Cerebral blood flow and metabolic changes from induction to onset of anesthesia with halothane or pentobarbital

Identifying causal relationships between EEG activity and intracranial pressure changes in neurocritical care patients

OBJECTIVE

To explore and assess the relationship between electroencephalography (EEG) activity and intracranial pressure (ICP) in patients suffering from traumatic brain injury (TBI) and subarachnoid hemorrhage (SAH) during their stay in an intensive care unit.

APPROACH

We performed an observational prospective cohort study of adult patients suffering from TBI or SAH. Continuous EEG-ECG was performed during ICP monitoring. In every patient, variables derived from the EEG were calculated and the Granger causality (GC) methodology was employed to assess whether, and in which direction, there is any relationship between EEG and ICP.

MAIN RESULTS

One-thousand fifty-five hours of continuous multimodal monitoring were analyzed in 21 patients using the GC test. During 37.88% of the analyzed time, significant GC statistic was found in the direction from the EEG activity to the ICP, with typical lags of 25-50 s between them. When recordings were adjusted by sedation-perfusion and/or bolus-and handling, these percentages hardly changed.

SIGNIFICANCE

Long-lasting, continuous and simultaneous EEG and ICP recordings from TBI and SAH patients provide highly rich and useful information, which has allowed for uncovering a strong relationship between both signals. The use of this relationship could lead to developing a medical device to measure ICP in a non-invasive way.

Functional and structural changes throughout the auditory system following congenital and early-onset deafness: implications for hearing restoration

The absence of auditory input, particularly during development, causes widespread changes in the structure and function of the auditory system, extending from peripheral structures into auditory cortex. In humans, the consequences of these changes are far-reaching and often include detriments to language acquisition, and associated psychosocial issues. Much of what is currently known about the nature of deafness-related changes to auditory structures comes from studies of congenitally deaf or early-deafened animal models. Fortunately, the mammalian auditory system shows a high degree of preservation among species, allowing for generalization from these models to the human auditory system. This review begins with a comparison of common methods used to obtain deaf animal models, highlighting the specific advantages and anatomical consequences of each. Some consideration is also given to the effectiveness of methods used to measure hearing loss during and following deafening procedures. The structural and functional consequences of congenital and early-onset deafness have been examined across a variety of mammals. This review attempts to summarize these changes, which often involve alteration of hair cells and supporting cells in the cochleae, and anatomical and physiological changes that extend through subcortical structures and into cortex. The nature of these changes is discussed, and the impacts to neural processing are addressed. Finally, long-term changes in cortical structures are discussed, with a focus on the presence or absence of cross-modal plasticity. In addition to being of interest to our understanding of multisensory processing, these changes also have important implications for the use of assistive devices such as cochlear implants.

Diagnosis of Intracranial Arterial Stenosis Using Transcranial Doppler Flowmetry

In this case report we describe the use of transcranial Doppler flowmetry during induction of anesthesia in a patient with a large pituitary tumor. In this patient, both IV anesthesia induction and onset hyperventilation were followed by severe decreases of flow velocity in the middle cerebral artery of the affected side. Transcranial Doppler detected critical blood flow reduction in response to anesthesia induction and onset of hyperventilation in a brain tumor patient.

Effect of halothane on the cerebral circulation in young children: a hysteresis phenomenon

To determine the effect of halothane on the cerebral blood flow velocity (CBFV) with increasing then decreasing concentrations, 11 children scheduled for minor surgery were studied. Anaesthesia consisted of halothane, vecuronium, nitrous oxide in oxygen and a caudal block. End-tidal carbon dioxide, temperature, heart rate and systolic arterial pressure were maintained constant. CBFV increased significantly between 0.5 and 1.0 MAC (p <0.001), and 0.5 and 1.5 MAC of halothane (p <0.001), but was not different after increasing concentration from 1.0 to 1.5 MAC. During the decreasing phase, CBFV decreased significantly from 1.5 to 1.0 MAC of halothane (p <0.001), whereas there was no difference in CBFV when decreasing halothane MAC from 1.0 to 0.5 MAC. In children, the decrease in CBFV during decreasing halothane concentration is not superimposable to the increase in CBFV seen when increasing halothane concentration, suggesting the presence of cerebrovascular hysteresis to halothane.

