Oxygen availability in critical illness ‐ An investigation using the oxygen status algorithm

Using the oxygen status algorithm of Siggaard-Andersen to derive 2, 3-diphosphoglycerate concentrations and parameters of oxygen extractivity, 143 arterial blood specimens from 73 ICU patients were compared with 119 venous blood specimens from 119 healthy outpatients. The venous extractivity parameters were calculated by arbitrarily assigning an oxygen tension of 90 mmHg to each specimen. There were no significant differences in 2, 3-diphosphoglycerate, but the mean concentration of extractable oxygen corrected for low haemoglobin values (cxlcHb) was significantly greater in the ICU patients (males: 0. 017 ± 0. 004 mmol/g, females: 0. 016 ± 0. 004 mmol/g) than in the healthy outpatients (males and females: 0. 014 ± 0. 001 mmol/g, p < 0. 05). This reduction in haemoglobin-oxygen affinity was attributed to the high incidence of acidaemia in the ICU specimens (mean pH 7. 36 ± 0. 08), despite a reduced mean 2, 3-diphosphoglycerate concentration in the ICU acidaemic specimens (when pH < 7. 35, mean 2, 3-diphosphoglycerate concentration was 4. 9 ± 1. 6 mmol/L; when 7. 35 < pH < 7. 45: mean 2, 3-diphosphoglycerate concentration was 5. 7 ± 1. 6 mmol/L, p < 0. 05). Hypophosphataemia had no demonstrable effect on 2, 3-diphosphoglycerate concentrations or extractivity parameters in the ICU patients. We conclude that oxygen release from haemoglobin to the tissues in critical illness is enhanced because of a tendency to acidaemia.