Maturational changes in cerebral lactate and acid clearance following ischemia measured in vivo using magnetic resonance spectroscopy and microdialysis.

Intraischemic hyperglycemia has different effects on neurologic outcome in mature vs. immature brain, and may reflect differences in the extent or duration of cerebral lactic acidosis. We examined the hypotheses that post-ischemic lactate and acid clearance rates depend on the severity of intraischemic cerebral acidosis, and that rates of clearance change as a function of brain maturation. In vivo 31P and 1H magnetic resonance spectroscopy (MRS) was used to compare intracellular acid and lactate clearance rates in newborn and 1-month old swine following a 14-min episode of transient near-complete global ischemia. In the same animals, in vivo microdialysis was used to determine if extracellular lactate clearance changed as a function of cerebral lactic acidosis or differed between age groups following ischemia. Plasma glucose concentration was altered in individual animals to study a range of intraischemic cerebral lactic acidosis. For both age-groups, maximal brain acidosis and lactosis occurred in the post-ischemia interval, indicating a delay in the re-establishment of oxidative metabolism following ischemia. Clearance half-lives of both cerebral acidosis and lactosis increase as a function of increased intraischemic cerebral acidosis. For either age group, the clearance half-life for acidosis was faster than the half-life for lactate. However, the subgroup of 1-month old swine who experienced severe cerebral acidosis (i.e., pH<6.1) had a longer cerebral lactate clearance half-life as compared to the subgroup of newborn animals with a similar severity of acidosis. In both age groups, there were comparable maximal increases in extracellular lactate concentrations in the post-ischemic period and similar rates of decline from the maximum. These results demonstrate that post-ischemic lactate and acid clearance are altered by the extent of intraischemic acidosis, and the extent of post-ischemic uncoupling between brain acid and lactate clearance increases with advancing age. The transmembrane clearance of lactate was not a prominent mechanism that differentiated lactate clearance rates between newborn and 1-month old swine.

Age-related differences in the effect of dichloroacetate on postischemic lactate and acid clearance measured in vivo using magnetic resonance spectroscopy and microdialysis.

Numerous studies using adult animal models suggest that dichloroacetate (DCA) may have neuroprotective properties by virtue of its ability to increase rates of metabolism and, therefore, clearance of brain lactic acidosis, which may accumulate during cerebral ischemia. We tested the hypothesis that postischemic DCA administration affects lactate and acid clearance to different extents in immature versus mature brain. 31P and 1H magnetic resonance spectroscopy were used to measure intracellular acid and lactate clearance rates in vivo in newborn and 1-month-old swine after a 14-min episode of transient near-complete global ischemia. Simultaneous monitoring of extracellular lactate efflux and clearance was measured in the same animals by in vivo microdialysis. Plasma glucose concentrations were elevated in order to study animals with severe cerebral lactic acidosis. Maximal levels of brain lactosis (16-20 micromol/g) and acidosis (PHintracellular 5.8-6.0) were reached during the first 10 min of recovery and were the same in age groups and in subgroups either acting as controls or treated with DCA (200 mg/kg) given from the last minute of ischemia to 5-7 min after ischemia. For newborns, DCA administration improved the postischemic clearance rate of cerebral acidosis and cerebral phosphocreatine, with similar trends for the clearance of lactosis and increased rates of recovery of nucleotide triphosphates, compared with controls. In contrast, DCA administration in 1-month-olds resulted in a modest trend for improvement of cerebral lactate clearance, but did not affect acid clearance or the recovery rate of phosphocreatine or nucleotide triphosphates. Extracellular brain lactate concentrations had similar relative increases and rates of decline for subgroups of either age treated with DCA versus controls. The results of this study indicate that postischemic DCA administration helps to resolve cerebral acidosis to a greater degree in immature than more mature brain, suggesting that DCA may have cerebroprotective properties for neonatal hypoxic-ischemic encephalopathy.

Modest hypothermia provides partial neuroprotection when used for immediate resuscitation after brain ischemia.