Sevoflurane (0.4 MAC) does not influence cerebral compliance in healthy individuals

The use of sevoflurane is favored for its rapid onset and offset of anesthesia as well as good intraoperative titratability of the anesthetic. With regard to neuroanesthesia, the reported effects of sevoflurane on cerebral hemodynamics and cerebrospinal fluid dynamics are inconsistent. We used phase-contrast magnetic resonance imaging measurement of systolic cerebrospinal fluid peak velocity (CSFVPeak) to evaluate the effect of sevoflurane on cerebral compliance in healthy individuals. During administration of 0.4 MAC sevoflurane, systolic CSFVPeak in the aqueduct of Sylvius remained unchanged, thereby indicating unaffected cerebral compliance: (CSFVPeak baseline: -3.1 +/- 1.0 cm/s vs. sevoflurane: -3.0 +/- 1.2 cm/s). We conclude that low-dose administration of sevoflurane does not influence cerebral compliance in healthy individuals, but the influence of coexisting intracranial pathology or comedications on cerebral compliance requires further clinical investigation.

A subanesthetic concentration of sevoflurane increases regional cerebral blood flow and regional cerebral blood volume and decreases regional mean transit time and regional cerebrovascular resistance in volunteers

Inhaled anesthetics exert metabolically mediated effects on cerebral blood vessels both directly and indirectly. We investigated the effects of a 0.4 minimum alveolar subanesthetic concentration of sevoflurane on regional cerebral blood flow (rCBF), regional cerebral blood volume (rCBV), regional cerebrovascular resistance (rCVR), and regional mean transit time (rMTT) in volunteers by means of contrast-enhanced magnetic resonance imaging perfusion measurement. Sevoflurane increased rCBF by 16% to 55% (control, 55. 03 +/- 0.33 to 148.83 +/- 1.9 mL. 100 g(-1). min(-1); sevoflurane, 71.75 +/- 0.36 to 193.26 +/- 2.14 mL. 100 g(-1). min(-1)) and rCBV by 7% to 39% (control, 4.66 +/- 0.03 to 10.04 +/- 0.12 mL/100 g; sevoflurane, 5.04 +/- 0.03 to 13.6 +/- 0.15 mL/100 g); however, sevoflurane decreased rMTT by 7% to 18% (control, 3.75 +/- 0.04 to 5. 39 +/- 0.04 s; sevoflurane, 3.4 +/- 0.03 to 4.44 +/- 0.03 s) and rCVR by 22% to 36% (control, 0.74 +/- 0.01 to 1.9 +/- 0.2 mm Hg/[mL. 100 g(-1). min(-1)]; sevoflurane, 0.54 +/- 0.01 to 1.41 +/- 0.01 mm Hg/[mL. 100 g(-1). min(-1)]). Interhemispheric differences in rCBF, rCBV, and rCVR were markedly reduced after the administration of sevoflurane. These findings are consistent with the known direct vasodilating effect of sevoflurane. The decrease in rMTT further shows that rCBF increases more than does rCBV. Furthermore, we can show that the observed increase in rCBF during inhalation of sevoflurane is not explained by vasodilation alone.

Intensity of halothane- and hypercapnia-induced cerebral hyperemia is strain-dependent in rats

Cerebrovascular responses to physiologic and pharmacologic stimuli vary between laboratories using different strains of the same species. We tested whether the cerebral blood flow (CBF) response to 1% halothane or hypercapnia is strain-dependent in rats. Age-matched adult male (n = 14 of each strain) Wistar, Wistar-Kyoto (WKY), and spontaneously hypertensive rats (SHR) were anesthetized with pentobarbital and mechanically ventilated. Under baseline conditions blood flow to cerebrum (microspheres) in WKY (66 +/- 5 mL.min-1.100 g-1) was less than (P < 0.05) in Wistar (88 +/- 5 mL.min-1.100g-1) and SHR (83 +/- 5 mL.min-1.100 g-1). Blood flow to brainstem was greater (P < 0.05) in Wistar (106 +/- 8 mL. min-1.100 g-1) than in WKY (71 +/- 5 mL.min-1. 100 g-1) and SHR (84 +/- 4 mL.min-1.100 g-1). In the halothane protocol (n = 8 each strain), administration of 1% halothane, during normocapnia, increased blood flow to the cerebrum in WKY (64 +/- 6 to 120 +/- 12 mL.min-1.100 g-1, P < 0.05) and SHR (78 +/- 6 to 115 +/- 8 mL.min-1.100 g-1, P < 0.05) but not Wistar rats (88 +/- 8 to 102 +/- 5 mL.min-1.100 g-1, not significant). Discontinuing halothane caused blood flow to return to baseline values. In the hypercapnia protocol (n = 6 each strain), exposure to 3% CO2 (to achieve a PaCO2 of 50-55 mm Hg) and 6% CO2 (to achieve a PaCO2 of 60-70 mm Hg) caused blood flow to the cerebrum to increase in Wistar (87 +/- 11 to 112 +/- 15 to 162 +/- 23) to similar amount as observed in WKY (69 +/- 7 to 115 +/- 13 to 162 +/- 23 mL.min-1. 100 g-1) but less than that observed in SHR (89 +/- 7 to 174 +/- 24 to 237 +/- 28 mL.min-1.100 g-1). These data demonstrate that the cerebral hyperemic response to vasodilator stimuli is strain-dependent in rats.

