ObjectiveTo validate a new application of the modified acetylene rebreathing method for pulmonary capillary blood flow in a swine extracorporeal membrane oxygenation (ECMO) model.DesignProspective, sequential measurements of pulmonary capillary blood flow, using a rebreathing technique, as affected by different flows through the ECMO circuit.SettingA cardiovascular hemodynamic research laboratory at a university medical center.SubjectsFifteen young mature farm swine (48 to 52 kg).InterventionsPulmonary capillary blood flow was measured using a modified rebreathing technique, and this measurement repeated at different flow rates through the extracorporeal membrane oxygenation circuit. Pulmonary artery flow rates were measured using both thermodilution and echo-Doppler techniques for comparison purposes.Measurements and Main ResultsPulmonary capillary blood flow measurements, as assessed by modified acetylene rebreathing, compared well with both the thermodilution cardiac output measurement during normal circulation and the pulmonary artery flow probe measurement while the subjects received ECMO. Mean pulmonary capillary blood flow measured by acetylene rebreathing decreased from 89.72 +/- 6.97 (baseline) to 43.59 +/- 5.66 mL/kg/min as ECMO flow was maximized to 56.22 +/- 3.62 mL/kg/min. Decreasing the ECMO flow rate by half (to 28.23 +/- 3.45 mL/kg/min) caused an increase in mean pulmonary capillary blood flow to 53.79 +/- 6.16 mL/kg/min. When ECMO flow was discontinued, pulmonary capillary blood flow returned to a near baseline value of 71.68 +/- 7.05 mL/kg/min (mean values of pooled data for both closed-and open-chest animals [n = 15]). These measurements correlated well with both thermodilution cardiac output and pulmonary artery ultrasonic flow probe measurements.ConclusionsThe modified acetylene rebreathing method is a valid and accurate method for the measurement of pulmonary capillary blood flow in the presence of ECMO flows. Pulmonary blood flow decreases as ECMO flow is increased, and the extent of decrease is directly proportional to the amount of flow through the extracorporeal circulation.(Crit Care Med 1995; 23:726-732)