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PubMed1991

Anesthetic and hemodynamic effects of dexmedetomidine during isoflurane anesthesia in a canine model.

Dexmedetomidine, an alpha 2-adrenergic agonist, was administered during isoflurane anesthesia to investigate its anesthetic sparing and hemodynamic effects in the canine model. Eleven healthy dogs were anesthetized with isoflurane, intubated, and allowed to breathe spontaneously. The animals were instrumented in order to measure or calculate mean arterial blood pressure (MABP), heart rate (HR), left ventricular end diastolic pressure (LVEDP), cardiac output (CO), and systemic vascular resistance (SVR). Isoflurane minimum alveolar concentration (MAC), baseline hemodynamic measurements, and plasma catecholamine levels were determined. A 20 micrograms/kg bolus of dexmedetomidine was injected intravenously (IV) and MAC and hemodynamic values were redetermined. When isoflurane requirements were 33% of baseline, isoflurane was returned to 90% MAC and the alpha 2-antagonist, atipamezole, was administered. All dogs were treated with 40 micrograms/kg glycopyrrolate throughout the experiment to prevent any bradycardic response. Dexmedetomidine reduced isoflurane MAC by 86%. SVR, MABP, and LVEDP were significantly increased while CO and catechloamine levels were reduced. Atipamezole fully reversed the reduction in anesthetic requirements. MABP and catecholamine levels returned to baseline. SVR and LVEDP increased while CO decreased. The dogs exhibited a profound reduction in anesthetic requirement, reduced catecholamine levels, and changes in hemodynamic parameters with dexmedetomidine administration. The anesthetic sparing effect appears to be mediated by the alpha 2-receptor, since atipamezole reversed the reduction in MAC. Hemodynamic changes may be species or dose related, or due to differences in existing sympathetic tone. The role of dexmedetomidine warrants further study as an adjunct anesthetic agent.

Adrenergic alpha-Agonists↗

PubMed1991

[Initial washout time of inhaled anesthetic agents].

Recovery of inhalation anesthesia is generally considered the inverse of induction. While other factors remain unchanged, the rate of elimination of an inhaled anesthetic agent with a lower solubility such as nitrous oxide (N2O) is faster than one with a high solubility. Therefore, at the end of surgery it is a common practice to discontinue the volatile agent prior to discontinuing N2O. However, the results of our routine monitoring of end-tidal concentrations of anesthetics and physiological gases by employing a mass spectrometer lead us to question the practice. We studied 24 non-premedicated ASA I/II patients requiring endotracheal intubation with an anesthesia time over 2 h. All patients were mechanically ventilated at an appropriate tidal volume and respiratory rate to maintain an ETCO2 of 35-38 mmHg with an anesthetic mixture of 40% O2 and 60% N2O and isoflurane (Forane). At the end of surgery, both N2O and isoflurane were discontinued simultaneously and total oxygen flow was increased to 6 or 10 l/min by random. The exhaled partial pressure of both inhaled anesthetic agents were assessed at the time of anesthetic termination, after 1, 2 min, and then every 2 min up to 20 min or at the time of extubation. The results obtained were plotted against real time using two different scales. Despite the marked difference in their plasma solubilities, these two inhaled anesthetic agents reached to low exhaled partial pressure that had no clinical significance about ten minutes after discontinuation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1991

Dual effects of baclofen on gastric acid secretion depending on the basal secretory activity in conscious and anesthetized rats.

