Correct assessment of vessel cross-sectional area (CSA) is essential for reliable cardiac output (CO) measurements by means of pulsed Doppler echocardiography. In 23 patients who underwent coronary artery bypass grafting (CABG) the main pulmonary artery CSA and diameter changes were assessed by epicardial two-dimensional echocardiography using a 7.5 MHz cm2 transducer. Our data indicate that the shape of the pulmonary artery changes over time. Time averaged CSA ranged from 3.51 to 8.29 cm2 (mean 5.04 cm2; SD 1.3 cm2) and the distensibility varied from 13% to 33% (mean 23%; SD 5%). The limits of agreement for the CSA calculated from the most suitable diameter (when assuming vessel circularity) and the corresponding traced CSA (reference value) during peak systole were -0.16 +/- 0.56 cm2 (mean +/- 2SD). In this idealized set-up (not clinically implementable) the maximal discrepancy between the calculated and traced CSA was 12% and 17% during peak systole and diastole, respectively. Therefore, a potential error in CO determination is incurred if CSA is assessed from a single diameter.