Minimum Alveolar Concentration Of Isoflurane In Dogs Research Articles

The effect of experimentally induced hypothyroidism on the isoflurane minimum alveolar concentration in dogs

ObjectiveTo determine the effect of experimentally induced hypothyroidism on isoflurane (ISO) minimum alveolar concentration (MAC) in dogs. Study designProspective experimental study. AnimalsEighteen adult female mongrel dogs, age 2–4 years and weighing 8.2–13.1 kg. MethodsHypothyroidism was induced in nine dogs by the intravenous administration of 1 mCi kg−1 of 131Iodine. The remaining nine dogs served as controls. Dogs were studied 9–12 months after the induction of hypothyroidism. Anesthesia was induced with ISO in oxygen via a mask. The trachea was intubated, and anesthesia was maintained using ISO in oxygen using a semi-closed rebreathing circle system. The dogs were mechanically ventilated to maintain an end-tidal carbon dioxide concentration between 35 and 45 mmHg. End-tidal ISO concentrations were measured with an infrared gas analyzer. The MAC was determined in duplicate using a tail clamp technique. The mean values for the groups were compared using a two sample t-test. ResultsThe mean ± SD MAC of isoflurane in the hypothyroid and euthyroid dogs was 0.98 ± 0.31% and 1.11 ± 0.26%, respectively. The mean MAC of isoflurane in hypothyroid dogs was not significantly different from the mean MAC of isoflurane in the control dogs (p=0.3553). Conclusion and clinical relevanceThe MAC of ISO in dogs was not significantly affected by experimentally induced hypothyroidism. The dose of ISO in dogs with hypothyroidism does not need to be altered.

The effect of buprenorphine on isoflurane minimum alveolar concentration in dogs

ObjectiveTo determine the effect of intravenous (IV) buprenorphine on the isoflurane (ISO) minimum alveolar concentration (ISOMAC) in dogs. Study designRandomized, crossover, design. AnimalsSix healthy, adult (2–3 years old), intact dogs (two males and four females) weighing 7.4–11.0 kg. MethodsEach dog was studied on three occasions, 1 week apart, and baseline ISOMAC (MACB) was determined on each occasion. ISOMAC was defined as the mean of the end-tidal ISO concentrations that prevented and allowed purposeful movement in response to a noxious stimulus. After MACB determination, dogs were randomly given buprenorphine (BUP) at either 0.01, 0.05 or 0.1 mg kg−1 IV, and ISOMAC was determined at two time periods after BUP administration. The first post-treatment determination (MACT1) was initiated 45 minutes after BUP administration and the second determination (MACT2) was initiated 4 hours after BUP administration. MAC values were determined in duplicate and the mean values were used for statistical analysis. ResultsIsoflurane minimum alveolar concentration was decreased at 141 minutes (the time of MACT1 determination) by 25%, 35%, and 27% after administration of BUP at 0.01, 0.05, and 0.1 mg kg−1, respectively (p ≤ 0.05). The MAC reductions were not statistically different among doses. The reductions in ISOMAC at 342 minutes (the time of MACT2 determination) ranged from 13 to 16%, and were not statistically different among doses. Conclusions and clinical significanceBuprenorphine at 0.01, 0.05, and 0.1 mg kg−1 significantly decreased ISOMAC in dogs at 141 minutes but not at 342 minutes. When using BUP for MAC reduction re-dosing may be required for procedures of long duration, and there may be no advantage to using the 0.1 mg kg−1 dose.

