EFFECT OF NITROUS OXIDE AND OF NARCOTIC PREMEDICATION ON THE ALVEOLAR CONCENTRATION OF HALOTHANE REQUIRED FOR ANESTHESIA

Antinociceptive Agents as General Anesthetic Adjuncts: Supra-additive and Infra-additive Interactions

The hypothesis "General anesthesia consists of producing both loss of consciousness and the inhibition of noxious stimuli reaching the brain and causing arousal" was used as a basis for the review of published data on general anesthetic interactions with antinociceptive agents: opioids, α 2 adrenergic agonists, and systemic sodium channel blockers. This review is focused on a specific type of anesthetic interaction-the transformation of antinociceptive agents into general anesthetic adjuncts. The primary aim is to answer 2 questions. First, how does an antinociceptive agent transform the effect of an anesthetic in providing a certain component of anesthesia-hypnosis, immobility, or hemodynamic response to noxious stimulation? Second, does a combination of an anesthetic with an adjunct result in a simple summation of their respective effects or in a supra-additive or infra-additive interaction? The Medline database was searched for data describing the interactions of antinociceptive agents and general anesthetics. The following classes of antinociceptive agents were considered: opioids, α 2 adrenergic agonists, and systemic sodium channel blockers. Drugs used in combination with antinociceptive agents were general anesthetics and benzodiazepines. The following terms related to drug interactions were used: anesthetic interactions, synergy, antagonism, isobolographic analysis, response surface analysis, and fractional analysis. The interactions of antinociceptive agents with general anesthetics result in a decrease of general anesthetic requirements, which differ for each of the components of general anesthesia: hypnosis, immobility, and hemodynamic response to noxious stimulation. Most studies of the nature of anesthetic interactions are related to opioid-general anesthetic combinations, and their conclusions usually confirm supra-additivity.

End-tidal Anesthetic Concentration: Monitoring, Interpretation, and Clinical Application

Age-adjusted fraction of minimum alveolar concentration derived from end-tidal anesthetic partial pressure measurement remains a useful drug advisory display to help prevent awareness if interpreted with proper understanding of the quantal and probabilistic nature of minimum alveolar concentration, semantics, drug interactions, and hysteresis.

Serendipity: Being in the Right Place at the Right Time

Effect of Nitrous Oxide and of Narcotic Premedication on the Alveolar Concentration Required for Anesthesia. By Saidman LJ, Eger EI II. Anesthesiology 1964; 25:302-6. Hyperthermia during Anesthesia. By Saidman LJ, Havard ES, Eger EI II. JAMA 1964; 190:1029-32. The minimum alveolar concentration (MAC) of an inhaled anesthetic preventing movement in response to a surgical incision as a measure of equipotency was "invented" in 1964 at the University of California, San Francisco. The principal advantage of MAC is that it allows the pharmacologic effects of inhaled anesthetics to be compared against each other at a similar anesthetic depth. Thus, if the hemodynamic effect (hypotension, decreased cardiac output) of anesthetic "A" is greater than that of anesthetic "B," the anesthesiologist may elect to use "A" in patients with myocardial dysfunction. A rare side effect of a volatile anesthetic is that in some patients, malignant hyperthermia may occur with or without succinylcholine use. This phenomenon was detected in a patient in whom halothane MAC was being measured. The availability of the Severinghaus blood gas device allowed for the first ever measurement of the metabolic and respiratory acidemia that accompanies malignant hyperthermia.

[Inhalational anesthetics]

Inhalational anesthetics have been used for induction and maintenance of general anesthesia for more than 150 years. All of the currently used inhalational anesthetics are chlorinated and fluorinated derivatives of ether. Dosing is carried out using the minimal alveolar concentration (MAC) concept. The pharmacokinetic properties of the various inhalational anesthetics are governed by the specific distribution coefficients. Mechanisms of action include specific modulations of various receptors of the central nervous system as well as an unspecific interaction with the cell membrane. Organ toxicity of modern inhalational anesthetics is considered to be minimal. The role of inhalational anesthetics in the context of postoperative nausea and vomiting (PONV) has been reassessed in recent years. The superiority of inhalational anesthetics over intravenous hypnotics with respect to intraoperative awareness is undisputed. The organ protective mechanism of preconditioning is an exclusive property of inhalational anesthetics among all the currently available hypnotics.

MAC-sparing effect of nitrous oxide in sevoflurane anesthetized sheep and its reversal with systemic atipamezole administration

INTRODUCTION

Nitrous oxide (N2O) is an anesthetic gas with antinociceptive properties and reduces the minimum alveolar concentration (MAC) for volatile anesthetic agents, potentially through mechanisms involving central alpha2-adrenoceptors. We hypothesized that 70% N2O in the inspired gas will significantly reduce the MAC of sevoflurane (MACSEVO) in sheep, and that this effect can be reversed by systemic atipamezole.

MATERIALS AND METHODS

Animals were initially anesthetized with SEVO in oxygen (O2) and exposed to an electrical current as supramaximal noxious stimulus in order to determine MACSEVO (in duplicates). Thereafter, 70% N2O was added to the inspired gas and the MAC re-determined in the presence of N2O (MACSN). A subgroup of sheep were anesthetized a second time with SEVO/N2O for re-determination of MACSN, after which atipamezole (0.2 mg kg-1, IV) was administered for MACSNA determinations. Sheep were anesthetized a third time, initially with only SEVO/O2 to re-determine MACSEVO, after which atipamezole (0.2 mg kg-1, IV) was administered for determination of MACSA.

RESULTS

MACSEVO was 2.7 (0.3)% [mean (standard deviation)]. Addition of N2O resulted in a 37% reduction of MACSEVO to MACSN of 1.7 (0.2)% (p <0.0001). Atipamezole reversed this effect, producing a MACSNA of 3.1 (0.7)%, which did not differ from MACSEVO (p = 0.12). MACSEVO did not differ from MACSA (p = 0.69). Cardiorespiratory variables were not different among experimental groups except a lower ETCO2 in animals exposed to SEVO/N2O.

CONCLUSIONS

N2O produces significant MACSEVO-reduction in sheep; this effect is completely reversed by IV atipamezole confirming the involvement of alpha2-adrenoreceptors in the MAC-sparing action of N2O.

