Inhalational Agent Dosing Behaviors of Anesthesia Practitioners Cause Variability in End-Tidal Concentrations at the End of Surgery and Prolonged Times to Tracheal Extubation.

INTRODUCTION: Prolonged times to tracheal extubation are intervals from the end of surgery to extubation ≥15 minutes. We examined why there are associations with the end-tidal inhalational agent concentration as a proportion of the age­adjusted minimum alveolar concentration (MAC fraction) at the end of surgery. METHODS: The retrospective cohort study used 11.7 years of data from one hospital. All p­values were adjusted for multiple comparisons. RESULTS: There was a greater odds of prolonged time to extubation if the anesthesia practitioner was a trainee (odds ratio 1.68) or had finished fewer than five cases with the surgeon during the preceding three years (odds ratio 1.12) (both P<0.0001). There was a greater risk of prolonged time to extubation if the MAC fraction was >0.4 at the end of surgery (odds ratio 2.66, P<0.0001). Anesthesia practitioners who were trainees and all practitioners who had finished fewer than five cases with the surgeon had greater mean MAC fractions at the end of surgery and had greater relative risks of the MAC fraction >0.4 at the end of surgery (all P<0.0001). The source for greater MAC fractions at the end of surgery was not greater MAC fractions throughout the anesthetic because the means during the case did not differ among groups. Rather, there was substantial variability of MAC fractions at the end of surgery among cases of the same anesthesia practitioner, with the mean (standard deviation) among practitioners of each practitioner's standard deviation being 0.35 (0.05) and the coefficient of variation being 71% (13%). CONCLUSION: More prolonged extubations were associated with greater MAC fractions at the end of surgery. The cause of the large MAC fractions was the substantial variability of MAC fractions among cases of each practitioner at the end of surgery. That variability matches what was expected from earlier studies, both from variability among practitioners in their goals for the MAC fraction given at the start of surgical closure and from inadequate dynamic forecasting of the timing of when surgery would end. Future studies should examine how best to reduce prolonged extubations by using anesthesia machines' display of MAC fraction and feedback control of end-tidal agent concentration.

Anesthesia Practitioners' Goals for Sevoflurane Minimum Alveolar Concentration at the End of Surgery and the Incidence of Prolonged Extubations: A Prospective and Observational Study.

BACKGROUND: Prolonged times to tracheal extubation (≥15 minutes from dressing on the patient) are consequential based on their clinical and economic effect. We evaluated the variability among anesthesia practitioners in their goals for the age-adjusted end-tidal minimum alveolar concentration of sevoflurane (MAC) at surgery end and achievement of their goals. METHODS: We prospectively studied a cohort of 56 adult patients undergoing general anesthesia with sevoflurane as the sole anesthetic agent, scheduled operating room time of at least 3 hours, and non-prone positioning. At the start of surgical closure, an observer asked the anesthesia practitioner their goal for MAC when the surgical drapes are lowered (i.e., the functional end of surgery for the studied procedures). When the drapes were lowered, the MAC achieved was recorded, and the values were compared. RESULTS: The standard deviation of the practitioners' MAC goal was large, 0.199 (N = 56 cases, 95% confidence interval 0.17-0.24), not significantly different from the standard deviation of the MAC achieved of 0.253, P = 0.071. The MAC goal and MAC achieved were correlated pairwise, Pearson r =0.65, P < 0.0001. There was no incremental effect of operating room conversation(s) related to case progress on the association (partial correlation ­0.01, P = 0.96). Differences among practitioners in the MAC achieved at surgery end were consequential. Specifically, for the N = 12 cases with prolonged extubation, the mean MAC was 0.60 (standard deviation 0.10) versus 0.48 (0.21) among the N = 44 cases without prolonged extubation (P = 0.0070). CONCLUSIONS: The standard deviation of the MAC goal among practitioners was sufficiently large to contribute significantly to the variability in the MAC achieved at the end of surgery. We confirmed prospectively that the age-adjusted end-tidal MAC at the end of surgery matters clinically and economically because differences of 0.60 versus 0.48 were associated with more prolonged extubations. Our novel finding is that the MAC achieved ≥0.60 were caused in part by the anesthesia practitioners' stated MAC goals when surgical closures started.

