Thoracic Epidural Analgesia

Thoracic epidural analgesia (TEA) offers a unique oppor tunity for the anesthesiologist to enhance postopera tive recovery for the thoracic surgery patient. By deliver ing analgesics to a limited dermatomal distribution, TEA can provide profound segmental analgesia and also serves to modulate neural outflow to improve cardiac and pulmonary parameters. The notable side-effects of hypotension and respiratory depression can be mini mized by using synergistic combinations of local anes thetic and opioids, and by adopting a continuous infu sion strategy. With thoughtful patient selection, careful technique, and a proactive approach to the recognition of the known hemodynamic and respiratory effects of epidural drugs, TEA can be administered safely. The significant benefits of TEA include better pain relief, increased FEV1, earlier extubation, and, perhaps, de creased morbidity and mortality.

Effective out-of-operating room airway management for physicians not traditionally trained in airway management

BACKGROUND

There is a dearth of literature about the safety and practicality of intubation performed by an internal medicine (IM) or any other nonanesthesia, nonemergency physician.

OBJECTIVES

The objectives of the study were to describe abbreviated airway management training guidelines for IM physicians staffing the emergency department and to compare the success rates between intubations performed by anesthesia and IM physicians, respectively.

METHODS

In this study, 272 consecutive out-of-operating room intubations performed by anesthesia and IM physicians were evaluated after creating and implementing an abbreviated intubation training protocol.

RESULTS

Of 165 intubations attempted by IM physicians and 107 intubations attempted by the anesthesia service, the rates of successful intubation were 93% and 99%, respectively (P = 0.02). There were no other statistically significant differences in outcomes.

CONCLUSIONS

Procedurally oriented IM fellows could provide a temporary solution to hospitals that currently do not have the resources to provide full-time, in-house anesthesiology or emergency physicians for management of the emergent airway.

Fine epitope mapping of monoclonal antibody 5F1 reveals anticatalytic activity toward the N domain of human angiotensin-converting enzyme

Angiotensin I-converting enzyme (ACE, peptidyl dipeptidase, EC 3.4.15.2) is a key enzyme in cardiovascular pathophysiology. A wide spectrum of monoclonal antibodies to different epitopes on the N and C domains of human ACE has been used to study different aspects of ACE biology. In this study we characterized the monoclonal antibody (mAb) 5F1, developed against the N domain of human ACE, which recognizes both the catalytically active and the denatured forms of ACE. The epitope for mAb 5F1 was defined using species cross-reactivity, synthetic peptide (PepScan technology) and phage display library screening, Western blotting, site-directed mutagenesis, and protein modeling. The epitope for mAb 5F1 shows no overlap with the epitopes of seven other mAbs to the N domain described previously and is localized on the other side of the N domain globule. The binding of mAb 5F1 to ACE is carbohydrate-dependent and increased significantly as a result of altered glycosylation after treatment with alpha-glucosidase-1 inhibitor, N-butyldeoxynojirimycin (NB-DNJ), or neuraminidase. Out of 17 species tested, mAb 5F1 showed strict primate ACE specificity. In addition, mAb 5F1 recognized human ACE in Western blots and on paraffin-embedded sections. The sequential part of the epitope for mAb 5F1 is created by the N-terminal part of the N domain, between residues 1 and 141. A conformational region of the epitope was also identified, including the residues around the glycan attached to Asn117, which explains the sensitivity to changes in glycosylation state, and another stretch localized around the motif 454TPPSRYN460. Site-directed mutagensis and inhibition assays revealed that mAb 5F1 inhibits ACE activity at high concentrations due to binding of residues on both sides of the active site cleft, thus supporting a hinge-bending mechanism for substrate binding of ACE.

Immunotargeting of catalase to lung endothelium via anti-angiotensin-converting enzyme antibodies attenuates ischemia-reperfusion injury of the lung in vivo

Limitation of reactive oxygen species-mediated ischemia-reperfusion (I/R) injury of the lung by vascular immunotargeting of antioxidative enzymes has the potential to become a promising modality for extension of the viability of banked transplantation tissue. The preferential expression of angiotensin-converting enzyme (ACE) in pulmonary capillaries makes it an ideal target for therapy directed toward the pulmonary endothelium. Conjugates of ACE monoclonal antibody (MAb) 9B9 with catalase (9B9-CAT) have been evaluated in vivo for limitation of lung I/R injury in rats. Ischemia of the right lung was induced for 60 min followed by 120 min of reperfusion. Sham-operated animals (sham, n = 6) were compared with ischemia-reperfused untreated animals (I/R, n = 6), I/R animals treated with biotinylated catalase (CAT, n = 6), and I/R rats treated with the conjugates (9B9-CAT, n = 6). The 9B9-CAT accumulation in the pulmonary endothelium of injured lungs was elucidated immunohistochemically. Arterial oxygenation during reperfusion was significantly higher in 9B9-CAT (221 +/- 36 mmHg) and sham (215 +/- 16 mmHg; P < 0.001 for both) compared with I/R (110 +/- 10 mmHg) and CAT (114 +/- 30 mmHg). Wet-dry weight ratio of I/R (6.78 +/- 0.94%) and CAT (6.54 +/- 0.87%) was significantly higher than of sham (4.85 +/- 0.29%; P < 0.05), which did not differ from 9B9-CAT (5.58 +/- 0.80%). The significantly lower degree of lung injury in 9B9-CAT-treated animals compared with I/R rats was also shown by decreased serum levels of endothelin-1 (sham, 18 +/- 9 fmol/mg; I/R, 42 +/- 12 fmol/mg; CAT, 36 +/- 11 fmol/mg; 9B9-CAT, 26 +/- 9 fmol/mg; P < 0.01) and mRNA for inducible nitric oxide synthase (iNOS) [iNOS-GAPDH ratio: sham, 0.15 +/- 0.06 arbitrary units (a.u.); I/R, 0.33 +/- 0.08 a.u.; CAT, 0.26 +/- 0.05 a.u.; 9B9-CAT, 0.14 +/- 0.04 a.u.; P < 0.001]. These results validate immunotargeting by anti-ACE conjugates as a prospective and specific strategy to augment antioxidative defenses of the pulmonary endothelium in vivo.

Monoclonal Antibodies 1G12 and 6A12 to the N-domain of human angiotensin-converting enzyme: fine epitope mapping and antibody-based detection of ACE inhibitors in human blood

Angiotensin I-converting enzyme (ACE), a key enzyme in cardiovascular pathophysiology, consists of two homologous domains (N- and C-), each bearing a Zn-dependent active site. ACE inhibitors are among the most prescribed drugs in the treatment of hypertension and cardiac failure. Fine epitope mapping of two monoclonal antibodies (mAb), 1G12 and 6A12, against the N-domain of human ACE, was developed using the N-domain 3D-structure and 21 single and double N-domain mutants. The binding of both mAbs to their epitopes on the N-domain of ACE is significantly diminished by the presence of the C-domain in the two-domain somatic tissue ACE and further diminished by the presence of sialic acid residues on the surface of blood ACE. The binding of these mAbs to blood ACE, however, increased dramatically (5-10-fold) in the presence of ACE inhibitors or EDTA, whereas the effect of these compounds on the binding of the mAbs to somatic tissue ACE was less pronounced and even less for truncated N-domain. This implies that the binding of ACE inhibitors or removal of Zn2+ from ACE active centers causes conformational adjustments in the mutual arrangement of N- and C-domains in the two-domain ACE molecule. As a result, the regions of the epitopes for mAb 1G12 and 6A12 on the N-domain, shielded in somatic ACE by the C-domain globule and additionally shielded in blood ACE by sialic acid residues in the oligosaccharide chains localized on Asn289 and Asn416, become unmasked. Therefore, we demonstrated a possibility to employ these mAbs (1G12 or 6A12) for detection and quantification of the presence of ACE inhibitors in human blood. This method should find wide application in monitoring clinical trials with ACE inhibitors as well as in the development of the approach for personalized medicine by these effective drugs.

