Minimum Alveolar Concentration Determination Research Articles

Effect of oral administration of gabapentin on the minimum alveolar concentration of isoflurane in cats.

To determine if oral gabapentin decreases the minimum alveolar concentration (MAC) of isoflurane in cats. Prospective, randomized, blinded, crossover, and experimental study. A total of six healthy adult cats (three male, three female) aged 18-42 months, weighing 3.31 ± 0.26 kg. Cats were randomly given oral gabapentin (100 mg cat-1) or placebo 2 h before starting MAC determination, with the crossover treatment given at least 7 days apart. Anesthesia was induced and maintained with isoflurane in oxygen. Isoflurane MAC was determined in duplicate using an iterative bracketing technique and tail clamp method. Hemodynamic and other vital variables were recorded at each stable isoflurane concentration and were compared between gabapentin and placebo treatments at lowest end-tidal isoflurane concentration when cats did not respond to tail clamping. A paired t-test was used to compare normally distributed data, and a Wilcoxon signed-rank test was applied for non-normally distributed data. Significance was set at p < 0.05. Data are mean ± standard deviation. Isoflurane MAC in the gabapentin treatment was 1.02 ± 0.11%, which was significantly lower than that in the placebo treatment (1.49 ± 0.12%; p < 0.001), decreasing by 31.58 ± 6.94%. No significant differences were found in cardiovascular and other vital variables between treatments. Oral administration of gabapentin 2 h before starting MAC determination had a significant isoflurane MAC-sparing effect in cats with no observed hemodynamic benefit.

Phenylpiperidine opioid effects on isoflurane minimum alveolar concentration in cats.

Different structurally related phenylpiperidine opioids exhibit different isoflurane-sparing effects in cats. Because minimum alveolar concentration (MAC) in cats is affected only by very high plasma concentrations of some phenylpiperidine opioids, we hypothesized these effects are caused by actions on nonopioid receptors. Using a prospective, randomized, crossover design, six cats were anesthetized with isoflurane, intubated, ventilated, and instrumented. Isoflurane MAC was measured in triplicate using a tail-clamp and bracketing technique. A computer-controlled intravenous infusion using prior pharmacokinetic models targeted plasma concentrations of 60ng/ml fentanyl, 10ng/ml sufentanil, or 500ng/ml alfentanil, and isoflurane MAC was measured in duplicate. Next, naltrexone 0.6mg/kg was administered to cats hourly during the opioid infusion, and isoflurane MAC was measured in duplicate. Blood was collected during MAC determinations to measure opioid concentrations. Responses were analyzed using repeated measures ANOVA with significance at p<.05. Alfentanil and sufentanil decreased isoflurane MAC by 16.4% and 6.4%, respectively, and these effects were completely reversed by naltrexone. Fentanyl had no significant effect on isoflurane MAC. Alfentanil and sufentanil modestly reduce isoflurane MAC via agonist effects on opioid receptors. However, these effects are too small to justify clinical use of phenylpiperidine opioids as single agents to reduce MAC in cats.

Effects of Standard and Sustained-release Buprenorphine on the Minimum Alveolar Concentration of Isoflurane in C57BL/6 Mice.

