Minimum Alveolar Concentration Multiples Research Articles

Experimental study on the effects of isoflurane with and without remifentanil or dexmedetomidine on heart rate variability before and after nociceptive stimulation at different MAC multiples in cats

BackgroundHeart rate variability (HRV) provides information about autonomic nervous system (ANS) activity and is therefore a possible tool with which to assess anaesthetic depth. The aim of the present study was to evaluate the effects of isoflurane, remifentanil and dexmedetomidine on HRV before and after nociceptive stimulation at different anaesthetic depths.Seven healthy domestic short-hair cats were used, and each cat was anaesthetized three times – group I with isoflurane alone, group IR with isoflurane and a constant rate infusion (CRI) of remifentanil (18 μg/kg/h), and group ID with isoflurane and a CRI of dexmedetomidine (3 μg/kg/h). Minimum alveolar concentration (MAC) values were determined via electrical supramaximal nociceptive stimulation for each treatment group. Nociceptive stimulation was repeated at 3 different MAC multiples (0.75, 1.0 and 1.5 MAC), and electrocardiographic recordings were performed for 3 min before and after stimulation. Only the 1 min epochs were used for further statistical analysis. Electrocardiographic data were exported for offline HRV analysis.ResultsThe mean isoflurane MAC ± standard deviation (SD) was 1.83 ± 0.22 vol% in group I, 1.65 ± 0.13 vol% in group IR and 0.82 ± 0.20 vol% in group ID. Nociception was indicated by several HRV parameters, however, with high variability between treatments. The best correlation with MAC was found for the SD of heart rate (STD HR) in group I (rs = − 0.76, p = 0.0001, r2 = 0.46). STD HR was also able to distinguish 0.75 MAC from 1.5 MAC and 1.0 MAC from 1.5 MAC in group I, as well as 0.75 MAC from 1.5 MAC in group ID.ConclusionsThe choice of anaesthetic protocol influences the HRV parameters in cats. Frequency domain parameters respond to nociception at lower MAC levels. The STD HR has the potential to provide additional information for the assessment of anaesthetic depth in isoflurane-anaesthetized cats. The utility of HRV analysis for the assessment of anaesthetic depth in cats is still questionable.

Effects on indicators of tissue perfusion in dogs anesthetized with isoflurane at two multiples of the minimum alveolar concentration.

To investigate the effects of isoflurane anesthesia administered at 2 multiples of the minimum alveolar concentration (MAC) on tissue perfusion in dogs. 8 healthy young adult Beagles. A randomized crossover design was used. Dogs were anesthetized with isoflurane at 1.5 or 2.0 times the MAC for 2 hours, a 7-day washout period was provided, and dogs were reanesthetized with the alternate treatment. Various physiologic variables were monitored before anesthesia (baseline), at 20-minute intervals during anesthesia, and after anesthetic recovery. Variable values were compared between MAC multiples by means of repeated-measures ANOVA, with the Tukey test used for multiple comparisons. During anesthesia, mean arterial blood pressure, cardiac output, and mixed venous oxygen saturation were significantly greater when isoflurane was administered at 1.5 versus 2.0 times the MAC. Cardiac output gradually increased during anesthesia at 1.5 times but not at 2.0 times the MAC. Arterial blood lactate concentration did not differ between MAC multiples at any point; however, this concentration decreased with increasing anesthetic duration at both MAC multiples. Oxygen delivery differed between MAC multiples, and oxygen consumption differed from baseline during anesthesia at 2.0 times the MAC. Oxygen extraction was higher at 2.0 versus 1.5 times the MAC. Heart rate differed between MAC multiples only after anesthetic recovery. Isoflurane anesthesia impaired tissue perfusion in dogs, but these changes would not be clinically relevant with oxygen delivery at 100%. Peripheral tissue perfusion was maintained or improved with time.

