Oxygen dissociation curve in the adult respiratory distress syndrome

P50 implies adverse clinical outcomes in pediatric acute respiratory distress syndrome by reflecting extrapulmonary organ dysfunction

Hypoxemia and multiple organ dysfunction are significant contributors to mortality in patients with pediatric acute respiratory distress syndrome (PARDS). P50, the oxygen tension at which hemoglobin is 50% saturated, is a measure of hemoglobin-oxygen affinity, and its alteration might have implications for tissue hypoxia and organ dysfunction. The purpose of this single-center, retrospective study was to evaluate P50 levels in PARDS and to determine the association between P50 and clinical outcomes. The study included 212 children diagnosed with PARDS according to the Pediatric Acute Lung Injury Consensus Conference definition who required invasive mechanical ventilation and had arterial blood gas results of hemoglobin oxygen saturation < 97% at the time of diagnosis. P50 levels were calculated using Doyle’s method, and organ dysfunction was assessed using the Pediatric Logistic Organ Dysfunction-2 score. Most patients exhibited more than one dysfunctional extrapulmonary organ at PARDS onset. P50 increased with increasing PARDS severity (mild (26.6 [24.9–29.6]), moderate (26.8 [25.0–29.5]), and severe PARDS (29.1 [26.1–32.4] mmHg; P = 0.025). Moreover, P50 demonstrated a significant positive association with extrapulmonary organ dysfunction score (β = 0.158, P = 0.007) and risk of mortality (adjusted hazard ratio, 1.056; 95% confidence interval, 1.015–1.098; P = 0.007), irrespective of initial PARDS severity. The relationship between P50 and mortality was largely mediated by extrapulmonary organ dysfunction. A high P50 value at the time of PARDS diagnosis may be associated with mortality via dysfunctional extrapulmonary organs. Future studies should consider P50 as a potential candidate index for risk stratification of PARDS patients.

Reduced red cell 2,3-diphosphoglycerate concentrations in critical illness without decreased in vivo P50

We investigated whether red cell 2,3-diphosphoglycerate (2,3-DPG) concentrations are reduced in critical illness, whether acidaemia, hypophosphataemia or anaemia influence 2,3-DPG, and whether there is any net effect on in vivo P50. Twenty healthy, non-smoking, male volunteers were compared with 20 male intensive care patients with APACHE 2 scores >20 on the preceding day. Those transfused in this time were excluded. Venous red cell 2,3-DPG concentrations were measured in both groups. In the patient group, routine multichannel biochemical profile and arterial blood gas analysis were also performed and 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 mmol/l vs 4.9+/-0.5 mmol/l, P=0.016). The 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 (median haemoglobin concentration 113 g/l, range 89 to 154 g/l). By linear regression of patient data, pH had a significant effect on 2,3-DPG concentrations (r=0.6, P=0.011). Haemoglobin and phosphate concentrations did not, but there were few abnormal phosphate values. There was no correlation between 2,3-DPG concentrations and in vivo P50 (r2 < or = 0.08). We conclude that 2,3-DPG concentrations were reduced in a broad group of critically ill patients. Although this would normally reduce the P50, the reduction was primarily linked with acidaemia, which increases the P50. Overall, there was no net effect on the P50 and thus no affinity-related decrease in tissue oxygenation.

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 (cx/cHb) 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 < or = pH < or = 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.