Brain energy metabolism in two kinds of total asphyxia: an in vivo phosphorus nuclear magnetic resonance spectroscopic study

Seizure susceptibility during recovery from hypercapnia in neonatal dogs

Seizure susceptibility during recovery from hypercapnia was investigated in seven anesthetized neonatal dogs; 13, 20, or 30% CO2 gas was administered for 30 min through a ventilator to result in three levels of hypercapnia in which measured PaCO2 values were approximately 70, 100, and 140 mm Hg. Thereafter, the animals were allowed to recover for 45 min; during this recovery phase, electrocorticography was performed. In five of seven dogs, approximately 1.5 Hz slow irregular spike and wave bursts appeared at 6 min after abrupt withdrawal from hypercapnia and lasted several minutes. This seizure activity was followed by a brief period of electrical suppression. This phenomenon was most often observed during the recovery from moderate hypercapnia and between the PaCO2 values of 100 and 50 mm Hg. When seizure activities appeared in the electrocorticogram, arterial blood pressure increased -40 mm Hg from the preseizure level. These results suggest that neonatal seizures may occur during recovery from hypercapnia.

Intracellular alkalosis during hypoxia in newborn mouse brain in the presence of systemic acidosis: a phosphorus magnetic resonance spectroscopic study

We investigated the in vivo changes in cerebral energy metabolism and pHi in newborn mice noninvasively during 8 h of hypoxia with FiO2 = 5%, using phosphorus magnetic resonance spectroscopy continuously. The intracellular brain pH (pHi) increased from 7.20 +/- 0.03 to 7.36 +/- 0.03 (P < 0.05) at 1 h of hypoxia and then decreased gradually. On the other hand, the mixed arterial and venous blood pH decreased gradually during hypoxia, reaching a minimum value of 7.16 +/- 0.01 at the end of the hypoxia. There was no significant difference in PCO2 between control (47.4 +/- 0.8 mm Hg) and 1-h hypoxic (49.0 +/- 1.1 mm Hg) mice. The blood glucose concentration was significantly increased at 1 h of hypoxia. These results indicate that the alkaline shift in pHi during hypoxia was caused neither by systemic alkalosis due to hypocapnia nor hypoglycemia.

Direct measurement of free radicals in the neonatal mouse brain subjected to hypoxia: an electron spin resonance spectroscopic study

Free radical generation in the neonatal mouse brain subjected to acute hypoxia was measured directly by using electron spin resonance spectroscopy (ESR). Free radical density, an index of free radical content, was investigated during exposure to nitrogen gas (N2 group) or carbon dioxide gas (CO2 group) of high purity, and its subsequent recovery in air. Free radical density in the N2 group declined during hypoxia, increased over the control level at 10 min of recovery, and then returned to the prehypoxic level. In the CO2 group, it increased during hypoxia, reached the maximum level at 20 min of recovery, and then returned to the control level. The main origin of the spectrum in control brain was considered as being the coenzyme Q10 (CoQ10) radical in the mitochondria. The change of free radical density during hypoxia and recovery in the N2 group and that during recovery in the CO2 group was thought to be correlated with changes in CoQ10 radicals indicating mitochondrial function. The increase of free radical density in the CO2 group during hypoxia suggested the generation of free radicals other than the CoQ10 radical. We conclude that the free radicals which appeared during CO2 hypoxia may play a role in producing the differences in brain injury between the two kinds of hypoxia.

Monitoring of immature rabbit brain during hypoxia with near-infrared spectroscopy

Continuous monitoring of the cerebral blood flow, oxyhemoglobin, deoxyhemoglobin, total hemoglobin, oxidized cytochrome a, a3, and tissue pH during prolonged CO2 or N2 loading in 2-week-old rabbits was performed by near-infrared spectroscopy, the thermocouple method, and a tissue pH meter. Near-infrared spectroscopy demonstrated decreases in oxyhemoglobin and oxidized cytochrome a, a3 and increases in deoxyhemoglobin and total hemoglobin in the early stage within 5 min, which gradually lessened with time on both 10% concentration of inspired O2 with CO2 and N2. CBF increased with venous retention in the early stage and then slowly decreased in parallel with blood pressure and oxidized cytochrome a, a3 on abolition of autoregulation. These changes were more remarkable during the 10% concentration of inspired O2 with CO2 than N2 which may be caused by marked acidosis and hypotension associated with hypercarbia. Oxidized cytochrome a, a3, however, demonstrated a gradual decrease in 10% concentration of inspired O2 with N2 rather than CO2; therefore, the continuous monitorings demonstrated hemodynamic and oxygenation changes despite the same extent of prolonged hypoxic loading. These changes in prolonged hypoxic conditions may occur in human intrapartum asphyxia which develops into postnatal hypoxic-ischemic encephalopathy.

Lipid peroxidation in neonatal mouse brain subjected to two different types of hypoxia

To elucidate the role of free radicals in the pathogenesis of neonatal hypoxic encephalopathy, we determined the content of thiobarbituric acid reactants (TBARs), as an index of lipid peroxidation related with a free radical reaction, in the brains of newborn mice during hypoxia and recovery from hypoxia. Hypoxic stress was induced by 100% nitrogen gas breathing (N2 group) or 100% carbon dioxide gas breathing (CO2 group). TBARs increased with 20 minutes of hypoxia and returned to the control level during the recovery period in both groups. The increase in TBARs in the CO2 group was greater than that in the N2 group. These results may suggest that free radical reaction occurs during the hypoxic period and that CO2 hypoxia is more effective on free radical production in the newborn brain than N2 hypoxia.

Response on near-infrared spectroscopy and of cerebral blood flow to hypoxemia induced by N2 and CO2 in young rabbits

Cerebral blood oxyhemoglobin (HbO2), deoxyhemoglobin (HbR) and total hemoglobin (Hb) were examined in N2 and CO2 induced hypoxemia by near-infrared spectroscopy and compared with CBF examined by the H2 clearance method. HbO2 and HbR changed more sensitively than total Hb, reflecting the blood volume. Low CO2-loading showed marked increase in CBF with little change of blood volume, and higher CO2-induced hypoxemia was less increased and followed by a crossed after-reaction, probably because of persistent arterial dilatation due to the marked hypercarbia and acidosis. Neck venous compression showed a specific pattern of increased total Hb and HbR with little change in CBF. Thus, for near-infrared spectroscopy the intracranial blood volume may be affected mainly by arterial dilatation with an increase in CBF and venous dilatation with congestion. And Hb fractions of HbO2 and HbR may be influenced by cerebral blood oxygenation as well as the arteriovenous blood volume.