Effects of baclofen, a GABAB agonist, on gastric acid secretion were studied from a viewpoint that baclofen may stimulate or inhibit gastric acid secretion depending on the basal secretory activity in conscious or anesthetized rats. In conscious pylorus-ligated rats, baclofen (2-16 mg/kg, s.c.) depressed acid secretion, while in urethane-anesthetized pylorus-ligated rats, baclofen (4-16 mg/kg, s.c.) stimulated acid secretion. Basal acid secretion was much higher in the conscious rats than in the urethane-anesthetized rats. When gastric secretion was measured by the stomach-perfusion method, baclofen (s.c.) significantly increased acid at 2-4 mg/kg in urethane-anesthetized rats but decreased acid at 0.5-2 mg/kg in pentobarbital- and alpha-chloralose-anesthetized rats. Basal acid secretion was markedly higher in the latter rats than in the former. Moreover, baclofen did not affect acid secretion elicited by peripheral vagal stimulation in alpha-chloralose-anesthetized rats. These results suggest that acid secretion is centrally stimulated by baclofen in rats with low basal secretory activity, and vice versa, and moreover, these dual effects are closely associated with tone of the central vagal center.

Anesthesia↗

PubMed1991

Anesthetics and automaticity in latent pacemaker fibers: I. Effects of halothane, enflurane, and isoflurane on automaticity and recovery of automaticity from overdrive suppression in Purkinje fibers derived from canine hearts.

Knowledge of arrhythmic or antiarrhythmic actions of anesthetics on automaticity of latent pacemaker fibers has relevance to the intraoperative management of patients with bradyarrhythmia due to sinus node dysfunction or heart block. The authors determined the effects of halothane, enflurane, and isoflurane on automaticity and recovery of automaticity from overdrive suppression in canine Purkinje fibers derived from normal hearts. Purkinje fibers were superfused with a modified Krebs' solution (37 degrees C) containing epinephrine (2 or 15 microM) and equilibrated with a 97% O2-3% CO2 gas mixture (control). Transmembrane action potentials (AP) were recorded using standard microelectrode techniques. Purkinje fibers were then exposed to anesthetics at vaporizer settings of 0.75 or 1.5% (halothane), 1.75 or 3.5% (enflurane), and 1 or 2% (isoflurane), which were equivalent to measured superfusate concentrations of 0.22 or 0.47 mM (halothane), 0.44 or 0.94 mM (enflurane), and 0.28 or 0.53 mM (isoflurane). Compared to control, there was no significant effect of either concentration of the anesthetics on upstroke (phase 0) depolarization, AP amplitude or duration (50% repolarization), or maximum diastolic potential. All three anesthetics increased spontaneous rate. The increase in rate with all three anesthetics was due to enhanced diastolic depolarization (rate dV/dt, phase-4 depolarization). Recovery times from overdrive suppression were determined after 30 or 60 s of pacing at drive cycle lengths of 800, 500, and 400 ms and only at higher anesthetic concentrations. Recovery of automaticity was shortened by halothane only in slowly paced fibers exposed to the lower concentration of epinephrine. Under all other conditions recovery times were not affected by halothane.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1985

Anesthetic induction with fentanyl.

The efficacy of fentanyl, 30 micrograms/kg, was evaluated as an anesthetic induction agent in 72 ASA I-III patients scheduled for 2-4-hr operations. The effect of preinduction pretreatment with pancuronium and/or diazepam and the incidence of loss of consciousness (anesthesia), recall, rigidity, abnormal muscle movements, and hemodynamic changes were documented. Seventy-four percent of all patients became anesthetized. Diazepam pretreatment enhanced but did not ensure success of anesthetic induction. There was a significant correlation between age and the incidence of unconsciousness (P = 0.0287) and all patients over 60 yr old were anesthetized with 30 micrograms/kg of fentanyl. The incidence and severity of rigidity was reduced by pancuronium (P = 0.0002) but not by diazepam pretreatment. However, pancuronium plus diazepam produced a significant reduction in the incidence of rigidity when compared to pancuronium alone (P = 0.031). A significant positive correlation between age and the incidence of rigidity (P = 0.003) was found. Six patients had focal and one patient global tonic-clonic abnormal muscle movements. Diazepam but not pancuronium significantly decreased both heart rate (P = 0.05) and blood pressure (P = 0.04). Seventeen patients required reversal of narcotic effect to restore adequate spontaneous respiration after surgery. No patient required postoperative mechanical ventilatory assistance. The results of this study demonstrate that 30 micrograms/kg of fentanyl is not a reliable anesthetic induction dose in patients less than 60 yr old. Both age and premedication enhance the anesthetic capabilities of induction with fentanyl.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗

PubMed1988

Local anesthetics potentiate spinal morphine antinociception.