Effects of isoflurane with and without dexmedetomidine or remifentanil on heart rate variability before and after nociceptive stimulation at different multiples of minimum alveolar concentration in dogs

To evaluate the influence of 3 anesthetic protocols and multiples of minimum alveolar concentration (MAC) on heart rate variability (HRV) with and without nociceptive stimulation in dogs. 6 healthy adult Beagles. Each dog was anesthetized 3 times: with isoflurane alone, with isoflurane and a constant rate infusion of dexmedetomidine (IsoD; 3 μg/kg/h, IV), and with isoflurane and a constant rate infusion of remifentanil (IsoR; 18 μg/kg/h, IV). Individual MAC was determined via supramaximal electrical stimulation. Sinus rhythm-derived intervals between 2 adjacent R-R intervals were exported from ECG recordings. Selected HRV time and frequency domain variables were obtained (at 2-minute intervals) and analyzed offline with signed rank tests before and after stimulation at 0.75, 1.0, and 1. 5 MAC for each anesthetic session. The isoflurane session had the overall lowest prestimulation SDNN (SD of all R-R intervals) values. Prestimulation SDNN values decreased significantly with increasing MAC in all sessions. For the IsoD session, SDNN (milliseconds) or high-frequency power (milliseconds(2)) was inversely correlated with MAC (Spearman rank correlation coefficient for both variables, -0.77). In the isoflurane and IsoR sessions, heart rate increased significantly after stimulation. In the IsoD session, poststimulation SDNN was increased significantly, compared with prestimulation values, at 0.75 and 1.0 MAC. On the basis of SDNN and high-frequency power values, anesthetic levels between 0.75 and 1.5 MAC within the same anesthetic protocol could be differentiated, but with a large overlap among protocols. Usefulness of standard HRV variables for assessment of anesthetic depth and nociception in dogs is questionable.

Correction: Intracranial pressure and cardiopulmonary variables during isoflurane or sevoflurane anesthesia at various minimum alveolar concentration multiples in normocapnic dogs

"Correction: Intracranial pressure and cardiopulmonary variables during isoflurane or sevoflurane anesthesia at various minimum alveolar concentration multiples in normocapnic dogs" published on 01 Apr 2013 by American Veterinary Medical Association.

Intracranial pressure and cardiopulmonary variables during isoflurane or sevoflurane anesthesia at various minimum alveolar concentration multiples in normocapnic dogs

To compare effects of isoflurane and sevoflurane on intracranial pressure and cardiovascular variables at 1.0, 1.5, and 2.0 times the minimum alveolar concentration (MAC) in mechanically ventilated normocapnic dogs. 6 healthy male Beagles. The individual MAC was determined for each agent with an electrical stimulus. After a minimum of 1 week, anesthetic induction by use of a mask with one of the inhalation anesthetics selected randomly was followed by mechanical ventilation and instrumentation for measurement of intracranial pressure and cardiovascular variables. Heart rate; systolic, mean, and diastolic arterial blood pressures; central venous pressure; mean pulmonary arterial pressure; pulmonary artery occlusion pressure; cardiac output; intracranial pressure (ICP); core body temperature; end-tidal inhalation anesthetic and carbon dioxide concentration; and arterial blood gas values were measured after attaining equilibrium at 1.0, 1.5, and 2.0 MAC of each inhalation anesthetic. Cardiac index, systemic vascular resistance, pulmonary vascular resistance, and cerebral perfusion pressure (CPP) were calculated. Mean ICP did not differ within and between anesthetics at any MAC. Compared with equipotent concentrations of isoflurane, the CPP and mean values for systolic, mean, and diastolic arterial blood pressures were increased at 2.0 MAC for sevoflurane, whereas mean values for mean and diastolic arterial blood pressures and systemic vascular resistance were increased at 1.5 MAC for sevoflurane. Although ICP was similar in healthy normocapnic dogs, CPP was better maintained during 2.0 MAC for sevoflurane, compared with isoflurane.