Intraischemic reduction in temperature of 2-3 degrees C (modest hypothermia) has been demonstrated to provide partial neuroprotection in neonatal animals. This investigation determined if modest hypothermia initiated immediately after brain ischemia provides neuroprotection. Piglets were studied with rectal temperature maintained during the 1st h after 15 min of brain ischemia at either 38.3 +/- 0.3 degrees C (normothermia, n = 11) or at 35.8 +/- 0.5 degrees C (modest hypothermia, n = 11). The severity of brain ischemia was similar between groups as indicated by equivalent reduction in mean blood pressure (90 +/- 15 to 24 +/- 3 versus 92 +/- 13 to 26 +/- 3 mm Hg), and changes in cerebral metabolites and intracellular pH (pH(i)) measured by magnetic resonance spectroscopy (beta-nucleoside triphosphate = 44 +/- 9 versus 42 +/- 18% of control, control = 100%, pH(i): 6.25+/- .15 versus 6.24 +/- 0.22 for normothermic and modestly hypothermic groups, respectively). In the first 90 min after ischemia, there were no differences between groups in the duration and extent of brain acidosis, and relative concentrations of phosphorylated metabolites. Categorical assessment of neurobehavior was evaluated at 72 h postischemia (n = 16), or earlier if an animal's condition deteriorated (n = 6). Postischemic hypothermia was associated with less severe stages of encephalopathy compared with normothermia (p = 0.05). Histologic neuronal injury was assessed categorically in 16 brain regions, and postischemic hypothermia resulted in less neuronal injury in temporal (p = 0.024) and occipital (p = 0.044) cortex at 10 mm beneath the cortical surface, and in the basal ganglia (p = 0.038) compared with that in normothermia. Modest hypothermia for 1 h immediately after brain ischemia provides partial neuroprotection and may represent an adjunct to resuscitative strategies.

Effect of hypoxia on glucose-modulated cerebral lactic acidosis, agonal glycolytic rates, and energy utilization.

Newborn and 1-mo-old swine were exposed to identical durations (18 min) and degrees of hypoxia (O2 content = 4 mL/dL), to examine the effects of hypoxia on cerebral energy metabolism and intracellular pH (pHi) in vivo, using 31P and 1H nuclear magnetic resonance spectroscopy. Hypoxia produced the same extent of reductions in phosphocreatine (PCr) (63 +/- 28% and 65 +/- 10%, newborns and 1-mo-olds, respectively) and pHi (6.93 +/- 0.06 and 6.89 +/- 0.06, respectively) for either age group. The magnitude of changes in PCr, lactate, and pHi was larger for subgroups of data collected when cardiovascular instability was present, suggesting that hypotension and possibly reduced cerebral perfusion contributed to cerebral energy failure and lactic-acidosis for either age group. There were no correlations between the blood plasma glucose concentration at 18 min of hypoxia and the extent of change in PCr, lactate, or pHi for either age group. During a subsequent period of complete ischemia induced via cardiac arrest after 20 min hypoxia, the decline in PCr and nucleoside triphosphate (NTP), and increase in lactate followed similar rates compared with previously studied age-matched animals that were normoxic before ischemia. The rate constants for the change in PCr, NTP, and lactate followed similar rates compared with previously studied age-matched animals that were normoxic before ischemia. The rate constants for the change in PCr, NTP, and lactate during ischemia showed no correlation with the blood plasma glucose concentration measured immediately before cardiac arrest. These results suggest that cerebral glycolytic rates and energy utilization during ischemia are unaffected by a preceding interval of hypoxia and that hyperglycemia does not delay cerebral energy failure during hypoxia or combined hypoxic-ischemia.

Quantitative relationship between brain temperature and energy utilization rate measured in vivo using 31P and 1H magnetic resonance spectroscopy.

In neonatal and adult animals, modest reduction in brain temperature (2-3 degrees C) during ischemia and hypoxia-ischemia provides partial or complete neuroprotection. One potential mechanism for this effect is a decrease in brain energy utilization rate with consequent preservation of brain ATP, as occurs with profound hypothermia. To determine the extent to which modest hypothermia is associated with a decrease in brain energy utilization rate, in vivo 31P and 1H magnetic resonance spectroscopy (MRS) was used to measure the rate of change in brain concentration of phosphocreatine, nucleoside triphosphate, and lactate after complete ischemia induced by cardiac arrest in 11 piglets (8-16 d). Pre-ischemia metabolite concentrations and MRS-determined rate constants were used to calculate the initial flux of high energy phosphate equivalents (d[approximately P]/dt, brain energy utilization rate). Baseline physiologic and MRS measurements were obtained at 38.2 degrees C and repeated after brain temperature was adjusted between 28 and 41 degrees C. This was followed by measurement of d[approximately P]/dt during complete ischemia at 1-2 degrees C increments within this temperature range. Adjusting brain temperature did not alter any systemic variable except for heart rate which directly correlated with brain temperature (r = 0.95, p < 0.001). Before ischemia brain temperature inversely correlated with phosphocreatine (r = -0.89, p < 0.001), and reflected changes in the phosphocreatine-ATP equilibrium, because brain temperature inversely correlated with intracellular pH (r = -0.77, p = 0.005). Brain temperature and d[approximately P]/dt were directly correlated and described by a linear relationship (slope = 0.61, intercept = -12, r = 0.92, p < 0.001). A reduction in brain temperature from normothermic values of 38.2 degrees C was associated with a decline in d[approximately P]/dt of 5.3% per 1 degree C, and therefore decreases in d[approximately P]/dt during modest hypothermia represent a potential mechanism contributing to neuroprotection.