Some investigators have postulated a synergistic analgesic effect of local anesthetic agents and opiates when given intrathecally or epidurally, but little objective evidence has been presented to quantitate such an effect. A study was therefore undertaken to compare in mice the antinociceptive effects of intrathecal injections of mixtures of morphine with bupivacaine or lidocaine with the effects of these agents when administered alone. The antinociceptive effects (tail-flick and hotp-late tests) of morphine (0.1-1.6 micrograms) with either bupivacaine, 25 micrograms, or lidocaine, 200 micrograms, were significantly greater than the effects of morphine or the local anesthetics when administered alone. When morphine was administered with the local anesthetics, the intensity and the duration of antinociception were greater, although the time courses of the effects resembled that of morphine administered alone. An enhanced effect was also observed when combinations of local anesthetics and low doses of morphine were used that by themselves had no or little effect. The addition of morphine did not affect the motor block produced by the local anesthetics. The results indicate a potentiating effect of local anesthetics on spinal morphine antinociception, a finding that may have important clinical implications.

Anesthesia, Endotracheal↗

PubMed1984

The influence of local anesthetics on molecular organization in phosphatidylethanolamine membranes.

The influence of the local anesthetics tetracaine (TTC) and procaine (PRC) on bilayers of specifically deuterated phosphatidylethanolamines (PE) has been studied by 2H and 31P NMR. Dimyristoylphosphatidylethanolamines (DMPE), deuterated at positions 2, 4, and 14 of the sn-2 chain, position 2 of the sn-1 chain, and in the ethanolamine headgroup, were mixed 1:1 with a semisynthetic egg PE and the effect of measured quantities of TTC and PRC on the 2H quadrupole splittings, spin-lattice relaxation times, and 31P chemical shift anisotropy were observed. Experiments were performed at pH 5.5, when the anesthetics are primarily charged, and at pH 9.5, when they are uncharged. Tetracaine was observed to disorder the hydrocarbon region of the bilayer and to induce a conformational change in the PE headgroup. Conversely, procaine had little or no effect on the hydrocarbon region and induced only a small change in the headgroup. These conformational changes and disordering effects, when adjusted for anesthetic partitioning, are essentially independent of the charge on the anesthetic. However, at pH 5.5 and low TTC/PE molar ratios (less than 0.1), the 2H NMR spectra showed two lipid environments--one corresponding to free PE and the other to PE in contact with TTC. Continued addition of TTC resulted in the eventual disappearance of the free PE signal and the corresponding growth of the signal from PE in contact with TTC. At pH 9.5, when TTC is uncharged, only one signal is observed. This indicates that at low pH, when TTC is primarily charged, it has a much slower rate of lateral diffusion in the PE bilayer. In mixtures of PE and phosphatidylserine, a conformational change in the headgroup was noted which was similar to that seen in the pure PE; however, there was no evidence for slow lateral diffusion of the anesthetics. The effects of TTC and PRC on the PE bilayer, when combined with our earlier study of the labeled anesthetics [Kelusky, E.C., and I.C.P. Smith, Biochemistry, 22:6011-6017 (1983)], indicate that TTC penetrates into the hydrocarbon portion of the bilayer whereas PRC sites only in the headgroup region.

Deuterium↗

PubMedDate not supplied

Anesthetic cartridge system under evaluation.