Sequential allocation trial design in anesthesia: an introduction to methods, modeling, and clinical applications

Estimation of the dose-response curve for new anesthetic protocols typically focuses on identifying minimum effective doses. The application of a sequential experimental method is appropriate, as it minimizes sample size requirements by updating dose assignments based on information accrued from successive subjects. One approach is the up-and-down method for estimating the median effective dose in a patient population (ED₅₀ ). Designs better suited for achieving greater than 50% effectiveness, include the biased coin approach, and continual reassessment method. In this review we introduce different sequential design methods, provide examples of their use, and show through simulation how the method employed influences sample size and the accuracy of the estimated dose. Simulation studies are presented to illustrate the effects of dose parameter and stopping rule choice for up-and-down method and biased coin approach. For continual reassessment method, the effects of assumed dose-response model, prior guess, and cohort size are simulated. A binary response regression curve was fit to the data in Saidman and Eger's endtidal halothane dose-finding study to provide a dose-response curve for generating simulations. A range of options exist when designing a study using sequential allocation with biased coin approach or continual reassessment method. Method choice influences the required sample size and confidence in estimated effect. In the halothane example, up-and-down method decreases the required sample size by 20-30% when the choice of design parameters is optimal. For both up-and-down method and biased coin approach designs, greater sample sizes, arising from adjusted stopping criteria, might be required to achieve reliable estimates. The continual reassessment method is only efficient if a limited range of doses can be chosen a priori. In conclusion the up-and-down method can be more efficient than nonsequential designs for the estimation of the median dose/intervention level for a given intervention (ED₅₀ ). The biased coin approach or continual reassessment method are preferred for the estimation of higher or lower tail quantiles such as ED₉₀ or ED₁₀ . Continual reassessment method may be superior if knowledge of the dose-response relationship is available for the drug of interest.

Changes in the Bispectral Index in Response to Loss of Consciousness and No Somatic Movement to Nociceptive Stimuli in Elderly Patients

Background:

Bispectral index (BIS) is considered very useful to guide anesthesia care in elderly patients, but its use is controversial for the evaluation of the adequacy of analgesia. This study compared the BIS changes in response to loss of consciousness (LOC) and loss of somatic response (LOS) to nociceptive stimuli between elderly and young patients receiving intravenous target-controlled infusion (TCI) of propofol and remifentanil.

Methods:

This study was performed on 52 elderly patients (aged 65–78 years) and 52 young patients (aged 25–58 years), American Society of Anesthesiologists physical status I or II. Anesthesia was induced with propofol administered by TCI. A standardized noxious electrical stimulus (transcutaneous electrical nerve stimulation, [TENS]) was applied (50 Hz, 80 mA, 0.25 ms pulses for 4 s) to the ulnar nerve at increasing remifentanil predicted effective-site concentration (Ce) until patients lost somatic response to TENS. Changes in awake, prestimulus, poststimulus BIS, heart rate, mean arterial pressure, pulse oxygen saturation, predicted plasma concentration, Ce of propofol, and remifentanil at both LOC and LOS clinical points were investigated.

Results:

BIS_LOC in elderly group was higher than that in young patient group (65.4 ± 9.7 vs. 57.6 ± 12.3) (t = 21.58, P < 0.0001) after TCI propofol, and the propofol Ce at LOC was 1.6 ± 0.3 μg/ml in elderly patients, which was significantly lower than that in young patients (2.3 ± 0.5 μg/ml) (t = 7.474, P < 0.0001). As nociceptive stimulation induced BIS to increase, the mean of BIS maximum values after TENS was significantly higher than that before TENS in both age groups (t = 8.902 and t = 8.019, P < 0.0001). With increasing Ce of remifentanil until patients lost somatic response to TENS, BIS_LOS was the same as the BIS_LOC in elderly patients (65.6 ± 10.7 vs. 65.4 ± 9.7), and there were no marked differences between elderly and young patient groups in BIS_awake, BIS_LOS, and Ce of remifentanil required for LOS.

Conclusion:

In elderly patients, BIS can be used as an indicator for hypnotic-analgesic balance and be helpful to guide the optimal administration of propofol and remifentanil individually.

Trial Registration:

CTRI Reg. No: ChiCTR-OOC-14005629; http://www.chictr.org.cn/showproj.aspx?proj=9875.

Comparison of C₅₀ for Propofol-remifentanil Target-controlled Infusion and Bispectral Index at Loss of Consciousness and Response to Painful Stimulus in Elderly and Young Patients

BACKGROUND

In this prospective randomized study, we compared the predicted blood and effect-site C 50 for propofol and remifentanil target-controlled infusion (TCI) and the bispectral index (BIS) values at loss of consciousness (LOC) and response to a standard noxious painful stimulus (LOS) in elderly and young patients, respectively. We hypothesized that the elderly patients will require lower target concentration of both propofol and remifentanil at above two clinical end-points.

METHODS

There were 80 American Society of Anesthesiologists (ASA) physical status I-II unpremedicated patients enrolled in this study, they were divided into elderly group (age ≥65 years, n = 40) and young group (aged 18-64 years, n = 40). Propofol was initially given to a predicted blood concentration of 1.2 μg/ml and thereafter increased by 0.3 μg/ml every 30 s until Observer's Assessment of Alertness and Sedation score was 1. The propofol level was kept constant, and remifentanil was given to provide a predict blood concentration of 2.0 ng/ml, and then increased by 0.3 ng/ml every 30 s until loss of response to a tetanic stimulus. BIS (version 3.22, BIS Quattro sensor) was also recorded.

RESULTS

In elderly group, the propofol effect-site C 50 at LOC of was 1.5 (1.4-1.6) μg/ml, was significantly lower than that of young group, which was 2.2 (2.1-2.3) μg/ml, the remifentanil effect-site C 50 at LOS was 3.5 (3.3-3.7) ng/ml in elderly patients, was similar with 3.7 (3.6-3.8) ng/ml in young patients. Fifty percent of patients lost consciousness at a BIS value of 57.3 (56.4-58.1), was similar with that of young group, which was 55.2 (54.0-56.3).

CONCLUSION

In elderly patients, the predicted blood and effect-site concentrations of propofol at LOC were lower than that of young patients. At same sedation status, predicted blood and effect-site concentrations of remifentanil required at LOS were similar in elderly and young patients. BIS were not affected by age. Low-propofol/high-opioid may be optional TCI strategy for elderly patients.

