Cardiovascular and respiratory parameters were obtained in 374 studies from 151 patients (64 septic or septic shock and 87 with nonseptic cardiogenic syndromes). Cardiac output and related measures, respiratory parameters, pulmonary blood volume (DV/m2), cardiac ejection fraction (EFx), left ventricular end-diastolic volume (LVEDV), peripheral resistance (TPR), pulmonary shunt (Qs/Qt), physiologic dead space (Vd/Vt) and mean ventilation perfusion ratios (Va/Qt) were calculated. Both physiologic evaluation and multivariable statistical analyses of the data were performed. For a given level of (A-aO2 gradient/PaO2) septic patients have a higher Qs/Qt than cardiogenic patients. Hyperdynamic septic patients have a larger Qs/Qt, a greater Vd/Vt, and a lower VA/Qt and TPR than cardiogenics. The decrease in Va/Qt and the rise in VD/VT are strong functions of the increased EFx, which raises cardiac index (CI) and reduces LVEDV and the mean dispersion of pulmonary blood volume, thus causing hemodynamic redistribution of flow. Qs/Qt is mainly a direct function of the increase in CI which also tends to reverse the EFx mediated rise in VD/VT. The role of the septic mediated decrease in TPR as a contributing pathologic feature and the use of volume infusion to raise LVEDV and Va/Qt in high-CI septics with large Qs/Qt is discussed.