Inhibitory antibodies to human angiotensin-converting enzyme: fine epitope mapping and mechanism of action

Angiotensin I-converting enzyme (ACE), a key enzyme in cardiovascular pathophysiology, consists of two homologous domains (N and C), each bearing a Zn-dependent active site. We modeled the 3D-structure of the ACE N-domain using known structures of the C-domain of human ACE and the ACE homologue, ACE2, as templates. Two monoclonal antibodies (mAb), 3A5 and i2H5, developed against the human N-domain of ACE, demonstrated anticatalytic activity. N-domain modeling and mutagenesis of 21 amino acid residues allowed us to define the epitopes for these mAbs. Their epitopes partially overlap: amino acid residues K407, E403, Y521, E522, G523, P524, D529 are present in both epitopes. Mutation of 4 amino acid residues within the 3A5 epitope, N203E, R550A, D558L, and K557Q, increased the apparent binding of mAb 3A5 with the mutated N-domain 3-fold in plate precipitation assay, but abolished the inhibitory potency of this mAb. Moreover, mutation D558L dramatically decreased 3A5-induced ACE shedding from the surface of CHO cells expressing human somatic ACE. The inhibition of N-domain activity by mAbs 3A5 and i2H5 obeys similar kinetics. Both mAbs can bind to the free enzyme and enzyme-substrate complex, forming E.mAb and E.S.mAb complexes, respectively; however, only complex E.S can form a product. Kinetic analysis indicates that both mAbs bind better with the ACE N-domain in the presence of a substrate, which, in turn, implies that binding of a substrate causes conformational adjustments in the N-domain structure. Independent experiments with ELISA demonstrated better binding of mAbs 3A5 and i2H5 in the presence of the inhibitor lisinopril as well. This effect can be attributed to better binding of both mAbs with the "closed" conformation of ACE, therefore, disturbing the hinge-bending movement of the enzyme, which is necessary for catalysis.

Isoflurane, but Not Sevoflurane, Increases Transendothelial Albumin Permeability in the Isolated Rat Lung

Background

Caveolae mediated transendothelial transport of albumin has recently been shown to be the primary mechanism regulating microvascular endothelial albumin permeability. The authors investigated the effects of isoflurane and sevoflurane on pulmonary endothelial albumin permeability and assessed the potential role of the caveolae scaffold protein, caveolin-1, in these effects.

Methods

Isolated rat lungs and cultured rat lung microvessel endothelial cells (RLMVECs) were exposed to 1.0 or 2.0 minimum alveolar concentration (MAC) isoflurane or sevoflurane for 30 min. I-albumin permeability-surface area product and capillary filtration coefficient were determined in the isolated lungs. In RLMVECs, uptake and transendothelial transport of I-albumin were measured in the absence and presence of pretreatment with 2 mm methyl-beta-cyclodextrin, a caveolae-disrupting agent. Uptake of fluorescent-labeled albumin, as well as phosphorylation of Src kinase and caveolin-1, was also determined. In Y14F-caveolin-1 mutant (nonphosphorylatable) expressing RLMVECs, uptake of I-albumin and phosphorylation of caveolin-1 were evaluated.

Results

In the isolated lungs, 2.0 MAC isoflurane increased I-albumin permeability-surface area product by 48% without affecting capillary filtration coefficient. In RLMVECs, isoflurane more than doubled the uptake of I-albumin and caused a 54% increase in the transendothelial transport of I-albumin. These effects were blocked by pretreatment with methyl-beta-cyclodextrin. The isoflurane-induced increase in uptake of I-albumin in wild-type RLMVECs was abolished in the Y14F-caveolin-1 mutant expressing cells. Isoflurane also caused a twofold increase in Src and caveolin-1 phosphorylation. Neither 1.0 MAC isoflurane nor 1.0 or 2.0 MAC sevoflurane affected any index of albumin transport or phosphorylation of caveolin-1.

Conclusion

Isoflurane, but not sevoflurane, increased lung transendothelial albumin permeability through enhancement of caveolae-mediated albumin uptake and transport in the isolated lung. This effect may involve an enhanced phosphorylation of caveolin-1.

Testicular Isoform of Angiotensin I-Converting Enzyme (ACE, CD143) on the Surface of Human Spermatozoa: Revelation and Quantification Using Monoclonal Antibodies

PROBLEM

The elucidation of the role of angiotensin-converting enzyme (ACE, CD143) in the male fertility has been hampered by the absence of highly specific antibodies to the native testicular isoform (tACE). The quantification of tACE expression on human-ejaculated spermatozoa was performed using a novel panel of monoclonal antibodies (mAbs).

METHOD OF STUDY

The expression of tACE on the surface of live and fixed human spermatozoa was analyzed by flow cytometry and immunocytochemistry using new mAbs to human tACE.

RESULTS

Monoclonal antibodies 1E10 and 4E3 similarly revealed tACE on the surface of live and fixed spermatozoa. The high percentage of tACE-positive spermatozoa (median 81%) was revealed in the swim-up fraction of sperm. Antibody-induced tACE shedding occurs preferentially from live sperm with defective function and/or morphology. Testicular ACE is located on the plasma membrane of the post-acrosomal region, the neck and midpiece of normal spermatozoa, but showed a variable distribution on the defective cells.

CONCLUSIONS

The new mAbs recognizing the C-terminal domain of human ACE are useful tools for quantification of tACE expression on human live and fixed spermatozoa and further adequate analysis of the tACE role in reproduction.

Localization of an N-domain region of angiotensin-converting enzyme involved in the regulation of ectodomain shedding using monoclonal antibodies

ACE chimeric proteins and N domain monoclonal antibodies (mAbs) were used to determine the influence of the N domain, and particular regions thereof, on the rate of ACE ectodomain shedding. Somatic ACE (having both N and C domains) was shed at a rate of 20%/24 h. Deletion of the C domain of somatic ACE generated an N domain construct (ACEDeltaC) which demonstrated the lowest rate of shedding (12%). However, deletion of the N domain of somatic ACE (ACEDeltaN) dramatically increased shedding (212%). Testicular ACE (tACE) having 36 amino acid residues (heavily O-glycosylated) at the N-terminus of the C domain shows a 4-fold decrease in the rate of shedding (49%) compared to that of ACEDeltaN. When the N-terminal region of the C domain was replaced with the corresponding homologous 141 amino acids of the N domain (N-delACE) the rate of shedding of the ACEDeltaN was only slightly decreased (174%), but shedding was still 3.5-fold more efficient than wild-type testicular ACE. Monoclonal antibodies specific for distinct, but overlapping, N-domain epitopes altered the rate of ACE shedding. The mAb 3G8 decreased the rate of shedding by 30%, whereas mAbs 9B9 and 3A5 stimulated ACE shedding 2- to 4-fold. Epitope mapping of these mAbs in conjunction with a homology model of ACE N domain structure, localized a region in the N-domain that may play a role in determining the relatively low rate of shedding of somatic ACE from the cell surface.

Monoclonal antibodies 1B3 and 5C8 as probes for monitoring the integrity of the C-terminal end of soluble angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a membrane-anchored ectoprotein that is proteolytically cleaved, yielding an enzymatically active soluble ACE. Two mouse monoclonal antibodies, MAbs 1B3 and 5C8, were generated to the C-terminal part of human soluble ACE. MAb 1B3 recognized the catalytically active ACE, as revealed by ELISA and precipitation assays, whereas Western blotting and immunohistochemisty on paraffin- embedded sections using MAb 5C8 detected denatured ACE. MAb 1B3 showed extensive cross-reactivity, recognizing 15 species out of the 16 tested. The binding of this MAb to ACE was greatly affected by conformational changes induced by adsorption on plastic, formalin fixation, and underglycosylation. Furthermore, MAb 1B3 binding to the mutated ACE (Pro1199Leu substitution in the juxtamembrane region, leading to a fivefold increase in serum ACE level) was markedly decreased. MAb 5C8 detected all the known expression sites of full-size ACE using formalin-fixed and paraffin-embedded human tissues. The sequential epitope for MAb 5C8 is formed by the last 11 amino acid residues of soluble ACE (Pro1193-Arg1203), whereas the conformational epitope for 1B3 is formed by a motif within these 11 amino acid residues and, in addition, by at least one stretch that includes Ala837-His839 located distal to the sequential epitope. Our findings demonstrated that MAbs 1B3 and 5C8 are very useful for the study of ACE shedding, for identification of mutations in stalk regions, and for studying alternatively spliced variants of ACE. In addition, binding of MAb 1B3 is a sensitive determinant of the integrity of soluble ACE.