Both standard and sustained-release injectable formulations of buprenorphine (Bup and BupSR, respectively) are used as preemptive analgesics, potentially affecting gas anesthetic requirements. This study tested the effects of Bup and BupSR on isoflurane requirements and confirmed that buprenorphine could reduce isoflurane requirements during a laparotomy in mice. We hypothesized that both Bup and BupSR would significantly decrease the required minimum alveolar concentration (MAC) of isoflurane. C57BL/6 mice received either isotonic crystalloid fluid (control), Bup (0.1 mg/kg), or BupSR (1.2 mg/kg) subcutaneously 10 min prior to the induction of anesthesia. Each anesthetized mouse was tested at 2 isoflurane concentrations. A 300-g noxious stimulus was applied at each isoflurane concentration, alternating between hindfeet. In addition, a subset of mice underwent terminal laparotomy or 60 min of anesthesia after injection with Bup, BupSR, or saline to ensure an appropriate surgical plane of anesthesia. Mice were maintained at the lowest isoflurane concentration that resulted in 100% of mice at a surgical plane from the aforementioned MAC experiments (control, 2.0%; Bup and BupSR, 1.7%). Analysis showed that both Bup and BupSR significantly decreased isoflurane requirements by 25.5% and 14.4%, respectively. The isoflurane MAC for the control injection was 1.80% ± 0.09%; whereas Bup and BupSR decreased MAC to 1.34% ± 0.08% and 1.54% ± 0.09%, respectively. Sex was not a significantly different between the injection groups during MAC determination. All of the mice that underwent surgery achieved a surgical plane of anesthesia on the prescribed regimen and recovered normally after discontinuation of isoflurane. Lastly, heart and respiratory rates did not differ between mice that underwent surgery and those that were anesthetized only. Bup and BupSR are MAC-sparing in male and female C57BL/6 mice and can be used for effective multimodal anesthesia.

Determination of minimum alveolar concentration and cardiovascular effects of desflurane in positive-pressure ventilated sheep.

OBJECTIVE To determine the minimum alveolar concentration of desflurane (MACDES) and effects on cardiovascular variables in positive-pressure ventilated sheep. ANIMALS 13 adult female sheep. PROCEDURES Anesthesia was induced with desflurane. After a 30-minute equilibration at an end-tidal concentration of desflurane (etDES) of 10.5%, an electrical stimulus (5 Hz/ms and 50 mA) was applied for 1 minute or until gross purposeful movement occurred. The etDES was then changed by 0.5% (modified up-down method), depending on whether a positive motor response had been elicited, and stimulation was repeated. The MACDES was the etDES midway between a positive and negative response. After MACDES was determined, etDES was increased to 1.3 and 1.6 MACDES. Animals were allowed to equilibrate for 15 minutes, and cardiovascular, blood gas, acid-base, and hematologic variables were measured. Times to induction of anesthesia, extubation, attainment of sternal position, and standing and duration of anesthesia were recorded. RESULTS Mean ± SD MACDES was 9.81 ± 0.79%. Times to intubation, extubation, and standing were 4.81 ± 2.21 minutes, 14.09 ± 4.05 minutes, and 32.4 ± 12.5 minutes, respectively. Duration of anesthesia was 226 ± 22 minutes. Heart rate increased significantly at induction of anesthesia but otherwise remained at preanesthetic rates. Arterial blood pressures progressively decreased with increasing etDES; pressures increased slightly only in response to noxious stimulation. CONCLUSIONS AND CLINICAL RELEVANCE The MACDES determined here compared favorably with that determined for other sheep populations and indicated similar anesthetic potency as in other species. Desflurane caused dose-dependent arterial hypotension, which indicated the need for careful blood pressure monitoring.

Effects of constant rate infusions of dexmedetomidine or MK-467 on the minimum alveolar concentration of sevoflurane in dogs

ObjectiveTo determine the effects of low and high dose infusions of dexmedetomidine and a peripheral α2-adrenoceptor antagonist, MK-467, on sevoflurane minimum alveolar concentration (MAC) in dogs. Study designCrossover experimental study. AnimalsSix healthy, adult Beagle dogs weighing 12.6±0.9 kg (mean±standard deviation). MethodsDogs were anesthetized with sevoflurane in oxygen. After a 60-minute instrumentation and equilibration period, the MAC of sevoflurane was determined in triplicate using the tail clamp technique. PaCO2 and temperature were maintained at 40±5 mmHg (5.3±0.7 kPa) and 38±0.5 ºC, respectively. After baseline MAC determination, dogs were administered two incremental loading and infusion doses of either dexmedetomidine (1.5 μg kg−1 then 1.5 μg kg−1 hour−1 and 4.5 μg kg−1 then 4.5 μg kg−1 hour−1) or MK-467 (90 μg kg−1 then 90 μg kg−1 hour−1 and 180 μg kg−1 then 180 μg kg−1 hour−1); loading doses were administered over 10 minutes. MAC was redetermined in duplicate starting 30 minutes after the start of drug administration at each dose. End-tidal sevoflurane concentrations were corrected for calibration and adjusted to sea level. A repeated-measures analysis was performed and comparisons between doses were conducted using Tukey's method. Statistical significance was considered at p<0.05. ResultsSevoflurane MAC decreased significantly from 1.86±0.3% to 1.04±0.1% and 0.57±0.1% with incremental doses of dexmedetomidine. Sevoflurane MAC significantly increased with high dose MK-467, from 1.93±0.3% to 2.29±0.5%. Conclusions and clinical relevanceDexmedetomidine caused a dose-dependent decrease in sevoflurane MAC, whereas MK-467 caused an increase in MAC at the higher infusion dose. Further studies evaluating the combined effects of dexmedetomidine and MK-467 on MAC and cardiovascular function may elucidate potential benefits of the addition of a peripheral α2-adrenergic antagonist to inhalation anesthesia in dogs.