Isoflurane MAC determination in dogs using three intensities of constant-current electrical stimulation

OBJECTIVES

To compare isoflurane minimum alveolar concentrations (MACs) in dogs determined using three intensities of constant-current electrical stimulation applied at the tail, and thoracic and pelvic limbs, and to compare isoflurane MACs obtained with all combinations of electrical stimulation and anatomic site with those obtained using the tail clamp as the noxious stimulus.

STUDY DESIGN

Randomized trial.

ANIMALS

Six mixed-breed, adult female dogs aged 1-2 years and weighing 11.1 ± 4.4 kg.

METHODS

In each dog, MAC was determined by the bracketing method with the tail clamp (MACTAILCLAMP ), and three electrical currents (10 mA, 30 mA, 50 mA) at three anatomic sites (thoracic limb, pelvic limb, tail). Each MAC achieved with electrical stimulation was compared with MACTAILCLAMP using a mixed-model anova and Dunnett's procedure for multiple comparisons. The effects of current intensity and anatomic site on isoflurane MAC were tested using a mixed-model anova followed by Tukey's test for multiple comparisons (p < 0.05).

RESULTS

Mean MACTAILCLAMP was 1.69%. MACs achieved with currents of 30 mA and 50 mA did not differ independently of anatomic site. When currents of 10 mA were applied to the tail and thoracic limb, resulting MACs were lower than those obtained using currents of 30 mA and 50 mA. Currents of 30 mA and 50 mA provided MACs that did not differ from those of MACTAILCLAMP , whereas a current of 10 mA achieved the same result only for the pelvic limb.

CONCLUSIONS AND CLINICAL RELEVANCE

Isoflurane MAC is affected by current intensity and anatomic site. Current intensities of 30 mA and 50 mA provided consistent results when applied to the tail, and thoracic and pelvic limbs that did not differ from those obtained using the tail clamp. Consequently, they can be used in place of the tail clamp in MAC studies in dogs.

Is a new paradigm needed to explain how inhaled anesthetics produce immobility?

A paradox arises from present information concerning the mechanism(s) by which inhaled anesthetics produce immobility in the face of noxious stimulation. Several findings, such as additivity, suggest a common site at which inhaled anesthetics act to produce immobility. However, two decades of focused investigation have not identified a ligand- or voltage-gated channel that alone is sufficient to mediate immobility. Indeed, most putative targets provide minimal or no mediation. For example, opioid, 5-HT3, gamma-aminobutyric acid type A and glutamate receptors, and potassium and calcium channels appear to be irrelevant or play only minor roles. Furthermore, no combination of actions on ligand- or voltage-gated channels seems sufficient. A few plausible targets (e.g., sodium channels) merit further study, but there remains the possibility that immobilization results from a nonspecific mechanism.

Inhaled anesthetics do not combine to produce synergistic effects regarding minimum alveolar anesthetic concentration in rats

BACKGROUND

We hypothesized that pairs of inhaled anesthetics having divergent potencies [one acting weakly at minimum alveolar anesthetic concentration (MAC); one acting strongly at MAC] on specific receptors/channels might act synergistically, and that such deviations from additivity would support the notion that anesthetics act on multiple sites to produce anesthesia.

METHODS

Accordingly, we studied the additivity of MAC for 11 anesthetic pairs divergently (one weakly, one strongly) affecting a specific receptor/channel at MAC. By "divergently," we usually meant that at MAC the more strongly acting anesthetic enhanced or blocked the in vitro receptor or channel at least twice (and usually more) as much as did the weakly acting anesthetic. The receptors/channels included: TREK-1 and TASK-3 potassium channels; and gamma-aminobutyric acid type A, glycine, N-methyl-D-aspartic acid, and acetylcholine receptors. We also studied the additivity of cyclopropane-benzene because the N-methyl-D-aspartic acid blocker MK-801 had divergent effects on the MACs of these anesthetics. We also studied four pairs that included nitrous oxide because nitrous oxide had been reported to produce infraadditivity (antagonism) when combined with isoflurane.

RESULTS

All combinations produced a result within 10% of that which would be predicted by additivity except for the combination of isoflurane with nitrous oxide where infraadditivity was found.

CONCLUSIONS

Such results are consistent with the notion that inhaled anesthetics act on a single site to produce immobility in the face of noxious stimulation.

Pharmakokinetische/pharmakodynamische Modelle für Inhalationsanästhetika

Pharmacokinetic models can be differentiated into two groups: physiological-based models and empirical models. Traditionally the pharmacokinetics of volatile anaesthetics are described using physiological-based models together with the respective tissue-blood distribution coefficients. The compartments of the empirical model have no anatomical equivalents and are merely the product of the mathematical procedure for parameter estimation. The end expiratory concentration of volatile anaesthetics is approximately equal to the arterial concentration and, therefore, the description of the transition between plasma and effect site for volatile anaesthetics plays a central role. The most important parameter here is the k(e0) value which is a time constant and describes the time delay for the transition from the central compartment to the calculated effect compartment. The k(e0) values for sevoflurane and isoflurane are the same but the concentration balance between the end-tidal concentration and the effect compartment occurs twice as quickly with desflurane. In clinical practice volatile anaesthetics are normally combined with N(2)O and/or opioids. This results in an additive interaction between volatile anaesthetics and N(2)O but a synergistic interaction of volatile anaesthetics with opioids. However, there are relatively few investigations on the interactions between the clinically widely used combination of volatile anaesthetics, N(2)O and opioids.

Minimum alveolar concentration of desflurane in llamas and alpacas

OBJECTIVE

To determine the minimum alveolar concentration (MAC) of desflurane in llamas and alpacas.

DESIGN

Prospective study. Animals Six healthy adult llamas and six healthy adult alpacas.

PROCEDURE

Anesthesia was induced with desflurane delivered with oxygen through a mask. An endotracheal tube was inserted, and a port for continuous measurement of end-tidal and inspired desflurane concentrations was placed between the endotracheal tube and the breathing circuit. After equilibration at an end-tidal-to-inspired desflurane concentration ratio >0.90 for 15 minutes, a 50-Hz, 80-mA electrical stimulus was applied to the antebrachium until a response was obtained (i.e. gross purposeful movement) or for up to 1 minute. The vaporizer setting was increased or decreased to effect a 10-20% change in end-tidal desflurane concentration, and equilibration and stimulus were repeated. The MAC was defined as the average of the lowest end-tidal desflurane concentration that prevented a positive response and the highest concentration that allowed a positive response.