Development and characterization of rat monoclonal antibodies to denatured mouse angiotensin-converting enzyme

Four new rat monoclonal antibodies, generated to denatured mouse somatic angiotensin-converting enzyme (ACE, CD143), detect mouse ACE with high sensitivity in Western blotting. Epitope mapping for the monoclonal antibodies--B12, 4G6 and 5C4--was also performed. Two monoclonal antibodies--B12 and 5C4--are directed to various epitopes on the N-domain--i.e., they recognized only the somatic isoform of mouse ACE. The monoclonal antibody H7 recognized an epitope on the C-domain of mouse ACE. The monoclonal antibody 4G6 was directed to a sequence on the N-domain of mouse ACE, which is homologous to a region of the C-domain and, as a result, also recognizes mouse testicular ACE (tACE) by means of Western blotting. In paraffin-embedded mouse tissues, all monoclonal antibodies detected all known expression sites of somatic ACE (sACE), e.g., the epithelial cells of the kidney proximal tubules, intestine and epididymis, and heterogeneously in endothelial cells. The monoclonal antibodies 4G6 and H7 additionally stained mouse tACE in spermatozoa and in mature spermatids. The monoclonal antibody 4G6 also demonstrated cross-reactivity with sACE from a broad spectrum of animal species, including human, rat, rabbit and bovine. However, this monoclonal antibody did not recognize the testicular isoform of ACE of these species. This set of monoclonal antibodies is useful for identifying even subtle changes in mouse ACE conformation because of denaturation. These monoclonal antibodies are also sensitive tools for the detection of mouse ACE in biological fluids and tissues by using proteomics approaches. Their high reactivity in paraffin-embedded tissues opens up opportunities to study possible changes in the pattern of ACE expression in knockout mouse models and may prove useful for correlating ACE expression in these models with human diseases.

Jet Injection of Local Anesthetic Decreases Pain of Arterial Cannulation in Awake Neurosurgical Patients

Arterial cannulation through the standard skin wheal of local anesthetic raised with a needle may be painful. The authors compared the efficacy of local anesthetic injected via a 25G needle versus a Bioject jet injector for arterial cannulation in awake neurosurgical patients. After institutional review board approval, 40 patients were randomized to receive 0.3 mL 1% lidocaine adjusted to pH 7.0 with NaHCO3 by Bioject with a 2-cm spacer between the syringe and skin or by 25G needle injection. Two pain assessments were used at the time of local anesthetic injection and at arterial cannulation. Patients rated their pain on a visual analog scale (VAS) (0 = no pain, 100 = worst pain). Observers scored patient response as 0 (no response), 1 (flinch), or 2 (withdrawal). The VAS at injection was 23 +/- 19 for the needle group and 3 +/- 6 for the Bioject group (P < 0.001). The VAS at arterial cannulation was 39 +/- 25 for the needle group and 15 +/- 22 for the Bioject group (P < 0.001). Median observer scores at injection and cannulation were 1 (range 0-2) for the needle group and 0 (range 0-2) for the Bioject group (P < 0.001). Patients in the Bioject group experienced significantly less pain during lidocaine administration and at the time of arterial cannulation by their own and by an observer's assessment than the needle injection group. Jet injection of local anesthetic should be considered prior to arterial cannulation in awake patients.

Comparison of adenosine, isoflurane, and desflurane on myocardial tissue oxygen pressure during coronary artery constriction in dogs

OBJECTIVE

To compare adenosine-, isoflurane-, or desflurane-induced hypotension with and without left anterior descending (LAD) coronary artery constriction for the effects on myocardial tissue oxygen pressure (PmO(2)) in dogs.

DESIGN

Prospective, randomized, nonblinded.

SETTING

University teaching hospital.

PARTICIPANTS

Male nonpurpose-bred dogs (n = 18).

INTERVENTIONS

Dogs were anesthetized with 1.5% isoflurane (n = 12) or 8% desflurane (n = 6). A flow probe and balloon occluder were placed on the LAD artery. A probe that measured myocardial oxygen pressure was inserted into the middle myocardium in the LAD region. Myocardial oxygen consumption (MVO(2)) was calculated as LAD flow x arterial minus coronary sinus oxygen content.

MEASURES AND MAIN RESULTS

Measures were made during hypotension produced by adenosine infusion, 2.8% isoflurane, or 14% desflurane with and without LAD constriction to decrease blood flow 30%. Without LAD artery constriction, adenosine infusion increased LAD flow 90% and MVO(2) 70%, 2.8% isoflurane produced no change in MVO(2), and 14% desflurane decreased MVO(2) 25%, but no treatment changed PmO(2). LAD artery constriction decreased PmO(2) 50% by itself. Adenosine infusion during LAD constriction decreased tissue oxygen pressure an additional 60%, 2.8% isoflurane produced no change, and 14% desflurane increased PmO(2) 100%.

CONCLUSION

There was an inverse relationship between the effect of adenosine, 2.8% isoflurane, and 14% desflurane on MVO(2) and PmO(2) during ischemia. This is consistent with reports that increasing oxygen demand worsens myocardial ischemia.

Epitope-dependent blocking of the angiotensin-converting enzyme dimerization by monoclonal antibodies to the N-terminal domain of ACE: possible link of ACE dimerization and shedding from the cell surface

In a biomembrane modeling system, reverse micelles, somatic ACE forms dimers via carbohydrate-mediated interaction, providing evidence for the existence of a carbohydrate-recognizing domain on the ACE molecule. We localized this putative region on the N-domain of ACE using monoclonal antibodies (mAbs) to seven different epitopes of ACE. Two mAbs, 9B9 and 3G8, directed to distinct, but overlapping, epitopes of the N-domain of ACE shielded the CRD. Only "simple" ACE-antibody complexes were found in the system. Five mAbs allowed the formation of "double" antibody-ACE-ACE-antibody complexes via carbohydrate-mediated interactions. The results were confirmed using the ACE N- and C-domains. Testicular ACE was unable to form carbohydrate-mediated ACE dimers in the reverse micelles, while the N-domain of ACE, obtained by limited proteolysis of the parent full-length ACE, retained the ability to form dimers. Furthermore, mAb 3G8, which blocked ACE dimerization in micelles, significantly inhibited ACE shedding from the surface of ACE-expressing cells. Galactose prevented ACE dimerization in reverse micelles and also affected antibody-induced ACE shedding in an epitope-dependent manner. Restricted glycosylation of somatic ACE, obtained by the treatment of CHO-ACE cells with the glucosidase inhibitor N-butyldeoxynojirimycin, significantly increased the rate of basal ACE shedding and altered antibody-induced ACE shedding. A chemical cross-linking approach was used to show that ACE is present (at least in part) as noncovalently linked dimers on the surface of CHO-ACE cells. These results suggest a possible link between putative ACE dimerization on the cell surface and the proteolytic cleavage (shedding) of ACE.

Treatment of life-threatening hyperkalemia using hemoconcentration in parallel to venovenous bypass during orthotopic liver transplantation

IMPLICATIONS

The elimination of potassium in patients with end-stage kidney failure is limited. An increase in potassium concentrations can lead to lethal arrhythmias. In the described case, a large potassium concentration was treated during a liver transplantation using a new technical approach.

Glyburide decreases myocardial oxygen pressure in dogs

BACKGROUND

Reports show that glyburide, an adenosine triphosphate sensitive potassium (K+ATP) channel blocker, will reverse the myocardial protective effect of inhalational anesthesia. We evaluated the effect of glyburide on myocardial tissue oxygen pressure (PmO2) in dogs anesthetized with desflurane.

METHODS

Twelve dogs were anesthetized with 8% end-tidal desflurane for baseline anesthesia. A flow probe was placed on the left anterior descending (LAD) artery. A probe that measured PmO2 was inserted into the middle myocardium in the LAD region. After baseline measures, six dogs received i.v. 1 mg kg(-1) of glyburide and six dogs received sham vehicle treatment. After the glyburide or sham treatment, each dog received an i.v. infusion of adenosine 0.1 microg kg(-1) x min(-1), sodium nitroprusside (SNP) 2-4 microg kg(-1) x min(-1) and 14% end-tidal desflurane in random order.

RESULTS

Glyburide decreased LAD artery flow from 59 +/- 9 ml min(-1) to 30 +/- 6 ml min(-1) (P < 0.05) and PmO2 from 44 +/- 16 mmHg to 30 +/- 9 mmHg (P < 0.05). Adenosine infusion increased LAD artery blood flow 180% in the sham-treated dogs but produced no change in the glyburide-treated dogs. Sodium nitroprusside infusion increased LAD artery flow and decreased PmO2 in both the glyburide- and sham-treated dogs. Desflurane (14%) did not reverse the glyburide-induced vasoconstriction but increased PmO2 to 38 +/- 20 mmHg (P < 0.05).