Effect of dexmedetomidine, morphine-lidocaine-ketamine, and dexmedetomidine-morphine-lidocaine-ketamine constant rate infusions on the minimum alveolar concentration of isoflurane and bispectral index in dogs

To determine the effect of dexmedetomidine, morphine-lidocaine-ketamine (MLK), and dexmedetomidine-morphine-lidocaine-ketamine (DMLK) constant rate infusions on the minimum alveolar concentration (MAC) of isoflurane and bispectral index (BIS) in dogs. 6 healthy adult dogs. Each dog was anesthetized 4 times with a 7-day washout period between anesthetic episodes. During the first anesthetic episode, the MAC of isoflurane (baseline) was established. During the 3 subsequent anesthetic episodes, the MAC of isoflurane was determined following constant rate infusion of dexmedetomidine (0.5 μg/kg/h), MLK (morphine, 0.2 mg/kg/h; lidocaine, 3 mg/kg/h; and ketamine, 0.6 mg/kg/h), or DMLK (dexmedetomidine, 0.5 μg/kg/h; morphine, 0.2 mg/kg/h; lidocaine, 3 mg/kg/h; and ketamine 0.6 mg/kg/h). Among treatments, MAC of isoflurane was compared by means of a Friedman test with Conover posttest comparisons, and heart rate, direct arterial pressures, cardiac output, body temperature, inspired and expired gas concentrations, arterial blood gas values, and BIS were compared with repeated-measures ANOVA and a Dunn test for multiple comparisons. Infusion of dexmedetomidine, MLK, and DMLK decreased the MAC of isoflurane from baseline by 30%, 55%, and 90%, respectively. Mean heart rates during dexmedetomidine and DMLK treatments was lower than that during MLK treatment. Compared with baseline values, mean heart rate decreased for all treatments, arterial pressure increased for the DMLK treatment, cardiac output decreased for the dexmedetomidine treatment, and BIS increased for the MLK and DMLK treatments. Time to extubation and sternal recumbency did not differ among treatments. Infusion of dexmedetomidine, MLK, or DMLK reduced the MAC of isoflurane in dogs.

Effects of intravenous administration of perzinfotel, fentanyl, and a combination of both drugs on the minimum alveolar concentration of isoflurane in dogs

OBJECTIVE-To determine the effects of IV administration of perzinfotel and a perzinfotel-fentanyl combination on the minimum alveolar concentration (MAC) of isoflurane in dogs. ANIMALS-6 healthy sexually intact Beagles (3 males and 3 females). PROCEDURES-All dogs were instrumented with a telemetry device for continuous monitoring of heart rate, arterial blood pressure, and core body temperature (at a femoral artery). Dogs were anesthetized with propofol (6 mg/kg, IV) and isoflurane. Isoflurane MAC values were determined in 3 experiments in each dog, separated by at least 7 days, before (baseline) and after the following treatments: no treatment (anesthetic only), perzinfotel (20 mg/kg, IV), fentanyl (5 microg/kg bolus, IV, followed by a continuous IV infusion at 0.15 microg/kg/min), and a fentanyl-perzinfotel combination (20 mg of perzinfotel/kg, IV, plus the fentanyl infusion). Bispectral index and oxygen saturation as measured by pulse oximetry were also monitored throughout anesthesia. RESULTS-Without treatment, the mean +/- SD isoflurane MAC for all 6 dogs was 1.41 +/- 0.10%. Baseline MAC was 1.42 +/- 0.08%. Intravenous administration of perzinfotel, fentanyl, and the perzinfotel-fentanyl combination significantly decreased the MAC by 39%, 35%, and 66%, respectively. Perzinfotel and perzinfotel-fentanyl administration yielded significant increases in the bispectral index. Mean, systolic, and diastolic arterial blood pressures significantly increased from baseline values when perzinfotel was administered. Systolic arterial blood pressure significantly increased from the baseline value when perzinfotel-fentanyl was administered. No adverse effects were detected. CONCLUSIONS AND CLINICAL RELEVANCE-IV administration of perzinfotel, fentanyl, or a perzinfotel-fentanyl combination reduced isoflurane MAC in dogs and increased arterial blood pressure.

Effects of Carprofen and Morphine on the Minimum Alveolar Concentration of Isoflurane in Dogs

The minimum alveolar concentration (MAC) of isoflurane in dogs was determined following carprofen (2.2 mg/kg per os) alone, morphine (1 mg/kg intravenously) alone, carprofen and morphine, and no drug control in eight healthy adult dogs. Isoflurane MAC following administration of morphine alone (0.81%+/-0.18%) or carprofen and morphine (0.68%+/-0.31%) was significantly less than the control MAC (1.24%+/-0.15%). Isoflurane MAC after carprofen alone (1.13%+/-0.13%) was not significantly different from the control value. Results indicated that administration of morphine alone or in combination with carprofen significantly reduced the MAC of isoflurane in dogs. The isoflurane MAC reduction was additive between the effects of carprofen and morphine.