Effects of adenosine and 2-chloroadenosine on cerebral collateral vessels

Adenosine is a potent cerebral vasodilator. We tested the hypothesis that dilatation of collateral vessels in cerebrum, in response to topical adenosine and 2-chloroadenosine (2-CAD), would increase blood flow to collateral-dependent cerebrum. In dogs anesthetized with halothane, a branch of the middle cerebral artery (MCA) was occluded proximally and cannulated distally. The collateral-dependent area at risk for infarction was perfused from a reservoir with microsphere-free blood, and blood flow to normal cerebrum and to cerebrum dependent on collateral flow was measured with radioactive microspheres injected into the left ventricle through a femoral artery catheter. Perfusion through the cannulated MCA branch was stopped, and flow to normal and collateral-dependent cerebrum was measured after adenosine (10(-2) M) or 2-CAD (10(-4) M) was added to the superfusate over the cerebrum. In normal cerebrum, topical application of adenosine increased flow to outer but not inner layers. Topical application of adenosine had little effect on flow to collateral-dependent tissue. In normal cerebrum, 2-CAD increased flow to outer layers, whereas flow to inner layers tended to increase. During 2-CAD, flow to outer cortical layers of collateral-dependent cerebrum increased from 140 +/- 20 ml/100 g/min (mean +/- SD) to 231 +/- 68, whereas flow to the inner collateral-dependent tissue did not change. The findings indicate that, after occlusion of a cerebral artery, topical 2-CAD increases blood flow to outer layers of collateral-dependent and normal cerebrum.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral blood flow and metabolic responses to sustained hypercapnia in awake sheep

This investigation determined the effects of sustained hypercapnia on cerebral blood flow (CBF; radiolabeled microspheres), cerebral metabolic rates for O2 and glucose (CMRO2 and CMRglc), and brain water content in conscious sheep instrumented with aortic, left ventricular, vena cava, and brain sagittal sinus catheters. PaCO2 was elevated from 38 +/- 3 to 53 +/- 3 (mean +/- SD) mm Hg and PaO2 from 109 +/- 7 to 131 +/- 4 mm Hg for 96 h in an environmental chamber. Hypercapnia did not alter sheep behavior, food and water intake, arterial pressures, core temperature, or brain lactate release. Total and regional CBF and CBF/CMRO2 reached peak values at 1 h and then readjusted, to stabilize at lower, but still elevated levels at 24 h and thereafter. CMRO2 and CMRglc increased at 6 h and thereafter during hypercapnia. PaCO2, CBF, CMRO2, and CMRglc remained elevated at 3 h after restoration to room air, while CBF/CMRO2 returned to the control value. Frontal and occipital lobe wet-to-dry weight ratios increased modestly but significantly after hypercapnic exposure. It is concluded that sustained hypercapnia induces stable and nonadapting increases in both CBF and brain metabolism that persist for at least 3 h after restoration to room air in association with hypoventilization and modest elevations of brain water.

A simple survival model of volume-controlled hemorrhagic shock in awake rats

A simple rat model was developed for the study of spontaneous survival after volume-controlled hemorrhage. The objective was to determine in awake, unrestrained rats the shed blood volume (SBV) in ml/100 g body weight that without fluid resuscitation, would result in either a high or a low percentage of survivors within 24 h. About 24 h after cannulation under light anesthesia, the awake rats were insulted with arterial blood withdrawal at a constant rate over 20 min, while mean arterial pressure (MAP) was monitored (N = 78). Then, the arterial catheter was removed, and the rats were observed for 24 h or until death. With increasing SBV, survival rate decreased, SBV of 2.50 ml/100 g (group I) resulted in 74% survivors at 2 h and 24 h; SBV of 2.75 ml/100 g (group II), in 67% survivors at 2 h and 46% at 24 h; SBV of 3.00 ml/100 g (group III), in 35% survivors at 2 h and 20% at 24 h; and SBV of 3.50 ml/100 g (group IV), in no survivors to 2 h. MAP declined similarly over 20 min blood withdrawal in the four insult groups, without difference between ultimate survivors vs. nonsurvivors. All rats survived to the end of 20 min hemorrhage (i.e. hemorrhagic shock [HS] time = 0 min). Deaths at HS time 0-2 h occurred after SBV of 2.50 ml/100 g, at HS time 56 +/- 35 min; after SBV of 2.75 ml/100 g, at HS 81 +/- 26 min; after SBV of 3.00 ml/100 g, at HS 37 +/- 33 min; and after SBV of 3.50 ml/100 g, at HS 11 +/- 2 min. Weight may have affected MAP response and survival. We conclude that a volume-controlled HS model in rats without anesthesia or restraint is feasible. SBV of 2.50 ml/100 g should be suitable for testing additional insults. SBV of 3.00 ml/100 g should be suitable for testing resuscitative therapies. The model should be modified to allow monitoring of key variables after hemorrhage.

