Hypoxia in fetal lambs: a study with (1)H-MNR spectroscopy of cerebrospinal fluid.

In fetal lambs, severe hypoxia (SH) will lead to brain damage. Mild hypoxia (MH) is thought to be relatively safe for the fetal brain because compensating mechanisms are activated. We questioned whether MH, leading to mild acidosis, induces changes in cerebral metabolism. Metabolites in cerebrospinal fluid (CSF) samples, as analyzed by proton magnetic resonance spectroscopy, were studied in two groups of seven anesthetized near-term fetal lambs. In group I, SH leading to acidosis with an arterial pH <7.1 was achieved. In group II, MH with an intended pH of 7.23--7.27 was reached [start of MH (SMH)], and maintained during 2 h [end of MH (EMH)]. During SH, choline levels in CSF, a possible indicator of cell membrane damage, were increased. Both during SH and at EMH, CSF levels of lactic acid, alanine, phenylalanine, tyrosine, lysine, branched chain amino acids, and hypoxanthine were increased compared with control values and with SMH, respectively. At EMH, the hypoxanthine CSF-to-blood ratio was increased as compared with SMH. These results indicate that prolonged MH leads to energy degradation in the fetal lamb brain and may not be as safe as assumed.