Effect of Graded Hyperventilation on Cerebral Metabolism in a Cisterna Magna Blood Injection Model of Subarachnoid Hemorrhage in Rats

Interaction of hypocapnia, hypoxia, brain blood flow, and brain electrical activity in voluntary hyperventilation in humans

Changes in various physiological measures in voluntary hyperventilation lasting three minutes or more in humans were studied and compared. Three-minute hyperventilation, in which the rate of external ventilation increased by an average factor of 4.5-5, produced similar phasic changes in central and brain hemodynamics. The rate of circulation, indicated by rheographic data, initially increased during hyperventilation, reaching a maximum at 1-2 min of the test; there was then a reduction, to a minimum 2-3 min after the end of the test; this was followed by a further slow increase. The rate of cerebral blood flow during all 3 min of hyperventilation remained elevated in most subjects as compared with baseline and decreased during the 5 min following the end of the test. Transcutaneous carbon dioxide tension changed differently - there was a decrease to a minimum (about 25 mmHg) by the end of the test, lasting 1 min from the end of the test, this being followed by an increase to a level of 90% of baseline at 5 min after the test. Blood oxygen saturation remained at 98-100% during the test, decreasing to about 90% 5 min after the test; this, along with the decrease in cerebral blood flow, was a factor producing brain hypoxia. In different subjects, changes in the spectral power of oscillations in different EEG ranges on hyperventilation were "mirrored" to different extents by the dynamics of transcutaneous carbon dioxide tension. The duration and repetition of hyperventilation were important factors for understanding the interaction between brain hemodynamics, hypocapnia, hypoxia, and brain electrical activity. After several repetitions of 3-min hyperventilation over a period of 1 h, the increasing brain blood flow could decrease significantly on the background of relatively small changes in brain electrical activity. The data presented here were assessed from the point of view of the important role of brain tissue oxygen utilization mechanisms in adaptation to hypoxia and hypocapnia.

Neuroanesthesiology review--2006

In this article, we will provide a review of the 2006 literature of interest to those readers who provide perioperative care to patients with neurologic disease. This evaluation of the literature is not intended to be comprehensive, nor were systematic criteria used to include or exclude articles. Instead, the authors attempted to highlight those articles of greatest clinical relevance or those that provided unique insights into the physiology, pharmacology, and pathomechanisms of neurologic function for practicing clinicians and clinician-investigators. This article focuses on intracranial hemorrhage, anesthetic considerations in neurosurgical patients, cerebral hemodynamics, electrophysiologic monitoring, neuroprotection, and traumatic brain injury.

Brain changes to hypocapnia using rapidly interleaved phosphorus-proton magnetic resonance spectroscopy at 4 T

Substantial controversy persists in the literature concerning the physiologic consequences hypocapnia, or low partial pressure of carbon dioxide (PaCO(2)). Invasive animal studies have demonstrated large pH increases (>0.25 U), phosphocreatine (PCr) decreases (>30%), and adenosine triphosphate (ATP) decreases (>10%) after hyperventilation (HV) (20 mm Hg PaCO(2)). However, using magnetic resonance spectroscopy, HV studies in awake humans have demonstrated only small pH changes ( approximately 0.05 U) and no changes in PCr or ATP. It remains important to ascertain whether this failure to detect PCr changes in human studies reflects a true absence of changes, or a limitation in data fidelity. The present study used a rapidly interleaved phosphorus-proton spectroscopy acquisition from large samples at high magnetic field (4 T), to measure pH, PCr, inorganic phosphate, beta-ATP, and lactate changes with high temporal and signal sensitivity. Five of six subjects had usable data. During 20 mins HV, PaCO(2) reached a minimum at 16 mins (17 mm Hg); however, the maximum pH change (+0.047) peaked earlier (14 mins). Maximal lactate increases were measured at 15 mins. By 10 mins, maximum changes were observed for PCr (-3.4%) and inorganic phosphate (+6.4%). No changes in beta-ATP were observed. The peak in pH, despite continued decreases in PaCO(2), suggests active buffering during HV. These data, and the small magnitude of early PCr and inorganic phosphate changes, do not support substantial energy compromise during HV. Other mitigating factors, such as anesthesia-induced deregulation of the cerebrovasculature, might have contributed to the exaggerated metabolic changes observed in previous animal investigations.