RESULTS

Mean +/- SD MAC of desflurane was 7.99 +/- 0.58% in llamas and 7.83 +/- 0.51% in alpacas.

CONCLUSIONS AND CLINICAL RELEVANCE

The MAC of desflurane in llamas and alpacas was in the range of that reported for other species.

The effect of addition of nitrous oxide to a sevoflurane anesthetic on BIS, PSI, and entropy

OBJECTIVE

N(2)O is a commonly used anesthetic that has amnestic and analgesic properties. Recently, devices that estimate depth of consciousness have been introduced in an attempt to better titrate anesthesia, however the effect of N(2)O on these monitors is unclear.

METHODS

General anesthesia was induced and titrated to maintain normal blood pressure and pulse in healthy adults. Data were collected in three 10 minute intervals (Sevo, Sevo + N(2)O, Sevo). In Phase A, sevoflurane concentration was held constant during the N(2)O trial in 60 subjects monitored with either BIS, PSI, or Entropy. In Phase B, sevoflurane concentration was reduced as N(2)O was added, maintaining a constant overall "MAC" in 20 subjects monitored concurrently with BIS and Entropy. Sample size for both phases was designed to detect a 10 unit change in measure of processed EEG with alpha = .05 and statistical power = .80.

RESULTS

In Phase A, supplementing sevoflurane with > 65% N(2)O increased MAC from 1.3 +/- 0.05 to 2.2 +/- 0.10, but did not significantly alter BIS nor PSI (p-value for differential MAC is < 0.05). Entropy, however, dropped significantly, with a change in state entropy (SE) from 31.1 +/- 7.3 to 18.9 +/- 3.7 and a corresponding rise when N(2)O was discontinued. In Phase B, supplementing sevoflurane with > 65% N(2)O with a concomitant reduction in sevoflurane resulted in an increase in both BIS (from 34 +/- 5 to 53.9 +/- 11.5) and SE (from 32 +/- 8.2 to 55.4 +/- 21.3).

CONCLUSION

Supplementing sevoflurane with > 65% N(2)O did not result in a significant change in either BIS or PSI when sevoflurane concentration was kept constant. Entropy, however, significantly decreased as anesthetic depth increased. When sevoflurane concentration was reduced during N(2)O administration, both BIS and Entropy rose despite maintenance of anesthetic depth, indicating a variable concentration effect between volatiles and N(2)O.

Nitrous oxide induces paradoxical electroencephalographic changes after tracheal intubation during isoflurane and sevoflurane anesthesia

In this randomized, double-blind, controlled study, we tested the hypothesis that nitrous oxide (N2O) affects bispectral index (BIS) and 95% spectral edge frequency (SEF95) in response to tracheal intubation during anesthesia with isoflurane and sevoflurane. In protocol 1, we randomly allocated 90 ASA physical status I patients to 6 groups (n = 15 each). Anesthesia was induced with isoflurane or sevoflurane with 0%, 33%, or 66% N2O. The concentration of isoflurane and sevoflurane was gradually increased and end-tidal concentrations were maintained at 1.1% and 1.7%, respectively. Tracheal intubation was performed 12 min after induction of anesthesia. BIS was significantly increased 1 min after tracheal intubation compared before laryngoscopy in patients receiving only isoflurane or sevoflurane (P = 0.001 and 0.007, respectively). In patients receiving 66% N2O-isoflurane or 66% N2O-sevoflurane, both BIS and SEF95 were significantly decreased after tracheal intubation and significantly lower than in those patients receiving only isoflurane or sevoflurane, respectively (P < 0.01 for both). In protocol 2, 3 microg/kg of IV fentanyl completely abolished the decrease of BIS and SEF95 after tracheal intubation during anesthesia with 66% N2O-isoflurane and 66% N2O-sevoflurane (n = 10). We conclude that 66% N2O induced a paradoxical decrease of BIS in response to tracheal intubation during anesthesia with isoflurane and sevoflurane.

Anesthetic potency of sevoflurane with and without nitrous oxide in mechanically ventilated Dumeril monitors

OBJECTIVE

To determine the minimum alveolar concentration (MAC) of sevoflurane and assess the sevoflurane-sparing effect of coadministration of nitrous oxide in mechanically ventilated Dumeril monitors (Varanus dumerili).

DESIGN

Prospective crossover study.

ANIMALS

10 healthy adult Dumeril monitors.

PROCEDURE

Anesthesia was induced with sevoflurane in 100% oxygen or sevoflurane in 66% nitrous oxide (N2O) with 34% oxygen, delivered through a face mask. Monitors were endotracheally intubated, and end-tidal and inspired isoflurane concentrations were measured continuously; MAC was determined by use of a standard bracketing technique. An electrical stimulus (50 Hz, 50 V) was delivered to the ventral aspect of the tail as the supramaximal stimulus. A blood sample for blood gas analyses was collected from the ventral coccygeal vessels at the beginning and end of the anesthetic period. An interval of at least 7 days was allowed to elapse between treatments.

RESULTS

The MAC +/- SDs of sevoflurane in oxygen and with N2O were 2.51 +/- 0.46% and 1.83 +/- 0.33%, respectively. There was a significant difference between the 2 treatments, and the mean MAC-reducing effect of N2O was 26.4 +/- 11.4%. Assuming simple linear additivity of sevoflurane and N2O, the MAC for N2O was estimated to be 244%. No significant differences in blood gas values--with the predictable exception of oxygen pressure--were detected between the 2 groups.

CONCLUSIONS AND CLINICAL RELEVANCE

The MAC of sevoflurane in Dumeril monitors is similar to that reported for other species. The addition of N2O significantly decreased the MAC of sevoflurane in this species.