CONCLUSION

Glyburide produced myocardial tissue hypoxia, which was not changed by adenosine, worsened by SNP and improved by 14% desflurane. The improvement in PmO2 with desflurane occurred without a change in myocardial blood flow.

Myocardial tissue oxygen during coronary artery constriction and hypotension in dogs

BACKGROUND

Sodium nitroprusside (SNP) may decrease myocardial tissue oxygenation in dogs with normal coronary arteries. We compared SNP- with desflurane-induced hypotension on myocardial tissue oxygen and pH in dogs with left anterior descending artery constriction.

METHODS

Twenty-four dogs were anesthetized with 8% desflurane for baseline anesthesia. Catheters were inserted into the femoral artery and vein and the coronary sinus. A flow probe and flow restriction device was placed on the left anterior descending (LAD) artery. A probe that measured myocardial oxygen pressure was inserted into the middle myocardium in the LAD region. Baseline measures were made of LAD artery flow, arterial and coronary sinus blood gases, and myocardial tissue gases. A 30% decrease in blood pressure was induced with SNP with unrestricted LAD flow (n=6) or when LAD artery flow was restricted by 30% from baseline (n=6). In separate dogs, a 30% decrease in blood pressure was produced with 14 +/- 1% desflurane with unrestricted LAD flow (n=6) or with baseline LAD artery flow restricted by 30% (n=6).

RESULTS

During SNP-induced hypotension with no LAD constriction, LAD artery flow and coronary sinus oxygen tension increased but myocardial tissue oxygen tension (PmO2) decreased by 40%. When baseline artery flow was decreased by 30% by LAD constriction, SNP-induced hypotension decreased tissue oxygen pressure by 80%, and ischemic acidosis was produced. During unrestricted LAD artery flow or with a 30% flow restriction, desflurane-induced hypotension produced no significant change from baseline myocardial tissue oxygen tension or pH.

CONCLUSION

During coronary artery constriction, desflurane-induced hypotension maintained myocardial tissue oxygenation and pH better than did SNP-induced hypotension. The divergence between tissue and coronary sinus oxygen tension during SNP suggests that arteriovenous shunting may occur.

Sodium nitroprusside-induced, but not desflurane-induced, hypotension decreases myocardial tissue oxygenation in dogs anesthetized with 8% desflurane

OBJECTIVE

To compare sodium nitroprusside (SNP)-induced hypotension with desflurane-induced hypotension for the effects on myocardial blood flow and tissue oxygenation in dogs.

DESIGN

Prospective, randomized, crossover, nonblinded.

SETTING

University teaching hospital.

PARTICIPANTS

Male nonpurpose-bred hounds (n = 8).

INTERVENTIONS

Dogs were anesthetized with 8% desflurane. Catheters were inserted into the femoral artery and coronary sinus. A flow probe was placed in the left anterior descending (LAD) branch of the coronary artery. A sensor that measured myocardial oxygen pressure (PmO(2)) was inserted into the myocardium of the left ventricle. Myocardial oxygen consumption (MVO(2)) was calculated as LAD flow x arterial - coronary sinus oxygen content.

MEASUREMENTS AND MAIN RESULTS

Measurements were made at baseline blood pressure levels of 99 mmHg (measure 1), during hypotension to 62 to 66 mmHg using intravenous SNP or 14% desflurane (measure 2), and during SNP or 14% desflurane with blood pressure support using phenylephrine (measure 3). Each dog randomly received both hypotensive treatments, separated by 1 hour. Baseline measures were PmO(2) = 46 +/- 9 mmHg, LAD flow = 43 +/- 11 mL/min, and MVO(2) = 2.47 +/- 0.73 mL O(2)/min. During hypotension induced with SNP, PmO(2) decreased 30% (p < 0.05), LAD flow increased 40% (p < 0.05), and MVO(2) did not change. During hypotension induced with 14% desflurane, PmO(2) did not change, and LAD flow and MVO(2) decreased 25% and 40% (p < 0.05). Blood pressure support with phenylephrine increased LAD flow and MVO(2) but did not change PmO(2) during SNP or 14% desflurane treatment.

CONCLUSION

SNP-induced hypotension produced myocardial vasodilation, but tissue oxygenation was impaired. PmO(2) was maintained during desflurane-induced hypotension.

Epitope-specific antibody-induced cleavage of angiotensin-converting enzyme from the cell surface

Angiotensin I-converting enzyme (ACE; CD143, EC 3.4.15.1) is a type-1 integral membrane protein that can also be released into extracellular fluids (such as plasma, and seminal and cerebrospinal fluids) as a soluble enzyme following cleavage mediated by an unidentified protease(s), referred to as ACE secretase, in a process known as "shedding". The effects of monoclonal antibodies (mAbs) to eight different epitopes on the N-terminal domain of ACE on shedding was investigated using Chinese hamster ovary cells (CHO cells) expressing an ACE transgene and using human umbilical vein endothelial cells. Antibody-induced shedding of ACE was strongly epitope-specific: most of the antibodies increased the shedding by 20-40%, mAbs 9B9 and 3A5 increased the shedding by 270 and 410% respectively, whereas binding of mAb 3G8 decreased ACE shedding by 36%. The ACE released following mAb treatment lacked a hydrophobic transmembrane domain anchor. The antibody-induced shedding was completely inhibited at 4 degrees C and by zinc chelation using 1,10-phenanthroline, suggesting involvement of a metalloprotease in this process. A hydroxamate-based metalloprotease inhibitor (batimastat, BB-94) was 15 times more efficacious in inhibiting mAb-induced ACE shedding than basal (constitutive) ACE release. Treatment of CHO-ACE cells with BB-94 more effectively prevented elevation in antibody-dependent (but not basal) ACE release induced by 3,4-dichloroisocoumarin and iodoacetamide. These data suggest that different secretases might be responsible for ACE release under basal compared with antibody-induced shedding. Further experiments with more than 40 protease inhibitors suggest that calpains, furin and the proteasome may participate in this process.

Brain compared to heart tissue oxygen pressure during changes in arterial carbon dioxide in the dog

Myocardial tissue oxygen pressure (PmO2 ) and left anterior descending (LAD) artery blood flow were measured in dogs anesthetized with 1.5% isoflurane, and were then compared to brain tissue oxygen pressure (PbO2 ) and middle cerebral artery (MCA) blood flow during normocapnia, hypocapnia, and hypercapnia. A craniotomy was performed and a tissue probe (Codman, Inc.) that measures PO2, PCO2, and pH was inserted into the brain cortex in the MCA region (n = 8). Separately, after a thoracotomy, a probe was inserted into the middle myocardium of the left ventricle, within the distribution of the LAD, in eight dogs. Blood flow probes were placed on the LAD or MCA. Blood flow and tissue gases were measured during normocapnia (PaCO2 = 38 mm Hg), hypocapnia (PaCO2 = 26 mm Hg), and hypercapnia (PaCO2 = 53 mm Hg). Mean arterial pressure, heart rate, arterial gases, and pH were not different between brain and heart measurements. PbO2 was 21 +/- 9 mm Hg (mean +/- SD ), 40 +/- 16 mm Hg, and 47 +/- 11 mm Hg. PmO2 was 35 +/- 12 mm Hg, 40 +/- 14 mm Hg, and 48 +/- 15 mm Hg during hypocapnia, normocapnia, and hypercapnia respectively. During hypercapnia, LAD and MCA flow increased 50% and tissue oxygenation increased 20% ( P < .05). During hypocapnia, MCA flow and PbO2 decreased 50% ( P < .05), but LAD flow and PmO2 did not significantly change. These results indicated that LAD flow and myocardial PO2 were less responsive to hypocapnia than MCA flow and PbO2.

Occurrence of potentially detrimental temperature alterations in hospitalized patients at risk for brain injury

OBJECTIVE

To ascertain the incidence and timing of fever in patients at risk for temperature modulation of brain injury resulting from ischemia or trauma.

DESIGN

We retrospectively reviewed the medical records of patients admitted between January 1991 and December 1994.

MATERIAL AND METHODS

We investigated three groups of hospitalized patients considered at risk for ongoing brain injury resulting from a prior cerebral insult: successful resuscitation from out-of-hospital cardiac arrest (CA), subarachnoid hemorrhage (SAH), or traumatic closed-head injury (CHI). Forty patients per condition were randomly selected from those who survived for more than 24 hours after hospital admission.

