Cerebral blood flow and cerebral energy state

Dependence of oxygen delivery on blood flow in rat brain: a 7 tesla nuclear magnetic resonance study

Magnetic resonance imaging (MRI) and spectroscopy (MRS) were used at a magnetic field strength of 7 T to measure CBF and CMRO2 in the sensorimotor cortex of mature rats at different levels of cortical activity. In rats maintained on morphine anesthesia, transitions to lower activity and higher activity states were produced by administration of pentobarbital and nicotine, respectively. Under basal conditions of morphine sulfate anesthesia, CBF was 0.75 +/- 0.09 mL x g(-1) x min(-1) and CMRO2 was 3.15 +/- 0.18 micromol x g(-1) x min(-1). Administration of sodium pentobarbital reduced CBF and CMRO2 by 66% +/- 16% and 61% +/- 6%, respectively (i.e., "deactivation"). In contrast, administration of nicotine hydrogen tartrate increased CBF and CMRO2 by 41% +/- 5% and 30% +/- 3%, respectively (i.e., "activation"). The resting values of CBF and CMRO2 for alpha-chloralose anesthetized rats were 0.40 +/- 0.09 mL x g(-1) x min(-1) and 1.51 +/- 0.06 micromol x g(-1) x min(-1), respectively. Upon forepaw stimulation, CBF and CMRO2 were focally increased by 34% +/- 10% and 26% +/- 12%, respectively, above the resting nonanesthetized values (i.e., "activation"). Incremental changes in CBF and CMRO2, when expressed as a percentage change for "deactivation" and "activation" from the respective control conditions, were linear (R2 = 0.997) over the entire range examined with the global and local perturbations. This tight correlation for cerebral oxygen delivery in vivo is supported by a recent model where the consequence of a changing effective diffusivity of the capillary bed for oxygen, D, has been hypothetically shown to be linked to alterations in CMRO2 and CBF. This assumed functional characteristic of the capillary bed can be theoretically assessed by the ratio of fractional changes in D with respect to changes in CBF, signified by omega. A value 0.81 +/- 0.23 was calculated for omega with the in vivo data presented here, which in turn corresponds to a supposition that the effective oxygen diffusivity of the capillary bed is not constant but presumably varies to meet local requirements in oxygen demand in a similar manner with both "deactivation" and "activation."

A high performance isolated rat brain preparation. Part I: Operative technique and recipient extracorporeal perfusion

The microsurgical preparation and high level performance with extracorporeal recipient perfusion of a stable, totally isolated rat brain model is presented. Emphasis has been placed on the use of the operating microscope, bipolar cautery and mechanical fixation units to affect a complete ablation of all non-osseous cephalic and vertebral tissues with minimal physiological disturbance of the animal as a whole. The simplified technique of recipient support circulation of the isolated brain organ achieves at least 5 h of tissue viability and function as measured by electrocortical activity, A-Vo2 differences and morphological appearance. To date, no other isolated rodent brain model has been described which is truly anatomically separated from all cephalic tissues. Additionally, other surgical attempts at isolation have required the use of abnormal physiological states, including deep hypothermia and circulatory arrest.

Cortical oxidative metabolism under conditions of ischemia, hypoxia, and asphyxia in the rabbit

The purpose of this investigation was to compare the effects of hypoxia, asphyxia, and ischemia on brain cortical oxidative metabolism. This study was carried out using 14 New Zealand White rabbits. The effects of episodic stress were measured simultaneously on brain functional metabolism by monitoring cortical oxygen tension (brain pO2), cortical cerebral blood flow (cCBF), cortical blood volume, and mitochondrial oxidative metabolism. During hypoxia (when the fraction of inspired O2 (FiO2) was reduced to 10%) and asphyxia (induced by turning the respirator off), there was a decrease of brain pO2 but an increase of cCBF and blood volume. Similarly, there was a reduction of cortical oxidative metabolism. In post-asphyxic conditions, an overshoot of brain pO2 and post-asphyxic oxidation of cytochrome (Cyt.) aa3 were usually shown. Under ischemic conditions (induced by sudden severe hypotension plus bilateral common carotid occlusion), cCBF and blood volume were decreased. There was also a decrease of brain pO2 and a reduction of Cyt. aa3 following ischemia. These techniques are applicable in intraoperative monitoring of patients.

Failure of brain cytochrome alpha , alpha 3 redox recovery after hypoxic hypotension as determined by in vivo reflectance spectrophotometry

Rats were subjected to a 30 minute period of combined hypoxia (F1o2 = .08) and hemorrhagic hypotension (MAP = 30 mm Hg), then resuscitated by restoration of F1o2 = .30 and reinfusion of shed blood and saline. Intracranial blood volume, hemoglobin saturation, and the cytochrome alpha, alpha 2 redox state were monitored through the intact skull during hypoxic hypotension and after resuscitation utilizing reflectance spectrophotometry. Although resuscitation returned arterial blood pressure, arterial pO2, and hemoglobin saturation toward normal, a sustained, significant (p less than .005) reduction in cytochrome alpha , alpha 2 remained. A parallel series of rats was subjected to identical hypoxic hypotension. At designated intervals the animals were sacrificed to determine brain ATP, ADP, and inorganic phosphate (P1). The data are discussed in terms of relationships between high energy phosphate metabolism and recorded changes in cerebral cytochrome alpha , alpha 2 redox state.

