Objective: To determine whether reoxygenation with an FIO2 of 0.21 (21% oxygen) is preferable to an FIO2 of 1.0 (100% oxygen) in normalizing brain and muscle hypoxia in the newborn.
Design: Prospective, randomized, animal study.
Setting: Hospital surgical research laboratory.
Subjects: Twenty-six anesthetized, mechanically ventilated, domestic piglets, 2 to 5 days of age.
Interventions: The piglets were randomized to control or hypoxemia groups. Hypoxemia was induced by ventilating the piglets with 8% oxygen in nitrogen, which was continued until mean arterial pressure decreased to <20 mm Hg. After hypoxemia, the piglets were further randomized to receive reoxygenation with an FIO2 of 0.21 (21% oxygen group, n = 9) or an FIO2 of 1.0 for 30 mins followed by an FIO2 of 0.21 (100% oxygen group, n = 9), and followed for 5 hrs. The piglets in the control group were mechanically ventilated with 21% oxygen (n = 8).
Measurements and main results: We measured extracellular concentrations of hypoxanthine in the cerebral cortex and femoral muscle (in vivo microdialysis), plasma hypoxanthine concentrations, cerebral arterial-venous differences for hypoxanthine, acid base balances, arterial and venous (sagittal sinus) blood gases, and mean arterial pressures. The lowest pH values of 6.91 +/- 0.11 (21% oxygen group, mean +/- SD) and 6.90 +/- 0.07 (100% oxygen group) were reached at the end of hypoxemia and then normalized during the reoxygenation period. Plasma hypoxanthine increased during hypoxemia from 28.1 +/- 9.3 to 119.1 +/- 31.9 micromol/L in the 21% oxygen group (p < .001) and from 32.6 +/0- 14.5 to 135.0 +/- 31.4 micromol/L in the 100% oxygen group (p <.001). Plasma hypoxanthine concentrations then normalized over the next 2 hrs in both groups. In the cerebral cortex, extracellular concentrations of hypoxanthine increased during hypoxemia from 3.9 +/- 2.8 to 20.2 +/- 7.4 micromol/L in the 21% oxygen group (p < .001) and from 5.9 +/- 5.0 to 25.1 +/- 7.1 micromol/L in the 100% oxygen group (p < .001). In contrast to plasma hypoxanthine, extracellular hypoxanthine in the cerebral cortex increased significantly further during early reoxygenation, and, within the first 30 mins, reached maximum values of 24.9 +/- 6.3 micromol/L in the 21% oxygen group (p < .01) and 34.8 +/- 10.9 micromol/L in the 100% oxygen group (p < .001). This increase was significantly larger in the 100% oxygen group than in the 21% oxygen group (9.7 +/- 4.7 vs. 4.7 +/- 2.6 micromol/L, p < .05). There were no significant differences between the two reoxygenated groups in duration of hypoxemia, hypoxanthine concentrations in femoral muscle, plasma hypoxanthine concentrations, pH, or mean arterial pressure. The cerebral arterial-venous difference for hypoxanthine was positive both at baseline, at the end of hypoxemia, and after 30 mins and 300 mins of reoxygenation, and no differences were found between the two reoxygenated groups.
Conclusions: Significantly higher extracellular concentrations of hypoxanthine were found in the cerebral cortex during the initial period of reoxygenation with 100% oxygen compared with 21% oxygen. Hypoxanthine is a marker of hypoxia, and reflects the intracellular energy status. These results therefore suggest a possibly more severe impairment of energy metabolism in the cerebral cortex or an increased blood-brain barrier damage during reoxygenation with 100% oxygen compared with 21% oxygen in this newborn piglet hypoxia model.