RESULTS

During the initial 72 hours of hospitalization, temperature increases to 38 degrees C or more (that is, temperatures previously reported to worsen neurologic outcome after brain injury) were noted in 83% of patients with CA, 70% of those with SAH, and 68% of those with CHI. Within the cohort of febrile patients, 18 to 44% of all temperature measurements were 38 degrees C or higher, and the febrile episodes occurred randomly throughout the study interval. Fewer than one-eighth of the febrile patients received drugs possessing antipyretic properties (such as aspirin or acetaminophen) in a dose appropriate to treat fever. No other method of temperature control (for example, physical means) was used in any patient. The fractions of patients who were dismissed from the hospital with permanent neurologic injury were as follows: CA, 20%; SAH, 45%; and CHI, 43%.

CONCLUSION

In these hospitalized patients at risk for ongoing brain injury, the incidence of temperature increases within the range reported to worsen neurologic outcome (elevations of 1.0 degree C or more) was very high. The characterization of these potentially injurious, randomly occurring, and traditionally undertreated temperature increases may have implications for the design of future protocols aimed at providing cerebral protection.

The influence of chronic hypokalemia on myocardial adrenergic receptor densities: enhanced sensitivity to epinephrine-induced arrhythmias

We studied the effects of a 30-day potassium (K+)-deficient diet on blood [K+] myocardial adrenergic receptor densities, serum catecholamines, and epinephrine arrhythmogenicity in adult laboratory rats (250 +/- 25 g). Within 3 days of beginning the K+-deficient diet, blood [K+] decreased by 50%. After 5 days, the myocardial alpha-1 density increased (62 +/- 2 vs 148 +/- 16 fmols/mg protein), and the total beta receptor increased (95 +/- 5 vs 273 +/- 49) without significant change in receptor affinity. However, 18-21 days of this diet was necessary to produce an increase in the duration of epinephrine arrhythmias (from 56 +/- 8 to 224 +/- 21 s). While prazosin block of the alpha-1 receptor in hypokalemic rats caused a significant, 42% reduction in arrhythmic duration and propranolol block caused a 62% reduction, both prazosin and propranolol were necessary to return arrhythmia times to normal (44 +/- 0.3 mmols/dL). Total serum catecholamines were reduced after 3 days of the diet (from 482 +/- 37 to 299 +/- 31 pg/ml) and remained depressed throughout the 30 days of the K+ diet. The results of this study indicate that prolonged restriction causes a reduction in serum catecholamines, an increase in myocardial alpha-1 and beta receptors densities, and an increase in epinephrine arrhythmogenicity. All of these changes were reversed within 5 days of initiating a normal dietary K+ intake.

Divergence of intracranial and central venous pressures in lightly anesthetized, tracheally intubated dogs that move in response to a noxious stimulus

BACKGROUND

Intracranial pressure (ICP) may increase in tracheally intubated subjects during periods of movement (e.g, "bucking" and coughing). Recent research has suggested that factors other than passive congestion of the cerebral vessels, resulting from increases in central venous pressure, may contribute to the ICP response. The current study evaluated this issue in a canine model of intracranial hypertension and additionally evaluated the relationship between ICP and static increases in superior vena caval pressure.

METHODS

Six dogs were lightly anesthetized with 0.65% end-expired halothane in oxygen and nitrogen, and ventilation was mechanically controlled. Intracranial pressure was increased to a stable baseline of 15-20 mmHg using a subarachnoid infusion of warm 0.9% saline solution. The following variables were quantified before, and for 6 min after, initiating a 1-min noxious stimulus to the trachea and skin: ICP, central venous pressure, electromyograms (masseter, deltoid, and intercostal muscles), intrathoracic pressure, and cerebral perfusion pressure (defined as mean arterial pressure -- ICP). Later, the protocol was repeated in the presence of neuromuscular block with pancuronium. Finally, in the same dogs, occlusion of the superior vena cava at its junction with the right atrium was used to increase superior vena caval pressure in 5-mmHg increments, from 5 to 30 mmHG, so that the resulting increases in ICP could be quantified.

RESULTS

In unparalyzed dogs whose heads were maintained at the level of the right atrium, there was a 22-mmHg increase in ICP at 1 min after initiating the noxious stimulus (P<0.05). The ICP increase was related to electromyogram activation and a 6-mmHg increase in central venous pressure; however, it was not associated with significant increases in intrathoracic pressure or cerebral perfusion pressure. Treatment with pancuronium abolished the electromyographic, ICP, and central venous pressure responses to noxious stimulus. When superior vena caval pressure was statically manipulated, the resulting ICP increase was only one half the magnitude of the superior vena caval pressure increase. After elevating the head 14 cm, the ratio of ICP to superior vena pressure increases was reduced to one third.

CONCLUSIONS

If these results apply to humans, it was concluded that increases in ICP that accompany movement in tracheally intubated patients may arise from two complementary factors: (1) cerebrovascular dilation that correlates with electromyographic activity and is mediated by ascending neural pathways that transmit proprioreceptive information, and (2) passive venous congestion that results from any increase in central venous pressure. The influence of the latter factor can be reduced by elevating the head. (Key words: Blood pressure, venous pressure; mean arterial pressure. Muscle: afferent activity; electromyograms, skeletal. Neuromuscular relaxants: pancuronium.)

The bispectral index during induction of anesthesia with midazolam and propofol

This study evaluated the bispectral index as an indicator of anesthetic depth in relation to the cardiovascular response to intubation. Two treatments were compared: group 1 (n = 8) received propofol for induction of anesthesia (2 mg/kg bolus followed by an infusion of 0.20 mg/kg-1/min-1, group 2 (n = 8) was given 90 micrograms/kg midazolam 2 min before, followed by anesthesia with half-strength propofol (1 mg/kg bolus with infusion of 0.10 mg/kg-1/min-1). The bispectral index of the electroencephalogram, blood pressure, and heart rate were measured under unanesthetized conditions, during anesthetic induction, intubation, and a 15-min period after intubation. The duration of anesthesia and the total propofol requirement were recorded. Midazolam pretreatment produced transient decreases in blood pressure and the bispectral index. During anesthetic induction with propofol, blood pressure decreased 20% in both groups, and the bispectral index decreased to lower levels in group 1 (29 +/- 9) than in group 2 (47 +/- 22). Intubation increased blood pressure more in group 2 (50 +/- 10 mm Hg) than in group 1 (30 +/- 12 mm Hg). Throughout the rest of the surgery, more propofol was used in group 1 (77 +/- 14 micrograms/kg-1/min-1) than in group 2 (42 +/- 14 micrograms/kg-1/min-1). These results show that the decrease in bispectral index provides an indication of the blood pressure increase to intubation during propofol anesthesia. Midazolam pretreatment did not attenuate the cardiovascular response to intubation but did decrease propofol use during surgery.

Role of nitric oxide, adenosine, N-methyl-D-aspartate receptors, and neuronal activation in hypoxia-induced pial arteriolar dilation in rats

In this study, we tested the hypothesis that nitric oxide (NO) and adenosine (ADO) are the principal mediators of severe hypoxia-induced vasodilation. In addition, we examined whether activation of N-methyl-D-aspartate (NMDA) receptors and/or perivascular nerves plays a role. A closed cranial window and intravital microscopy system was used to monitor diameter changes in pial arterioles (approximately 40 microns) in anesthetized rats. The relative contributions of ADO, NMDA, NO, and neuronal activation to hypoxic cerebrovasodilation were assessed using the blockers 8-sulfophenyltheophylline (8-SPT), MK-801, nitro-L-arginine methylester (L-NAME), and tetrodotoxin (TTX). Two experimental series were studied. In the first, we tested the effects of NOS inhibition, via topical L-NAME (1 mM), on moderate (PaO2 approximately 46 mmHg) then severe (PaO2 approximately 34 mmHg) hypoxia-induced dilation. To confirm that L-NAME was affecting specifically NO-dependent responses, we also examined, in each experiment, the vasodilatory responses to topical applications of NOS-dependent (adenosine diphosphate (ADP); acetylcholine (ACh)) and -independent (sodium nitroprusside (SNP)) agents, in the presence of L-NAME or, in controls, the presence of D-NAME or no added analogue. In the second series, topical suffusions of ADP, ADO, and NMDA were sequentially applied, followed by 5 min exposure to severe hypoxia (PaO2 approximately 32 mmHg). Following return to normoxia, a suffusion of either 8-SPT (10 microM), MK-801 (10 microM), TTX (1 microM), or 8-SPT+MK-801 was initiated (or, in controls, application of a drug-free suffusate was maintained), and the above sequence repeated. In control, TTX, and 8-SPT+MK-801 experiments, baseline conditions were then restored and hypercapnia (PaCO2 = 70-85 mmHg) was imposed. In the series 1 control groups, moderate and severe hypoxia elicited approximately 20% and 35-40% increases in diameter, respectively. L-NAME attenuated ADP- and ACh-induced dilations, did not alter the arteriolar responses to SNP or moderate hypoxia, but prevented further dilation upon imposition of severe hypoxia. This suggested that 45-50% of the severe hypoxia response was NO-dependent. In series 2, 8-SPT blocked the adenosine response and reduced severe hypoxia-induced dilation by 46%. MK-801 predictably blocked NMDA-induced relaxation and reduced the hypoxic response by 42%. When combined, 8-SPT and MK-801 affected hypoxic vasodilation additively. After TTX, the ADP and ADO responses were normal, but NMDA and hypoxia responses were completely blocked. Hypercapnia-induced dilation was unaffected by TTX or 8-SPT+MK-801. The results imply that severe hypoxia-induced release of NO and ADO, and the accompanying pial arteriolar dilation, are wholly dependent on the capacity to generate action potentials in perivascular nerves. The similarity of the L-NAME and MK-801 effects on hypoxic cerebrovasodilation suggests that the NO-dependency, to a large degree, derives from NMDA receptor activation.