INHALATION ANESTHESIA IN DUMERIL'S MONITOR (VARANUS DUMERILI) WITH ISOFLURANE, SEVOFLURANE, AND NITROUS OXIDE: EFFECTS OF INSPIRED GASES ON INDUCTION AND RECOVERY

Induction and recovery from inhalation anesthesia of Dumeril's monitors (Varanus dumerili) using isoflurane, sevoflurane, and nitrous oxide (N2O) were characterized using a randomized crossover design. Mean times to induction for isoflurane in 100% oxygen (O2), sevoflurane in 100% O2, sevoflurane in 21% O2:79% nitrogen (N2; room air), and sevoflurane in 66% N2O:34% O2 were 13.00 +/- 4.55, 11.20 +/- 3.77, 10.40 +/- 2.50, and 9.40 +/- 2.80 min, respectively, at 26 degrees C (n = 10). Mask induction with sevoflurane was significantly faster than with isoflurane. There was no significant difference between the induction time for sevoflurane in O2 or in room air, but sevoflurane combined with N2O resulted in significantly faster inductions than were obtained with sevoflurane in 100% O2. All treatments resulted in a significantly higher respiratory rate than in undisturbed animals. There were no significant differences in respiratory rate among lizards receiving O2, isoflurane in 100% O2, sevoflurane in room air, and sevoflurane combined with N2O, but animals receiving sevoflurane in O2 had a lower respiratory rate than those receiving pure O2. The sequence of complete muscle relaxation during induction was consistent and not significantly different among the four treatments: front limbs lost tone first, followed by the neck and the hind limbs; then the righting reflex was lost and finally tail tone. There were no significant differences in recovery times between isoflurane and sevoflurane or between sevoflurane in 100% O2 and sevoflurane combined with N2O. Similar recovery times were observed in animals recovering in 100 and 21% O2.

Sevoflurane decreases bispectral index values more than does halothane at equal MAC multiples

At the minimum alveolar concentration (MAC) of inhaled anesthetics, 50% of subjects move in response to noxious stimulation. Similarly, at MAC-awake, 50% of subjects respond appropriately to command. The bispectral index (BIS) nominally measures the effect of anesthetics on wakefulness or consciousness. We postulated that the use of halothane with a larger MAC-awake/MAC ratio than sevoflurane would produce higher BIS values at comparable levels of MAC. We studied 33 unpremedicated patients anesthetized by inhalation, 18 with sevoflurane and 15 with halothane. We measured BIS before and during anesthesia at 1 MAC, both before and after tracheal intubation facilitated by fentanyl and rocuronium and then at 1.5 MAC. BIS measurements were made after meeting steady-state conditions. No surgery was performed during this study. BIS values in awake patients did not differ between the sevoflurane and halothane groups (96 +/- 2 and 96 +/- 2, mean +/- sd, respectively). At 1 MAC without and with neuromuscular blockade and at 1.5 MAC, BIS values for patients anesthetized with halothane (54 +/- 7, 56 +/- 7, and 49 +/- 7, respectively) exceeded those for patients anesthetized with sevoflurane (34 +/- 6, 34 +/- 6, and 29 +/- 5, respectively) (P < 0.0001). This finding adds to other evidence indicating that BIS is drug specific.

An animal model for surgical anesthesia and analgesia: characterization with isoflurane anesthesia and remifentanil analgesia

With a traditional clamp test alone, quantitative evaluation of the level of surgical anesthesia/analgesia is not easy. We have developed a rabbit model that allows for repeated quantification of the varying level of surgical anesthesia/analgesia using both mechanical and electrical stimulation as simulated surgical stimuli. After tracheostomy and intravascular cannulations under isoflurane anesthesia, eight rabbits were placed on a sling that allowed for free movement of the head and extremities. The inspired isoflurane concentration was reduced from 3% to 1.5% and then to 0%. Remifentanil was then infused at 4 graded infusion rates (0.1-0.8 microg. kg(-1) x min(-1)). At each drug dose, analgesic variables were determined including the number of animals behaviorally unresponsive to clamping the forepaw (nonresponders) and threshold voltage of subcutaneous electrical stimulation (2 Hz, 5 Hz, and 50 Hz) required to evoke the head lift (HLT, pain detection/arousal threshold) and escape movement responses (EMT, pain tolerance threshold). With increasing drug doses, HLTs and EMTs at 5 Hz increased dose-dependently and most proportionately to increases in the number of nonresponders, a standard indicator of the anesthetic/analgesic level. Therefore, using the HLT and EMT at 5 Hz combined with a clamp test, this rabbit model allows for quantitative evaluation of the varying level of surgical anesthesia/analgesia.

IMPLICATIONS

We have developed a rabbit model of surgical anesthesia and analgesia using both mechanical and electrical stimulation as simulated surgical stimuli, which allows for repeated, quantitative, and qualitative evaluation of the varying level of surgical anesthesia and analgesia, differentiation between sedative/hypnotic and analgesic components of drug actions, and simultaneous monitoring of all the clinically relevant physiological variables including cardiovascular and respiratory variables.

Detection of Acute Tolerance to the Analgesic and Nonanalgesic Effects of Remifentanil Infusion in a Rabbit Model

Although acute tolerance to analgesia develops rapidly with remifentanil, it is unknown whether acute tolerance also develops to its nonanalgesic effects. We investigated the analgesic and cardiorespiratory effects of remifentanil during a continuous infusion in a rabbit model. Ten tracheotomized New Zealand White rabbits with arterial and venous accesses were placed on a sling that allowed for reasonably free movement. In spontaneously breathing conscious animals, remifentanil was infused IV at a constant-rate of 0.3 microg kg(-1)x min(-1) for 360 min. Sedative/analgesic and cardiorespiratory variables were assessed repeatedly during remifentanil infusion, including the number of animals behaviorally unresponsive to clamping the forepaw (nonresponders) and subcutaneous electrical stimulation thresholds required to elicit head lift (HLT: pain detection/arousal threshold) and escape movement responses (EMT: pain tolerance threshold). Within 60-120 min of starting the infusion, the number of nonresponders, HLT, EMT, and PaCO(2) increased significantly, whereas blood pressure, heart rate, and respiratory rate decreased. Thereafter, all variables returned towards preinfusion levels despite continuing infusion. These results indicate that during a remifentanil infusion acute tolerance develops for both its analgesic and cardiorespiratory effects.

