Requirements:
(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.
Performance:
• 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%).
Limitations:
• 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.