Transcranial Doppler sonography indicates critical brain perfusion during hemorrhagic hypotension in dogs

This study investigated the effects of hemorrhagic hypotension on cerebral blood flow velocity and brain electrical activity (by electroencephalogram [EEG]). Eleven mongrel dogs were anesthetized with isoflurane (1 minimum alveolar anesthetic concentration [MAC]) and catheters were placed into both femoral arteries and veins for mean arterial blood pressure (MAP) measurement, blood withdrawal, and drug administration. Brain temperature, arterial blood gases, and pH were maintained constant. EEG was recorded from temporoparietal recording sites versus a frontal reference. A pulsed transcranial Doppler (TCD) probe (2 MHz, Transpect, Medasonics) was placed on the dura via a temporal bone window to measure mean (Vmean, cm/s) and diastolic blood flow velocity (Vdiast, cm/s) in the middle cerebral artery. At the end of the surgical preparation, isoflurane was discontinued and all animals received fentanyl (bolus, 25 micrograms/kg intravenously (IV); infusion, 50 micrograms.kg-1.h-1 IV) plus 50% N2O/O2 during 30 min of equilibration. After recordings of baseline data, the dogs were hemorrhaged at a rate of 80-100 mL/min. The observation interval was 14 min. EEG spectral edge frequency (SEF 95%) and Vmean did not change when MAP was decreased from 109 +/- 10 to 63 +/- 7 mm Hg. This indicates preserved neuronal function and intact autoregulation of cerebral blood flow. Below MAP of 49 +/- 9 mm Hg, a shift of the EEG to lower frequencies was associated with decreases in Vmean and Vdiast. EEG burst suppression occurred at a MAP of 31 +/- 7 mm Hg, paralleled by a loss of the diastolic flow velocity pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of halothane effects on myocardial force-interval relationships at anesthetic concentrations depressing twitches but not tetanic contractions

BACKGROUND

Tetanic contractions in rat myocardium depend solely on cellular Ca2+ uptake, whereas twitches depend on Ca2+ release from the sarcoplasmic reticulum. Because halothane may cause loss of sequestered Ca2+, the anesthetic was tested for its differential effects on twitch and tetanic forces. The in vitro effects of halothane on the twitch force-interval relationship were then evaluated, using a mathematical model that relates twitch contractile force to the Ca2+ content of intracellular compartments.

METHODS

Isometric contractile force was measured in paced (0.4 Hz) rat atrial preparations. The sarcoplasmic reticulum was functionally eliminated using ryanodine (10(-6) M), abolishing twitches. Rapid pacing (20 Hz, 10 s) caused tetanic contractions. The effects of identical halothane exposures on twitches and tetanic contractions were compared. Ca2+ compartment model parameters were extracted from twitch force-interval data, according to a previously employed quantitative procedure.

RESULTS

Halothane (0.5-1%) depressed normal twitches, but not tetanic contractions. The anesthetic decreased the amplitude of the steady-state twitch force-frequency relationship, and accelerated the course of mechanical recovery. Halothane (0.5-1%) also accelerated the decay constant for the decline in amplitude of a series of rest-potentiated contractions. The modeling showed that a 20-30% decrease in the recirculating fraction of activator Ca2+ accounts for 0.5% halothane-induced negative inotropy and acceleration of the decay constant.

CONCLUSIONS

The differential effect of halothane on twitches and tetanic contractions implies that a functioning sarcoplasmic reticulum is required for halothane-induced negative inotropy. The effects of halothane on the force-interval relationship suggest that halothane reduces the sequestered pool of activator Ca2+.

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.

The role of nitric oxide in modulating brain activity and blood flow during seizure

The role played by nitric oxide (NO) in modulating seizure activity and cerebral blood flow (CBF) during seizures was investigated in rats. Seizures were induced with bicuculline (a GABA antagonist, 1.2 mg kg-1, i.v.). Each animal was subjected to an initial bicuculline-induced seizure followed by treatment with either L-nitroarginine (L-NA, a NO synthase inhibitor) or its less active enantiomer D-NA as a 50 mg kg-1 bolus followed by an infusion of 1 mg kg-1 min-1. The animals then received a second bicuculline treatment. Seizure duration was monitored using EEG and CBF was measured with laser-Doppler. There was no difference in seizure duration before or after D-NA administration. Seizure duration doubled from (6 +/- 1 to 12 +/- 2 min p < 0.05) following inhibition of NO synthase with L-NA. The increase in CBF that accompanied the seizure activity paralleled the seizure duration. Our data support the concept that (1) NO acts as an endogenous anticonvulsant, with seizure duration doubling when NO synthase is acutely inhibited, and (2) that NO is not the messenger that couples CBF to metabolism during bicuculline-induced seizures.

Nitrous oxide added to isoflurane increases brain artery blood flow and low frequency brain electrical activity

Although changes in cerebral blood flow (CBF) and the electroencephalogram (EEG) have been reported with nitrous oxide (N2O) administration, the interaction of these parameters is unclear. The purpose of this study was to measure CBF and EEG during N2O administration in eight patients. A craniotomy was performed and CBF was measured in major brain arteries using a transit time Doppler flowmeter. EEG was recorded bilaterally from frontooccipital leads. Power spectrum analysis was performed on the EEG and power for delta, theta, alpha, and beta frequency bands analyzed over time. Arterial blood pressure was recorded continuously. N2O (66%) was added to the inspired gases during isoflurane anesthesia (0.8% end tidal) under hypocapnic (Paco2 = 29 mm Hg) and normocapnic conditions (Paco2 = 39 mm Hg). During hypocapnia, N2O administration decreased alpha EEG activity and increased delta activity but did not change CBF. During normacapnia, N2O produced similar but greater changes in EEG and increased CBF 39%. In three patients, the isoflurane concentration was increased to 1.6% end tidal during normocapnia. N2O administration in these patients also enhanced delta EEG activity and increased CBF. The slowing of EEG activity with N2O is temporally related to increases in CBF during normocapnia. Hypocapnia abolished the increase in CBF during N2O and attenuated the shift of EEG to delta activity.