Determination of minimum alveolar concentration of isoflurane in dogs and cats using the up-and-down method. A preliminary study

Minimum alveolar concentration (MAC) is a reliable measurement of the potency of inhaled anesthetic agents. The determination of MAC in different species has followed a fairly consistent methodology. In people, MAC is determined with the up-and-down method, whereas in animal the bracketing technique is commonly used. The objectives of this study were to determine the MAC value of isoflurane in dogs and cats using the up-and-down method and to determine the MAC value of isoflurane at extubation (MACex). General anesthesia was induced in 13 dogs and 5 cats with 5% isoflurane in oxygen. An initial end-tidal isoflurane concentration of 1.3% was used for the first dog and of 1.6% for the first cat and maintained constant for ≥20min. A noxious stimulus in the form of Carmalt forceps was applied to the base of the tail for no more than one minute or until movement was noticed. After stimulation, the response was recorded as positive (movement) or negative (no movement) and the animal was recovered. The end-tidal isoflurane concentration of the following animal was increased or decreased by 0.1% if the response of the previous animal to the stimulus was negative or positive, respectively. Isoflurane MAC values in dogs were 1.27% and 1.23%. Isoflurane MAC value in cats was 1.58%. MACex value was 0.45% in dogs and in cats. The up-and-down method for MAC determination achieved similar results when compared to MAC values of isoflurane in dogs and cats reported in the literature using the bracketing technique.

Effects of oxymorphone hydrochloride or hydromorphone hydrochloride on minimal alveolar concentration of desflurane in sheep.

To establish the minimum alveolar concentration (MAC) of desflurane and evaluate the effects of 2 opioids on MAC in sheep. 8 adult nulliparous mixed-breed sheep. A randomized crossover design was used. Each sheep was evaluated individually on 2 occasions (to allow assessment of the effects of each of 2 opioids), separated by a minimum of 10 days. On each occasion, sheep were anesthetized with desflurane in 100% oxygen, MAC of desflurane was determined, oxymorphone (0.05 mg/kg) or hydromorphone (0.10 mg/kg) was administered IV, and MAC was redetermined. Physiologic variables and arterial blood gas and electrolyte concentrations were measured at baseline (before MAC determination, with end-tidal desflurane concentration maintained at 10%) and each time MAC was determined. Timing of various stages of anesthesia was recorded for both occasions. Mean ± SEM MAC of desflurane was 8.6 ± 0.2%. Oxymorphone or hydromorphone administration resulted in significantly lower MAC (7.6 ± 0.4% and 7.9 ± 0.2%, respectively). Cardiac output at MAC determination for desflurane alone and for desflurane with opioid administration was higher than that at baseline. No difference was identified among hematologic values at any point. Effects of oxymorphone and hydromorphone on durations of various stages of anesthesia did not differ significantly. MAC of desflurane in nulliparous adult sheep was established. Intravenous administration of oxymorphone or hydromorphone led to a decrease in MAC; however, the clinical importance of that decrease was minor relative to the effect in other species.

