Changes In Cerebral Oxygen Extraction Research Articles

Functional Magnetic Resonance Imaging: From Acquisition to Application

Functional magnetic resonance imaging (fMRI) is a technique that exploits magnetic resonance imaging (MRI) to detect regional brain activity through measurement of the hemodynamic response that is coupled to electrical neuronal activity. The most common fMRI method detects blood oxygen level dependent (BOLD) contrast. The BOLD effect represents alteration in the ratio of deoxygenated to oxygenated hemoglobin within brain tissue following neuronal activity. Alterations in this hemoglobin ratio result from changes in cerebral oxygen extraction, cerebral blood flow, and cerebral blood volume that occur in response to neuronal activity. The small, but detectable, change in magnetics resonance signal intensity is due to the sensitivity of magnetic resonance (MR) images to the paramagnetic deoxygenated state of hemoglobin that is the basis of contrast in fMRI applications. This review describes the physical and physiological bases of the MR signal, the principle of the BOLD effect, technical issues related to fMRI implementation, and fMRI experimental design. Research and clinical applications of fMRI are presented, including the use of fMRI in neurosurgical planning. Since it provides an individualized map of brain function, fMRI enables accurate localization of eloquent brain regions prior to surgery, allowing assessment of surgical risk and prognosis, as well as planning surgical approach.

Postnatal changes in cerebral oxygen extraction in the preterm infant are associated with intraventricular hemorrhage and hemorrhagic parenchymal infarction but not periventricular leukomalacia.

Fluctuations in cerebral hemodynamics have been implicated in the pathogenesis of acquired brain damage in babies born prematurely. This study examined the changes in cerebral fractional oxygen extraction (FOE) over the first 3 d after birth in 25 very-low-birth-weight preterm infants. Twelve infants had no major cerebral injury and 13 had acquired brain injury; cystic periventricular leukomalacia (PVL) was present in 4 and intraventricular hemorrhage (IVH) in 9, of whom 2 also had hemorrhagic parenchymal infarction (HPI). Normal values (median, 5(th)-95(th) centiles) for cerebral FOE in very-low-birth-weight infants with no cerebral injury were 0.38 (0.23-0.53) on d 1, 0.31 (0.18-0.45) on d 2, and 0.28 (0.17-0.38) on d 3. Infants who developed cystic PVL had no significant change in cerebral FOE during the first 3 d after birth. By contrast, cerebral FOE fluctuated in infants with IVH over the 3 d of measurement, decreasing from d 1 to d 2 (p = 0.03) and increasing from d 2 to d 3 (p = 0.02). The highest cerebral FOE values were seen in the two infants with HPI. The different patterns of change in cerebral FOE with HPI and cystic PVL provide additional evidence that the pathogenesis of these two conditions is different. Because high cerebral FOE is likely to be a consequence of low cerebral oxygen delivery, probably because of low cerebral blood flow, our results indicate that fluctuations in cerebral blood flow may occur when there is IVH or HPI.

Hypothermic acid-base management does not affect cerebral metabolic rate for oxygen at 27 degrees C. A study during cardiopulmonary bypass in rabbits.

It has been contended that, during cardiopulmonary bypass at 27 degrees C, pH-stat management decreases cerebral metabolic rate for oxygen (CMRO2) more than alpha-stat management. In contrast, other studies have not found CMRO2 to differ between techniques. Using each animal as its own control, the authors assessed the effect of alpha-stat versus pH-stat management of CMRO2, cerebral blood flow (CBF), and brain oxygen extraction during cardiopulmonary bypass at 27 degrees C. Fourteen New Zealand White rabbits, anesthetized with fentanyl and diazepam, underwent cardiopulmonary bypass at 27 degrees C (membrane oxygenator, centrifugal pump, and bifemoral arterial perfusion). Group 1 animals (n = 7) had alpha-stat management for the initial 65-70 min of bypass, and were then changed to pH-stat management for the remaining 30 min of bypass. Group 2 animals (n = 7) had pH-stat management for the initial 65-70 min of bypass, and were then changed to alpha-stat management for the remaining 30 min. Measurement of CBF (radiolabeled microspheres), CMRO2 (CBF x brain arterial-venous oxygen content difference), brain temperature, systemic hemodynamics, and arterial blood gases were made in each animal under both alpha-stat and pH-stat conditions. CMRO2 did not differ between alpha-stat and pH-stat conditions (1.4 +/- 0.3 ml.100 g-1.min-1; median +/- quartile deviation), and was independent of order of determination. Changes in CBF between alpha-stat and pH-stat conditions were associated with proportional opposite changes in cerebral oxygen extraction. Cerebral blood flow was significantly greater with pH-stat management than with alpha-stat management (37 +/- 5 vs. 30 +/- 3 ml.100 g-1.min-1, respectively). The CBF response to changing PaCO2 was significantly greater when going from alpha-stat to pH-stat conditions (group 1) than in the reverse order (group 2). During cardiopulmonary bypass at 27 degrees C, hypothermic acid-base management has no measurable effect on CMRO2. CMRO2 was neither extraction limited nor dependent on either PaCO2, CBF, or hemoglobin oxygen affinity differences between alpha-stat and pH-stat management. Cerebral blood flow responses to changing CMRO2 depend on the "starting" conditions, with alpha-stat management appearing to better preserve CBF reactivity than pH-stat management.

Continuous monitoring of cerebral hemodynamic reserve in acute brain injury: relationship to changes in brain swelling.

A new concept of cerebral hemodynamic and metabolic physiology, cerebral hemodynamic reserve (CHR), was evaluated in 20 comatose adults with acute traumatic brain swelling who were undergoing continuous monitoring of the arteriojugular difference in oxyhemoglobin saturation, along with cerebral perfusion pressure and expired PCO2. The CHR was measured as the ratio of relative (percent) changes in cerebral oxygen extraction to relative changes in cerebral perfusion pressure during spontaneous increases in intracranial pressure. In patients with initially severe brain swelling, the CHR was more frequently abnormal (compromised) on the first day, and tended to improve on the second day. In patients with initially moderate brain swelling, the CHR was more frequently normal (preserved) on the first day, but tended to become compromised on the second day if the brain swelling exacerbated. It is concluded that cerebral hemodynamic reserve abnormalities very closely associate with signs of increased intracranial "tightness" on computed tomographic scans of the head. Cerebral hemodynamic reserve could therefore become an important guide in the functional evaluation and management of acute brain swelling (focusing on cerebral oxygenation and perfusion pressure) in a variety of predominantly diffuse acute intracranial disorders.