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

Objective: To determine if oral gabapentin decreases the minimum alveolar concentration (MAC) of isoflurane in cats. Study design: Prospective, randomized, blinded, crossover, and experimental study. Animals: A total of six healthy adult cats (three male, three female) aged 18-42 months, weighing 3.31 ± 0.26 kg. Methods: 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. Results: 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. Conclusion and clinical relevance: Oral administration of gabapentin 2 h before starting MAC determination had a significant isoflurane MAC-sparing effect in cats with no observed hemodynamic benefit.

The Effect of Pregabalin on the Minimum Alveolar Concentration of Sevoflurane: A Randomized, Placebo-Controlled, Double-Blind Clinical Trial.

Background: Pregabalin is commonly used perioperatively to reduce post-operative pain and opioid consumption and to prevent the development of chronic pain. It has been shown to reduce anesthetic consumption in balanced anesthesia, but studies investigating its effect on the minimum alveolar concentration (MAC) of volatile anesthetics are lacking. The aim of this study was to investigate the effect of two different doses of pregabalin on the MAC of sevoflurane. Methods: In a randomized, double-blinded, placebo controlled clinical study, 75 patients were assigned to receive placebo, 300 mg pregabalin, or 150 mg pregabalin, as a capsule 1 h before anesthesia induction with sevoflurane only. After equilibration, the response to skin incision (movement vs. non-movement) was monitored. The MAC was assessed using an up- and down-titration method. Results: The MAC of sevoflurane was estimated as 2.16% (95% CI, 2.07-2.32%) in the placebo group, 1.44% (95% CI, 1.26-1.70%) in the 300 mg pregabalin group, and 1.81% (95% CI, 1.49-2.13%) in the 150 mg pregabalin group. We therefore report a 33% reduction in the MAC of sevoflurane in the 300 mg pregabalin group as compared to placebo. The MAC of the 150 mg pregabalin group was reduced by 16% as compared to placebo but was not statistically significant. Conclusions: The administration of 300 mg pregabalin reduced the MAC of sevoflurane by 33%, while the administration of 150 mg pregabalin did not significantly reduce the MAC of sevoflurane. Pregabalin use led to a small reduction in post-operative pain levels but increased side effects in a dose-dependent manner.

Minimum Alveolar Concentration of Isoflurane in Rats Chronically Treated with the Synthetic Cannabinoid WIN 55,212-2.

The minimum alveolar concentration MAC of isoflurane was measured in rats chronically treated with WIN 55,212-2. METHODS: The MAC of isoflurane was determined in 24 male rats from expiratory samples at time of tail clamping under the following conditions: without treatment MAC(ISO), in rats treated for 21 days with WIN 55,212-2 MAC(ISO + WIN55), and in rats 8 days after stopping treatment with WIN 55,212-2 (MACISO + WIN55 + 8D). RESULTS: The MAC(ISO) was 1.32 ± 0.06. In the MAC(ISO + WIN55) group, the MAC increased to 1.69 ± 0.09 (28%, p-value ≤ (0.0001). Eight days after stopping treatment with WIN55, the MAC did not decrease significantly, 1.67 ± 0.07 (26%, p-value ≤ 0.0001). CONCLUSIONS: The administration of WIN 55,212-2 for 21 days increases the MAC of isoflurane in rats. This effect does not disappear 8 days after discontinuation of treatment with the synthetic cannabinoid.

Optimal anesthetic conduct regarding immediate and short-term outcomes after liver transplantation - Systematic review of the literature and expert panel recommendations.

