Laboratory analysis Blood was withdrawn from superior vena cava approximately 2cm above right atrium and pulmonary artery was performed at the time of each StO2 measurement to determine ScvO2 (%) and SvO2(%) respectively. In view of the known problems arising during sampling from pulmonary artery, including the possibility of contaminating artery blood with pulmonary capillary blood, all samples from this site were withdrawn over 30s, using a low-negative pressure technique, and never with the balloon inflated . A standard volume of 1mL of blood was obtained from each side after withdrawn of dead-space blood and flushing fluid. All measurements were made using a cooximeter (RapidLab 1265, Bayer HealthCare, Germany).