Purine metabolites and lactate as parameters of hypoxia in the newborn infant

In order to evaluate possible biochemical parameters of tissue hypoxia, arterial samples from 29 sick neonates were taken at 6-h intervals for lactate and blood gas analyses, as well as assays of purine metabolites in plasma. The state of the infant at each sampling was arbitrarily classified as severe, moderate, or mild on the basis of PO2, pH, FiO2, and transcutaneous PO2. Of the total of 289 samples, 72 were in the mild, 178 in the moderate, and 39 in the severe category. Elevated values for hypoxanthine (greater than 36 mumol/L), xanthine (greater than 11 mumol/L), and lactate (greater than 3.6 mmol/L) were defined as greater than 2 SD above the mean of infants with slight or no hypoxia. Hypoxanthine values were elevated in 7% of mild and in 13% of severe cases whereas the corresponding incidences for elevated xanthine values were 1 and 8%, and for elevated lactate values 7 and 39%. Conversely, when hypoxanthine was elevated (n = 21), mild hypoxia was present in 24% and severe in the same percentage of cases whereas the corresponding incidences for elevated xanthine (n = 7) were 14 and 43% and for lactic acid (n = 41) 12 and 37%. No consistent differences in plasma uric acid levels were noted between the mild and severe cases of hypoxia. We conclude that neither purine metabolites nor lactic acid concentrations in intermittent samples of arterial blood are reliable indicators of tissue oxygenation in the newborn.