The problem of glass breakage in the local anesthetic cartridge system was evaluated under laboratory conditions with a mechanical testing machine. The anticipated breakage of the glass did not occur with any frequency, as the rubber stopper produced more uniform failures of the system. The glass cartridge appeared to be quite reliable and resistant to breakage.Local anesthetics have been used for many years to provide patients temporary freedom from pain. Local anesthetic solutions are in wide use in both dentistry and medicine and are the most frequently used drugs in dentistry. Various estimates place the number of injections at approximately one half million daily or 125 million injections per year.These drugs and the armamentarium necessary to administer them have proven to be safe and reliable. Only rarely are there reports of sensitivity to the anesthetic solution or breakage of needles.. Sterility of the solutions has not been a problem as they are carefully processed and evaluated at the factory. Although there are sporadic reports of loss of sterility, this has been attributed to the reuse of the anesthetic cartridges on more than one patient. Monheim states "The success of the cartridge system in dentistry has been due to the sincerity, honesty, and high standards of the manufacturers in giving the profession a near-perfect product." However, on occassion a glass cartridge will break or shatter when inserting the harpoon into the rubber stopper or even during injection. Cooley et al reported on eye injuries occurring in the dental office, one of which was due to glass from a local anesthetic cartridge that exploded and propelled particles into the patient's eye. Forrest evaluated syringes, needles, and cartridges and reported that one brand (made in Britain) fractured more often than any other, but that the fracture rate was too low to be of any consequence.It is apparent that glass cartridges will fracture or burst from time to time. This study evaluates the cartridge system with carefully controlled laboratory procedures. The cartridges were tested under various pressures and conditions in an attempt to determine the causes of failure and when such failure may be anticipated.

Anesthesia, Local↗

PubMed1992

Anesthetic activity of the lipospheres bupivacaine delivery system in the rat.

The Lipospheres Bupivacaine Delivery System (bupivacaine-lipospheres) is a novel sustained-release local anesthetic preparation that has recently been made available for research purposes. This investigation compared the local anesthetic efficacy and safety of 2% bupivacaine-lipospheres, 0.5% bupivacaine plus 1:200,000 epinephrine, lipospheres plain, and physiologic saline following subcutaneous tail injection in the rat. A modified tail-flick paradigm was used to assess local anesthetic efficacy. Animals treated with 2% bupivacaine-lipospheres or 0.5% bupivacaine with epinephrine displayed significant antinociception (P < 0.05) compared to saline or lipospheres plain with 5 min of injection. Bupivacaine with epinephrine had an anesthetic duration of 30 min, whereas 2% bupivacaine-lipospheres had a duration of 3 hr. The local anesthetic blockade produced by both active solutions was completely reversible. All animals gained weight normally during the 1-wk course of the study, and there were no signs of local tissue toxicity at the injection sites. We conclude that 2% bupivacaine-lipospheres is a safe and efficacious local anesthetic preparation in this particular animal model. It possesses an onset of action that is a rapid as 0.5% bupivacaine with 1:200,000 epinephrine, and a duration that is six times longer.

Analysis of Variance↗

PubMed1999

Neurologic complications after placement of cerebrospinal fluid drainage catheters and needles in anesthetized patients: implications for regional anesthesia. Mayo Perioperative Outcomes Group.

UNLABELLED: Subarachnoid or epidural needle placement in an anesthetized patient is controversial because general anesthesia and muscle relaxation may mask neural trauma. However, placement of a needle or catheter in the subarachnoid space for the purpose of cerebral spinal fluid (CSF) drainage is frequently performed in anesthetized patients undergoing neurosurgery. The records from 530 consecutive transsphenoidal surgeries performed with lumbar CSF drainage were reviewed to determine the types of neurologic complications attributable to spinal drainage and their rates of occurrence. All patients were anesthetized during CSF drain placement. A 19-gauge malleable needle was placed in 473 (89%) patients. Subarachnoid catheters (20- or 16-gauge catheters placed via 18- or 14-gauge epidural needles, respectively) were placed in 17 (3%) patients. In 40 (8%) patients, the type of drain was unspecified. No new neurologic deficits attributable to spinal drain insertion were detected in the immediate postoperative period or within 1 yr of surgery. Thirteen patients developed postdural puncture headache (2.5%, exact 95% confidence interval 1.3%-4.2%); seven required epidural blood patch (1.3%, 0.5%-2.7%). The low incidence (0%, 0.0%-0.7%) of neurologic injury from spinal drain insertion in anesthetized patients from this study is similar to the incidence of neurologic complications historically reported for both CSF drain insertion and spinal anesthesia. IMPLICATIONS: The performance of regional anesthesia in an anesthetized patient is controversial due to the possibility of unrecognized nerve injury. We report no cases of nerve injury caused by the placement of cerebrospinal fluid drainage needles and catheters in 530 anesthetized patients undergoing neurosurgery.

