Relationships between mixed venous oxygen hemoglobin saturation and its components. To create these curves, we modeled a standard ICU population of 1,000 patients (Excel, Microsoft) with normal distribution of arterial oxygen hemoglobin saturation (SaO2; 0.95 ± 0.05 limited to 1) and normal distributions of total body oxygen consumption (VO2) needs (140 ± 30 ml/minute.m2), cardiac output (CO; 3.0 ± 0.5 L/minute.m2) and hemoglobin concentration (Hb; 100 ± 15 g/L). Only one of the four components was sequentially changed (Y-variables) to look at its specific relationship with mixed venous oxygen saturation (SvO2) (X-variable). When unchanged, variables were set to their mean value. The horizontal arrows indicate the fluctuations of the Y-variable around its mean value (±2 standard deviations). The vertical arrows show the corresponding fluctuations of SvO2. We can see that reference ranges (mean ±2SD) of SaO2, VO2 needs, CO, and Hb are compensated for by a 26%, 50%, 47%, and 40% change in SvO2, respectively. Thus, CO is not necessarily the predominant component of SvO2 except when low (left, flat part of the relationship).