IMPLICATIONS

Using a new rabbit model, we found that during continuous, constant-rate remifentanil infusion acute tolerance developed within the first few hours, not only to its analgesic but also to its cardiovascular and respiratory effects, albeit in slightly different time courses.

Minimum alveolar concentration of sevoflurane in spontaneously breathing llamas and alpacas

OBJECTIVE

To determine the minimum alveolar concentration (MAC) of sevoflurane in spontaneously breathing llamas and alpacas.

DESIGN

Prospective study.

ANIMALS

6 healthy adult llamas and 6 healthy adult alpacas.

PROCEDURE

Anesthesia was induced with sevoflurane delivered with oxygen through a mask. An endotracheal tube was inserted, and a port for continuous measurement of end-tidal and inspired sevoflurane concentrations was placed between the endotracheal tube and the breathing circuit. After equilibration at an end-tidal-to-inspired sevoflurane concentration ratio > 0.90 for 15 minutes, a 50-Hz, 80-mA electrical stimulus was applied to the antebrachium until a response was obtained (ie, gross purposeful movement) or for up to 1 minute. The vaporizer setting was increased or decreased to effect a 10 to 20% change in end-tidal sevoflurane concentration, and equilibration and stimulus were repeated. The MAC was defined as the mean of the lowest end-tidal sevoflurane concentration that prevented a positive response and the highest concentration that allowed a positive response.

RESULTS

Mean +/- SD MAC of sevoflurane was 2.29 +/- 0.14% in llamas and 2.33 +/- 0.09% in alpacas.

CONCLUSIONS AND CLINICAL RELEVANCE

The MAC of sevoflurane in llamas and alpacas was similar to that reported for other species.

Validation of several types of noxious stimuli for use in determining the minimum alveolar concentration for inhalation anesthetics in dogs and rabbits

OBJECTIVE

To compare 3 types of noxious stimuli applied to various anatomic areas of anesthetized dogs and rabbits for determination of the minimum alveolar concentration (MAC).

ANIMALS

10 dogs and 10 rabbits.

PROCEDURE

Dogs were anesthetized with isoflurane and halothane in a randomized order. Rabbits were anesthetized with isoflurane. The MAC was determined by skin incision on the lateral aspect of the chest; clamping of the tail, paw of the forelimb, and paw of the hind limb; and application of electrical current to the oral mucosa (dogs only), forelimb, and hind limb. The MAC was the end-tidal concentration midway between the value permitting and preventing purposeful movement in response to noxious stimuli.

RESULTS

In dogs, mean +/- SEM MAC for isoflurane was 1.27 +/- 0.05% for clamping stimuli, 1.36 +/- 0.04% for oral electrical stimulation, 1.35 +/- 0.04% for electrical stimulation to the limbs, and 1.01 +/- 0.07% for surgical incision. The MAC for halothane was 0.97 +/- 0.03% for tail clamping, 0.96 +/- 0.03% for clamping of the limbs, 1.04 +/- 0.03% for electrical stimulation, and 0.75 +/- 0.06% for surgical incision. In rabbits, MAC for isoflurane was 2.08 +/- 0.02% for clamping stimuli, 2.04 +/- 0.02% for electrical stimulation, and 0.90 +/- 0.02% for surgical incision. The MAC for surgical incision was significantly lower than values for the other methods in both species.

CONCLUSIONS AND CLINICAL RELEVANCE

Use of electrical current and clamping techniques resulted in similar MAC values. Surgical incision underestimated MAC values in dogs and rabbits.

Interactions of morphine and isoflurane in horses

OBJECTIVE

To quantitate dose- and time-related magnitudes of interactive effects of morphine (MOR) and isoflurane (ISO) in horses and to characterize pharmacokinetics of MOR in plasma and the ventilatory response to MOR during administration of ISO.

ANIMALS

6 adult horses.

PROCEDURE

Horses were anesthetized 3 times to determine the minimum alveolar concentration (MAC) of ISO in O2 and then to characterize the change in anesthetic requirement as defined by the alteration in ISO MAC following IV administration of saline (0.9% NaCl) solution and 2 doses of MOR (low dose, 0.25 mg/kg; high dose, 2.0 mg/kg). Arterial blood samples were obtained before and after MOR and analyzed.

RESULTS

Mean +/- SD baseline ISO MAC was 1.43 +/- 0.06%. The ISO MAC did not change with time after administration of saline solution. Effects of MOR on ISO MAC varied. Maximal change in MAC ranged from -20.2 to +28.3% and -18.9 to +56.2% after low and high doses of MOR, respectively. Typical half-life of MOR in plasma was 40 to 60 minutes and related to dose. Mean PaCO2 increased from 70 mm Hg before MOR to 88 to 102 mm Hg for 30 to 240 minutes after the high dose of MOR. Recovery from anesthesia after administration of the high dose of MOR was considered undesirable and dangerous.

CONCLUSIONS AND CLINICAL RELEVANCE

Our results do not support routine clinical use of MOR administered IV at dosages of 0.25 or 2.0 mg/kg as an adjuvant to anesthesia in horses administered ISO.

Propofol effective concentration 50 and its relationship to bispectral index

Sixty unpremedicated healthy adult patients were studied during induction of anaesthesia with intravenous propofol delivered by a 'Diprifusor' target-controlled infusion. Bispectral index (BIS) and spectral edge frequency (SEF95) were measured concurrently with the predicted blood and effect site propofol concentrations. Logistic regression was used to calculate the predicted propofol blood and effect site concentrations required to produce unconsciousness and no response to a noxious stimulus in 50% and 95% of patients and to correlate BIS with these end-points. The Diprifusor TCI software produces anaesthesia at consistent target concentrations. Bispectral index correlates well with clinical end-points and may be useful during propofol anaesthesia.

Age, minimum alveolar anesthetic concentration, and minimum alveolar anesthetic concentration-awake

Two defining effects of inhaled anesthetics (immobility in the face of noxious stimulation, and absence of memory) correlate with the end-tidal concentrations of the anesthetics. Such defining effects are characterized as MAC (the concentration producing immobility in 50% of patients subjected to a noxious stimulus) and MAC-Awake (the concentration suppressing appropriate response to command in 50% of patients; memory is usually lost at MAC-Awake). If the concentrations are monitored and corrected for the effects of age and temperature, the concentrations may be displayed as multiples of MAC for a standard age, usually 40 yr. This article provides an algorithm that might be used to produce such a display, including provision of an estimate of the effect of nitrous oxide.

