The presence of a normal alveolar dead-space fraction and a sensitive plasma D-dimer assay can exclude pulmonary embolism in most patients seen in the Emergency Department. This can help identify a subset of patients for whom imaging studies are warranted. The primary author is from the Carolinas Medical Center in Charlotte, North Carolina.



(1) respiratory parameters: CosmoPlus! from Novametrix Medical Systems

(2) special mouthpiece connected to a CO2 flow sensor

(3) blood gas analyzer

(4) plasma D-dimer assay demonstrated sensitive for pulmonary embolism detection


Parameters for alveolar dead space fraction:

(1) tidal volume (VT)

(2) PaCO2

(3) PECO2 (mixed expired CO2)

(4) airway dead space volume (VDSaw)


According to the Enghoff modification of the Bohr equation (assumes PACO2 = PaCO2):


(physiologic dead space volume) / (tidal volume) =

= ((PaCO2) - (PECO2)) / (PaCO2)


alveolar dead-space fraction in percent =

= (alveolar dead space volume) / (tidal volume) =

= (((physiologic dead space volume) / (tidal volume)) - ((airway dead space volume) / (tidal volume))) * 100%


A normal alveolar dead-space fraction is <= 20%.


The presence of a normal alveolar dead-space fraction and a normal plasma D-dimer can be used to exclude pulmonary embolism in the Emergency Department.



• In the 2001 report (conducted at 6 urban teaching hospitals in the USA), the sensitivity was 98.4% with a specificity of 51.6% (63 of 64 patients with pulmonary embolism were identified, as were 153 of 316 without).

• Posterior probability of a pulmonary embolism with normal results in both tests was 0.75% (with 95% CI 0.0% to 3.4%).



• Pulmonary emboli come in all sizes. This should be able to exclude a pulmonary embolism large enough to bring a person to the Emergency Department, but would be insensitive for small emboli.


To read more or access our algorithms and calculators, please log in or register.