Non-invasive pulmonary blood flow measurement by means of CO2 analysis of expiratory gases.

Two different methods of CO2-derived non-invasive assessment of the pulmonary blood flow were evaluated. The principle of the formula, as proposed by Gedeon et al., is based on a rapid change in arterial CO2 content and subsequent changes in endtidal PCO2 and CO2 elimination. Both methods were compared to thermodilution cardiac output in 44 postoperative patients after CABG. The first method consisted of a short period of hyperventilation followed by hypoventilation. Comparison with the thermodilution cardiac output showed a low correlation coefficient: using a measured arterial--end-tidal PCO2 difference (E) r = 0.397 was found. Entering a fixed E of 0.53 kPa resulted in r = 0.454. These disappointing figures may be explained by procedural mistakes. The second method, based on partial rebreathing by means of adding an additional dead space of 220 ml for 30-45 s, correlated very well with the thermodilution findings. Correlation coefficients of r = 0.925 (measured E) and r = 0.925 (fixed E) were found. Considering the simplicity of the method, the additional dead space approach seems to be an easy and reliable way to determine pulmonary blood flow.