BACKGROUND: In the era of enhanced recovery after surgery, there is significant discussion regarding the impact of intraoperative anesthetic management on short-term outcomes following liver transplantation (LT), with no clear consensus in the literature. OBJECTIVES: To identify whether or not intraoperative anesthetic management affects short-term outcomes after liver transplantation. DATA SOURCES: Ovid MEDLINE, Embase, Scopus, Google Scholar, and Cochrane Central. METHODS: A systematic review following PRISMA guidelines was undertaken. The systematic review was registered on PROSPERO (CRD42021239758). An international expert panel made recommendations for clinical practice using the GRADE approach. RESULTS: After screening, 14 studies were eligible for inclusion in this systematic review. Six were prospective randomized clinical trials, three were prospective nonrandomized clinical trials, and five were retrospective studies. These manuscripts were reviewed to look at five questions regarding anesthetic care and its impact on short term outcomes following liver transplant. After review of the literature, the quality of evidence according to the following outcomes was as follows: intraoperative and postoperative morbidity and mortality (low), early allograft dysfunction (low), and hospital and ICU length of stay (moderate). CONCLUSIONS: For optimal short term outcomes after liver transplantation, the panel recommends the use of volatile anesthetics in preference to total intravenous anesthesia (TIVA) (Level of Evidence: Very low; Strength of Recommendation: Weak) and minimum alveolar concentration (MAC) versus bispectral index (BIS) for depth of anesthesia monitoring (Level of Evidence: Very low; Strength of Recommendation: Weak). Regarding ventilation and oxygenation, the panel recommends a restrictive oxygenation strategy targeting a PaO2 of 70-120 mmHg (10-14 kPa), a tidal volume of 6-8 ml/kg ideal body weight (IBW), administration of positive end expiratory pressure (PEEP) tailored to patient intraoperative physiology, and recruitment maneuvers. (Level of evidence: Very low; Strength of Recommendation: Strong). Finally, the panel recommends the routine use of antiemetic prophylaxis. (Level of evidence: low; Strength of Recommendation: Strong).

Comparison of the median effective concentration (EC50) and effect of sevoflurane with or without remifentanil in cesarean section with supreme laryngeal mask under narcotrend monitoring: a randomized trial.

BACKGROUND: Remifentanil combined with sevoflurane is a standard protocol for obstetric general anesthesia (GA). METHODS: In this study, we performed a randomized clinical trial to evaluate whether remifentanil has an effect on the median effective concentration (EC50) of sevoflurane and compare anesthetic outcomes of them in cesarean section with Supreme™ laryngeal mask airway (SLMA) under narcotrend monitoring. Ninety parturients with singleton births undergoing elective cesarean delivery (CD) with initial inhaled 1.0 minimum alveolar concentration (MAC) sevoflurane for anesthesia maintenance were assigned to three groups randomly and evenly: Group A (0.05 µg·kg-1·min-1 remifentanil combined with sevoflurane), Group B (0.1 µg·kg-1·min-1 remifentanil combined with sevoflurane), and Group C (normal saline combined with sevoflurane). Narcotrend was used to monitor the depth of anesthesia during the operation, with the level of anesthesia depth controlled within the D-E stage. The EC50 of sevoflurane was determined by Dixon's sequential method. The Narcotrend index, amount of bleeding, neonatal Apgar score, and corresponding treatment measures in the three groups were recorded. RESULTS: The results showed that the estimated EC50 of sevoflurane for obstetric GA was 0.80 MAC (95% CI: 0.63-0.95 MAC) in group A, 0.82 MAC (95% CI: 0.63-0.96 MAC) in group B, and 0.80 MAC (95% CI: 0.63-0.95 MAC) in group C. There was no statistically significant difference in the estimated EC50 of sevoflurane, time to wakefulness, Apgar score, amount of intraoperative bleeding, and postoperative bleeding within 24 hours between the three groups (all P>0.05). CONCLUSIONS: The addition of remifentanil at 0.05-0.1 µg·kg-1·min-1 did not change the EC50 of sevoflurane and anesthetic quality. The concentration of inhaled anesthetics can be minimized with Narcotrend monitoring. TRIAL REGISTRATION: Chinese Clinical Trial Registry ChiCTR2000034512.

Effects of different plasma target concentrations of remifentanil on the MACBAR of sevoflurane in children with laparoscopic surgery.

BACKGROUND: To investigate the effects of different plasma target concentrations of remifentanil on the minimum alveolar concentration (MAC) for blocking adrenergic response (BAR) of sevoflurane in children with laparoscopic herniorrhaphy. METHODS: Seventy-five children with 3-7 years old scheduled for laparoscopic herniorrhaphy were randomly divided into group R0, group R1, and group R2 according to different remifentanil plasma target concentration (0, 1, and 2 ngml-1), respectively. The MACBAR of sevoflurane was determined by the up-and-down and sequential method in each group. The concentrations of epinephrine and noradrenaline were also determined at corresponding time points. RESULTS: A total of 52 child patients were used among the anticipated 75 patients. In groups R0, R1, and R2, the MACBAR of sevoflurane was (3.29 ± 0.17) %, (2.12 ± 0.10) % and (1.29 ± 0.11) %, respectively, and a significant difference was found among the three groups (P<0.05). The changes of epinephrine and noradrenaline concentrations in each group before and after insufflation of carbon dioxide pneumoperitoneum showed no significant differences. CONCLUSION: Remifentanil by target-controlled infusion can effectively reduce the MACBAR of sevoflurane during laparoscopic surgery in children. At a similar effect of MACBAR, both the changes of epinephrine and noradrenaline concentrations are not affected by the infusion of different remifentanil target concentrations. TRIAL REGISTRATION: The trial was registered at http://www.chictr.org.cn ( ChiCTR1800019393 , 8, Nov, 2018).

