Critical oxygen delivery in conscious humans is less than 7.3 ml O2 · kg-1 · min-1

Abstract

Background: The 'critical' level of oxygen delivery (DO2) is the value below which DO2 fails to satisfy the metabolic need for oxygen. No prospective data in healthy, conscious humans define this value. The authors reduced DO2 in healthy volunteers in an attempt to determine the critical DO2. Methods: With Institutional Review Board approval and informed consent, the authors studied eight healthy, conscious volunteers, aged 19-25 yr. Hemodynamic measurements were obtained at steady state before and after profound acute isovolemic hemodilution with 5% albumin and autologous plasma, and again at the reduced hemoglobin concentration after additional reduction of DO2 by an infusion of a β-adrenergic antagonist, esmolol. Results: Reduction of hemoglobin from 12.5 ± 0.8 g/dl to 4.8 ± 0.2 g/dl (mean ± SD) increased heart rate, stroke volume index, and cardiac index, and reduced DO2 (14.0 ± 2.9 to 9.9 ± 2.0 ml O2 · kg-1 · min-1; all P < 0.001). Oxygen consumption (VO2; 3.0 ± 0.5 to 3.4 ± 0.6 ml O2 · kg-1 · min-1; P < 0.05) and plasma lactate concentration (0.50 ± 0.10 to 0.62 ± 0.16 mM; P < 0.05; n = 7) increased slightly. Esmolol decreased heart rate, stroke volume index, and cardiac index, and further decreased DO2 (to 7.3 ± 1.4 ml O2 · kg-1 · min-1; all P < 0.01 vs. before esmolol). VO2 (3.2 ± 0.6 ml O2 · kg-1 · min-1; P > 0.05) and plasma lactate (0.66 ± 0.14 mM; P > 0.05) did not change further. No value of plasma lactate exceeded the normal range. Conclusions: A decrease in DO2 to 7.3 ± 1.4 ml O2 · kg-1 · min-1 in resting, healthy, conscious humans does not produce evidence of inadequate systemic oxygenation. The critical DO2 in healthy, resting, conscious humans appears to be less than this value.