Responses to barbiturates of isolated dog cerebral and mesenteric arteries contracted with KCl and prostaglandin F2 alpha

In helical strips of dog cerebral and mesenteric arteries previously contracted with KCl or prostaglandin F2 alpha (PGF2 alpha), the addition of pentobarbital (10(-5) to 10(-3) mol.l-1) caused dose-related relaxation, whereas thiamylal and thiopental in a low concentration (10(-5) to 10(-4) mol.l-1) caused further contraction and in a high concentration (10(-3) mol.l-1) profound relaxation. Thiobarbiturate-induced contractions were greater in mesenteric than in cerebral arteries previously contracted with PGF2 alpha. In cerebral and mesenteric arteries exposed to Ca(++)-free media for 60 min and treated with KCl or PGF2 alpha, reintroduction of Ca++ produced a transient contraction, a transient relaxation and a persistent contraction. The Ca(++)-induced persistent contraction was attenuated by pretreatment with pentobarbital (10(-4) to 10(-3) mol.l-1) and thiamylal (10(-3) mol.l-1); the attenuation was greater in arteries treated with KCl than with PGF2 alpha. The Ca(++)-induced contractions of mesenteric artery treated with PGF2 alpha were potentiated by 10(-4) mol.l-1 thiamylal. It is concluded that pentobarbital possesses only a vasodilator effect, whereas thiamylal and thiopental have both constrictor and dilator effects on vascular smooth muscle. The vasodilator effect of barbiturates is associated in part with inhibition of transmembrane influx of Ca++; the inhibition is more predominant on the influx evoked by KCl-induced depolarization than by a stimulation of PGF2 alpha receptors. Thiamylal in low concentrations appears to enhance Ca++ influx through a receptor-operated Ca++ channel for PGF2 alpha.

Preoperative characteristics predicting intraoperative hypotension and hypertension among hypertensives and diabetics undergoing noncardiac surgery

We prospectively studied patients with hypertension and diabetes undergoing elective noncardiac surgery with general anesthesia to test the hypothesis that patients at high risk for prognostically significant intraoperative hemodynamic instability could be identified by their preoperative characteristics. Specifically we hypothesized that patients with a low functional capacity, decreased plasma volume, or significant cardiac comorbidity would be at high risk for intraoperative hypotension and those with a history of severe hypertension would be at risk for intraoperative hypertension. Patients who had a preoperative mean arterial pressure (MAP) greater than or equal to 110, a walking distance of less than 400 m, or a plasma volume less than 3000 cc were at increased risk of intraoperative hypotension (i.e., more than 1 hour of greater than or equal to 20 mmHg decreases in the MAP). Hypotension was also more common among patients having intra-abdominal or vascular surgery, and among those who had operations longer than 2 hours. Patients older than 70 years or with a decreased plasma volume were at increased risk of having more than 15 minutes of intraoperative elevations of greater than or equal to 20 mmHg over the preoperative MAP in combination with intraoperative hypotension; this was also more common when surgery lasted more than 2 hours. Patients who had intraoperative hypotension tended to have an immediate decrease in MAP at the onset of anesthesia and were often purposefully maintained at MAPs less than their usual level during surgery with fentanyl and neuromuscular blocking agents. Patients who had intraoperative hyper/hypotension tended to have repeated elevations in MAP above their preoperative levels during the course of surgery, and such elevations precipitated interventions with neuromuscular blocking agents and/or fentanyl. Neither pattern was more common among patients who developed net intraoperative negative fluid balances. Both hypotension and hyper/hypotension were associated with increased renal and cardiac complications after operation. Patients with cardiac disease, especially diabetics, and those with negative fluid balances also had increased complications. Preoperative characteristics influence the susceptibility to intraoperative hypotension and hypertension, which are related to postoperative complications.