Effects of different remifentanil target concentrations on MACBAR of sevoflurane in patients with liver dysfunction under carbon dioxide pneumoperitoneum stimulus: A randomized controlled trial.

WHAT IS KNOWN AND OBJECTIVE: Remifentanil can effectively decrease the sevoflurane concentration to block sympathetic adrenergic response to CO2 pneumoperitoneum stimulus,and liver dysfunction will significantly reduce the MACBAR (minimum alveolar concentration for blocking adrenergic response) of sevoflurane. However, the effects of different remifentanil concentrations on the MACBAR of sevoflurane in patients with liver dysfunction are unclear. The aim of this study was to observe the effects of different remifentanil concentrations by intravenous target-controlled infusion on the MACBAR of sevoflurane in patients with grade B liver dysfunction under carbon dioxide pneumoperitoneum stimulus. METHODS: Seventy-five patients with grade B liver dysfunction undergoing elective laparoscopic surgery were selected, and randomly divided into three groups with remifentanil plasma target concentrations of 0 (group R0 ), 1 (group R1 ) and 2 (group R2 ) ng/ml. Anaesthesia was induced by intravenous injection of propofol 2-3 mg/kg, remifentanil 2 µg/kg and cisatracurium 0.15 mg/kg. All groups were inhaled different concentrations of sevoflurane. The determination of sevoflurane MACBAR in each group was used a method of sequential-allocation technique, and venous blood samples were taken before and after the creation of carbon dioxide pneumoperitoneum to determine plasma adrenaline and noradrenaline concentrations. RESULTS AND DISCUSSIONS: The MACBAR of sevoflurane in groups R0 , R1 and R2 was 4.83%, 3.00% and 2.10%, respectively. The MACBAR of sevoflurane was significantly difference among the three groups. When a similar effect of MACBAR had achieved in each group, no significant differences were found in the changes of plasma adrenaline and noradrenaline concentrations before and after the creation of pneumoperitoneum. What is new and conclusion Target-controlled infusion of different concentrations of remifentanil can reduce sevoflurane MACBAR during pneumoperitoneum stimulation in patients with liver dysfunction in some degree. However, the changes of plasma adrenaline and noradrenaline concentrations are consistent in the three groups when patient's stress response was inhibited at the same degree.

The effect of cannabidiol on sevoflurane minimum alveolar concentration reduction produced by morphine in rats.

OBJECTIVE: To investigate the effect of cannabidiol (CBD) on sevoflurane minimum alveolar concentration (MACSEV) reduction produced by morphine in rats. STUDY DESIGN: Randomized, blinded trial. ANIMALS: A total of 75 male Wistar Han rats weighing 276 ± 23 g (mean and standard deviation), aged 3 months. METHODS: Cannabidiol (CBD) was prepared in an ethanol-solutol-saline vehicle. Animals were randomly divided into 15 groups and given an intraperitoneal bolus of 1, 3, 5, 6.5, 7.5 or 10 mg kg-1 of CBD alone (CBD1, CBD3, CBD5, CBD6.5, CBD7.5 and CBD10 respectively) or combined with 5 mg kg-1 of morphine (MOR+CBD1, MOR+CBD3, MOR+CBD5, MOR+CBD6.5, MOR+CBD7.5 and MOR+CBD10). While three controls groups: MOR+saline, MOR+vehicle and vehicle were given an intraperitoneal bolus of morphine with saline, morphine with vehicle or vehicle alone respectively. The MACSEV was determined from alveolar gas samples at the time of tail clamp application. The MACSEV reduction was analyzed using a one-way ANOVA followed by Tukey's test. Additionally, Kruskal-Wallis test for non-normally-distributed data was performed. Data are presented as mean ± standard deviation. P < 0.05 RESULTS: The mean MACSEV was not reduced by the action of CBD administered alone, but the addition of morphine to the different doses of CBD significantly reduced the MACSEV. That reduction was greatest in the MOR+CBD1, MOR+CBD7.5 and MOR+CBD10 groups (29 ± 5%, 32 ± 5% and 30 ± 6% respectively), less in MOR+CBD3 and MOR+CBD6.5 groups (24 ± 3% and 26 ± 4% respectively) and least in MOR+CBD5 group (17 ± 2%). However, only the MOR+CBD5 group was statistically significantly different from MOR+CBD1, MOR+CBD7.5 and MOR+CBD10 groups. CONCLUSIONS AND CLINICAL RELEVANCE: MACSEV in rat was unaltered by the action of CBD alone, the reduction in MACSEV produced by morphine was not enhanced by the addition of CBD at the doses studied.

