Cerebral metabolic changes in arterial hypocapnia of short duration

Mechanisms of injury to the newborn brain

Preterm and ill term infants are at risk for brain injury and subsequent neurodevelopmental delay as a result of many perinatal factors. Outlined in this article are the basic science mechanisms by which hypoxia, hypocapnia, and hypercapnia may result in neuronal injury in the newborn brain.

Effect of moderate hypocapnic ventilation on nuclear DNA fragmentation and energy metabolism in the cerebral cortex of newborn piglets

Previous studies have shown that severe hypocapnic ventilation [arterial carbon dioxide partial pressure (PaCO(2)) 7-10 mm Hg] in newborn animals results in decreased cerebral blood flow and decreased tissue oxidative metabolism. The present study tests the hypothesis that moderate hypocapnic ventilation (PaCO(2) 20 mm Hg) will result in decreased cerebral oxidative metabolism and nuclear DNA fragmentation in the cerebral cortex of normoxemic newborn piglets. Studies were performed in 10 anesthetized newborn piglets. The animals were ventilated for 1 h to achieve a PaCO(2) of 20 mm Hg in the hypocapnic (H) group (n = 5) and a PaCO(2) of 40 mm Hg in the normocapnic, control (C) group (n = 5). Tissue oxidative metabolism, reflecting tissue oxygenation, was documented biochemically by measuring tissue ATP and phosphocreatine (PCr) levels. Cerebral cortical nuclei were purified, nuclear DNA was isolated, and DNA content was determined. DNA samples were separated, stained, and compared with a standard DNA ladder. Tissue PCr levels were significantly lower in the H group than the C group (2.32 +/- 0.66 versus 3.73 +/- 0.32 micromol/g brain, p < 0.05), but ATP levels were preserved. Unlike C samples, H samples displayed a smear pattern of small molecular weight fragments between 100 and 12,000 bp. The density of DNA fragments was eight times higher in the H group than the C group, and DNA fragmentation varied inversely with levels of PCr (r = 0.93). These data demonstrate that moderate hypocapnia of 1 h duration results in decreased oxidative metabolism that is associated with DNA fragmentation in the cerebral cortex of newborn piglets. We speculate that hypocapnia-induced hypoxia results in increased intranuclear Ca(2+) flux, which causes protease and endonuclease activation, DNA fragmentation, and periventricular leukomalacia in newborn infants.

1H and 31P NMR measurement of cerebral lactate, high-energy phosphate levels, and pH in humans during voluntary hyperventilation: associated EEG, capnographic, and Doppler findings

In order to explore the sensitivity of spatially resolved 1H and 31P NMR spectroscopy on a whole-body NMR instrument, cerebral metabolic changes in human volunteers were measured during hyperventilation provocation. During hyperventilation the flow velocity in the middle cerebral artery decreased significantly and the EEG showed a marked increase in slow activity. 1H NMR spectra revealed an increase in cerebral lactate concentration. 31P NMR spectra showed no changes in ATP or PCr peak heights, but a shift toward tissue alkalosis was derived from changes in Pi chemical shift. During subsequent recovery, lactate concentration decreased and a slight intracellular acidosis was detected. In three experiments broadening of the lactate resonance peak resulted in separation into two components at 1.32 and 1.48 ppm, in which the latter signal possibly arose from alanine.

