Reduced red cell 2,3-diphosphoglycerate concentrations in critical illnesswithout decreased in vivo P50

We investigated whether red cell 2,3-diphosphoglycerate (2,3-DPG) concentra tions are reduced in critical illness, whether acidaemia, hypophosphataemia or anaemia influence 2,3-DPG, and whether there is any net effect on in vi vo P50. Twenty healthy, non-smoking, male volunteers were compared with 20 male int ensive care patients with APACHE 2 scores > 20 on the preceding day. Those transfused in this time were excluded. Venous red cell 2,3-DPG concentratio ns were measured in both groups. In the patient group, routine multichannel biochemical profile and arterial blood gas analysis were also performed an d in vivo P50 calculated. The mean 2,3-DPG concentration was significantly lower in the patient group than in the controls (4.2 +/-1.3 mmoll/l vs 4.9 +/-0.5 mmol/l, P=0.016). T he patients were well oxygenated (lowest arterial PO2=75 mm Hg) and showed a tendency to acidaemia (median pH 7.37, range 7.06 to 7.48) and anaemia (m edian haemoglobin concentration 113 g/l, range 89 to 154 g/l). By linear re gression of patient data, pH had a significant effect on 2,3-DPG concentrat ions (r=0.6, P=0.011). Haemoglobin and phosphate concentrations did not, bu t there were few abnormal phosphate values. There was no correlation betwee n 2,3-DPG concentrations and in vivo P50 (r(2) less than or equal to 0.08). We conclude that 2,3-DPG concentrations were reduced in a broad group of cr itically ill patients. Although this would normally reduce the P50, the red uction was primarily linked with acidaemia, which increases the P50. Overal l, there was no net effect on the P50 and thus no affinity-related decrease in tissue oxygenation.