Determination of the minimum alveolar concentration (MAC) and cardiopulmonary effects of sevoflurane in sheep.

OBJECTIVE: To determine sevoflurane's minimum alveolar concentration (MACSEVO) and its cardiopulmonary effects in sheep. STUDY DESIGN: Prospective experimental study. ANIMALS: A group of 10 female nonpregnant Sardinian milk sheep. METHODS: Anesthesia was induced in each sheep twice with sevoflurane in oxygen. After a 30 minute equilibration at end-tidal sevoflurane concentration (Fe'Sevo) of 2.8%, an electrical stimulus (5 Hz/1 ms/50 mA) was applied to the right thoracic limb for 1 minute or until gross purposeful movement occurred. The Fe'Sevo was then changed using a 0.2% up-and-down protocol, dependent on whether or not the response was positive, and then noxious stimulation was repeated. The MACSEVO was defined as the mean Fe'Sevo between that allowing purposeful movement and that not. The group of 10 sheep were re-anesthetized and MACSEVO was re-determined. Thereafter, Fe'Sevo was maintained for 15 minutes each at concentrations corresponding to 1.0, 1.3, 1.6, 1.9 and 0.75 MACSEVO multiples, and cardiopulmonary, blood gas, acid-base variables and plasma electrolytes were determined. Also, time to induction of anesthesia, extubation and recovery were recorded. RESULTS: The mean ± standard deviation of the MACSEVO was 2.74 ± 0.38%. Median (interquartile range) time to intubation was 3.13 (2.98-3.33) minutes, time to extubation was 6.85 ± 2.65 minutes and time to recovery was 13.4 ± 5.2 minutes. With increasing Fe'Sevo, arterial blood pressures progressively decreased as did minute ventilation, which in turn caused end-tidal carbon dioxide, arterial partial pressure of carbon dioxide and bicarbonate values to steadily increase without significantly affecting arterial partial pressure of oxygen. CONCLUSIONS AND CLINICAL RELEVANCE: The reported MACSEVO agrees with published data in this and other species. Administration of sevoflurane in sheep caused marked hemodynamic and respiratory depression, but soon after turning off the vaporizer, sheep could be extubated and recovered rapidly and event-free.

The effect of remifentanil on the minimum alveolar concentration (MAC) and MAC derivatives of sevoflurane in dogs.

Remifentanil is an ultra-short-acting µ-opioid receptor agonist. The purpose of this study was to determine the relationship of the minimum alveolar concentration (MAC) of sevoflurane and other MAC derivatives, including the MAC for blocking adrenergic responses (MAC-BAR) and the MAC at which tracheal extubation is occurred (MAC-extubation), with or without remifentanil infusion. Six healthy adult beagle dogs were randomly anesthetized three times for determining the MAC-BAR (SEVMAC-BAR), MAC (SEVMAC), and MAC- extubation (SEVMAC-extubation) of sevoflurane under infusion of saline and remifentanil at rates of 0.15, 0.30, 0.60, 1.20, and 2.40 µg/kg/min. The ratio of the SEVMAC-BAR and SEVMAC and that of the SEVMAC-extubation and SEVMAC were not significantly different at baseline and during treatment. The MAC-BAR95 and MAC95 decreased in a dose-dependent manner until reaching 1.20 µg/kg/min, and the MAC-extubation5 decreased in a dose-dependent manner until reaching 0.60 µg/kg/min. The percentage reduction of SEVMAC-BAR, SEVMAC, and SEVMAC-extubation increased in a dose-dependent manner during remifentanil infusion. The heart rate significantly decreased in the MAC-BAR and MAC groups, and the systolic and mean arterial pressures increased after remifentanil infusion compared with the baseline values. Remifentanil infusion caused reduction of the SEVMAC-BAR, SEVMAC, and SEVMAC-extubation in a dose-dependent manner, and ceiling effects were observed in the dogs. Higher doses of remifentanil and sevoflurane were necessary for blocking the sympathetic response to the supramaximal stimulus to prevent movement and extubation in dogs.

