Verna L. Baughman

Estrogen Provides Neuroprotection in Transient Forebrain Ischemia Through Perfusion-Independent Mechanisms in Rats

BACKGROUND AND PURPOSE Estrogen-related neuroprotection in association with animal models of transient forebrain and focal ischemia has been documented in several recent reports. Some of those studies indicated that part of that benefit was a function of improved intraischemic vasodilating capacity. In the present study we examined whether chronic estrogen depletion and repletion affected ischemic neuropathology through perfusion-independent mechanisms. METHODS Normal, ovariectomized (OVX), and OVX female rats treated with 17beta-estradiol (E2) were subjected to 30 minutes of transient forebrain ischemia (right common carotid occlusion plus hemorrhagic hypotension) and reperfusion. Neurological function and brain histopathology were assessed over the 72-hour recovery period. In all rats, preischemic and intraischemic cortical cerebral blood flow (CBF) levels were monitored with laser-Doppler flowmetry. In additional rats, CBF changes in the striatum and hippocampus were also monitored with laser-Doppler flowmetry probes and radiolabeled microspheres. In each experiment, the level of ischemia was targeted to a 75% to 80% reduction in cortical CBF. RESULTS The similarity in ischemic severity among groups was supported by measurements of comparable patterns of electroencephalographic power changes during the ischemic period. Compared with normal females, OVX rats showed diminished neurological outcomes and more severe histopathology in the hippocampus and striatum. Two-week treatment of OVX rats with E2 was accompanied by postischemic neuropathological changes similar to those seen in normal females. Intraischemic CBF reductions in the hippocampus and striatum were similar in all groups (to 35% to 50% of the preischemic value) but significantly less than the cortical CBF reductions. CONCLUSIONS These findings indicate that estrogen provides ischemic neuroprotection through mechanisms unrelated to improvement of intraischemic cerebral perfusion.

The role of neuronal nitric oxide synthase in regulation of cerebral blood flow in normocapnia and hypercapnia in rats

The nitric oxide synthase (NOS) inhibitors, nitro-L-arginine, its methyl ester, and N-monomethyl-L-arginine, have been shown to attenuate resting CBF and hypercapnia-induced cerebrovasodilation. Those agents nonspecifically inhibit the endothelial and neuronal NOS (eNOS and nNOS). In the present study, we used a novel nNOS inhibitor, 7-nitroindazole (7-NI) to examine the role of nNOS in CBF during normocapnia and hypercapnia in fentanyl/N2O-anesthetized rats. CBF was monitored using laser-Doppler flowmetry. Administration of 7-NI (80 mg kg−1 i.p.) reduced cortical brain NOS activity by 57%, the resting CBF by 19–27%, and the CBF response to hypercapnia by 60%. The 60% reduction was similar in magnitude to the CBF reductions observed in previous studies in which nonspecific NOS inhibitors were used. In the present study, 7-NI did not increase the MABP. Furthermore, the CBF response to oxotremorine, a blood–brain barrier permeant muscarinic agonist that induces cerebrovasodilation via endothelium-derived NO, was unaffected by 7-NI. These results confirmed that 7-NI does not influence eNOS; they also indicated that the effects of 7-NI on the resting CBF and on the CBF response to hypercapnia in this study were solely related to its inhibitory action on nNOS. The results further suggest that the NO synthesized by the action of nNOS participates in regulation of basal CBF and is the major, if not the only, category of NO contributing to the hypercapnic CBF response.

Cerebral vasodilating capacity during forebrain ischemia: effects of chronic estrogen depletion and repletion and the role of neuronal nitric oxide synthase.

THE effects of chronic 17β-estradiol (E2) depletion, via ovariectomy (OVX), and its repletion, on cortical cerebral blood flow (CBF) and EEG activities during forebrain ischemia, as well as post-ischemic recovery and neuropathology, were assessed and compared with results obtained in normal female rats. We also examined whether neuronal nitric oxide synthase (nNOS) activity is affected by OVX and E2 replacement and whether NOS-derived NO supports vasodilation during ischemia. OVX females displayed a significantly lower CBF during ischemia (10% of baseline) than did normal females (23% of baseline). In OVX rats, given chronic low-dose E2 treatment (0.1 mg kg−1 day−1), intra-ischemic CBF was similar to normal females (25% of baseline). However, at supraphysiologic E2 doses (≥ 0.5 mg kg−1 day−1), that benefit was diminished or lost. Intra-ischemic EEG power reductions and post-ischemic survival rates, neurological dysfunction, and histopathology displayed similar relative differences among groups as the CBF findings. Intra-ischemic CBF was reduced by nNOS inhibition, with ARL 17477, in normal and low-dose E2-treated OVX rats (4–8% baseline). The repressed intra-ischemic vasodilating function in OVX rats may be due to reductions in nNOS activity, because untreated OVX rats showed a 50% lower cortical nNOS activity than that in normal rats and in rats treated with low or high dose (5 mg kg−1 day−1) E2. However, the inability to restore vasodilating function despite normalization of nNOS activity indicates that another mechanism is responsible for the repression of vasodilatory function in the high-dose group. These findings suggest that E2, at levels within the physiological range, promotes ischemic neuroprotection via improving vasodilating capacity. One possible mechanism may relate to E2 enhancing brain nNOS expression and activity.