Effects of adenosine infusion on the minimum alveolar concentration of isoflurane in dogs

ObjectiveTo determine the effects of adenosine infusion on the minimum alveolar concentration (MAC) of isoflurane in dogs. Study designProspective, randomized crossover study.Animals Seven adult male and female Beagles weighing 10.9 (7.5, 13.6) kg [median (minimum, maximum)]. MethodsEach dog was anesthetized with isoflurane in oxygen and randomly assigned to receive either an intravenous (IV) adenosine (0.3 mg kg−1 minute−1) or saline (6 mL kg−1 hour−1 IV) infusion. After an interval of 7 days or more, each dog was re-anesthetized and treated with the alternative infusion. Using a tail-clamp technique, MAC was determined before (pre-infusion), during (infusion), and 2 hours after the infusions (post-infusion). ResultsThe pre-infusion MAC of isoflurane was 1.25 (1.15, 1.35) [median (minimum, maximum)] vol.% for the saline treatment group and 1.25 (1.05, 1.45) vol.% for the adenosine treatment group, and did not differ significantly between the two treatments. The infusion MAC values were not significantly different (p = 0.16) and were 1.25 (0.95, 1.35) vol.% and 1.05 (1.00, 1.25) vol.%, respectively. The post-infusion MAC values differed significantly (p = 0.016); MAC was 1.15 (1.15, 1.35) vol.% and 1.05 (1.05, 1.25) vol.% for the saline and adenosine treatment groups, respectively. During infusion, mean arterial blood pressure decreased significantly (p = 0.008) during adenosine treatment compared with the saline 66 mmHg (52, 72) and 91 mmHg (68, 110), respectively. End-tidal CO2 (Pe′CO2), urine production, hematocrit, and plasma total solids did not differ significantly between the two treatments at any time (all p > 0.05). ConclusionAlthough the MAC of isoflurane in dogs was not decreased significantly during infusion with adenosine (0.3 mg kg−1 minute−1), it was significantly decreased post-infusion, but only by 0.1 vol.%, an amount not considered clinically important. Adenosine infusion decreased mean arterial pressure by 27% and did not adversely affect renal function.

Effects of oxymorphone and hydromorphone on the minimum alveolar concentration of isoflurane in dogs

ObjectivesTo quantify the change in the minimum alveolar concentration (MAC) of isoflurane (ISO) associated with oxymorphone (OXY) or hydromorphone (HYDRO) in dogs. DesignRandomized crossover study with at least 1 week between assessments. AnimalsSix young, healthy, mixed-breed dogs (1–3 years old), weighing 24.7 ± 4.70 kg. MethodsFollowing mask induction, anesthesia was maintained with ISO in 100% O2 using mechanical ventilation. The dogs received 0.05 mg kg−1 OXY, 0.1 mg kg−1 HYDRO, or 1 mL saline (control) IV. Following equilibration (15 minutes) at each percentage ISO tested, a supramaximal electrical stimulus was applied to the toe web and the response was assessed. Two separate MAC determinations were carried out during 4.5 hours of anesthesia, with completion of the evaluations at 1.5–2 and 4–4.5 hours after drug administration. A two-factor anova was used to determine whether there was a time or treatment effect on MAC and a Tukey test compared the drug effects at each time. Significance is reported at p < 0.05. ResultsThe mean MAC values (±SD) were 1.2 ± 0.18 and 1.2 ± 0.16% for control, 0.7 ±0.15 and 1.0 ± 0.15% for OXY, and 0.6 ± 0.14 and 0.8 ± 0.17% for HYDRO. The initial MAC with OXY and the MAC determined at both times with HYDRO were significantly different from the control MAC values. ConclusionsBoth OXY and HYDRO significantly reduced the MAC of ISO in dogs at 2 hours. At approximately 4.5 hours, HYDRO had a significant MAC-sparing effect, whereas OXY did not. Clinical relevanceAlthough both OXY and HYDRO resulted in a significant reduction in the MAC of ISO at approximately 2 hours, HYDRO may be preferred for procedures of long duration and rarely needs repeated dosing before 4.5 hours.