Comparison of the cardiovascular effects of equipotent anesthetic doses of sevoflurane alone and sevoflurane plus an intravenous infusion of lidocaine in horses

To compare cardiovascular effects of sevoflurane alone and sevoflurane plus an IV infusion of lidocaine in horses. Animals-8 adult horses. Each horse was anesthetized twice via IV administration of xylazine, diazepam, and ketamine. During 1 anesthetic episode, anesthesia was maintained by administration of sevoflurane in oxygen at 1.0 and 1.5 times the minimum alveolar concentration (MAC). During the other episode, anesthesia was maintained at the same MAC multiples via a reduced concentration of sevoflurane plus an IV infusion of lidocaine. Heart rate, arterial blood pressures, blood gas analyses, and cardiac output were measured during mechanical (controlled) ventilation at both 1.0 and 1.5 MAC for each anesthetic protocol and during spontaneous ventilation at 1 of the 2 MAC multiples. Cardiorespiratory variables did not differ significantly between anesthetic protocols. Blood pressures were highest at 1.0 MAC during spontaneous ventilation and lowest at 1.5 MAC during controlled ventilation for either anesthetic protocol. Cardiac output was significantly higher during 1.0 MAC than during 1.5 MAC for sevoflurane plus lidocaine but was not affected by anesthetic protocol or mode of ventilation. Clinically important hypotension was detected at 1.5 MAC for both anesthetic protocols. Lidocaine infusion did not alter cardiorespiratory variables during anesthesia in horses, provided anesthetic depth was maintained constant. The IV administration of lidocaine to anesthetized nonstimulated horses should be used for reasons other than to improve cardiovascular performance. Severe hypotension can be expected in nonstimulated horses at 1.5 MAC sevoflurane, regardless of whether lidocaine is administered.

Relationship of bispectral index to hemodynamic variables and alveolar concentration multiples of sevoflurane in puppies

The relationships between bispectral index (BIS), cardiovascular variables and minimum alveolar concentration (MAC) multiples of sevoflurane in puppies were determined. Five puppies were anesthetized with sevoflurane on two occasions. First, the individual sevoflurane MAC values were determined for each puppy. Secondly, dogs were anesthetized with sevoflurane at each of 5 MAC multiples, 0.75, 1, 1.25, 1.5 and 1.75 MAC administered in random order. Hemodynamic parameters and BIS data were collected for 20 min. Somatic stimulus was then applied and the same parameters and data were collected for 6 min. Correlation between BIS and end tidal sevoflurane and between BIS and hemodynamic parameters were studied. We found positive significant correlation in both cases. BIS is lower in puppies that in adults at the same alveolar anesthetic concentrations and sevoflurane appears to be a safe anesthetic in puppies.

Comparison of variability in cardiorespiratory measurements following desflurane anesthesia at a multiple of the minimum alveolar concentration for each dog versus a multiple of a single predetermined minimum alveolar concentration for all dogs in a group

To determine whether the variability of cardiorespiratory measurements is smaller when administering desflurane at a multiple of the individual's minimum alveolar concentration (MAC) or at a predetermined, identical concentration in all subjects. 10 dogs. Desflurane was administered at 1.5 times the individual's MAC (iMAC) and 1.5 times the group's MAC (gMAC). The order of concentrations was randomly selected. Heart rate, respiratory rate, arterial blood pressure, central venous pressure, mean pulmonary artery pressure, pulmonary artery occlusion pressure, arterial and mixed-venous blood gas tensions and pH, and cardiac output were measured. The desflurane concentration required to achieve a mean arterial pressure (MAP) of 60 mm Hg was then determined. Finally, the desflurane concentration required to achieve an end-tidal PCO(2) of 55 mm Hg was measured. Variances when administering 1.5 iMAC or 1.5 gMAC were not significantly different for any variable studied. Differences between the MAC multiples needed to reach an MAP of 60 mm Hg and the mean of the sample were significantly larger when gMAC was used, compared with iMAC, indicating that a multiple of iMAC better predicted the concentration resulting in a MAP of 60 mm Hg. Results suggest that, in a small group of dogs, variability in cardiorespiratory measurements among dogs is unlikely to differ whether an inhalant anesthetic is administered at a multiple of the iMAC in each dog or at an identical gMAC in all dogs.

