Ricardo González-Méndez

Cerebral intracellular changes during supercarbia: an in vivo 31P nuclear magnetic resonance study in rats

31P nuclear magnetic resonance (NMR) spectroscopy was used noninvasively to measure in vivo changes in intracellular pH and intracellular phosphate metabolites in the brains of rats during supercarbia (Paco2 ⩾ 400 mm Hg). Five intubated rats were mechanically ventilated with inspired gas mixtures containing 70% CO2 and 30% O2. Supercarbia in the rat was observed to cause a greater reduction in cerebral intracellular pH (pHi) and increase in Pco2 than observed in other experiments with rats after 15 min of global ischemia. Complete neurologic and metabolic recovery was observed in these animals, despite an average decrease in pH; of 0.63 ± 0.02 pH unit during supercarbia episodes that raised Paco2 to 490 ± 80 mm Hg. No change was observed in cerebral intracellular ATP and only a 25% decrease was detected in phosphocreatine. The concentration of free cerebral intracellular ADP, which can be calculated if one assumes that the creatine kinase reaction is in equilibrium, decreased to approximately one-third of its control value. The calculated threefold decrease in the concentration of free ADP and twofold increase in the cytosolic phosphorylation potential suggest that there is increased intracellular oxygenation during supercarbia. Because a more than fourfold increase in intracellular hydrogen ion concentration was tolerated without apparent clinical injury, we conclude that so long as adequate tissue oxygenation and perfusion are maintained, a severe decrease in intracellular pH need not induce or indicate brain injury.

Effects of Hypoxic Hypoxia on Cerebral Phosphate Metabolites and pH in the Anesthetized Infant Rabbit

The effects of hypoxic hypoxia on high-energy phosphate metabolites and intracellular pH (pHi) in the brain of the anesthetized infant rabbit were studied in vivo using 31P nuclear magnetic resonance spectroscopy. Five 10- to 16-day-old rabbits were anesthetized with 1.5% halothane. Ventilation was controlled to maintain normocarbia. Inspired O2 fraction was adjusted to produce three states of arterial oxygenation: hyperoxia (Pao2 > 250 mm Hg), normoxia (Pao2 ∼ 100 mm Hg), and hypoxia (Pao2 25–30 mm Hg). During hypoxia, blood pressure was kept within 20% of control values with a venous infusion of epinephrine. During hyperoxia, the phosphocreatine-to-ATP ratio was 0.86, a value that is 2–2.5 times less than that reported for adults. During normoxia, ATP decreased by 20% and Pi increased by 90% from hyperoxia values. During 60 min of hypoxia, the concentrations of high-energy phosphate metabolites did not change, but intracellular and arterial blood pH (pHa) decreased significantly. When hyperoxia was reestablished, pHi returned to normal and pHa remained low. These results suggest that during periods of hypoxemia, the normotensive infant rabbit maintains intracellular concentrations of cerebral high-energy phosphates better than has been reported for adult animals.

Spin-echo fluorine magnetic resonance imaging at 2 T: In vivo spatial distribution of halothane in the rabbit head

Spin-echo 19F magnetic resonance imaging was performed at 2.0 T to explore the in vivo spatial distribution of halothane in the rabbit head. Because the halothane concentration is low in vivo, and because the measured relaxation times of the 19F resonance peak for halothane were T1 approximately equal to 1.0 sec and T2 approximately equal to 3.5-65 msec, 1-3-h imaging times were required (TR = 1 sec, TE = 9 msec) in order to obtain adequate images with a 64 X 256 raw data matrix and a 20-mm slice thickness. With this technique, halothane was primarily detected in lipophilic regions of the rabbit head, but little or no halothane was observed in brain tissue. Because T2 was shorter in brain tissue than in surrounding fat, a shorter TE than we could obtain is needed for optimal spin-echo imaging of brain halothane.