IMPLICATIONS

Two defining effects of inhaled anesthetics (immobility in the face of noxious stimulation, and absence of memory) correlate with the end-tidal concentrations of the anesthetics. Thus, these defining effects may be monitored and the results displayed if the concentrations are known and corrected for the effects of age and temperature.

Nitrous oxide prevents movement during orotracheal intubation without affecting BIS value

We sought to determine whether the addition of nitrous oxide (N(2)O) to an anesthetic with propofol and remifentanil modifies the bispectral index (BIS) during the induction of anesthesia and orotracheal intubation. Thirty ASA physical status I or II patients were randomly allocated to receive either 50% air in oxygen (control group) or 60%-70% N(2)O in oxygen (N(2)O group) that was commenced via a mask simultaneously with the induction of anesthesia. Anesthesia was performed in all the patients with IV propofol at the target effect compartment site concentration of 4 microg/mL throughout the study. A target-controlled infusion (TCI) of remifentanil was initiated 3 min after the TCI of propofol and maintained at the effect-site concentration of 4 ng/mL until the end of the study. After loss of consciousness, and before the administration of vecuronium 0.1 mg/kg, a tourniquet was applied to one arm and inflated to a value more than the systolic blood pressure. An examiner, blinded to the presence of N(2)O, sought to detect any gross movement within the first minute after tracheal intubation, which was performed 10 min after remifentanil TCI began. Inspired and expired oxygen, N(2)O, and carbon dioxide were continuously monitored. A BIS value was generated every 10 s. Arterial blood pressure and heart rate (HR) were measured noninvasively every minute. Measures of mean arterial pressure (MAP), HR, and BIS were obtained before the induction, before the start of the remifentanil TCI, before laryngoscopy, and 5 min after intubation. No significant intergroup differences were seen in BIS, HR, and MAP throughout the study. Maximum changes in BIS, HR, and MAP with intubation were significant (P < 0.01) for both groups but comparable. Six patients in the control group and none in the N(2)O group moved after intubation (P < 0.05).

IMPLICATIONS

We demonstrated that 0.6 minimal alveolar concentration of nitrous oxide combined with a potent anesthetic and an opioid prevents movement after orotracheal intubation without affecting the bispectral index. This demonstrates that the bispectral index is not a useful neurophysiologic variable to monitor the level of anesthesia when nitrous oxide is added to a general anesthetic regimen using propofol and remifentanil.

Clinical and economic factors important to anaesthetic choice for day-case surgery

Clinical and economic factors that are important to consider when selecting anaesthesia for day-case surgery can differ from those for inpatient anaesthesia. Patients undergoing day-case surgery tend to be healthier and have shorter durations of surgery. They expect less anxiety before surgery, amnesia for the surgical experience, a rapid return to normal (normal mentation with minimal pain and nausea) after surgery, and lower expenses. However, the latter 2 expectations can conflict; older generic drugs have lower acquisition costs but often impose longer recovery times. Longer recovery periods can increase costs by prolonging the time to discharge from labour-intensive areas such as the operating suite or the post-anaesthesia recovery unit. The challenge for today's anaesthetist is to use newer drugs judiciously to minimise their expense without compromising the rate or quality of recovery. Several approaches can secure these aims. Most apply the least anaesthetic needed. 'Least anaesthetic' may mean the particular form of anaesthetic (e.g. local infiltration with monitored anaesthesia care versus a general anaesthetic), or may mean the delivery of the smallest effective dose, perhaps guided by anaesthetic monitors such as end-tidal analysers or the bispectral index. For patients requiring general anaesthesia, a combination of several drugs usually secures the closest approach to the ideal. Drug combinations used usually include a short-acting properative anxiolytic (e.g. midazolam), intravenous propofol (a short-acting potent anxiolytic and amnestic agent) for induction of anaesthesia (and sometimes for maintenance) and primary maintenance of anaesthesia with inhaled nitrous oxide combined with a poorly soluble (low solubility produces rapid recovery; the least soluble is desflurane) potent inhaled anaesthetic delivered at a low inflow rate (to minimise cost). Although old, nitrous oxide is inexpensive and has favourable pharmacokinetic and cardiovascular advantages; however, it is limited in its anaesthetic/amnestic potency, and has the capacity to increase nausea. In children, induction of anaesthesia is often accomplished with sevoflurane rather than desflurane; although sevoflurane is modestly more soluble than desflurane, it is non-pungent whereas desflurane is pungent. Moderate- or short-acting opioids (fentanyl is popular) or nonsteroidal anti-inflammatory agents (especially ketorolac), or local anaesthetics are added to secure analgesia during and after surgery. Similarly, when needed, moderate- or short-acting muscle relaxants are selected. Before the end of anaesthesia, an intravenous antiemetic may be given. With this drug combination, patients usually awaken within minutes after anaesthesia and can often move themselves to the vehicle for transport to the recovery unit. These combinations of anaesthetics and techniques minimise use of expensive drugs while expediting recovery (again minimising cost) with minimal or no compromise in the quality of recovery.

A method for overcoming the ceiling effect of bounded pain scales

OBJECTIVE

The Verbal Numerical Scale (VNS) for rating pain is bounded between 0 (= no pain) and 10 (= worst pain imaginable). We hypothesized that the limitations inherent to this boundary when rating extremely painful stimuli may be identified by integrating the VNS with an unbounded score such as magnitude estimation of relative change.

METHODS

Volunteers received stimuli of increasing current via cutaneous electrodes until they rated >5 on the VNS scale. This stimulus, termed S, was arbitrarily assigned a magnitude estimate of 100%. Then, stimuli of varying currents were delivered; two were 10 mA and 20 mA higher than S (S(+10) and S(+20)), two were 1/2 of the current for the S stimulus (S(1/2)), and one was at the original current (Srepeat). The pain elicited by each stimulus was scored in proportion to the S stimulus. The extrapolated VNS score (VNSext) was determined by multiplying this magnitude estimate (%) by the VNS score for S.

