The threshold of oxygen delivery (the level below which evidence of hypoxia is produced), also defined as the “critical” DO2, has been determined in anesthetized dogs, rats, and pigs. In an attempt to define the critical DO2in conscious, healthy adults, Lieberman et al.  recruited eight paid volunteers, aged 19–25 yr, and reduced oxygen delivery from 14.0 ± 2.9 ml O2· kg−1· min−1to 7.3 ± 1.4 ml O2· kg−1· min−1by acute hemodilution followed by β-blockade.

As blood was removed, isovolemia was maintained by infusion of 5% human serum albumin and the subjects’ platelet-rich plasma. Cardiovascular measurements were made 30 min after insertion of invasive cannulae and before removal of blood; after hemodilution to a hemoglobin concentration of 5 g/dl (which reduced oxygen delivery to 9.9 ± ml O2· kg−1· min−1); and again after a further reduction in oxygen delivery to 7.3 ml O2· kg−1· min−1, achieved by the infusion of esmolol. Arterial and mixed venous blood was also sampled for measurement of pH, oxygen content, oxyhemoglobin saturation, and arterial plasma lactate concentration. At the conclusion of the experiment, all erythrocytes were transfused and subjects were thoroughly examined by a physician before discharge.

Despite increased heart rate and increased stroke volume and cardiac indices, there was no evidence of inadequate systemic oxygenation in any of the subjects. Oxygen consumption and plasma lactate concentrations increased slightly. In addition, one woman subject had a single transient ST-segment change during the study period. However, the incident was not symptomatic and resolved despite a further reduction in DO2. The researchers were not able to determine the critical level of systemic DO2, which appears to be less than the value achieved in this study.