The effect of increasing filling pressures on cardiac performance was examined in 15 patients undergoing fluid resuscitation for hypovolemic and septic shock. The initial significant increase in pulmonary artery wedge pressure (WP) from 7.0 +/- 2.0 to 11.9 +/- 1.6 mm Hg was associated with an increase in stroke volume index (SVI) from 24.2 +/- 9.8 to 34.7 +/- 12.4 ml/M2 (p less than 0.01), left ventricular stroke work index (LVSWI) from 16.9 +/- 7.8 to 28.5 +/- 11.6 g x m/M2 (p less than 0.01) and cardiac index (CI) from 2.25 +/- 0.68 to 3.06 +/- 0.85 L/min x M2 (p less than 0.01). Further significant increase in WP to 16.0 +/- 0.9 mm Hg did not result in any increase in SVI, LVSWI, or CI. The correlation between central venous pressure (CVP) and WP during fluid loading was only fair and the changes in CVP vs WP did not significantly correlate. We suggest that the optimum left heart filling pressure during fluid resuscitation of patients with hypovolemic and septic shock may not exceed a WP of 12 mm Hg.