Critical oxygen delivery in conscious humans is less than 7.3 ml O-2 center dot kg(-1) center dot min(-1)

Background: The "critical" level of oxygen delivery (DO2) is the value belo w which DO, fails to satisfy the metabolic need for oxygen. No prospective data in healthy, conscious humans define this value. The authors reduced DO 2 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, Hemody namic measurements were obtained at steady state before and after profound acute isovolemic hemodilution with 5% albumin and autologous plasma, and ag ain at the reduced hemoglobin concentration after additional reduction of D O2 by an infusion of a beta-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 O-2.kg(-1).min(-1); all P < 0,001). Oxygen consumption (VO2; 3.0 +/- 0.5 to 3.4 +/- 0.6 ml O-2.kg(-1 ).min(-1); P < 0.05) and plasma lactate concentration (0.50 +/- 0.10 to 0.6 2 +/- 0.16 mM; P < 0.05; n = 7) increased slightly. Esmolol decreased heart rate, stroke volume index, and cardiac index, and further decreased DO2 (t o 7.3 +/- 1.4 ml O-2.kg(-1).min(-1); all P < 0.01 vs. before esmolol). VO2 (3.2 +/- 0.6 ml O-2.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 excee ded the normal range. Conclusions: A decrease in DO2 to 7.3 +/- 1.4 ml O-2 kg(-1) min(-1) in rest ing, healthy, conscious humans does not produce evidence of inadequate syst emic oxygenation. The critical DO, in healthy, resting, conscious humans ap pears to be less than this value.