Pulmonary capillary blood flow by partial CO₂ rebreathing: Importance of the regularity of the respiratory pattern

The partial CO₂ rebreathing technique has been shown to be a reliable non-invasive method for measurement of pulmonary capillary blood flow (Q(c)), but experience with this technique has been limited to controlled mechanical ventilation. In this study, we evaluated this technique during spontaneous and oriented ventilation in nine subjects without known cardiopulmonary disease. Subjects underwent 10 consecutive (Q(c)) measurements with both spontaneous and oriented ventilation. Breath-by-breath gas exchange was measured and (Q(c)) was calculated from changes in CO₂ elimination and P(ET)CO₂, which were achieved by sudden increase of the apparatus deadspace (rebreathing period). An exponential curve was fitted to the P(ET)CO₂ values in the rebreathing period in order to estimate P(ET)CO₂ at equilibrium. We found that mean (Q(c)) values were not influenced by the ventilation pattern (P = 0.51), but that the intra-individual variability with oriented ventilation (median = 16.0%) was significantly lower than with spontaneous ventilation (median = 31.8%, P = 0.039). Accordingly, the curve fitting for rebreathing P(ET)CO₂ rise failed in 4.4% of measurements with oriented ventilation vs. 18.9% of measurements with spontaneous ventilation (P = 0.039). Our results suggest that the performance of the partial CO₂ rebreathing technique is adversely affected by spontaneous ventilation and, consequently, that this method should be reserved for patients with regular respiratory patterns.