MAIN RESULTS

Seventy percent of the stimuli with higher intensity than S generated a VNSext score above 10. The mean magnitude estimations for S(+10) and S(+20) were 186% and 242%: they generated mean (median) VNSext values of 12.4 and 16.2, respectively (p = 0.019 for the difference between them by Wilcoxon signed rank test).

CONCLUSIONS

The combined use of VNS and magnitude estimation confirmed that the ceiling of the bounded pain scale may significantly limit a patient's ability to describe a new pain stimulus. VNSext may provide a means of overcoming this limitation.

The effects of subanesthetic concentrations of sevoflurane and nitrous oxide, alone and in combination, on analgesia, mood, and psychomotor performance in healthy volunteers

We studied the effects of subanesthetic concentrations of sevoflurane and nitrous oxide, alone and in combination, on analgesia, mood, and psychomotor performance in human volunteers. We hypothesized that nitrous oxide and sevoflurane would produce both opposing and potentiating effects within the same study. Over the course of three sessions, 20 subjects inhaled 0%, 0.2%, or 0.4% end-tidal sevoflurane for a 68-min period that was divided into four 17-min blocks. During either the second or fourth block, 30% end-tidal nitrous oxide was added to the concentration of sevoflurane being inhaled. Pain response, psychomotor performance, and mood were evaluated during the second and fourth blocks. Pain ratings were higher when sevoflurane and nitrous oxide were administered together than when nitrous oxide was administered alone, which indicates that sevoflurane attenuated the analgesic effects of nitrous oxide. Sevoflurane increased self-reported ratings of sleepiness, and the addition of nitrous oxide decreased these ratings. Nitrous oxide potentiated psychomotor impairment that was induced by sevoflurane. The combination of sevoflurane and nitrous oxide produced both opposing and potentiating effects within the same study. The results suggest that nitrous oxide and sevoflurane may act through different neurochemical mechanisms on some end points, such as analgesia and sleepiness.

IMPLICATIONS

Healthy volunteers inhaled subanesthetic concentrations of sevoflurane and nitrous oxide. Sevoflurane made nitrous oxide less effective as an analgesic, and nitrous oxide made sevoflurane less effective as a sedative. The two drugs may work at cross purposes on different end points of anesthesia.

Esmolol potentiates reduction of minimum alveolar isoflurane concentration by alfentanil

Esmolol, a short-acting beta1-receptor antagonist, decreases anesthetic requirements during propofol/N2O/morphine anesthesia. This study was designed to determine whether esmolol affects the volatile anesthetic (isoflurane) required to prevent movement to skin incision in 50% patients (minimum alveolar anesthetic concentration [MAC]) with or without an additional opioid (alfentanil). One hundred consenting adult patients were randomly divided into five treatment groups: isoflurane alone (I), I with continuous large-dose (250 microg x kg(-1) x min(-1)) esmolol (E), I with alfentanil (effect site target of 50 ng/mL) via a continuous computer-controlled infusion (A), A plus continuous small-dose (50 microg x kg(-1) x min(-1)) esmolol (A1), or A plus large-dose esmolol (A2). Anesthesia was induced via a face mask, and steady-state target end-tidal isoflurane concentrations were maintained before incision. The MAC of isoflurane alone was 1.28% +/- 0.13%. Large-dose esmolol did not significantly alter the isoflurane MAC (1.23% +/- 0.14%). Alfentanil alone significantly decreased isoflurane MAC by 25% (0.96% +/-0.09%). Adding small-dose esmolol did not further decrease MAC with alfentanil (0.96% +/- 0.13%). However, large-dose esmolol significantly decreased isoflurane MAC with alfentanil (0.74% +/- 0.09%). Esmolol and alfentanil both significantly reduced the increases in heart rate and mean arterial pressure associated with endotracheal intubation and incision. The mechanism of this effect is unknown.

IMPLICATIONS

Most anesthetic techniques rely on a balance of several highly selective medications. The current results define a new anesthetic-sparing effect when volatile anesthetic, analgesic, and beta-adrenergic blocking drugs are combined.

Nonlinear additivity of nitrous oxide and isoflurane potencies in rats

PURPOSE

To test the hypothesis that the MAC values of nitrous oxide (N2O) and isoflurane were not linearly additive, as theorized by the postulated mode of action based on lipid solubility, in a rat model.

METHODS

Eight Long Evans rats were randomly assigned to order of measurement of MAC for isoflurane and N2O alone and in combination using standard 45 sec supramaximal electrical stimulation (50 volts x 10 msec duration pulses at 50.sec-1 applied for 45 sec s.c. to the lower abdominal groin area). The MAC of N2O was measured at hyperbaric compression to 2.25 atmospheres absolute, 1710 mmHg.

RESULTS

The MAC values found were: isoflurane -0.98 +/- 0.12 and N2O - 159 +/- 12 volume (vol)%, or 1.59 +/- 0.12 atmospheres absolute (ATA) (All values are mean +/- standard deviation). The linear additivity theory suggests % MAC agent A + % MAC agent B = 1.0. However, % MAC isoflurane + % MAC N2O = 1.37 +/- 0.15 (P < .001).

CONCLUSION

Nonlinear additivity was demonstrated with direct MAC measurement for both isoflurane and N2O in rats. This suggests an agonist-antagonist relationship.

Influence of analgesic supplementation on the target propofol concentrations for anaesthesia with 'Diprifusor' TCI

Forty healthy patients undergoing orthopaedic surgery were randomly allocated to receive an initial blood propofol target concentration of either 4 micrograms.ml-1 or 6 micrograms.ml-1 for induction of anaesthesia with a 'Diprifusor' target controlled infusion system for propofol, and analgesic supplementation with either nitrous oxide 67% in oxygen or alfentanil 15-20 micrograms.kg-1.h-1. Anaesthesia was induced within 3 min in 80% and 95% of patients with propofol target concentrations of 4 micrograms.ml-1 and 6 micrograms.ml-1, respectively. The frequency of discomfort on infusion was similar for both target concentrations. During maintenance, supplementary doses of alfentanil were required to provide adequate surgical conditions in approximately half of the patients receiving nitrous oxide. There was no statistically significant difference between the target concentration [mean (SD)] of propofol for total intravenous anaesthesia [5.1 (2.0) micrograms.ml-1] compared with a technique using nitrous oxide [4.6 (1.2) micrograms.ml-1] supplemented as needed with small doses of alfentanil.