Comparisons of the effects of acupuncture, electroacupuncture, and transcutaneous cranial electrical stimulation on the minimum alveolar concentration of isoflurane in dogs

To compare the effects of acupuncture (AP), electroacupuncture (EA), and transcutaneous cranial electrical stimulation (TCES) with high-frequency intermittent currents on the minimum alveolar concentration (MAC) of isoflurane and associated cardiovascular variables in dogs. 8 healthy adult female Beagles. Each dog was anesthetized with isoflurane on 4 occasions, allowing a minimum of 10 days between experiments. Isoflurane MAC values were determined for each dog without treatment (controls) and after treatment with AP and EA (AP points included the Large Intestine 4, Lung 7, Governing Vessel 20, Governing Vessel 14, San Tai, and Baihui) and TCES. Isoflurane MAC values were determined by use of noxious electrical buccal stimulation. Heart rate, mean arterial blood pressure (MAP), arterial blood oxygen saturation (Spo2) measured by use of pulse oximetry, esophageal body temperature, inspired and expired end-tidal isoflurane concentrations, end-tidal carbon dioxide concentration, and bispectral index (BIS) were monitored. Blood samples were collected for determination of plasma cortisol concentration. Mean +/- SD baseline MAC of isoflurane was 1.19 +/- 0.1%. Acupuncture did not significantly change MAC of isoflurane. Treatments with EA and TCES significantly lowered the MAC of isoflurane by 10.1% and 13.4%, respectively. The Spo2, heart rate, MAP, BIS, esophageal body temperature, and plasma cortisol concentration were not significantly different after AP, EA, TCES, and control treatments at any time interval. Use of EA and TCES decreased MAC of isoflurane in dogs without inducing adverse hemodynamic effects. However, the reduction in isoflurane MAC by EA andTCES treatments was not considered clinically relevant.

Effects of morphine, lidocaine, ketamine, and morphine-lidocaine-ketamine drug combination on minimum alveolar concentration in dogs anesthetized with isoflurane.

To determine the effects of constant rate infusion of morphine, lidocaine, ketamine, and morphine-lidocaine-ketamine (MLK) combination on end-tidal isoflurane concentration (ET-Iso) and minimum alveolar concentration (MAC) in dogs anesthetized with isoflurane and monitor depth of anesthesia by use of the bispectral index (BIS). 6 adult dogs. Each dog was anesthetized with isoflurane on 5 occasions, separated by a minimum of 7 to 10 days. Individual isoflurane MAC values were determined for each dog. Reduction in isoflurane MAC, induced by administration of morphine (3.3 microg/kg/min), lidocaine (50 microg/kg/min), ketamine (10 microg/kg/min), and MLK, was determined. Heart rate, mean arterial blood pressure, oxygen saturation as measured by pulse oximetry (Spo2), core body temperature, and BIS were monitored. Mean +/- SD isoflurane MAC was 1.38 +/- 0.08%. Morphine, lidocaine, ketamine, and MLK significantly lowered isoflurane MAC by 48, 29, 25, and 45%, respectively. The percentage reductions in isoflurane MAC for morphine and MLK were not significantly different but were significantly greater than for lidocaine and ketamine. The Spo2, mean arterial pressure, and core body temperature were not different among groups. Heart rate was significantly decreased at isoflurane MAC during infusion of morphine and MLK. The BIS was inversely related to the ET-Iso and was significantly increased at isoflurane MAC during infusions of morphine and ketamine, compared with isoflurane alone. Low infusion doses of morphine, lidocaine, ketamine, and MLK decreased isoflurane MAC in dogs and were not associated with adverse hemodynamic effects. The BIS can be used to monitor depth of anesthesia.