Effects of continuous intravenous infusion of morphine and morphine-tramadol on the minimum alveolar concentration of sevoflurane and electroencephalographic entropy indices in dogs

ObjectiveTo compare the effects of continuous rate infusions (CRIs) of intravenous (IV) morphine and morphine-tramadol on the minimum alveolar concentration (MAC) of sevoflurane, and on electroencephalographic entropy indices in dogs. DesignProspective study. AnimalsEight young, healthy German shepherds, weighing 26.3 ± 3.1 kg (mean ± SD). MethodsAnaesthesia was induced and maintained with sevoflurane. A standard tail-clamp technique was used for MAC determination. Within one anaesthetic period, MAC was first determined during sevoflurane anaesthesia alone (MACB); then during morphine infusion (MACM), (loading dose 0.5 mg kg−1IM; CRI, 0.2 mg kg−1hour−1) then finally during morphine-tramadol infusion (tramadol loading dose 1.5 mg kg−1IV; CRI, 2.6 mg kg−1 hour−1) (MACMT). At each change, periods of 45 minutes were allowed for equilibration. Stated entropy (SE), response entropy (RE), and RE-SE differences were measured five minutes prior to and during tail clamping. ResultsThe MACB was 2.1 ± 0.3vol%. The morphine and morphine-tramadol infusions reduced MAC to 1.6 ± 0.3vol% and 1.3 ± 0.3vol%, respectively. MAC was decreased below baseline more during morphine-tramadol than during morphine alone (39 ± 9% versus 25 ± 6%, respectively; p = 0.003). All SE and RE and most RE-SE differences were increased significantly (p < 0.05) over pre-stimulation in all groups when the dogs responded purposefully to noxious stimulation. When no response to noxious stimulation occurred, the entropy indices did not change. Conclusion and clinical relevanceIn dogs, combined morphine-tramadol CRI decreased sevoflurane MAC more than morphine CRI alone. Entropy indices changed during nociceptive responses in anaesthetized animals, suggesting that entropy measurements may be useful in determining anaesthetic depth in dogs.

Effects of fentanyl on isoflurane minimum alveolar concentration in New Zealand White rabbits (Oryctolagus cuniculus).

To determine effects of increasing plasma fentanyl concentrations on the minimum alveolar concentration (MAC) of isoflurane in rabbits. 6 adult female New Zealand White rabbits (Oryctolagus cuniculus). Rabbits were anesthetized with isoflurane in oxygen; ventilation was controlled and body temperature maintained between 38.5° and 39.5°C. Fentanyl was administered IV by use of a computer-controlled infusion system to achieve 6 target plasma concentrations. Isoflurane MAC was determined in duplicate by use of the bracketing technique with a supramaximal electrical stimulus. Blood samples were collected for measurement of plasma fentanyl concentration at each MAC determination. The MAC values were analyzed with a repeated-measures ANOVA followed by Holm-Sidak pairwise comparisons. Mean ± SD plasma fentanyl concentrations were 0 ± 0 ng/mL (baseline), 1.2 ± 0.1 ng/mL, 2.2 ± 0.3 ng/mL, 4.4 ± 0.4 ng/mL, 9.2 ± 0.4 ng/mL, 17.5 ± 2.6 ng/mL, and 36.8 ± 2.4 ng/mL. Corresponding mean values for isoflurane MAC were 1.92 ± 0.16%, 1.80 ± 0.16%, 1.60 ± 0.23%, 1.46 ± 0.22%, 1.12 ± 0.19%, 0.89 ± 0.14%, and 0.70 ± 0.15%, respectively. Isoflurane MAC for plasma fentanyl concentrations ≥ 2.2 ng/mL differed significantly from the baseline value. In 3 rabbits, excessive spontaneous movement prevented MAC determination at the highest plasma fentanyl concentration. Fentanyl reduced isoflurane MAC by approximately 60% in New Zealand White rabbits. Further studies will be needed to investigate the cardiorespiratory effects of isoflurane and fentanyl combinations in rabbits; however, fentanyl may prove to be a useful adjunct to inhalation anesthesia in this species.