The role of endothelium and nitric oxide in rat pial arteriolar dilatory responses to CO2 in vivo

Using a closed cranial window system and intravital microscopy/videometry, we studied the rat pial arteriolar (30-60 microns) responses to CO2 before and following a light/dye (L/D) endothelial injury or topical application of the nitric oxide synthase (NOS) inhibitor, nitro-L-arginine (L-NA) or its inactive form, D-NA. L/D treatment consisted of intravenous injection of sodium fluorescein and the illumination (for 90 s) of arteriolar discrete segments on the cortical surface with light from a mercury lamp. Functional changes in pial arteriolar endothelium were characterized by evaluating responses to topical application of acetylcholine (Ach, 5 x 10(-4) M) and to intravenous (i.v.) oxotremorine (OXO, a stable blood-brain barrier permeant muscarinic agonist, 1 microgram kg-1 min-1). After the L/D injury, dilation to Ach was absent whereas dilations to the NO donor, S-nitrosoacetyl-penicillamine (SNAP, 10(-5) M) and to CO2 (5%) were unchanged (PaCO2 = 70 mm Hg). Loss of Ach response but intact SNAP response confirmed functional endothelial injury and intact smooth-muscle function. The global endothelium-dependent vasodilation induced by i.v. OXO was markedly attenuated when expanding the L/D injury field from 300 microns to 6 mm in diameter. However, the global vasodilation induced by inhalation of CO2 was still unaffected by this increase in the area of light exposure. This provides evidence that the expanded exposure was capable of impairing global vasodilation resulting from endothelium-dependent stimuli but not from inhalation of CO2. The intact CO2 response despite an endothelial dysfunction suggests that the reported NO dependence of hypercapnia-induced cerebral hyperemia in rats cannot be attributed to an endothelial NO source.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of nitric oxide and endothelium in rat pial vessel dilation response to isoflurane

Isoflurane induces cerebral hyperemia. We sought to assess whether isoflurane induces cerebral microvessel dilation in vivo, and if so, to determine whether nitric oxide (NO) and endothelium are involved. By using a rat closed cranial window model, pial arterioles and venules of 30-70 microns in diameter were measured using intravital microscopy. The cerebral microvascular dilatory response was recorded as percent change of diameter from baseline. The pial vessels were suffused with sodium nitroprusside (SNP) or S-nitroso-acetyl-penicillamine (SNAP) to verify intact vascular smooth muscle relaxation function, and with adenosine diphosphate (ADP) and/or acetylcholine (ACh) to verify endothelial NO-generating capability. To isolate NO's role in the cerebral microvascular effects of isoflurane (Protocol I), microvessels were studied with and without nitric oxide synthase (NOS) inhibition by topically applied nitro-L-arginine methyl ester (L-NAME). In controls, L-NAME was replaced by its inactive enantiomer, nitro-D-arginine methyl ester (D-NAME). Mercury light plus fluorescein dye (LD) endothelial injury (Protocol II) was used to delineate an endothelium-mediated mechanism. Subsequently, vasodilator applications were repeated to verify the desired effects of the interventions and followed by suffusion of isoflurane 1%, 2%, and 3% (Protocol I) or isoflurane 3% (Protocol II). Suffusions of SNP, ADP, and ACh induced diameter increases of 15%-30%. NOS inhibition with L-NAME greatly attenuated ADP and ACh responses, but did not alter the SNP response, confirming that NO generation was blocked, but not NO action. These responses were unaffected in D-NAME-suffused rats. Isoflurane dilated arterioles 17% and venules 6% in the presence of D-NAME suffusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide (NO) is an endogenous anticonvulsant but not a mediator of the increase in cerebral blood flow accompanying bicuculline-induced seizures in rats

Neurons synthesize NO, which may act as a retrograde messenger, involved in either potentiating or depressing neuronal excitability. NO may also play a role in the cerebral vasodilatory response to increased neuronal activity (i.e., seizures). In this study, two questions were asked: (1) is NO an endogenous anticonvulsant or proconvulsant substance? and (2) is the cerebral blood flow (CBF) increase accompanying bicuculline (BC)-induced seizures mediated by NO? The experiments were performed in 300-400-g Wistar rats anesthetized with 0.6% halothane and 70% N2O/30% O2. CBF was measured using the intracarotid 133Xe clearance method or laser-Doppler flowmetry. EEG activity was recorded. Chronic treatment (4 days) with nitro-L-arginine (L-NA), a potent NO synthase (NOS) inhibitor (400 mg/kg total), suppressed brain NOS by > 97% and prolonged seizure duration from 6 +/- 1 (saline-treated controls) to 12 +/- 2 min. In the L-NA-treated group, the CBF increase was sustained as long as seizure activity remained, indicating that CBF was still tightly coupled to seizure activity. Interestingly, the supposed inactive enantiomer of L-NA, D-NA, also showed an inhibition of brain NOS activity, ranging from 87 to 100%. The duration of seizures in this group (average 8 +/- 2 min) corresponded directly to the magnitude of reduction in NOS activity (r = 0.83, P < 0.05). Specifically, the D-NA results indicated that NOS inhibition had to exceed 95% before any effect on seizure duration could be seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C suppresses receptor-mediated pial arteriolar relaxation in the diabetic rat

Cerebral vasodilatory responses are selectively impaired in chronically hyperglycemic, diabetic rats. In this study, we tested the hypothesis that chronic hyperglycemia-induced protein kinase C (PKC) activation can account for the suppression of 2 separate receptor-mediated vascular relaxation processes: (1) endothelium-derived nitric oxide (NO) release, and (2) NO-independent beta-adrenergic receptor (beta-AR) activation. The in vivo reactivity of pial arterioles was evaluated in anesthetized rats (streptozotocin-treated diabetics and controls) using a closed cranial window and intravital microscopy. Compared with controls, diabetic rats showed a substantial attenuation or loss of the arteriolar relaxation response accompanying suffusion of the receptor-linked, NO-dependent agonists, acetylcholine (Ach) and adenosine diphosphate (ADP), and the beta-AR-agonist, isoproterenol (ISO). The vasodilatation induced by the direct NO donor, sodium nitroprusside (SNP), was the same in both groups. In the presence of the PKC inhibitor, staurosporine (STAURO), the Ach, ADP, and ISO responses were, largely restored and the SNP response was unaffected. STAURO produced no changes in Ach, ADP, ISO, or SNP responses in non-diabetic rats. These results suggest that PKC activation in chronically hyperglycemic, diabetic rats suppresses receptor-dependent NO release and desensitizes beta-ARs.

Interaction of catecholamines and nitrous oxide ventilation during incomplete brain ischemia in rats

The interaction of plasma catecholamines and nitrous oxide (N2O) ventilation was examined during brain ischemia in rats. Group 1 (n = 19) was anesthetized with 50 micrograms.kg-1 x h-1 of fentanyl and ventilated with 70% nitrogen in oxygen. Group 2 (n = 19) was anesthetized with intravenous fentanyl (25 micrograms.kg-1 x h-1) and 70% N2O ventilation in oxygen. Group 3 (n = 10) received 25 micrograms.kg-1 x h-1 of fentanyl and 70% N2O ventilation and 100 micrograms/kg of dexmedetomidine, an alpha 2-adrenergic receptor agonist that decreases sympathetic activity. Incomplete brain ischemia was produced by right carotid ligation combined with hemorrhagic hypotension to 30 mm Hg for 30 min. Plasma catecholamines were measured during ischemia. Cerebral blood flow (CBF) was evaluated by using laser Doppler. Neurologic outcome was evaluated for 3 days after ischemia. Plasma epinephrine and norepinephrine and were decreased 20% and neurologic outcome was significantly worse in Group 2 ventilated with N2O compared with fentanyl-anesthetized controls (P < 0.05). Dexmedetomidine-treated rats had lower plasma catecholamines (20% of control) and larger decreases in CBF during ischemia compared with controls. Dexmedetomidine (Group 3) improved outcome from ischemia in comparison to both Groups 1 and 2 (P < 0.05). These results suggest that catecholamines play a major role in worsening ischemic outcome. N2O ventilation may increase neuronal injury by enhancing the sympathetic response to ischemia.

The effects of midazolam and sufentanil sedation on middle cerebral artery blood flow velocity in awake patients

Midazolam and sufentanil are commonly used for sedation. Cerebrovascular effects of low-dose midazolam have not been studied previously, and cerebrovascular effects of sufentanil remain controversial. Forty ASA I and II patients were studied preoperatively. These patients were given midazolam (20 or 40 micrograms/kg) or sufentanil (0.1 or 0.2 micrograms/kg) i.v. Transcranial Doppler recordings of middle cerebral artery mean blood flow velocity (Vm) were recorded before administration of the study drug and for the 5-min investigation period. Mean arterial pressure, heart rate, and end-tidal CO2 remained constant during the investigation and did not vary between treatment groups. Vm decreased 17 to 21% with both midazolam doses (p < 0.05), returning to baseline within 5 min. Vm did not change with either sufentanil dose. These results suggest that midazolam decreases cerebral blood flow (CBF) by increasing cerebral vascular resistance (CVR). The low sedative doses of sufentanil used in this study did not affect Vm over 5 min in unanesthetized patients.

