In the daily clinical routine at the bedside, information on effective pulmonary 
blood flow (PBF) is limited and requires invasive monitoring, including a 
pulmonary artery catheter, to determine both cardiac output and intrapulmonary 
shunt. Therefore we evaluated a non-invasive method for the measurement of PBF 
in a clinical setting, including 12 patients with acute respiratory failure 
(acute respiratory distress syndrome) undergoing prone positioning. PBF was 
determined before (baseline), during and after prone positioning, by using a 
foreign gas rebreathing method with a new photoacoustic gas analyser. Values 
were compared with the cardiac output corrected for intrapulmonary shunt 
(COeff). Responders to prone positioning were defined according to the 
improvement of arterial oxygenation. A total of 84 measurements were performed. 
PBF values correlated well with COeff (R2=0.96; P<0.0001). Bias and limits of 
agreement (+/- 2 S.D.) for all measurements were -0.11 +/- 0.76 litre/min. At 
baseline, responders showed significantly lower PBF levels than non-responders 
(4.8 +/- 1.0 compared with. 6.4 +/- 1.2 litre/min; P=0.03). During prone 
positioning, PBF increased continuously in responders and remained high after 
patients had been returned to the supine position. PBF was unaffected in 
non-responders. Mean total increase in PBF was 1.2 +/- 0.2 litre/min in 
responders compared with -0.4 +/- 0.2 litre/min in non-responders (P<0.0001). In 
conclusion, the investigated rebreathing system allows for a non-invasive 
determination of PBF at the bedside. The accuracy of the measurements is 
comparable with the thermodilution method. It is able to reliably reflect 
changes in PBF induced by prone positioning. Moreover, measuring PBF might be a 
promising tool to identify responders to prone therapy.