Fig. 2. Example of the effect of a rapid volume expansion on superior vena caval (SVC) size and Doppler pulmonary artery flow velocity in a group 2 patient. At baseline (top ), combined two-dimensional and M-mode echocardiography with simultaneous recording of tracheal pressure (TP) show a vena caval partial collapse (left ) associated with a major inspiratory decrease in pulmonary artery flow velocity (right ; group 2A). After volume expansion (bottom ), only slight respiratory changes in SVC diameter and in pulmonary artery flow velocity persisted (group 2B).