Adolescent↗

PubMed1988

Teratogen update: anesthetic agents.

Although many pregnant women undergo anesthesia and many others are occupationally exposed to anesthetics every year, assessment of the teratogenic risk of anesthetic agents in humans must currently be made on the basis of very limited data. Available studies suggest that administration of an anesthetic to a pregnant woman will usually not have a deleterious effect on embryonic or fetal development. The risk of congenital anomalies does not appear to be substantially increased among children of women who have chronic occupational exposure to anesthetic gases during pregnancy, either, but miscarriages may occur more frequently than expected among such women. There is an urgent need for publication of good human epidemiologic and experimental animal studies on the teratogenicity of anesthetics.

Anesthesia, General↗

PubMed1997

Anesthetic interactions following bolus injections.

The article presents two main principles of anesthetic interactions: (1) In contrast to the interactions of inhalational anesthetics (when deviations from additivity are debatable), profound synergistic and antagonistic anesthetic interactions between different classes of intravenous drugs are possible due to the difference in the mechanism of their action, (2) A combination of intravenous anesthetics may provide different outcomes (addition, synergism, or antagonism) regarding different components of anesthesia (eg, unconsciousness or movement to noxious stimulation) because the components, even if induced by one anesthetic drug, have different underlying mechanisms.

Anesthetics, Combined↗

PubMed2006

Photoactive analogues of the haloether anesthetics provide high-resolution features from low-affinity interactions.

The difficulty in obtaining binding target and site information for low-affinity drugs, like the inhaled anesthetics, has limited identification of their molecular effectors. Because such information can be provided by photoactive analogues, we designed, synthesized, and characterized a novel diazirnyl haloether that closely mimics isoflurane, the most widely used clinical general anesthetic. This compound, H-diaziflurane, is a nontoxic, potent anesthetic that potentiates GABA-gated ion channels in primary cultures of hippocampal neurons. Calorimetric and structural characterizations show that H-diaziflurane binds a model anesthetic host protein with similar energetics as isoflurane and forms photoadducts with residues lining the isoflurane binding site. H-diaziflurane will be immediately useful for identifying targets and sites important for the molecular pharmacology of the inhaled haloether anesthetics.

Anesthetics↗

PubMed2001

Pharmacologic rationale for anesthetic agents in ambulatory practice.

Patients undergoing surgery in the ambulatory setting require anesthetic agents that expedite postoperative recovery, minimize adverse side effects, and contribute to patient satisfaction. The newer anesthetic agents that are currently used in today's practice offer the flexibility needed to provide anesthesia care for a wide variety of diagnostic and therapeutic procedures performed on an outpatient basis. It is important for the perianesthesia nurse to be familiar with the anesthetic agents used in the operating room to fully understand the influence of these drugs on the patient's recovery process. The role of the perianesthesia nurse is vital in assessing the residual effects of the anesthetic agents and instituting proper nursing interventions during the patient's postanesthesia experience. This article focuses on the rationale for the use of a variety of anesthetic and related agents necessary for the provision of ambulatory anesthesia. The commonly used agents used in ambulatory anesthesia care and their influences on the continuum of care is reviewed.

