AbstractA pulmonary artery cannula allows the determination of free and wedge pulmonary artery pressures and mixed venous oxygen tension. These indices have been reported to provide useful information in the assessment of the haemodynamic status of the ill patient. The purpose of this study was to compare them with systemic arterial and central venous pressures as predictors of cardiac output during acute continuous haemorrhage in the dog. Pulmonary artery pressure changed almost linearly with cardiac output, and the percentage changes in each were similar; by contrast, systemic arterial pressure was an inferior predictor of cardiac output. Pulmonary artery wedge pressure fell rapidly in the initial phase of bleeding, but right atrial pressure more gradually. The oxygen tension of blood in the pulmonary artery fell steadily during haemorrhage. These findings suggest that data derived from the use of a pulmonary artery cannula may be more useful than systemic arterial and central venous pressures in the detection of hypovolaemia and reduced cardiac output; more frequent use of a pulmonary artery cannula should be made in patients in whom blood volume may fluctuate rapidly.