Differences in need for hemodynamic support in horses anesthetized with sevoflurane as compared to isoflurane

Objective To study whether hemodynamic function in horses, particularly mean arterial blood pressure (MAP), is better maintained with sevoflurane than isoflurane, thus requiring less pharmacological support.Study design Prospective randomized clinical investigation.Animals Thirty-nine racehorses undergoing arthroscopy in lateral recumbency.Methods Horses were assigned to receive either isoflurane (n = 20) or sevoflurane (n = 19) at 0.9–1.0 minimum alveolar concentration (MAC) for maintenance of anesthesia. Besides routine clinical monitoring, cardiac output (CO) was measured by lithium dilution. Hemodynamic support was prescribed as follows: when MAP decreased to <70 mmHg, patients were to receive infusion of 0.1% dobutamine, which was to be discontinued at MAP >85 mmHg or heart rate >60 beats minute−1. Statistical analysis of results, given as mean ± SD, included a clustered regression approach.Results Average inhalant anesthetic time [91 ± 35 (isoflurane group) versus 97 ± 26 minutes (sevoflurane group)] and dose (in MAC multiples), volume of crystalloid solution infused, and cardiopulmonary parameters including CO were similar in the two groups, except heart rate was 8% higher in isoflurane than sevoflurane horses (p < 0.05). To maintain MAP >70 mmHg, isoflurane horses received dobutamine over a significantly longer period (55 ± 26 versus 28 ± 21% of total anesthetic time, p < 0.01) and at a 51% higher dose than sevoflurane horses (41 ± 19 versus 27 ± 23 μg kg−1 MAC hour−1; p = 0.058), with 14/20 isoflurane animals and only 9/19 sevoflurane horses being infused with dobutamine at >30 μg kg−1 MAC hour−1 (p < 0.05). Dobutamine infusion rates were consistently lower in the sevoflurane as compared to the isoflurane group, with differences reaching significance level during the 0–30 minutes (p < 0.01) and 61–90 minutes periods (p < 0.05).Conclusions and clinical relevance Horses under sevoflurane anesthesia may require less pharmacological support in the form of dobutamine than isoflurane-anesthetized horses. This could be due to less suppression of vasomotor tone.

The Effects of Stimulation Pattern and Sevoflurane Concentration on Intraoperative Motor-Evoked Potentials

The usefulness of intraoperative monitoring of motor-evoked potentials (MEPs) during inhaled anesthesia is limited by the suppressive effects of volatile anesthetics on MEP signals. We investigated the effects of different stimulation patterns and end-tidal concentrations of sevoflurane on intraoperative transcranial electrical MEPs. In 12 patients undergoing craniotomy, stimulation patterns (300-500 V, 100-1000 Hz, 1-5 stimuli) and multiples (0.5, 0.75, and 1.0) of minimum alveolar concentration (MAC) of sevoflurane were varied randomly while remifentanil was administered at a constant rate of 0.2 microg x kg(-1) x min(-1). MEPs were recorded from thenar and hypothenar muscles and analyzed without knowledge of the respective MAC. Three-way analysis of variance revealed significant main effects for increasing stimulation intensity, frequency, and number of stimuli on MEP amplitude (P < 0.05). Maximum MEP amplitudes and recording success rates were observed during 4 stimuli delivered at 1000 Hz and 300 V. A significant main effect of sevoflurane concentration (0.5 versus 0.75 and 1 MAC multiple) on MEP amplitude was observed at the thenar recording site only (P < 0.05). In conclusion, MEP characteristics varied significantly with changes in stimulation pattern and less so with changes in sevoflurane concentration. The results suggest that high frequency repetitive stimulation allows intraoperative use of MEP monitoring during up to 1 MAC multiple of sevoflurane and constant infusion of remifentanil up to 0.2 microg x kg(-1) x min(-1).

Effects of sevoflurane dose and mode of ventilation on cardiopulmonary function and blood biochemical variables in horses

