Variations in Respiratory Excretion of Carbon Dioxide Can Be Used to Calculate Pulmonary Blood Flow

BACKGROUND: A non-invasive means of measuring pulmonary blood flow (PBF) would have numerous benefits in medicine. Traditionally, respiratory-based methods require breathing maneuvers, partial rebreathing, or foreign gas mixing because exhaled CO(2) volume on a per-breath basis does not accurately represent alveolar exchange of CO(2). We hypothesized that if the dilutional effect of the functional residual capacity was accounted for, the relationship between the calculated volume of CO(2) removed per breath and the alveolar partial pressure of CO(2) would be reversely linear. METHODS: A computer model was developed that uses variable tidal breathing to calculate CO(2) removal per breath at the level of the alveoli. We iterated estimates for functional residual capacity to create the best linear fit of alveolar CO(2) pressure and CO(2) elimination for 10 minutes of breathing and incorporated the volume of CO(2) elimination into the Fick equation to calculate PBF. RESULTS: The relationship between alveolar pressure of CO(2) and CO(2) elimination produced an R(2) = 0.83. The optimal functional residual capacity differed from the “actual” capacity by 0.25 L (8.3%). The repeatability coefficient leveled at 0.09 at 10 breaths and the difference between the PBF calculated by the model and the preset blood flow was 0.62 ± 0.53 L/minute. CONCLUSIONS: With variations in tidal breathing, a linear relationship exists between alveolar CO(2) pressure and CO(2) elimination. Existing technology may be used to calculate CO(2) elimination during quiet breathing and might therefore be used to accurately calculate PBF in humans with healthy lungs.