Validation of a noninvasive method to measure brain temperature in vivo using 1H NMR spectroscopy.

The goal of this study was to evaluate the potential of using the difference between the 1H NMR frequencies of water and N-acetylaspartic acid (NAA) to measure brain temperature noninvasively. All water-suppressed and non-water-suppressed 1H NMR spectra were obtained at a field strength of 4.7 T using a surface coil. Experiments performed on model solutions revealed a decrease in the difference between NMR frequencies for NAA and water as a linear function of increasing temperature from 14 to 45 degrees C. Changing pH in the range 5.5-7.6 produced no discernible trends for concurrent changes in the slope and intercept of the linear relationship. There were minor changes in slope and intercept for solutions containing 80 or 100 mg of protein/ml versus no protein, but these changes were not considered to be of sufficient magnitude to deter the use of this approach to measure brain temperature. The protein content of swine cerebral cortex was found to remain constant from newborn to 1 month old (78 +/- 12 mg/g; n = 41). Therefore, data collected for the model solution containing 80 mg of protein/ml were used as a calibration curve to calculate brain temperature in eight swine during control, hypothermia, ischemia, postischemia, or death, over a temperature range of 23-40 degrees C. A plot of 61 temperatures determined from 1H NMR versus temperatures measured from an optical fiber probe sensor implanted 1 cm into the cerebral cortex showed excellent linear agreement (slope = 1.00 +/- 0.03, r2 = 0.96). We conclude that 1H NMR spectroscopy presents a practical means of making noninvasive measurements of brain temperature with an accuracy of better than +/- 1 degree C.

Modest hypothermia provides partial neuroprotection for ischemic neonatal brain.

Hypothermia is a frequent occurrence in newborns, and thermoregulatory management is a fundamental part of medical stabilization. Although modest reduction in brain temperature (2-3 degrees C) before ischemia provides neuroprotection in adults, the effect of modest hypothermia on immature brain has not been examined. Nine-day-old swine were exposed to 15 min of incomplete global brain ischemia, with intraischemic rectal temperatures of either 38.3 +/- 0.4 degrees C (n = 10, normothermic) or 35.4 +/- 0.5 degrees C (n = 10, hypothermic). The relationship between rectal and brain temperature was delineated in preliminary experiments on four swine. Animals with intraischemic rectal temperatures maintained at either 39.5 degrees C or 35.5 degrees C were associated with a similar magnitude of difference in brain temperature. Therefore, rectal temperature was used to monitor brain temperature for 20 animals studied subsequently. Ischemia was induced by combining neck compression with hemorrhagic hypotension and resulted in similar group values for mean arterial pressure and changes in pH and blood gases at the completion of ischemia. A clinical overall performance score and brain tissue structure were evaluated after 72 h (or earlier if animals died prematurely). Hypothermic animals had less severe stages of impairment compared with the normothermic group (p = 0.023). Hypothermic piglets had less histologic damage in the neocortex at 0.5 cm beneath the brain surface (p = 0.048), the caudate nucleus (p = 0.038), and the pons/midbrain (p = 0.04) and the same direction of effect in neocortex at 1 cm beneath the surface (p = 0.07) and the cerebellum (p = 0.07) as compared with normothermic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of hypercarbia on age-related changes in cerebral glucose transport and glucose-modulated agonal glycolytic rates.

This study examined the effect of hypercarbia on cerebral agonal glycolytic rates and brain lactate accumulation after complete ischemia induced by cardiac arrest. Before cardiac arrest, the blood plasma glucose concentration in seven newborn (113 d postconception; normal gestation, 115 d) and seven 1-mo-old (144 d postconception) piglets was adjusted to a specific value (range, 1 to 64 mM), and then inspired ventilation gases were changed to 10:50:40 CO2:O2:N2 for 20 min. The agonal glycolytic rate was measured by monitoring the rate of cerebral lactate formation in vivo using proton nuclear magnetic resonance spectroscopy, and postmortem brain lactate concentrations were measured biochemically in tissue extracts obtained 40 to 45 min after cardiac arrest. These data were compared with 21 normocarbic piglets of similar age, nine examined as part of the present study and 12 examined previously (Corbett RJT, Laptook AR, Ruley JI, Garcia D: Pediatr Res 30:579-586, 1991). There was a nonlinear relationship between the final postmortem brain lactate concentration and preischemia blood plasma glucose concentration that was most prominent in newborn piglets and previously had gone unnoticed. When analyzed using a steady-state model for glucose transport, this relationship revealed that normocarbic newborns had a lower preischemia affinity constant for the transport mechanism for glucose (2.8 +/- 1.5 mM) and lower cerebral glucose utilization rate relative to transport rate (0.12 +/- 0.04), compared with 1-mo-olds (4.5 +/- 1.4 mM and 0.30 +/- 0.03, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)