Adjuvants, Anesthesia↗

PubMed2000

Sevoflurane versus propofol for anesthetic induction: a meta-analysis.

UNLABELLED: We performed this meta-analysis to compare the characteristics of sevoflurane and propofol for the induction of routine anesthesia and for laryngeal mask airway (LMA) insertion. The variables assessed were 1) time to loss of consciousness, 2) incidence of apnea during induction, 3) induction complications, 4) time for successful LMA insertion, 5) success with LMA insertion on first attempt, 6) patient dissatisfaction, and 7) postoperative nausea and vomiting. MEDLINE, Embase, and the Cochrane library databases between January 1992 and October 1999 were reviewed for randomized, controlled trials comparing anesthetic induction between sevoflurane/nitrous oxide and propofol. Data from the 12 randomized, controlled studies were used for the meta-analysis. Sevoflurane induction was associated with a trend toward higher patient dissatisfaction and higher first-time success with LMA. Apnea was less common in the sevoflurane group. The incidence of postoperative nausea and vomiting was significantly more frequent in the sevoflurane group (P < 0.05). This effect was still present when all other variables, except the induction methods, were controlled. The other pooled variables did not show a significant difference between sevoflurane and propofol. Sevoflurane and propofol had similar efficacy for anesthetic induction. However, for routine outpatient surgery, propofol may still be the preferred induction anesthetic because of its favorable induction of anesthesia characteristics, high patient satisfaction, and less frequent incidence of postoperative nausea and vomiting. IMPLICATIONS: Sevoflurane and propofol had similar efficacy for anesthetic induction. However, for routine outpatient surgery, propofol may still be the preferred induction anesthetic because of its favorable induction of anesthesia characteristics, high patient satisfaction, and less frequent incidence of postoperative nausea and vomiting.

Anesthesia↗

PubMed2004

Intravenous emulsified halogenated anesthetics produce acute and delayed preconditioning against myocardial infarction in rabbits.

BACKGROUND: Preconditioning against myocardial infarction by volatile anesthetics is well known. The authors tested the hypothesis that new emulsified formulations of halogenated anesthetics administered intravenously reduce myocardial infarct size when administered either 1 or 24 h before prolonged ischemia and reperfusion. METHODS: Pentobarbital-anesthetized rabbits (n = 39) were instrumented for measurement of hemodynamics and randomly assigned to receive intravenous saline (control), lipid vehicle, or infusions (3.5 ml . kg . h for 30 min) of emulsified isoflurane (6.9%), enflurane (7.1%), or sevoflurane (7.5%). Infusions were discontinued 30 min before a 30-min coronary occlusion and 3 h of reperfusion. In three additional groups, conscious rabbits (n = 21) received saline, lipid vehicle, or emulsified sevoflurane (7.5%) infusions (3.5 ml . kg . h for 30 min) 24 h before ischemia and reperfusion. Infarct size was determined using triphenyltetrazolium staining. RESULTS: Lipid vehicle produced transient increases in heart rate, whereas emulsified volatile anesthetics had no effect on hemodynamics before coronary occlusion. Lipid vehicle did not affect infarct size (38 +/- 2% of the area at risk; mean +/- SEM) as compared with saline control (41 +/- 4%). In contrast, emulsified isoflurane, enflurane, and sevoflurane reduced infarct size (20 +/- 3%, 20 +/- 3%, and 21 +/- 2% of the area at risk, respectively; P < 0.05). Administration of lipid vehicle or emulsified sevoflurane did not produce sedation or respiratory depression in conscious rabbits. Emulsified sevoflurane (18 +/- 2%) but not lipid vehicle (44 +/- 2%) reduced infarct size as compared with control in delayed preconditioning experiments. CONCLUSIONS: Intravenous emulsified halogenated anesthetics produce acute and delayed preconditioning against myocardial infarction.