To quantitate effects of dose of sevoflurane and mode of ventilation on cardiovascular and respiratory function in horses and identify changes in serum biochemical values associated with sevoflurane anesthesia. 6 healthy adult horses. Horses were anesthetized twice: first, to determine the minimum alveolar concentration (MAC) of sevoflurane and second, to characterize cardiopulmonary and serum biochemical responses of horses to 1.0, 1.5, and 1.75 MAC multiples of sevoflurane during controlled and spontaneous ventilation. Results-Mean (+/- SEM) MAC of sevoflurane was 2.84 +/- 0.16%. Cardiovascular performance during anesthesia decreased as sevoflurane increased; the magnitude of cardiovascular depression was more severe during mechanical ventilation, compared with spontaneous ventilation. Serum inorganic fluoride concentration increased to a peak of 50.8 +/- 7.1 micromol/L at the end of anesthesia. Serum creatinine concentration and sorbitol dehydrogenase activity reached their greatest values (2.0 +/- 0.8 mg/dL and 10.2 +/- 1.8 U/L, respectively) at 1 hour after anesthesia and then returned to baseline by 1 day after anesthesia. Serum creatine kinase, aspartate aminotransferase, and alkaline phosphatase activities reached peak values by the first (ie, creatine kinase) or second (ie, aspartate aminotransferase and alkaline phosphatase) day after anesthesia. Sevoflurane causes dose-related cardiopulmonary depression, and mode of ventilation further impacts the magnitude of this depression. Except for serum inorganic fluoride concentration, quantitative alterations in serum biochemical indices of liver- and muscle-cell disruption and kidney function were considered clinically unremarkable and similar to results from comparable studies of other inhalation anesthetics.

Application of Bispectral Index and Narcotrend index to the measurement of the electroencephalographic effects of isoflurane with and without burst suppression.

The Narcotrend monitor (MonitorTechnik, Bad Bramstedt, Germany) has recently been introduced as an intraoperative monitor of anesthetic state, based on a classification scheme originally developed for visual assessment of the electroencephalogram. The authors compared the performance of the Narcotrend index (software version 4.0) to the Bispectral Index (BIS, version XP; Aspect Medical Systems, Natick, MA) as electroencephalographic measures of isoflurane drug effect during general anesthesia. The authors observed 15 adult patients scheduled to undergo radical prostatectomy with a combined epidural-isoflurane general anesthesia technique. At least 45 min after induction of general anesthesia, during a phase of constant surgical stimulation, end-tidal isoflurane concentrations were varied between 0.5 and 2.0 multiples of minimum alveolar concentration, and the BIS and the Narcotrend index were recorded. The prediction probability (PK) was calculated for the BIS and the Narcotrend index to predict isoflurane effect compartment concentration for each measure. The correlation analysis of the BIS and the Narcotrend index with the isoflurane effect compartment concentration was obtained by pharmacodynamic modeling based on two sigmoidal curves to account for the discontinuity in both indices with the onset of burst suppression. The prediction probabilities were indistinguishable (BIS PK = 0.72 +/- 0.07 (mean +/- SD); range, 0.61-0.84; Narcotrend index PK = 0.72 +/- 0.10; range, 0.51-0.87), as were the correlations between the electroencephalographic measures and isoflurane effect compartment concentrations (BIS R = 0.82 +/- 0.12; Narcotrend index R = 0.85 +/- 0.09). The pharmacodynamic models for the BIS and the Narcotrend index yielded nearly identical results. The BIS and the Narcotrend index detected the electroencephalographic effects of isoflurane equally. Combining two fractional sigmoid Emax models adequately described the data before and after the onset of burst suppression.

The cardiovascular dose–response effects of isoflurane alone and combined with butorphanol in the green iguana (Iguana iguana)