Efficacy of repeat intravenous dosing of ondansetron in controlling postoperative nausea and vomiting: a randomized, double-blind, placebo-controlled multicenter trial

STUDY OBJECTIVES To compare repeat intravenous (i.v.) dosing of ondansetron 4 mg with placebo for the treatment of postoperative nausea and vomiting (PONV) in patients for whom prophylactic, preoperative ondansetron 4 mg i.v. was inadequate DESIGN Randomized, double-blind, placebo-controlled study. SETTING Ten outpatient surgical centers in the United States. PATIENTS 2,199 male and female ASA physical status I, II, and III patients > or = 12 years old scheduled to undergo outpatient surgical procedures and receive nitrous oxide-based general anesthesia. INTERVENTIONS Ondansetron 4 mg i.v. was administered to all patients before induction of general anesthesia. Patients who experienced PONV or requested antiemetic therapy within 2 hours after discontinuation of inhaled anesthesia were randomized (1:1) to either a repeat i.v. ondansetron 4 mg dose or placebo. MEASUREMENTS AND MAIN RESULTS Of the 2,199 patients prophylactically treated with ondansetron 4 mg before anesthesia induction, 1,771 (80.5%) did not experience PONV or request antiemetic therapy during the 2 hours following discontinuation of anesthesia. Of the 428 patients who experienced PONV or requested antiemetic therapy during the same period, and were randomized to additional treatment (214 randomized to ondansetron, 214 randomized to placebo), the incidence of complete response (no emesis, no rescue medication, no study withdrawal) was similar for both ondansetron-randomized and placebo-randomized groups for the 2-hour (34% and 43%, respectively, p = 0.074) and 24-hour (28% and 32%, respectively, p = 0.342) postrandomization study periods. Repeat ondansetron dosing was not more effective than placebo in controlling either postoperative emesis or the severity/duration of postoperative nausea. The administration of an additional dose of ondansetron 4 mg postoperatively did not result in an increased incidence of adverse effects. CONCLUSIONS In patients for whom preoperative prophylaxis with ondansetron 4 mg i.v. is not successful, a repeat dose of ondansetron 4 mg i.v. in the postanesthesia care unit does not appear to offer additional control of PONV.

Neurologic Outcome in Rats Following Incomplete Cerebral Ischemia during Halothane, Isoflurane, or N2O

Using rats in which incomplete cerebral ischemia was induced, the authors evaluated the effects of halothane (H) and isoflurane (I) on neurologic outcome compared to nitrous oxide (N2O) controls. Incomplete cerebral ischemia was produced by right carotid artery occlusion combined with hemorrhagic hypotension. Neurologic outcome was evaluated using a graded deficit score from 0 to 5 (0 = normal, 5 = death associated with stroke). Two levels of cerebral ischemia were tested. At moderate ischemia with hypotension of 30 mmHg, an FIO2 of 0.3, and ischemic periods of 30 or 45 min, N2O produced a deficit of 4.7–5.0 and a mortality rate of 90–100%. In contrast, halothane (1 MAC) and isoflurane (1 MAC) resulted in similar deficit scores (H = 1.1–1.8, I = 1.4–1.6) and mortality rates (H = 17–30%, I = 17–20%). Cerebral blood flow (CBF) measured with radioactive microspheres showed a 60–65% decrease in the ischemic hemisphere at this level of hypotension. With severe ischemia with hypotension = 25 mmHg, FIO2 = 0.2, and a 30-min period of ischemia, deficit scores increased to 3.0 and 3.9 with 1 MAC halothane and 1 MAC isoflurane, respectively. Morality rates also increased to 40% with halothane and 70% with isoflurane. Increasing the concentration of halothane or isoflurane to 2 MAC did not significantly improve outcome. Brain histology demonstrated extensive neuronal damage in striatal, hippocampal, and neocortical regions of N2O control treated rats, and less damage with little difference between H-and I-treated rats at each level of ischemia. Using this model of incomplete cerebral ischemia, halothane and isoflurane provided significantly better neurologic and histologic outcomes when compared to N2O controls, with little difference between the two volatile anesthetics.