Effects of lidocaine administration via continuous rate infusion on the minimum alveolar concentration of isoflurane in New Zealand White rabbits (Oryctolagus cuniculus)

To evaluate the effect of a continuous rate infusion (CRI) of lidocaine on the minimum alveolar concentration (MAC) of isoflurane in rabbits. Five 12-month-old female New Zealand White rabbits (Oryctolagus cuniculus). Rabbits were anesthetized with isoflurane. Baseline isoflurane MAC was determined by use of the tail clamp technique. A loading dose of lidocaine (2.0 mg/kg, IV) was administered followed by a CRI of lidocaine at 50 μg/kg/min. After 30 minutes, isoflurane MAC was determined. Another loading dose was administered, and the lidocaine CRI then was increased to 100 μg/kg/min. After 30 minutes, isoflurane MAC was determined again. Plasma samples were obtained for lidocaine analysis after each MAC determination. Baseline isoflurane MAC was 2.09%, which was similar to previously reported values in this species. Lidocaine CRI at 50 and 100 μg/kg/min induced significant reductions in MAC. The 50 μg/kg/min CRI resulted in a mean plasma lidocaine concentration of 0.654 μg/mL and reduction of MAC by 10.5%. The 100 μg/kg/min CRI of lidocaine resulted in a mean plasma concentration of 1.578 μg/mL and reduction of MAC by 21.7%. Lidocaine also induced significant decreases in arterial blood pressure and heart rate. All cardiopulmonary variables were within reference ranges for rabbits anesthetized with inhalation anesthetics. No adverse effects were detected; all rabbits had an uncomplicated recovery from anesthesia. Lidocaine administered as a CRI at 50 and 100 μg/kg/min decreased isoflurane MAC in rabbits. The IV administration of lidocaine may be a useful adjunct in anesthesia of rabbits.

Effect of mepivacaine in an infraorbital nerve block on minimum alveolar concentration of isoflurane in clinically normal anesthetized dogs undergoing a modified form of dental dolorimetry

To evaluate the effects of a routinely used infraorbital nerve block, performed for dental procedures, on the anesthetic requirement for isoflurane in dogs. Prospective controlled study. 8 healthy adult Beagles. Dogs were anesthetized with isoflurane, and the minimum alveolar concentration (MAC) of isoflurane was established. A modification of a well-established method of stimulating the dental pulp, dental dolorimetry, was used to deliver a noxious stimulus (electrical stimulation) for isoflurane MAC determination. Once the isoflurane MAC was established, an infraorbital nerve block was performed with mepivacaine. The isoflurane MAC was then determined with the addition of the nerve block. Measurements of heart rate and mean arterial blood pressure were obtained at specified time points (baseline and prevention and elicitation of purposeful movement) during the determination of MAC and in response to the noxious stimulus. The mean ± SD isoflurane MAC without an infraorbital nerve block was 1.12 ± 0.13%. Isoflurane MAC with the regional mepivacaine anesthesia was 0.86 ± 0.11%. A significant reduction in isoflurane MAC (23%) was seen after the infraorbital nerve block, compared with results before the nerve block. With the exception of baseline measurements, no significant differences were found between treatments (isoflurane alone vs isoflurane with regional mepivacaine anesthesia) in heart rate or mean arterial blood pressure before or after the noxious stimulus. The significant reduction in MAC of isoflurane supported the practice of the addition of regional anesthesia for painful dental procedures to reduce the dose-dependent cardiorespiratory effects of general anesthesia.

Minimum alveolar concentration (MAC) for sevoflurane and xenon at normothermia and hypothermia in newborn pigs