Halothane vasodilation and nitric oxide in rat pial vessels

We investigated whether halothane (HAL), administered via cerebral cortical suffusion at concentrations of 1, 2, and 3%, could induce cerebral microvascular dilatation in vivo and whether the vasodilatory response was dependent on nitric oxide (NO) synthesis. The studies were performed using N2O/fentanyl-anesthetized, paralyzed, and mechanically ventilated rats. A closed cranial window and an intravital microscopy technique were employed. This system permitted the controlled delivery of various vasoactive agents in an artificial cerebrospinal fluid (aCSF) solution and the measurement of diameters of pial arterioles and venules. Each experiment included evaluations of (a) the direct smooth muscle relaxing action of NO, using sodium nitroprusside (SNP), and (b) the capacity for generation and release of endogenous NO, using adenosine diphosphate (ADP). Following confirmation of an intact NO-relaxing and generating capacity, HAL (in aCSF) was suffused at increasing concentrations. Nitric oxide synthase (NOS) inhibition was established with topical nitro-L-arginine (L-NA) or its methyl ester (L-NAME) and the above sequence repeated. The results for rats treated with L-NA (n = 5) or L-NAME (n = 5) were analyzed separately and as a combined group. No significant differences in vascular responses were observed when comparing the two groups. Initially, both SNP and ADP produced significant diameter increases (all groupings) in arterioles (14-28% change) and venules (14-25% change). For all groups, suffusions of 1 to 3% HAL produced arteriolar dilation, ranging from a 10 to 25% increase over baseline diameter. A statistically significant dose dependency was only observed with the combined data.(ABSTRACT TRUNCATED AT 250 WORDS)

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

BACKGROUND

A 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.

METHODS

Baseline 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 micrograms.kg-1.min-1) or alfentanil (1.6 and 3.2 micrograms.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.

RESULTS

Both 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.

CONCLUSIONS

These results show that the cardiovascular and cerebral effects of remifentanil and alfentanil are similar but that recovery of these parameters occurs sooner following remifentanil.

Sympathetic stimulation with physostigmine worsens outcome from incomplete brain ischemia in rats

BACKGROUND

It has been suggested that anesthetics may protect the brain during incomplete cerebral ischemia by inhibition of sympathetic activity. This study evaluated whether physostigmine may increase plasma epinephrine and norepinephrine during carotid occlusion with hypotension and worsen ischemic outcome in rats and if this effect could be reversed by dexmedetomidine, an alpha 2-adrenergic agonist.

METHODS

Anesthesia was maintained with fentanyl (25 micrograms.kg-1.h-1) combined with 70% N2O ventilation in oxygen. Ischemia was produced by right carotid ligation combined with hemorrhagic hypotension to 30 mmHg for 30 min. Plasma epinephrine and norepinephrine were measured during ischemia. Neurologic outcome was evaluated for 3 days after ischemia. There were three groups: control (n = 10), physostigmine (1 mg/kg intraperitoneal 3 min before the start of ischemia, n = 10), and physostigmine plus dexmedetomidine (100 micrograms/kg intraperitoneally 15 min before the start of ischemia, n = 10). Brain tissue glutamate concentrations were measured by microdialysis in separate studies.

RESULTS

Compared to control rats, physostigmine increased plasma epinephrine and norepinephrine 10-fold and worsened neurologic outcome. The increases in epinephrine and norepinephrine were blocked by dexmedetomidine before treatment, and neurologic outcome was improved. Outcome was not correlated with blood glucose during ischemia (r = 0.11). Ischemia increased brain tissue glutamate from < 100 microM to 400 microM during ischemia. This increase was not altered by physostigmine treatment.

CONCLUSIONS

These results suggest that physostigmine worsens ischemic outcome by a mechanism that is associated with increases in plasma epinephrine and norepinephrine.

Sympathetic activity enhances glucose-related ischemic injury in the rat

BACKGROUND

Studies have shown that increased sympathetic activity or increased blood and brain glucose concentration worsen postischemic brain damage. The authors evaluated the interaction of plasma glucose with epinephrine and norepinephrine during incomplete cerebral ischemia in the rat using ganglionic blockade.

METHODS

Rats were anesthetized with 25 micrograms.kg-1.h-1 fentanyl and 70% nitrous oxide in oxygen. Ganglionic blockade was produced in 30 rats using 8 mg/kg hexamethonium intravenously. Three plasma glucose ranges, low < 150 mg/dl, moderate = 150-300 mg/dl, and high > 300 mg/dl, were produced in each group. Ischemia was induced by unilateral carotid ligation and hemorrhagic hypotension to 30 mmHg for 30 min. Plasma norepinephrine and epinephrine were measured by radioimmunoassay. Neurologic outcome was evaluated daily for 3 days after ischemia.

RESULTS

Ganglionic blockade decreased blood pressure before the start of ischemia and plasma epinephrine and norepinephrine during ischemia (P < 0.05). Neurologic outcome was significantly worse in rats with high glucose compared with low glucose concentrations with and without ganglionic blockade (P < 0.05). Neurologic outcome and stroke-related mortality were worse in rats with increased plasma epinephrine and norepinephrine compared with rats with ganglionic blockade when plasma glucose was less than 300 mg/dl (P < 0.05).

CONCLUSIONS

These results indicate that increased concentration of catecholamines enhance glucose-related injury during incomplete ischemia in rats.

Cerebral blood flow velocity in relation to cerebral blood flow, cerebral metabolic rate for oxygen, and electroencephalogram analysis during isoflurane anesthesia in dogs

The purpose of this study was to correlate changes in cerebral blood flow velocity (Vmean) with cerebral blood flow (CBF) during isoflurane anesthesia in dogs. The relation between cerebral oxygen consumption (CMRO2) and electroencephalogram (EEG) analysis also was investigated. Blood flow velocity was measured in the middle cerebral artery using a pulsed transcranial Doppler (TCD). CBF was measured with radioactive microspheres. EEG was measured over both hemispheres and median EEG frequency (median frequency) was calculated after fast Fourier transformation. Baseline anesthesia was maintained with 50% nitrous oxide in oxygen and 50 micrograms.kg-1 x h-1 fentanyl. Animals of Group I (control, n = 6) were not given isoflurane. Data were recorded at baseline, and at 30, 60, and 90 min. There was no significant change in any variable over time. In Group II (n = 7), data were recorded at baseline and at 1%, 2%, and 3% end-tidal isoflurane. Mean arterial pressure was maintained at baseline levels by phenylephrine infusion. CBF increased from 70.8 +/- 10.6 mL.100g-1 x min-1 at baseline to 146.1 +/- 36.9 mL.100 g-1 x min-1 with 3% isoflurane (P < 0.01). Vmean increased from 38.3 +/- 6.7 cm/s to 65.6 +/- 9.7 cm/s (P < 0.01). The correlation between relative changes in CBF and Vmean was r = 0.94 (P < 0.01). With 1% isoflurane the EEG shifted to slow-wave, high-voltage activity, and median frequency decreased from 5.9 +/- 0.7 Hz to 1.4 +/- 0.4 Hz (P < 0.05). Median frequency was not decreased further during 2% and 3% isoflurane anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of propofol on cerebral and spinal cord blood flow in rats

The effects of low and high doses of propofol on global cerebral blood flow (CBF) and spinal cord blood flow (SCBF) as a function of mean arterial blood pressure were investigated. CBF and SCBF during propofol infusion were compared to the levels in rats anesthetized with nitrous oxide (N2O) and fentanyl. Rats in the fentanyl/N2O group (control, n = 13) received 70% N2O in O2 plus fentanyl (bolus: 10 micrograms/kg; infusion: 25 micrograms.kg-1 x h-1). Rats in the low-dose propofol group (n = 10) received 30% O2 in air and propofol infusion (0.5 mg.kg-1 x min-1). Rats in the high-dose propofol group (n = 8) received 30% O2 in air and propofol infusion (2.0 mg.kg-1 x min-1). Blood flow autoregulation was tested by manipulating the mean arterial blood pressure with phenylephrine infusion or trimethaphan infusion and blood withdrawal by measuring CBF and SCBF using radioactive microspheres. Arterial blood gases, pHa, and skull temperature were controlled. Cerebral and spinal cord vasculature showed autoregulation in all treatment groups with a pressure range of 50-140 mm Hg. Within this pressure range, when compared to fentanyl/N2O, propofol decreased cortical CBF 60% (P < 0.001), subcortical CBF 40% (P < 0.001), midbrain blood flow 30% (P < 0.001), and SCBF 20% (P < 0.05). These results indicate that propofol maintains CBF and SCBF autoregulation.