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J. Paul Muizelaar, Anthony Marmarou, John D. Ward, Hermes A. Kontos, Sung C. Choi, Donald P. Becker, Hans Gruemer and Harold F. Young

✓ There is still controversy over whether or not patients should be hyperventilated after traumatic brain injury, and a randomized trial has never been conducted. The theoretical advantages of hyperventilation are cerebral vasoconstriction for intracranial pressure (ICP) control and reversal of brain and cerebrospinal fluid (CSF) acidosis. Possible disadvantages include cerebral vasoconstriction to such an extent that cerebral ischemia ensues, and only a short-lived effect on CSF pH with a loss of HCO3 buffer from CSF. The latter disadvantage might be overcome by the addition of the buffer tromethamine (THAM), which has shown some promise in experimental and clinical use. Accordingly, a trial was performed with patients randomly assigned to receive normal ventilation (PaCO2 35 ± 2 mm Hg (mean ± standard deviation): control group), hyperventilation (PaCO2 25 ± 2 mm Hg: HV group), or hyperventilation plus THAM (PaCO2 25 ± 2 mm Hg: HV + THAM group). Stratification into subgroups of patients with motor scores of 1–3 and 4–5 took place. Outcome was assessed according to the Glasgow Outcome Scale at 3, 6, and 12 months. There were 41 patients in the control group, 36 in the HV group, and 36 in the HV + THAM group. The mean Glasgow Coma Scale score for each group was 5.7 ± 1.7, 5.6 ± 1.7, and 5.9 ± 1.7, respectively; this score and other indicators of severity of injury were not significantly different. A 100% follow-up review was obtained. At 3 and 6 months after injury the number of patients with a favorable outcome (good or moderately disabled) was significantly (p < 0.05) lower in the hyperventilated patients than in the control and HV + THAM groups. This occurred only in patients with a motor score of 4–5. At 12 months posttrauma this difference was not significant (p = 0.13). Biochemical data indicated that hyperventilation could not sustain alkalinization in the CSF, although THAM could. Accordingly, cerebral blood flow (CBF) was lower in the HV + THAM group than in the control and HV groups, but neither CBF nor arteriovenous difference of oxygen data indicated the occurrence of cerebral ischemia in any of the three groups. Although mean ICP could be kept well below 25 mm Hg in all three groups, the course of ICP was most stable in the HV + THAM group. It is concluded that prophylactic hyperventilation is deleterious in head-injured patients with motor scores of 4–5. When sustained hyperventilation becomes necessary for ICP control, its deleterious effect may be overcome by the addition of THAM.

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Lennart Rabow, Antonio A. F. DeSalles, Donald P. Becker, Mildred Yang, Hermes A. Kontos, John D. Ward, Richard J. Moulton, Guy Clifton, Hanns D. Gruemer, J. Paul Muizelaar and Anthony Marmarou

✓ The posttraumatic creatine kinase-BB isoenzyme (CKBB) activity and lactate concentration in ventricular cerebrospinal fluid (CSF) have been studied in 29 patients with severe head injuries. The CKBB activity reaches its maximum a few hours after trauma, and has a monoexponential drop with a half-time of approximately 10 hours. Ventricular CSF lactate concentration continues to rise in patients with a poor outcome, and decreases only slowly and inconsistently in most of the other patients. Thus, increase of lactate in the ventricular CSF is not, like CKBB, a direct one-stage consequence of the trauma but is due to continuous production from a derangement of metabolism caused by the trauma. Since even higher ventricular CSF lactate levels can be survived when not caused by head injury, and since no significant pH changes were related to the ventricular CSF lactic acidosis in these artificially ventilated patients, it is concluded that ventricular CSF lactic acidosis is indicative of a severe, although not necessarily intractable, disturbance of brain function associated with intracellular lactate production and acidosis.

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J. Paul Muizelaar, Harry A. Lutz III and Donald P. Becker

✓ In a previous paper, the authors showed that mannitol causes cerebral vasoconstriction in response to blood viscosity decreases in cats. The present paper describes the changes in intracranial pressure (ICP) and cerebral blood flow (CBF) after mannitol administration in a group of severely head-injured patients with intact or defective autoregulation. The xenon-133 inhalation method was used to measure CBF. Autoregulation was tested by slowly increasing or decreasing the blood pressure by 30% and measuring CBF again. Mannitol was administered intravenously in a dose of 0.66 gm/kg; 25 minutes later, CBF and ICP were measured once again. In the group with intact autoregulation, mannitol had decreased ICP by 27.2%, but CBF remained unchanged. In the group with defective autoregulation, ICP had decreased by only 4.7%, but CBF increased 17.9%. One of the possible explanations for these findings is based on strong indications that autoregulation is mediated through alterations in the level of adenosine in response to oxygen availability changes in cerebral tissue. The decrease in blood viscosity after mannitol administration leads to an improved oxygen transport to the brain. When autoregulation is intact, more oxygen leads to decreased adenosine levels, resulting in vasoconstriction. The decrease in resistance to flow from the decreased blood viscosity is balanced by increased resistance from vasoconstriction, so that CBF remains the same. This might be called blood viscosity autoregulation of CBF, analogous to pressure autoregulation. Vasoconstriction also reduces cerebral blood volume, which enhances the effect of mannitol on ICP through dehydration of the brain. When autoregulation is not intact there is no vasoconstriction in response to increased oxygen availability; thus, CBF increases with decreased viscosity. With the lack of vasoconstriction, the effect on ICP through dehydration is not enhanced, so that the resulting decrease in ICP is much smaller. Such a mechanism explains why osmotic agents do not change CBF but decrease ICP in normal animals or patients with intact vasoconstriction, but do (temporarily) increase CBF in the absence of major ICP changes after stroke.

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J. Paul Muizelaar, Enoch P. Wei, Hermes A. Kontos and Donald P. Becker

✓ There is no proof that osmotic agents such as mannitol lower intracranial pressure (ICP) by decreasing brain water content. An alternative mechanism might be a reduction in cerebral blood volume through vasoconstriction. Mannitol, by decreasing blood viscosity, would tend to enhance cerebral blood flow (CBF), but the cerebral vessels would constrict to keep CBF relatively constant, analogous to pressure autoregulation. The cranial window technique was used in this study to measure the pial arteriolar diameter in cats, together with blood viscosity and ICP changes after an intravenous bolus of 1 gm/kg of mannitol. Blood viscosity decreased immediately; the greatest decrease (23%) occurred at 10 minutes, and at 75 minutes there was a “rebound” increase of 10%. Vessel diameters decreased concomitantly, the largest decrease being 12% at 10 minutes, which is exactly the same as the 12% decrease in diameter associated with pronounced hyperventilation (PaCO2 30 to 19 mm Hg) in the same vessels; at 75 minutes vessel diameter increased by 12%. With hyperventilation, ICP was decreased by 26%; 10 minutes after mannitol was given, ICP decreased by 28%, and at 75 minutes it showed a rebound increase of 40%. The correlation between blood viscosity and vessel diameter and between vessel diameter and ICP was very high. An alternative explanation is offered for the effect of mannitol on ICP, the time course of ICP changes, the “rebound effect,” and the absence of influence on CBF, all with one mechanism.