Response of the cerebral circulation to profound hypocarbia in neonatal lambs

Hyperventilation to extremely low arterial carbon dioxide tension (PaCO2) has been used in the management of persistent pulmonary hypertension in newborn infants. With progressive hypocarbia, cerebral vasoconstriction occurs, raising the concern that extreme hypocarbia may result in cerebral oxygen deprivation. Therefore, I evaluated regulation of the cerebral circulation during acute hypocarbia in 10 newborn lambs. Whole-brain and regional blood flows measured using radioactive microspheres, arterial and venous (sagittal sinus) blood gases, and oxygen contents were measured in each lamb at four arterial carbon dioxide tensions. Whole-brain oxygen delivery, oxygen consumption, and fractional oxygen extraction were calculated. Finally, arterial and venous lactate concentrations were measured to assess cerebral lactate production. Whole-brain blood flow (CBF) decreased in a nonlinear fashion as PaCO2 ranged from 46 to 12 mm Hg [In(CBF) = 0.025(PaCO2) + 3.38; r = 0.70, p less than 0.001]. Similar responses were demonstrated for all regional blood flows examined. Cerebral fractional oxygen extraction (E) increased in a nonlinear fashion [In(1-E) = 0.023(PaCO2)-1.37; r = 0.80, p less than 0.001], and cerebral metabolic rate for oxygen was unchanged with hypocarbia. Cerebral venous lactate concentration increased significantly (3.49 +/- 0.23 vs. 2.01 +/- 0.22 mM, p less than 0.001) during severe hypocarbia (PaCO2 of less than 22 mm Hg), and the arterial-venous lactate concentration difference became negative. These results demonstrate uniform responses of whole-brain and regional blood flows and stable cerebral oxygen consumption during moderate and severe hypocarbia. Although there is evidence for cerebral lactate production during severe hypocarbia, this is not likely to indicate cerebral hypoxia as oxygen consumption does not change.

Effects of hyperventilation on cerebral blood flow and brain tissue metabolism in normotensive and spontaneously hypertensive rats

Cerebral vascular carbon dioxide (CO2) reactivities were compared in normotensive (NTR) and hypertensive (SHR) rats. Cerebral blood flow (CBF) in cortex and thalamus were evaluated before and during one hour of hyperventilation. After one hour of hyperventilation brain lactate, pyruvate, and ATP concentrations were also determined. Significant and similar reductions of CBF due to hyperventilation induce hypocapnia were found in both NTR and SHR groups. In contrast the percent increase in cerebrovascular resistance (CVR) per unit decrease in paCO2 was significant, indicating that hypocapnia induced vasoconstriction is greater in NTR than in SHR groups. During hyperventilation the average value for lactate in the NTR group was 3.98 mM/kg. In contrast it was 3.15 mM/kg in the SHR group, a significant difference (p less than 0.05). When paCO2 fell below 15 mm Hg the cerebral lactate increased strikingly in the NTR group and cortical CVR was reduced suggesting that an accumulation of the ischemic metabolites caused dilatation of the constricted cerebral vessels. In contrast the SHR group disclosed no such changes. The increase CVR characteristic of SHR appeared to diminish the cerebral vasoconstrictive response to hypocapnia. As a result ischemic metabolites in the brain do not increase in this group to the degree that they do in NTR.

Interregional differences in brain intracellular pH and water compartmentation during acute normoxic and hypoxic hypocapnia in the anesthetized dog

Interregional differences in intracellular pH (pHi) in brain tissue, and its regulation following 1 and 5 h of respiratory alkalosis (with and without hypoxemia) were determined in N2O anesthetized dogs. Two techniques for pHi estimation were used (TCO2 and 14C-DMO) and included corrections for measured extracellular fluid (35SO4(2-)) space (ECS). Cortical pHi by the two techniques agreed closely in control and in 3 of the 4 experimental conditions, suggesting: (a) our estimation of extracellular fluid (ECF) [HCO3-] from measured CSF [HCO3-] was a valid assumption; and (b) our method had sufficient resolution to determine the magnitude of brain pHi regulation during respiratory acid-base disturbances. When moderate normoxic respiratory alkalosis (PaCO2 approximately 25 mm Hg) was imposed for 5 h, pHi (in most brain regions) was well regulated and always exceeded the incomplete regulation noted in bulk CSF. When moderate hypoxemia (PaO2 approximately 45 mm Hg) accompanied hypocapnia, pHi was more closely regulated during the early phase (1 h) of respiratory alkalosis. Increased levels of metabolic acids (especially lactic acid) were critical to brain pHi regulation during the initial hour of respiratory alkalosis and accounted for much of the independent effect of hypoxemia on pHi regulation. However, these metabolic acids remained unchanged as pHi was more completely regulated between 1 and 5 h of continued hypocapnia or hypoxic hypocapnia. This time-dependent regulation of pHi may involve some regulatory role for changed transmembrane fluxes of H+ and/or HCO3-. Significant interregional differences were observed in both pHi and in ECS; with tendencies toward more alkaline pHi and lower ECS in brain stem and white matter. With respiratory alkalosis ECS fell and intracellular fluid increased in both cortex and caudate nucleus, possibly reflecting an osmotic effect of increased metabolic acid levels or reduction in cell membrane ion pumping.