Effect of intravenous lidocaine on isoflurane MAC in dogs

Lidocaine decreases minimum alveolar concentration (MAC) of inhalational anesthetics. This study determined the influence of a low dose, 50 µg kg−1 minute−1 (LDI) and high dose, 200 µg kg−1 minute−1 (HDI) constant rate infusion of lidocaine on the MAC of isoflurane (I) in dogs. Ten mongrel dogs were anesthetized with I in oxygen and mechanically ventilated. End-tidal anesthetic (Fe′A) and CO2 (Pe′CO2) concentrations were monitored at the endotracheal tube adaptor with an infrared gas analyzer calibrated before each experiment with a standardized calibration gas mixture designed for the analyzer. Pe′CO2 and body temperature were maintained within normal limits. Noxious stimuli included clamping the hindlimb paw (HC) and electrical current (50 V at 50 cycles second−1 for 10 milliseconds pulse−1) applied subcutaneously to the forelimb (FE) at the level of the ulna. After an initial equilibration period of at least 40 minutes at an Fe′A of 1.7%, the Fe′A was decreased to a value close to the estimated MAC for dogs. MAC was defined as the Fe′A mid-way between the value permitting and preventing purposeful movement. Following baseline MAC, a loading dose of 2 mg kg−1 of lidocaine IV was administered over 3 minutes followed by the LDI, and MAC determinations for the combination started after 30 minutes of infusion. Once determined, the lidocaine infusion was stopped for 30 minutes and the dog maintained at the ETC that prevented movement without the lidocaine. Following this period, a second loading dose of lidocaine was given (2 mg kg−1) over 3 minutes followed by the HDI, and the MAC determination procedure repeated after 30 minutes of infusion. Data were analyzed using an anova for repeated measures. MAC of I was 1.34 ± 0.035% (mean ± SEM) for both the FE and HC stimuli. The LDI significantly decreased MAC to 1.09 ± 0.043% (18.7% reduction) and HDI to 0.76 ± 0.030% (43.3% reduction). In conclusion, lidocaine infusions decreased the MAC of isoflurane in a dose-dependent manner.

Effects of diazepam and flumazenil on minimum alveolar concentrations for dogs anesthetized with isoflurane or a combination of isoflurane and fentanyl.

To determine the effect of a constant-rate infusion of fentanyl on minimum alveolar concentration (MAC) of isoflurane and to determine the interaction between fentanyl and a benzodiazepine agonist (diazepam) and antagonist (flumazenil) in isoflurane-anesthetized dogs. 8 mixed-breed adult dogs. Dogs were anesthetized with isoflurane 3 times during a 6-week period. After a 30-minute equilibration period, each MAC determination was performed in triplicate, using standard techniques. Fentanyl was administered as a bolus (10 microg/kg of body weight, IV) that was followed by a constant infusion (0.3 microg/kg per min, IV) throughout the remainder of the experiment. After determining isoflurane-fentanyl MAC in triplicate, each dog received saline (0.9% NaCl) solution, diazepam, or flumazenil. After 30 minutes, MAC was determined again. Fentanyl significantly decreased isoflurane MAC (corrected to a barometric pressure of 760 mm Hg) from 1.80+/-0.21 to 0.85+/-0.14%, a reduction of 53%. Isoflurane-fentanyl-diazepam MAC (0.48+/-0.29%) was significantly less than isoflurane-fentanyl-saline MAC (0.79+/-0.21%). Percentage reduction in isoflurane MAC was significantly greater for fentanyl-diazepam (74%), compared with fentanyl-saline (54%) or fentanyl-flumazenil (61%). Mean fentanyl concentrations for the entire experiment were increased over time and were higher in the diazepam group than the saline or flumazenil groups. Fentanyl markedly decreased isoflurane MAC in dogs. Diazepam, but not flumazenil, further decreased isoflurane-fentanyl MAC. Our results indicate that diazepam enhances, whereas flumazenil does not affect, opioid-induced CNS depression and, possibly, analgesia in dogs.