The observation that calves with brisket disease and markedly reduced cardiac output often also have marked reduction in resting oxygen consumption stimulated comparison of oxygen consumption index in cardiac patients having normal and severely reduced cardiac indices. Group I consisted of 29 patients whose resting cardiac index was ≤1.5 liters/min/M2(mean:1.4 ± 0.1 standard deviation). Group II consisted of 100 cardiac patients selected only for cardiac index between 3.0 and 4.0 liters/min/M2(mean:3.3 ±0.3). The resting oxygen consumption index in Group I was 115.9 ± 17.3 ml/min/M2compared to 144.5±15.6 in Group II (p<.001). Similar reductions in oxygen consumption index have been observed in dogs with experimental hemorrhagic shock. Oxygen is the most nearly flow-limited of all physiologically important constituents of the blood in man. As cardiac index decreases, oxygen consumption may initially be maintained by increased oxygen extraction and compensatory changes in the oxygen-carrying capacity of blood and position of the oxyhemoglobin dissociation curve. With very severe reductions in cardiac index these mechanisms may become inadequate. Reduced oxygen consumption in patients with severely reduced cardiac index may represent obligatory adjustment at the tissue level to critically reduced oxygen delivery.