Pulmonary capillary blood flow and cardiac output measurement by partial carbon dioxide rebreathing in patients with acute respiratory distress syndrome receiving lung protective ventilation.

Background: Partial carbon dioxide rebreathing noninvasively measures the pulmonary capillary blood flow and estimates the cardiac output with the use of a predicted shunt value. It has been reported that the accuracy of the method is decreased in patients with high pulmonary shunt. The aim of this study was to investigate the agreement between partial rebreathing and thermodilution for the determination of pulmonary capillary blood flow and cardiac output in the setting of acute respiratory distress syndrome.

Methods: Twenty consecutive patients with the acute respiratory distress syndrome were enrolled. Ventilator settings include low tidal volume (6 ml x kg(-1)) and positive end-expiratory pressure + 2 cm H2O higher than the lower inflection point if present or 10 cm H2O if not. Seven pairs of cardiac output and pulmonary capillary blood flows were recorded every 20 min over a 2-h period. The authors determined bias, SD, limit of agreement (95% confidence interval) and percentage error.

Results: Bias and agreement for cardiac output measurement were 0.8 +/- 1.2 l x min(-1) (-2.1 to 3.7 l x min(-1)), and percentage error was 36%. Bias and agreement for pulmonary capillary blood flow measurement were -0.1 +/- 0.8 l x min(-1) (-2.1 to 1.9 l x min(-1)), and percentage error was 35%. Dead space, arteriovenous oxygen content difference, mean pulmonary arterial pressure, and baseline cardiac output were independently associated with differences between methods.

Conclusions: In patients with the acute respiratory distress syndrome, partial rebreathing cannot yet replace thermodilution for measuring pulmonary capillary blood flow or cardiac output. However, accuracy of the method is close to the boundary of clinical relevance.