Circulatory and metabolic effects in the brain induced by amphetamine sulphate

Cerebral circulatory and metabolic effects of amphetamine sulphate (0.25-25 mg.kg-1 i.v. or 5-10 mg.kg-1 i.p.) were studied in anesthetized, paralyzed and artifically ventilated rats. Cerebral blood flow (CBF) was measured with a modification of the Kety and Schmidt (1948) technique, and oxygen consumption (CMRO2) was calculated from CBF and arteriovenous differences in oxygen content. Regional CBF was evaluated from the uptake of 14C-ethanol. Cortical metabolites were analysed following freezing of tissue in situ. Amphetamine administration gave rise to a marked increase in CBF that was doubled following 0.25 mg.kg-1 and increased 4-fold following 15 mg.kg-1. However, such excessive increases in flow were confined to frontoparietal cortical regions, while other cortical or subcortical areas showed more moderate hyperemia. The increase in CBF was unrelated to changes in arterial PCO2, blood pressure, or tissue lactate content. CMRO2 increased by 30% to 95% depending on dose and rat strain used. At all doses employed, amphetamine gave rise to glycogenolysis in cerebral cortex but, in animals studied within the first 30 min after 5 mg.kg-1, or less, the only other changes were increases in glucose-6-phosphate and alpha-ketoglutarate concentrations. When the dose was increased to 15 mg.kg-1, there were moderate increased in lactate concentration and lactate/pyruvate ratio. Sixty min after 5 mg.kg-1 there were increases in tissue concentrations of pyruvate, citric acid cycle intermediates and alanine, as well.

Intravenous sodium lactate administration in respiratory alkalosis secondary to severe brain injuries

In acute cerebrovascular accidents the uptake of lactate by the brain tissue is enhanced, possibly in an attempt to buffer the increased local lactic acidosis. In this paper the results obtained by the administration of the lactate in 78 cases of hyperventilatory syndromes secondary to acute brain injuries, and the biochemistry of the cerebrospinal fluid and blood, are reported.

Ventricular fluid lactate, pyruvate, bicarbonate and ph in unconscious brain-injured patients subjected to controlled ventilation

In 35 unconscious patients suffering from traumatic brain injury and subjected to controlled hyperventilation within 12 hours of the acute trauma, we measured continuously the intraventricular pressure (IVP), ventricular fluid lactate, pyruvate, L/P ratio, pH and bicarbonate during the first 4 posttraumatic days, and related the findings to the clinical course. In patients in whom the follow-up study did not reveal severe mental impairment or dementia, the ventricular fluid lactate never exceeded 4 mmol/litre. In this group, the IVP level never exceeded 40 mmHg/hour, and the mean pressure/day never exceeded 20 mmHg. In half of the patients in whom dementia, vegetative survival or death occurred, ventricular fluid lactate exceeded 4 mmol/litre; the IVP level/hour exceeded 40 mmHg in two of 23 patients, and the mean pressure/day exceeded 20 mmHg in nine of 23 patients. The prognostic value of pyruvate was of less importance, although a significant increase in patients with a poor outcome was seen. No prognostic conclusions could be drawn from ventricular fluid, L/P ratio, pH and bicarbonate.