Regional cerebral blood flow and intraventricular pressure in acute head injuries

The effects of PaCO2 reduction on regional cerebral blood flow in the acute phase of brain injury

In 26 unconscious patients with brain injuries, regional cerebral blood flow (rCBF) was measured with a 16-channel Cerebrograph before and after acute reduction of PaCO2. The intra-arterial 133xenon washout technique was used, and CBF was calculated regionally as initial slope index or stochastic flow. The CO2 reactivity was calculated as deltaln CBV/deltaPaCO2. In supratentorial cortical lesions, an acute fall in PaCO2 increased the homogeneity of the regional flow pattern (decrease in the standard deviation of the regional flow values), and reduced the number of focal hyperaemic regions (tissue peaks). The CO2 reactivity in tissue peak regions was generally higher than in regions without tissue peaks. In severely injured patients with a poor outcome (dementia, vegetative survival or death), inverse steal reaction was accounted for in 11% of all regions, but only in 3% of the regions in patients who survived without dementia. Inverse steal reaction was most frequently seen during the first 3 days after the trauma. In repeated CBF studies, an increase in the CO2 reactivity with time was observed after the acute trauma. In comparison with the CO2 reactivity found in normocapnic awake subjects, this increase was higher than expected in several cases.

Experimental head injury in the rat. Part 3: Cerebral blood flow and oxygen consumption after concussive impact acceleration

Cerebral blood flow (CBF) and oxygen consumption (CMRO2) were determined during timmediate posttraumatic period in rats subjected to concussive impact acceleration. According to previous studies an impact of 9 m/sec velocity elicited typical and marked symptoms of experimental concussion and often a prolonged comatose state, accompanied by cerebral metabolic signs of energy failure. During the immediate concussive response there was an increase of the CBF, followed within the next few minutes by a decrease to about one-third of normal flow, and then by a tendency toward normalization of flow 20 to 40 minutes posttrauma. Simultaneous measurements of cerebral oxygen extraction indicated an increase of the CMRO2 during the first minute. During the ischemic phase oxygen extraction increased but the lowest CBF values were only partially compensated for, and normal oxygen availability could not be maintained. The combined data, including cerebrospinal fluid pressure measurements, indicated primary cerebrovascular effects of the concussive trauma. These vasomotor effects may induce critical cerebral ischemia and thus profoundly influence posttraumatic cerebral function, and cause irreversible damage.

Exerimental head injury in the rat. Part 2: Regional brain energy metabolism in concussive trauma

A standardized model of acceleration concussion in the rat was used for the study of cerebral energy metabolism during the acute concussive reaction. Impact velocities of 7 and 9 m/sec were used, and the cerebral metabolic state was determined 1, 4, and 15 minutes after the impact. A concussive response could be sustained with a normal energy state in the tissue, but with the more intense reaction to a 9 m/sec impact, energy depletion usually occurred. At 1 minute these changes were most pronounced in the brain-stem regions. At 4 minutes the reactions were more varied but a progression usually occurred during this time, while at 15 minutes restitution was indicated. Hypoxia due to neurogenic pulmonary edema aggravated the state. The findings are compatible with a high metabolic rate during concussion, but progressive changes indicate the rapid appearance of complicating factors, including hypoxemia and probably also ischemia.

The cerebrovascular CO2 reactivity during the acute phase of brain injury

Using the intra-arterial 133xenon (133Xe) method, the cerebrovascular response to acute Paco2 reduction was studied in 26 unconscious, brain-injured patients subjected to controlled ventilation. The CO2 reactivity was calculated as delta in CBF/delta Paco2. The perfusion pressure was defined as the difference between mean arterial pressure and mean intraventricular pressure. Although the CO2 reactivities did not differ significantly from that in awake, normocapnic subjects, it was low in the acute phase of injury, especially in those patients with severe outcome in whom the brain-stem reflexes were often affected. An increase of the CO2 reactivity with time was observed, indicating normal response after 1-2 weeks. Chronic hypocapnia in six unconscious patients resulted in sustained CSF pH adaptation. The question whether a delay in CSF pH adapation exerts an influence on the CO2 reactivity, and the influence of cerebral lactacidosis on the CO2 response are discussed.