Brain energy metabolism in global brain oedema

Different degrees of severity in global brain oedema were induced by varying amounts of water intoxication (50, 100, 150, and 200 ml Aqua dest./kg b.wt. intravenously) in groups of six cats, which were functionally nephrectomized. Animals loaded with physiological saline and sham-operated served as controls. Two hours following the water load, the tissue concentrations of CrP, ATP, ADP, AMP, pyruvate, glucose, and lactate were determined by optical enzymatic analysis. The results show disturbances in brain energy metabolism dependent on the severity of the brain oedema. The high energy compounds and in consequence the ATP/ADP-ratio, and respectively the energy charge potential, fall in direct relationship to the severity of the brain oedema. Lactate and lactate-pyruvate ratio increase. The energy source of the cell, glucose as well as pyruvate, significantly falls in the group with severe brain oedema. The results of the brain energy metabolism were compared with our previous study concerning the brain water content, rCBF and CPP in global brain oedema (Meinig et al. 1973). The results show that the disturbances of energy metabolism are directly related to the rCBF and are not dependent on CPP over a wide range.

Recovery of brain energy metabolism following a period of combined hypoxia and hypotension

Rats were "stressed" by a 30-min period of hypoxia (FIO2 = 7.5%) and hypotension (x arterial pressure = 30 mm Hg), and then "resusciated" by restoring FIO2 = 30% and reinfusing shed blood to restore arterial pressure toward baseline values. Concentrations of brain phosphocreatine, ATP and lactate were measured after "stress" and 20, 60, and 120 min after "resuscitation". A biphasic response was noted in which ATP was initially restored to baseline values by "resuscitation", and then progressively decreased. Physiologic mechanisms to explain the observed data are presented.

Changes in local cerebral blood flow following profound systemic hypotension

The authors studied local cerebral blood flow in monkeys rendered hypotensive by infusion of a ganglionic blocking agent. Application of the 14C-antipyrine method demonstrated that the blood flow: 1) normally varies reproducibly from one structure to another within the brain; 2) appears at its lowest level in all structures during the early minutes of a rapid-onset hypotension; 3) maintains the same general rank order of blood flow rate during hypotension as was present during normotension; and 4) returns to supranormal levels immediately following the rapid restoration of blood pressure. The values for local cerebral blood flow remain close-to-normal in some animals and diminish significantly in others during late recovery from hypotension. The close-to-normal values accompany uncomplicated recoveries while the diminished values appear in those animals which became neurologically depressed. Areas of the brain considered predisposed to hypotensive injury did not exhibit depressions in blood flow rate during hypotension more markedly than did other brain areas. The present results are interpreted as strong evidence against the "border zone" hypothesis.

Reduced nicotinamide adenine dinucleotide fluorescence and cortical blood flow in ischemic and nonischemic squirrel monkey cortex. 2. effects of alterations in arterial carbon dioxide tension, blood pressure, and blood volume

The fluorescence of reduced nicotinamide adenine dinucleotide (NADH) from cerebral cortex was measured before, during, and after middle cerebral artery (MCA) occlusion and then at death of the animal. In normal cortex, NADH remained constant throughout a wide range of variations in blood pressure and Paco2. In ischemic cortex, NADH levels were higher in hypovolemic hypotensive animals than in normotensive normovolemic animals. Neither hypercapnia nor hypocapnia was effective in decreasing NADH in regions of ischemia, but the latter was associated with a degree of hypotension that interfered with interpretation of data. NADH returned to normal with restoration of flow, supporting the reversibility of this degree of ischemia. The high levels of NADH at death, compared to those during ischemia, are consistent with incomplete ischemia in this model of cerebral infarction.

Cerebral circulatory and metabolic effects of hypotension produced by deep halothane anaesthesia

Hypotension to a mean blood pressure of 33 mmHg for periods of 70 to 187 minutes was induced by increasing the inspired halothane concentration in 11 baboons which were already anaesthetized with 0·5% halothane, nitrous oxide, and oxygen. During hypotension, cerebral blood flow, measured by Xenon clearance and by a carotid electromagnetic flowmeter, decreased by more than half, and sagittal sinus oxygen saturation was 46%. Cerebral oxygen uptake fell from 5·15 to 3·56 ml./100 g/min at this deeper level of halothane anaesthesia. Cerebral hyperaemia developed after hypotension in those animals which regained a mean blood pressure greater than 70 mmHg. Acidbase measurements on CSF from the cisterna magna revealed no metabolic acidosis during or after hypotension. In all four animals with intact autoregulation before hypotension, this was absent or impaired afterwards.