Effect of cardiac index and hematocrit changes on oxygen consumption in resuscitated patients

Oxygen consumption (Vo2) has been found to depend on oxygen delivery (Do2) following resuscitation from hemorrhage in both humans and animals. The relative influence of blood flow and arterial oxygen (O2) content, the components of Do2, has not been separately assessed. To determine the relative contribution of content and flow, we determined Do2 and Vo2 while making systematic changes in cardiac index (CI) and hematocrit (HCT). Fourteen patients were studied within 36 hr of hypotension from which they were resuscitated to a HCT of 27.9 +/- 0.4% (mean +/- SEM). Following initial hemodynamic measurements, CI was manipulated by changing end expiratory pressure by increments of +/- 5 cm H2O and measurements were repeated. Patients were then transfused overnight to raise their HCT to 36.7 +/- 0.5% and measurements were repeated, varying CI in the same manner. The increase in HCT resulted in significant (P less than 0.05) increases in O2 delivery (+ 130 +/- 33 ml/min/m2), arterial O2 content (+ 3.9 +/- 0.3 vol%), and mixed venous O2 content (+ 3.7 +/- 0.4 vol%). O2 extraction decreased by 6 +/- 1% from 30 +/- 2%. The change in HCT did not alter Vo2 (143 +/- 7 ml/min/m2), CI (3.6 +/- 0.2 L/min/m2), or intrapulmonary shunt (18.1 +/- 1.7%). However, as CI was changed at both levels of HCT, there were changes in Vo2 directly dependent on Do2. We conclude that oxygen consumption in patients resuscitated from hemorrhage may be influenced by oxygen delivery and that this influence is related more to flow than to arterial content.