Effects of different remifentanil target concentrations on MAC BAR of sevoflurane in gynaecological patients with CO2 pneumoperitoneum stimulus.

BACKGROUND: To investigate the effects of different target plasma remifentanil concentrations on the minimum alveolar concentration of sevoflurane (MAC) for blocking adrenergic response (BAR) during laparoscopic gynaecological surgery with carbon dioxide insufflation. METHODS: Seventy-five gynaecological patients with ASA I-II undergoing laparoscopic surgery were randomly assigned to three groups. Anaesthesia was induced by sevoflurane, and 0.1 mg kg(-1) of vecuronium i.v. was injected to facilitate tracheal intubation. After intubation the target plasma concentrations of remifentanil in Groups 1, 2, and 3 were adjusted to 0, 1, and 2 ng ml(-1), respectively. The changes in haemodynamics were observed before and after the creation of carbon dioxide pneumoperitoneum. The MAC BAR of sevoflurane in each group was determined by using an up-and-down sequential-allocation technique, and blood samples were collected at corresponding time points to determine the concentrations of remifentanil, norepinephrine, and epinephrine. RESULTS: In Groups 1, 2 and 3, the MAC BAR of sevoflurane was 4.6% (CI 95%: 4.3-4.9%), 2.4% (CI 95%: 2.2-2.6%), and 1.7% (CI 95%: 1.4-2.1%), respectively. No significant differences were found in the increase of norepinephrine, epinephrine, and mean arterial pressure after compared with before insufflation of pneumoperitoneum among the three groups. CONCLUSIONS: Remifentanil can effectively decrease the sevoflurane concentration to block sympathetic adrenergic response to CO2 pneumoperitoneum stimulus. At similar MAC BAR the haemodynamic and adrenergic response is not affected by the infused remifentanil concentration. CLINICAL TRIAL REGISTRATION: The number of this clinical trial registry is ChiCTR-TRC-13004005, and the Universal Trial Number is U1111-1151-5630.

Anaesthetic potency of isoflurane in cattle: determination of the minimum alveolar concentration.

OBJECTIVE: To determine the minimum alveolar concentration (MAC) of isoflurane in cattle. STUDY DESIGN: Prospective study. ANIMALS: Sixteen healthy adult female Holstein-Friesian cattle weighing 612 ± 17 kg (× ± SEM) and aged 5.7 ± 0.9 years old. METHODS: The unsedated cattle were restrained in right lateral recumbency using a rope harness technique. Anaesthesia was induced with isoflurane (ISO) in oxygen via a face mask connected to a large-animal, semiclosed anaesthetic circle system. Each cow was intubated with a cuffed orotracheal tube (25 mm ID). Inspired and end-tidal ISO were monitored using a calibrated infra red analyser with a methane filter. The MAC of ISO that prevented gross purposeful movement in response to a tail and dewclaw clamp was determined. The time from the start of ISO administration to intubation, the time interval between discontinuance of ISO and the time the animal regained sternal recumbency, were recorded. Time to standing and quality of recovery were also recorded. RESULTS: The time from the start of ISO administration to tracheal intubation was 18.68 ± 2.77 minutes. The MAC of ISO in these cattle was 1.27 ± 0.03% (1.14 ± 0.01% corrected to sea level). Time to sternal recumbency after 90 ± 16 minutes of anaesthesia from intubation was 4.60 ± 0.58 minutes and time to standing was 6.70 ± 1.02 minutes. All cattle were extubated when they regained sternal recumbency. CONCLUSION: The MAC of isoflurane in these cattle was 1.27 ± 0.03% (1.14 ± 0.01% corrected to sea level). ISO provided a smooth induction to, and rapid recovery from, anaesthesia. CLINICAL RELEVANCE: Knowledge of the MAC of ISO in cattle will facilitate its appropriate clinical use.