Effect of GM1 ganglioside after focal cerebral ischemia in halothane-anesthetized cats

The effect of the ganglioside GM1 was studied in a focal cerebral ischemia model in 30 cats consisting of 2 hours of middle cerebral artery occlusion followed by 4 hours of recirculation. The cerebrocortical electrical activity, extracellular potassium activity, and microcirculation indicated by NAD/NADH fluorescence were measured during occlusion as well as during recirculation in the core of the middle cerebral artery territory, while the cerebral metabolic rate for glucose (ICMRgl) was measured at the end of recirculation. The cats were classified into either mildly or moderately severe stroke groups based on the depression of the cerebrocortical electrical activity on the occluded side. Of 12 cats with only a mild stroke, six were administered GM1 intravenously 30 minutes after occlusion, while six cats were not treated. Of 12 cats with a moderate stroke, six were treated and six were left untreated. In six additional cats, only a sham insult was undertaken. In the cats with mild stroke, GM1 treatment significantly increased lCMRgl in the peripheral middle cerebral artery territory compared with the untreated cats; for the six treated cats, lCMRgl was normalized toward the control level, whereas it was depressed in the six untreated cats. There were no other significant effects of GM1 treatment on the other measured parameters. A potential protective effect of anesthesia is discussed.

Pentobarbital pharmacokinetics in patients with severe head injury

Intravenous administration of high-dose pentobarbital has been proposed as a treatment for elevated intracranial pressure refractory to other measures in brain-injured patients. The purpose of this clinical study was to examine the pharmacokinetics of high-dose continuous intravenous infusion of pentobarbital in this critical care setting. Six patients received a 25-34 mg/kg intravenous loading dose followed by a 1-3 mg/kg/h continuous infusion for 61-190 hours. Dosing rates were adjusted based on the patient's clinical status. The mean clearance was 0.72 ml/min/kg, with a volume of distribution (Vd) of 1.03 L/kg and a terminal half-life of 19.1 h. Considerable variation in individual patient parameters was observed. In addition, a change in clearance was suggested in patients requiring a longer infusion duration.

Photochemically induced cortical infarction in the rat. 1. Time course of hemodynamic consequences

Alterations in local CBF (LCBF) were assessed autoradiographically in the rat at several time points following photochemically induced cortical infarction. Cortical infarction of consistent size and location was produced by irradiating the brain with green light through the intact skull for 20 min following the systemic injection of rose bengal. A consistent pattern of altered LCBF was recorded in both ipsilateral and contralateral brain regions over the course of the study. At 30 min, a severely ischemic zone surrounded by regions of cortical hyperemia was apparent. LCBF was also depressed relative to control values in ipsilateral cortical regions remote from the irradiated area, while contralateral cortical structures were mildly hyperemic. By 4 h, the zone of severe ischemia had enlarged and its margins were no longer hyperemic. Ipsilateral cortical and some subcortical structures demonstrated significantly depressed levels of LCBF. At 5 days, LCBF throughout both ipsilateral and contralateral cortices was depressed compared with control values. By 15 days, LCBF had returned to control levels in most brain structures shown histopathologically not to be irreversibly damaged. The temporal sequence and magnitude of these hemodynamic alterations are consistent with findings in clinical studies in which repeated measurements of CBF have been carried out in patients with acute stroke. The ability to produce a cortical infarct that results in a consistent pattern of altered CBF should facilitate the investigation of stroke mechanisms responsible for these hemodynamic abnormalities.

Omental transposition or transplantation to the brain and superficial temporal artery--middle cerebral artery anastomosis in preventing experimental cerebral ischaemia

Transposition of lengthened omentum to the brain surface, transplantation of an omental graft, or superficial temporal artery--middle cerebral artery anastomosis were performed in dogs prior to transcranial occlusion of the ipsilateral middle cerebral artery at its origin, including the bifurcation of the internal carotid artery. Both omentum and by-pass were able to reduce the changes in cerebral blood flow, somatosensory evoked responses, cerebral water and electrolyte content, consequent to the ischaemic insult. In the experimental conditions adopted in this study, the effect of omental transposition in maintaining high levels of flow throughout the entire occluded hemisphere was more marked compared to that resulting from the other methods, while the onset of ischaemic cerebral oedema was affected approximately at the same degree by all procedures. The results point to the role of the transposed omentum in providing an effective collateral circulation to the ischaemic brain.