✓ Intravascular volume expansion has been successfully employed to promote blood flow in ischemic brain regions. This effect has been attributed to both decreased blood viscosity and increased cardiac output resulting from volume expansion. The physiological mechanism by which changes in cardiac output would affect cerebral blood flow (CBF), independent of blood pressure variations, is unclear, but impaired cerebral autoregulation is believed to play a role. In order to evaluate the relationship between cardiac output and CBF when autoregulation is either intact or defective, 135 simultaneous measurements of cardiac output (thermodilution method) and CBF (by the 133Xe inhalation or intravenous injection method) were performed in 35 severely head-injured patients. In 81 instances, these measurements were performed after manipulation of blood pressure with phenylephrine or Arfonad (trimethaphan camsylate), or manipulation of blood viscosity with mannitol. Autoregulation was found to be intact in 55 of these cases and defective in 26. A wide range of changes in cardiac output occurred after administration of each drug. No correlation existed between the changes in cardiac output and the changes in CBF, regardless of the status of blood pressure autoregulation. A significant (40%) increase in CBF was found after administration of mannitol when autoregulation was defective. These data support the hypothesis that, within broad limits, CBF is not related to cardiac output, even when autoregulation is impaired. Thus, the effect of intravascular volume expansion appears to be mediated by decreased blood viscosity rather than cardiac output augmentation.
Gerrit J. Bouma and J. Paul Muizelaar
Gerrit J. Bouma, J. Paul Muizelaar, Kuniaki Bandoh, and Anthony Marmarou
✓ Increased brain tissue stiffness following severe traumatic brain injury is an important factor in the development of raised intracranial pressure (ICP). However, the mechanisms involved in brain tissue stiffness are not well understood, particularly the effect of changes in systemic blood pressure. Thus, controversy exists as to the optimum management of blood pressure in severe head injury, and diverging treatment strategies have been proposed. In the present study, the effect of induced alterations in blood pressure on ICP and brain stiffness as indicated by the pressure-volume index (PVI) was studied during 58 tests of autoregulation of cerebral blood flow in 47 comatose head-injured patients. In patients with intact autoregulation mechanisms, lowering the blood pressure caused a steep increase in ICP (from 20 ± 3 to 30 ± 2 mm Hg, mean ± standard error of the mean), while raising blood pressure did not change the ICP. When autoregulation was defective, ICP varied directly with blood pressure. Accordingly, with intact autoregulation, a weak positive correlation between PVI and cerebral perfusion pressure was found; however, with defective autoregulation, the PVI was inversely related to cerebral perfusion pressure. The various blood pressure manipulations did not significantly alter the cerebral metabolic rate of oxygen, irrespective of the status of autoregulation.
It is concluded that the changes in ICP can be explained by changes in cerebral blood volume due to cerebral vasoconstriction or dilatation, while the changes in PVI can be largely attributed to alterations in transmural pressure, which may or may not be attenuated by cerebral arteriolar vasoconstriction, depending on the autoregulatory status. The data indicate that a decline in blood pressure should be avoided in head-injured patients, even when baseline blood pressure is high. On the other hand, induced hypertension did not consistently reduce ICP in patients with intact autoregulation and should only be attempted after thorough assessment of the cerebrovascular status and under careful monitoring of its effects.
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.
Robert F. Berman, Bon H. Verweij, and J. Paul Muizelaar
Object. Abnormal accumulation of intracellular calcium following traumatic brain injury (TBI) is thought to contribute to a cascade of cellular events that lead to neuropathological conditions. Therefore, the possibility that specific calcium channel antagonists might exert neuroprotective effects in TBI has been of interest. The focus of this study was to examine whether Ziconotide produces such neuroprotective effects.
Methods. The authors report that the acceleration—deceleration model of TBI developed by Marmarou, et al., induces a long-lasting deficit of neuromotor and behavioral function. The voltage-sensitive calcium channel blocker Ziconotide (also known as SNX-111 and CI-1009) exerts neuroprotective effects in this model of diffuse brain injury (DBI) in rats. The dose and time of injection of Ziconotide chosen for the present study was based on the authors' previous biochemical studies of mitochondria. Rats were trained in a series of motor and memory tasks, following which they were subjected to DBI using the Marmarou, et al., model. At 3, 5, and 24 hours, all rats were injected with 2 mg/kg Ziconotide for a total cumulative dose of 6 mg/kg Ziconotide. Control brain-injured animals were injected with an equal volume of saline vehicle at each of these time points. The rats were tested for motor and cognitive performance at 1, 3, 7, 14, 21, 28, 35, and 42 days postinjury. Saline-treated rats displayed severe motor and cognitive deficits after DBI. Compared with saline-treated control animals, rats treated with Ziconotide displayed better motor performance during inclined plane, beam balance, and beam walk tests; improved memory while in the radial arm maze; and improved learning while in the Morris water maze.