In situ brain metabolism

As an understanding of cerebral metabolism and circulation may have practical consequences for the treatment of brain injury and for surgery, a considerable body of knowledge has been gathered on the subject over a period of a t least twenty years.’-* Probably the most striking aspect of the subject is its complexity. The interplay of biochemical and physiological events when cerebral ischemia and hypoxia occur has still not been elucidated. Ischemia, i.e., either partial or total restriction of cerebral blood flow, presents the major medical problem of stroke. Hypoxia (low oxygen levels with normal cerebral blood flow) poses a concern during pulmonary failure, anesthetic malfeasance, and in high altitudes. The effects of ischemia and hypoxia are not identical. These cerebral insults exert various interrelated effects on morphological structure, function, and chemistry that are not simply reversed with reperfusion or restoration of oxygen. Since N M R spectroscopy has the potential for following some metabolic processes noninvasively, there has been some effort made to develop N M R as a technology to examine cerebral metabolism. Much is known about the mechanism of injury associated with cerebral ischemia. Gross physiological problems include brain edema, increased intracranial pressure, microcirculatory compromise, and post-ischemic recirculation problems, such as the “no-reflow” phenomenon and “loss of reperfusion” syndrome. Biochemical and in tracellular physiological aspects include low intracellular pH; the calcium-induced arachidonic acid cascade, excitotoxins, preischemic glucose excess, and oxygenderived free radical toxicity. Although much is known, optimum clinical stratagems for “brain protection” and “brain resuscitation” remain to be developed. It is suspected that much of the injury secondary to ischemia occurs during reperfusion and that an optimum regimen for reperfusion has yet to be developed. Many patients must also tolerate unavoidable periods of cerebral ischemia during surgery. Regulation of cellular energy metabolism and the consequences of lack of regulation are central to the problems of ischemia and hypoxia. The important factors fur regulation have been r e ~ i e w e d . ~ ATP is the bridge between the metabolic reactions that produce energy (glycolysis in the cytosol and oxidative phosphorylation in the mitochondria) and the energy-requiring functions of the cell including biosynthesis (gluconeogenesis, lipogenesis, protein synthesis, and nucleic acid synthesis), muscle contraction, and ion transport (to maintain ion gradients across cell membranes, transepithelial transport, and nerve conduction). The vast majority of ATP is produced

Comparison of in vivo 31P-MR spectra of the brain, liver, and kidney of adult and infant animals.

In vivo31P-magnetic resonance spectroscopy (MRS) was used to determine the phosphorus metabolite levels in the brain and kidney of infant rabbits and adult rats and in the liver of infant rabbits and adult and infant rats. For31P-MRS of the brain, a surface, radiofrequency coil was placed on the anterosuperior region of the head; for31P-MRS of the liver and kidney, a radiofrequency coil was chronically implanted either between the hepatic lobes or around the kidney.31P-MR spectra were found to show large variations in the levels of the phosphorus metabolites depending on the species, the organ, and the age of the animal. The phosphate monoester (MP)/adenosine triphosphate (ATP) ratio was significantly higher and the phosphocreatine (PCr)/ATP ratio was significantly lower in the brains of infant rabbits than in the brains of adult rats. Comparison of these data with data reported for humans and other animals suggests that these differences are due mainly to differences in age and not to differences among species. The phosphodiester (PD)/ATP ratio was found to be significantly higher in the livers of infant rabbits than in the livers of adult and infant rats — a difference more likely related to the species than to age. The kidneys of the infant rabbits showed a higher PCr/ATP radio than the kidneys of the adult rats, but this difference might be due to the influence of PCr in the surrounding muscle.

Cerebral Intracellular ADP Concentrations during Hypercarbia: An in vivo 31P Nuclear Magnetic Resonance Study in Rats

Qualitatively different responses of ADP levels have previously been observed in the brain during hypercarbia. One investigation has found that cerebral ADP stayed constant during hypercarbia in rats that were anesthetized with halothane, while another observed that ADP decreased during supercarbia in rats that received no supplemental anesthesia. This article reports an in vivo 31P nuclear magnetic resonance study to test the hypothesis that halothane anesthesia accounts for the discrepant observations. Isoflurane anesthesia was also studied in a second group of rats to see if a different general anesthetic agent would cause the same effects that halothane causes. The two groups of five rats underwent dual episodes of hypercarbia that were separated by a 45-min recovery period. General anesthesia, either 0.5% halothane or 1.0% isoflurane, was administered during the first episode but not during the second. Hypercarbia during halothane anesthesia caused the measured phosphocreatine (PCr) to decrease by 40%, while the calculated change in ADP was 10%, in agreement with the former investigation. In contrast, hypercarbia during either isoflurane anesthesia or no anesthesia caused a decrease of only 10% in PCr, which meant that the calculated decrease in ADP was 60%, in agreement with the results of the second investigation. We conclude that during hypercarbia, clinical concentrations of halothane, unlike clinical concentrations of isoflurane, interfere with the regulation of ATP metabolism.