Neuroprotection from therapeutic hypothermia increases when combined with the anaesthetic gas xenon in animal studies. A clinical feasibility study of the combined treatment has been successfully undertaken in asphyxiated human term newborns. It is unknown whether xenon alone would be sufficient for sedation during hypothermia eliminating or reducing the need for other sedative or analgesic infusions in ventilated sick infants. Minimum alveolar concentration (MAC) of xenon is unknown in any neonatal species. Eight newborn pigs were anaesthetised with sevoflurane alone and then sevoflurane plus xenon at two temperatures. Pigs were randomised to start at either 38.5°C or 33.5°C. MAC for sevoflurane was determined using the claw clamp technique at the preset body temperature. For xenon MAC determination, a background of 0.5 MAC sevoflurane was used, and 60% xenon added to the gas mixture. The relationship between sevoflurane and xenon MAC is assumed to be additive. Xenon concentrations were changed in 5% steps until a positive clamp reaction was noted. Pigs' temperature was changed to the second target, and two MAC determinations for sevoflurane and 0.5 MAC sevoflurane plus xenon were repeated. MAC for sevoflurane was 4.1% [95% confidence interval (CI): 3.65-4.50] at 38.5°C and 3.05% (CI: 2.63-3.48) at 33.5°C, a significant reduction. MAC for xenon was 120% at 38.5°C and 116% at 33.5°C, not different. In newborn swine sevoflurane, MAC was temperature dependent, while xenon MAC was independent of temperature. There was large individual variability in xenon MAC, from 60% to 120%.

Effects of a Single Bolus Intravenous Dose of Tramadol on Minimum Alveolar Concentration (MAC) of Sevoflurane in Dogs

Tramadol is an atypical opioid analgesic widely used in small animal practice. This study was designed to determine the effect of a single intravenous (IV) dose of tramadol on the minimum alveolar concentration (MAC) of sevoflurane in dogs. Six beagle dogs were anesthetized twice to determine the sevoflurane MAC with or without an administration of tramadol (4 mg/kg, IV) at 7 days interval. The sevoflurane MAC was determined using a tail clamp method in each dog ventilated with positive pressure ventilation. The tramadol administration produced a significant reduction in the sevoflurane MAC by 22.3 ± 12.2% (1.44 ± 0.28% with tramadol versus 1.86 ± 0.30% without tramadol, P=0.010). This MAC reduction had been determined from 122 ± 19 to 180 ± 41 min following the tramadol administration. During this period, the plasma concentrations of tramadol and its metabolite, O-desmethyltramadol (M1), decreased from 429 ± 64 to 332 ± 55 ng/ml and from 136 ± 24 to 114 ± 68 ng/ml, respectively, but these changes were not statistically significant. There was no significant difference in heart rate, mean arterial blood pressure and SpO2 between the control and tramadol treatment. The dogs that received tramadol treatment sometimes breathed spontaneously. Therefore, their respiratory rates significantly increased, and PETCO2 decreased during the MAC determination. In conclusion, the single IV dose of tramadol produced a significant reduction in the sevoflurane MAC in dogs.

Isoflurane but not halothane minimum alveolar concentration-sparing response of dexmedetomidine is enhanced in rats chronically treated with selectiveα2-adrenoceptor agonist

Halothane minimum alveolar concentration (MAC)-sparing response is preserved in rats rendered tolerant to the action of dexmedetomidine. It has been shown that halothane and isoflurane act at different sites to produce immobility. The authors studied whether there was any difference between halothane and isoflurane MAC-sparing effects of dexmedetomidine in rats after chronic administration of a low dose of this drug. Twenty-four female Wistar rats were randomly allocated into four groups of six animals: two groups received 10 μg/kg intraperitoneal dexmedetomidine for five days (treated groups) and the other two groups received intraperitoneal saline solution for five days (naive groups) prior to halothane or isoflurane MAC determination (one treated and one naive group of halothane and one treated and one naive group of isoflurane). Halothane or isoflurane MAC determination was performed before (basal) and 30 min after an intraperitoneal dose of 30 μg/kg of dexmedetomidine (post-dex) from alveolar gas samples at the time of tail clamp. Administration of an acute dose of dexmedetomidine to animals that had chronically received dexmedetomidine resulted in a MAC-sparing effect that was similar to that seen in naive animals for halothane; however, the same treatment increased the MAC-sparing response of dexmedetomidine for isoflurane. Isoflurane but not halothane MAC-sparing response of acutely administered dexmedetomidine is enhanced in rats chronically treated with this drug.