Effects of Remifentanil, a New Short-acting Opioid, on Cerebral Blood Flow, Brain Electrical Activity, and Intracranial Pressure in Dogs Anesthetized with Isoflurane and Nitrous Oxide

BackgroundA new short-acting opioid, remifentanil, is metabolized by esterase activity in blood and tissue. It is important to know whether remifentanil may decrease the time to recovery of opioid-induced cardiovascular and cerebral effects compared to that of other short-acting agents such as alfentanil. MethodsBaseline measures were made during 1% end-tidal isoflurane and 50% N2O in oxygen in dogs. Approximately equipotent low- and high-dose remifentanil (0.5 and 1.0

Dose Comparison of Remifentanil and Alfentanil for Loss of Consciousness

mUg. kg−1. min−1) or alfentanil (1.6 and 3.2

Clonidine Decreases Plasma Catecholamines and Improves Outcome From Incomplete Ischemia in the Rat

mUg. kg−1. min−1) were infused for 30 min each (total infusion time 60 min) followed by a 30-min recovery period. Blood pressure, heart rate, and intracranial pressure were recorded continuously. Electroencephalogram measurements were made using aperiodic analysis, and regional cerebral blood flow using radioactive microspheres. ResultsBoth remifentanil and alfentanil decreased blood pressure and heart rate 25–30%. Cortex, hippocampus, and caudate blood flow decreased 40–50% during opioid infusion, but flow changes in lower brain regions were modest or absent. The electroencephalogram showed a shift from low-amplitude, high-frequency activity during baseline to high-amplitude, low-frequency activity during opioid infusion. During a 30-min recovery period, heart rate, electroencephalogram, and regional cerebral blood flow recovered to baseline levels in remifentanil- but not in alfentanil-treated dogs. Blood pressure and intracranial pressure decreased during opioid infusion and increased above baseline levels during the recovery period in remifentanil-treated dogs. ConclusionsThese results show that the cardiovascular and cerebral effects of remifentanil and alfentanil are similar but that recovery of these parameters occurs sooner following remifentanil.

Effects of sufentanil on cerebral blood flow, cerebral blood flow velocity, and metabolism in dogs.

Background: This study evaluated the efficacy and safety of remifentanil, a potent mu agonist opioid with a rapid onset and offset of effect, as a sole induction agent for loss of consciousness (LOC) and compared it with alfentanil. Methods: Remifentanil and alfentanil were administered intravenously over 2 min in ascending doses (remifentanil 2, 3, 4, 5, 6, 8, 10, 15, 20 micro gram/kg; alfentanil 40, 60, 80, 100, 120, 160, 200 micro gram/kg) to unpremedicated healthy patients. Patients were observed for rigidity and LOC for 30 s after the end of infusion. If patients had not lost consciousness, 2 mg [center dot] kg sup ‐1 [center dot] min sup ‐1 thiopental was administered until LOC was achieved. Arterial blood samples, obtained at specified time intervals, were analyzed for remifentanil and alfentanil whole‐blood concentration. Blood pressure and heart rate were also recorded at preset time intervals. Results: Neither drug could reliably produce LOC. With both drugs, there was a dose‐dependent decrease in thiopental requirements and a dose‐dependent increase in the incidence and severity of rigidity (P <0.05). The median effective dose (ED50) for LOC with remifentanil was 12 micro gram/kg, and for alfentanil it was 176 micro gram/kg. The median effective concentration (EC50; whole‐blood concentration) of remifentanil was 53.8 ng/ml and for alfentanil it was 1,012 ng/ml. Minimal hemodynamic changes were observed after either drug was given. Conclusions: Remifentanil is 15 times more potent than alfentanil, based on the ED50 to achieve loss of response to a verbal command and 20 times more potent than alfentanil based on the EC50. Neither opioid is suitable as a sole induction agent.

Comparison of Methohexital and Isoflurane on Neurologic Outcome and Histopathology Following Incomplete Ischemia in Rats

Clonidine decreases central sympathetic activity and anesthetic requirement. We tested whether clonidine improves outcome from incomplete ischemia of the brain in rats. Control rats were anesthetized with 25 μg·kg−1·h−1 of intravenous fentanyl and inhalation of 70% nitrous oxide (N2O). Clonidine-treated rats received fentanyl/N2O and 10 μg/kg of intravenous clonidine 10 min before ischemia, which was produced by right carotid ligation combined with hemorrhagic hypotension to 35 mm Hg for 30 min. Clonidine increased plasma glucose before ischemia and decreased blood catecholamine concentrations during ischemia compared with the control group. Neurologic outcome was evaluated daily for 3 days after ischemia and histopathology was performed at the end of this period. Clonidine significantly improved neurologic outcome on each of the 3 days after ischemia. Histopathology was severe in the control group but not enough rats survived in this group for statistical analysis. The authors conclude mat donidine decreases sympathetic activity during ischemia and that this is associated with an improvement in outcome from incomplete ischemia.