Anesthetics, Inhalation↗

PubMed2006

Intraoperative motor-evoked potential monitoring in scoliosis surgery: comparison of desflurane/nitrous oxide with propofol total intravenous anesthetic regimens.

STUDY DESIGN: A prospective, randomized study in a large general hospital setting. BACKGROUND: During spinal surgery, monitoring motor-evoked potentials (MEPs) is a means of assessing the intraoperative integrity of corticospinal pathways. However, MEPs are known to be sensitive to the effects of anesthetic agents. OBJECTIVE: To compare the use of desflurane or total intravenous anesthetic regimens (TIVA) with multipulse cortical stimulation for intraoperative monitoring (IOM). METHODS: Twenty consecutive patients (10 in each arm) undergoing scoliosis correction surgery were randomly assigned to 2 equal groups receiving desflurane or TIVA. Inhalational anesthesia was maintained using 66% nitrous oxide in oxygen and a mean end-tidal desflurane concentration of 3.4%. For TIVA, continuous intravenous infusion of propofol was used. For analgesia, fentanyl and morphine were given when required for both groups. Cortical stimulation was achieved with 2 bipolar direct current stimulators connected in parallel by jumper cables. Five equivalent pulses 0.5 ms in duration at 4 ms intervals were delivered at C1C2 positions. MEP recordings were made in the abductor hallucis (AH) and tibialis anterior (TA) with needle electrodes. RESULTS: Reproducible MEPs were obtained throughout the operation in all 20 cases, with up to 80 mA per stimulator. Before insertion of pedicle screws, mean MEP amplitudes (SD) obtained were 85 (19) and 21.7 (10.8) mV for AH and TA, respectively, using desflurane. With TIVA, amplitudes were 56.7 (28.4) and 59.1 (24.5) mV, respectively. Both muscle MEP amplitudes were significantly different using different anesthetic regimens (P < 0.05 for all). AH MEP amplitudes obtained with desflurane were significantly larger than TA amplitudes (P < 0.0001). No complications were reported intraoperatively and postoperatively. CONCLUSIONS: This is the first study comparing the use of desflurane and TIVA showing that both anesthetic regimens allowed successful intraoperative monitoring useage throughout the procedures. For MEP recording, the AH was the preferred muscle with a desflurane anesthetic regimen.

Adolescent↗

PubMed2001

Toward a theory of the general-anesthetic-induced phase transition of the cerebral cortex. I. A thermodynamics analogy.

In a recent paper the authors developed a stochastic model for the response of the cerebral cortex to a general anesthetic agent. The model predicted that there would be an anesthetic-induced phase change at the point of transition into unconsciousness, manifested as a divergence in the electroencephalogram spectral power, and a change in spectral energy distribution from being relatively broadband in the conscious state to being strongly biased towards much lower frequencies in the unconscious state. Both predictions have been verified in recent clinical measurements. In the present paper we extend the model by calculating the equilibrium distribution function for the cortex, allowing us to establish a correspondence between the cortical phase transition and the more familiar thermodynamic phase transitions. This correspondence is achieved by first identifying a cortical free energy function, then by postulating that there exists an inverse relationship between an anesthetic effect and a quantity we define as cortical excitability, which plays a role analogous to temperature in thermodynamic phase transitions. We follow standard thermodynamic theory to compute a cortical entropy and a cortical "heat capacity," and we investigate how these will vary with anesthetic concentration. The significant result is the prediction that the entropy will decrease discontinuously at the moment of induction into unconsciousness, concomitant with a release of "latent heat" which should manifest as a divergence in the analogous heat capacity. There is clear clinical evidence of heat capacity divergence in historical anesthetic-effect measurements performed in 1977 by Stullken et al. [Anesthesiology 46, 28 (1977)]. The discontinuous step change in cortical entropy suggests that the cortical phase transition is analogous to a first-order thermodynamic transition in which the comatose-quiescent state is strongly ordered, while the active cortical state is relatively disordered.

Anesthesia, General↗