Concentration-dependent isoflurane effects on withdrawal reflexes in pigs and the role of the stimulation paradigm

In this prospective two-phase experimental trial, 10 pigs were anaesthetized twice with isoflurane only. In the first phase, the individual minimum alveolar concentration (MAC) was determined and in the second phase the effects on withdrawal reflexes of increasing end-tidal isoflurane concentrations (from 1.6% to 2.8%) were assessed. Single, 10 and 60 repeated electrical stimulations were used to evoke withdrawal reflexes which were recorded and quantified by electromyography. Recruitment curves for reflex amplitude for increasing stimulation intensities and isoflurane concentrations were constructed. Isoflurane MAC was 1.9 ± 0.3%. Reflexes evoked by repeated stimulation were suppressed at isoflurane concentrations significantly higher than those which suppressed complex movements during MAC determination (P=0.014 and P=0.006 for 10 and 60 repeated stimuli respectively). Isoflurane up to 2.8% was still not able to abolish reflex activity evoked by repeated stimulations in all pigs. Single stimulation reflexes were suppressed at significantly lower concentrations than repeated stimulation reflexes (P=0.008 and P=0.004 for 10 and 60 repeated stimuli, respectively). Reflex amplitude was significantly correlated with isoflurane concentration (P<0.001, r=-0.85) independent of the individual MAC. The findings indicate that the level at which isoflurane suppresses withdrawal reflexes is dependent on the stimulation paradigm (single vs. repeated electrical stimulation), and there is limited value in expressing reflex withdrawal suppression in terms of MAC as purposeful and reflex movements are independently affected by isoflurane in individual animals.

Effect of epidural and intravenous use of the neurokinin-1 (NK-1) receptor antagonist maropitant on the sevoflurane minimum alveolar concentration (MAC) in dogs

ObjectiveTo determine the effect of maropitant, an NK-1 receptor antagonist on the minimum alveolar concentration (MAC) of sevoflurane after intravenous and epidural administration to dogs. Study designProspective experimental study. AnimalsSeven, adult, spayed-female dogs (24.8 ± 1.9 kg). MethodsEach dog was anesthetized twice with sevoflurane in oxygen, with at least 10 days separating the anesthetic events. The minimum alveolar concentration (MAC) of sevoflurane was determined using the tail-clamp technique. During the first anesthetic event, the MAC of sevoflurane was determined initially and again after intravenous administration of maropitant (5 mg kg−1) and an infusion (150 μg kg−1 hour−1). During the second anesthetic event, an epidural catheter was advanced to the 4th lumbar vertebra and MAC was determined after administration of saline and maropitant (1 mg kg−1) epidurally. All MAC determinations were done in duplicate. The MAC values were adjusted to sea level and compared using student's t-test. ResultsThe baseline MAC for sevoflurane was 2.08 ± 0.25%. Intravenous maropitant decreased (p < 0.05) MAC by 16% (1.74 ± 0.17%). In contrast, epidural administration of either saline or maropitant did not change (p > 0.05) the MAC (2.17 ± 0.34% and 1.92 ± 0.12%, respectively). Conclusion and clinical relevanceMaropitant decreased the MAC of sevoflurane when administered intravenously to dogs but not after epidural administration.

Determination of the sevoflurane sparing effect of methadone in cats

ObjectiveTo determine the magnitude and duration of sevoflurane minimum alveolar concentration (MAC) reduction following a single intravenous (IV) dose of methadone in cats. Study designProspective experimental study. AnimalsEight (four females and four males) healthy mixed-breed adult (1–2 years) cats weighing 5.82 ± 0.42 kg. MethodsAnesthesia was induced and maintained with sevoflurane. Intravenous catheters facilitated administration of methadone and lactated Ringer’s solution. After baseline MAC determination in triplicate using a tail clamp technique, 0.3 mg kg−1 of methadone was administered IV. End-tidal sevoflurane concentration (e′SEVO) was reduced and MAC was redetermined. In an effort to determine the duration of MAC reduction, measurements were repeated in a stepwise manner until MAC values returned to baseline. After the last stimulation, the e′SEVO was increased to 1.2 individual MAC for 15 minutes, then sevoflurane was discontinued and cats were allowed to recover from anesthesia. ResultsBaseline sevoflurane MAC was 3.18 ± 0.06%. When compared with baseline the sevoflurane MAC after methadone administration was significantly reduced by 25, 15 and 7% at 26, 76 and 122 minutes, respectively. The final MAC value (3.09 ± 0.07%) determined 156 minutes after methadone administration was not significantly different from baseline. Conclusions and clinical relevanceIntravenous methadone (0.3 mg kg−1) significantly decreased MAC of sevoflurane in cats but the effect was short-lived.

