possible thromboembolism. Patients with confirmed pulmonary embolism have significantly lower values. 

“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 0 mmHg. As a result, CO2 content measured at end expiration, which represents admixture of all alveolar gas, decreases in proportion to dead space ventilation,” explains first author Dr Anna Hemnes.

While there are many potential aetiologies of increased dead space ventilation, e.g. 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 pulmonary embolism, e.g. unstable angina and gastro-oesophageal reflux. 

In the study, a PET,CO2 level of 36 mmHg showed optimal sensitivity and specificity (87.2 and 53.0%, respectively) with a negative predictive value of 96.6% (95% CI 92.3–98.5). 

“Using a cut-off of 36 mmHg, we were able to achieve a negative predictive value of 96.6%, which is similar to that reported with D-dimer testing.”

This novel test may provide a noninvasive, simple, inexpensive and reliable bedside means for excluding pulmonary embolism. 

Reference:

Hemnes AR et al. Eur Respir J 2010 Apr; 35: 735–741.