Conclusions. These results demonstrated that the acceleration—deceleration model, which had been developed by Marmarou, et al., induces severe motor and cognitive deficits. We also demonstrated that Ziconotide exhibits substantial neuroprotective activity in this model of TBI. Improvement was observed in both motor and cognitive tasks, even though treatment was not initiated until 3 hours after injury. These findings support the development of neuronal N-type calcium channel antagonists as useful therapeutic agents in the treatment of TBI.
Bon H. Verweij, J. Paul Muizelaar, and Federico C. Vinas
Object. The poor prognosis for traumatic acute subdural hematoma (ASDH) might be due to underlying primary brain damage, ischemia, or both. Ischemia in ASDH is likely caused by increased intracranial pressure (ICP) leading to decreased cerebral perfusion pressure (CPP), but the degree to which these phenomena occur is unknown. The authors report data obtained before and during removal of ASDH in five cases.
Methods. Five patients who underwent emergency evacuation of ASDH were monitored. In all patients, without delaying treatment, a separate surgical team (including the senior author) placed an ICP monitor and a jugular bulb catheter, and in two patients a laser Doppler probe was placed.
The ICP prior to removing the bone flap in the five patients was 85, 85, 50, 59, and greater than 40 mm Hg, resulting in CPPs of 25, 3, 25, 56, and less than 50 mm Hg, respectively. Removing the bone flap as well as opening the dura and removing the blood clot produced a significant decrease in ICP and an increase in CPP. Jugular venous oxygen saturation (SjvO2) increased in four patients and decreased in the other during removal of the hematoma. Laser Doppler flow also increased, to 217% and 211% compared with preevacuation flow.
Conclusions. Intracranial pressure is higher than previously suspected and CPP is very low in patients with ASDH. Removal of the bone flap yielded a significant reduction in ICP, which was further decreased by opening the dura and evacuating the hematoma. The SjvO2 as well as laser Doppler flow increased in all patients but one immediately after removal of the hematoma.
Marc L. Schröder, J. Paul Muizelaar, and A. John Kuta
✓ The authors report two cases of severe head injury with acute subdural hematoma, in which cerebral blood flow (CBF) and cerebral blood volume (CBV) measurements were obtained prior to evacuation of the subdural hematoma and again immediately after removal. The first patient, a 21-year-old man with a motor response localizing to pain, had a global CBF of 18.2 ml/100 gm/min and a decreased global CBV of 3.7 ml/100 gm at 2.3 hours after injury. Immediately after removal of the subdural hematoma (8.1 hours after injury), CBF and CBV measurements revealed increases to 35.5 ml/100 gm/min and 5.8 ml/100 gm, respectively. The second patient, a 49-year-old woman with a normal flexor motor response to pain, had preoperative global values of 15.8 ml/100 gm/min for CBF and 2.0 ml/100 gm for CBV at 3 hours after injury. Postoperatively (9.3 hours after injury), the CBF and CBV values increased to 41.6 ml/100 gm/min and 4.0 ml/100 gm, respectively. The first patient, with only borderline ischemia and removal of the subdural hematoma within 3 hours, made a good recovery, while the second patient, with prolonged lower levels of CBF, remained in a persistent vegetative state. The low values of preoperative CBV argue for compression of the microcirculation as the cause of ischemia.
Krista Keachie, Kiarash Shahlaie, and J. Paul Muizelaar
Significant progress has been made in lumbar and cervical disc replacement therapy. Several cervical disc prostheses have recently gained FDA approval. Although arthroplasty has not been previously described in the thoracic spine, selected patients with long-segment fusion to the level of C-7 have altered cervicothoracic and upper thoracic biomechanics and may benefit from motion-preservation therapy for T1–2 disc herniation. Currently, FDA-approved prostheses are indicated only for patients with single-level degenerative disc disease between C-3 and C-7 and no history of cervical arthrodesis.
The authors describe a 52-year-old woman who had previously undergone C3–7 fusion and returned 4 years later with symptoms of C-8 myeloradiculopathy and radiological evidence of T1–2 degenerative disc disease. She underwent T1–2 arthroplasty in which a Prestige artificial cervical disc was placed via an anterior cervicothoracic approach. Motion at C7–T1 and T1–2 was preserved, and the patient made an excellent clinical recovery.