Effects of fentanyl on isoflurane minimum alveolar concentration and cardiovascular function in mechanically ventilated goats

The effects of fentanyl on the minimum alveolar concentration (MAC) of isoflurane and cardiovascular function in mechanically ventilated goats were evaluated using six healthy goats (three does and three wethers)....

Effects of intravenous administration of lidocaine on the minimum alveolar concentration of sevoflurane in horses

To determine effects of a continuous rate infusion of lidocaine on the minimum alveolar concentration (MAC) of sevoflurane in horses. 8 healthy adult horses. Horses were anesthetized via IV administration of xylazine, ketamine, and diazepam; anesthesia was maintained with sevoflurane in oxygen. Approximately 1 hour after induction, sevoflurane MAC determination was initiated via standard techniques. Following sevoflurane MAC determination, lidocaine was administered as a bolus (1.3 mg/kg, IV, over 15 minutes), followed by constant rate infusion at 50 μg/kg/min. Determination of MAC for the lidocaine-sevoflurane combination was started 30 minutes after lidocaine infusion was initiated. Arterial blood samples were collected after the lidocaine bolus, at 30-minute intervals, and at the end of the infusion for measurement of plasma lidocaine concentrations. IV administration of lidocaine decreased mean ± SD sevoflurane MAC from 2.42 ± 0.24% to 1.78 ± 0.38% (mean MAC reduction, 26.7 ± 12%). Plasma lidocaine concentrations were 2,589 ± 811 ng/mL at the end of the bolus; 2,065 ± 441 ng/mL, 2,243 ± 699 ng/mL, 2,168 ± 339 ng/mL, and 2,254 ± 215 ng/mL at 30, 60, 90, and 120 minutes of infusion, respectively; and 2,206 ± 329 ng/mL at the end of the infusion. Plasma concentrations did not differ significantly among time points. Lidocaine could be useful for providing a more balanced anesthetic technique in horses. A detailed cardiovascular study on the effects of IV infusion of lidocaine during anesthesia with sevoflurane is required before this combination can be recommended.

Effects of high plasma fentanyl concentrations on minimum alveolar concentration of isoflurane in horses

To verify the isoflurane anesthetic minimum alveolar concentration (MAC)-sparing effect of a previously administered target plasma fentanyl concentration of 16 ng/mL and characterize an anticipated further sparing in isoflurane MAC associated with higher target plasma fentanyl concentrations. 8 horses. Horses were assigned 2 of 3 target plasma fentanyl concentrations (16, 24, and 32 ng/mL), administered in ascending order. Following determination of baseline MAC, horses received a loading dose of fentanyl followed by a constant rate infusion; MAC determination was performed in triplicate at baseline and at each fentanyl concentration. Venous blood samples were collected throughout the study for determination of actual plasma fentanyl concentrations. Recovery from anesthesia was monitored, and behaviors were rated as excellent, good, fair, or poor. Mean + or - SD fentanyl plasma concentrations were 13.9 + or - 2.6 ng/mL, 20.1 + or - 3.6 ng/mL, and 24.1 + or - 2.4 ng/mL for target concentrations of 16, 24, and 32 ng/mL, respectively. The corresponding changes in the MAC of isoflurane were -3.28%, -6.23%, and +1.14%. None of the changes were significant. Recovery behavior was variable and included highly undesirable, potentially injurious excitatory behavior. Results of the study did not verify an isoflurane-sparing effect of fentanyl at a plasma target concentration of 16 ng/mL. Furthermore, a reduction in MAC was not detected at higher fentanyl concentrations. Overall, results did not support the routine use of fentanyl as an anesthetic adjuvant in adult horses.