ObjectiveTo assess the cardiovascular effects (arterial blood pressure, heart rate, and metabolic acid–base status) of three doses (MAC multiples) of isoflurane alone and combined with butorphanol in the green iguana (Iguana iguana). Study designProspective randomized double-blind, two-period cross-over trial. AnimalsSix mature healthy green iguanas (Iguana iguana). MethodsThe iguanas received each of two treatments, saline 0.1 mL kg−1 (SAL) and butorphanol 1.0 mg kg−1 (BUT) during isoflurane anesthesia. Treatments were separated by at least 1 week. The iguanas were exposed to each of the three minimum alveolar concentration (MAC) multiples (1.0, 1.5, and 2.0) in random order. Anesthesia was induced with isoflurane and maintained using controlled ventilation. Instrumentation included use of an ECG, airway gas monitor, cloacal thermometer, esophageal pulse oximeter, and the placement of a femoral arterial catheter. Body temperature was stabilized and maintained at 32 °C. The treatment was administered, and the animals were equilibrated for 20 minutes at each MAC multiple. At each concentration, the heart rate, blood pressure (systolic, mean, diastolic), end-tidal CO2, and SpO2 were measured. At 1.0 and 2.0 MAC, simultaneous blood samples were drawn from the tail vein/artery complex and femoral catheter for blood gas analysis. Data were analyzed using a two-way analysis of variance for repeated measures looking for differences between treatments and among MAC multiples. ResultsThere were no significant differences in any of the cardiovascular variables between the treatments. Significant differences among isoflurane MAC multiples were observed for HR, mean, diastolic, and systolic blood pressures. Blood pressure and heart rate decreased with an increasing dose of anesthetic. There were no significant differences between treatments or MAC multiples for any of the blood gas variables. The blood pH, PCO2, HCO3−, and hemoglobin saturation differed significantly between sites. Pulse oximetry values measured from the carotid complex did not correlate with and were significantly different from the calculated hemoglobin saturation values determined using the gas analyzer. Conclusion and clinical relevanceCardiovascular depression associated with isoflurane anesthesia in the green iguana is dose dependent. The degree of cardiovascular depression was not significantly different when isoflurane was combined with butorphanol. This finding suggests that the pre-emptive or intraoperative use of butorphanol is unlikely to be detrimental to cardiovascular function. Butorphanol may be a useful anesthetic adjunct to isoflurane anesthesia in the green iguana.

Comparison of sevoflurane and isoflurane anesthetic index in unpremedicated dogs

Anesthetic respiratory effects of sevoflurane (SEVO) were compared with isoflurane (ISO) in unpremedicated dogs. Minimum alveolar concentration (MAC), apneic concentration (AC), and anesthetic index (AI) of SEVO and ISO were determined in eight 1-year-old healthy dogs, weighing 19 ± 3 kg (mean ± SEM) in a randomized complete block multiple cross-over design. Dogs were mask-induced with either SEVO or ISO in 100% oxygen. Following endotracheal intubation, dogs were instrumented, mechanically ventilated, and MAC was determined using a tail-clamp method. Next, spontaneous ventilation was re-established, and anesthetic concentration was increased to determine the AC. Throughout the anesthetic event, heart rate (HR), systolic blood pressure (SAP), mean blood pressure (MAP), diastolic blood pressure (DAP), respiratory rate (RR), end-tidal carbon dioxide (Pe′CO2), and oxyhemoglobin saturation (SpO2) were recorded at 3-minute intervals. Following AC determination, AI was calculated as AC/MAC, and dogs were allowed to recover. Each dog was anesthetized four times (twice with ISO and SEVO each) at 1-week intervals. All data were analyzed using the two-way anova. Multiple comparisons were performed between ISO and SEVO treatments. Statistical significance was set at p < 0.05. Significant differences were noted between agents for MAC (SEVO, 2.13 ± 0.10%; ISO, 1.38 ± 0.14%; p < 0.0001), AC (SEVO, 7.34 ± 0.13%; ISO, 3.60 ± 0.13%; p < 0.0001), and AI (SEVO, 3.46 ± 0.22; ISO, 2.63 ± 0.14; p = 0.0002). Physiologic parameters were compared between SEVO and ISO at 1MAC, 2MAC, 3MAC, and AC. No differences were noted between SEVO and ISO treatments for cardiovascular parameters (HR, SAP, MAP, DAP). Significant differences were noted, favoring SEVO, for all respiratory parameters (RR, Pe′CO2, SpO2) at increasing MAC multiples. Additionally, regression analysis was conducted for physiologic variable data points. Analysis of Pe′CO2 data points demonstrated a significant slope difference of −6.47 ± 1.02 (BSEVO − BISO; p < 0.0001; r2 = 0.6042) favoring SEVO. While expected dose-related ventilatory depression was noted for both agents, all the respiratory parameters for SEVO demonstrated less respiratory depression than ISO at equipotent doses. These results indicated that SEVO caused less dose-dependent ventilatory depression than ISO, having a significantly higher AI and causing less detrimental change in pulmonary parameters at increasing levels of MAC.

Quantification of dose-dependent respiratory depression in isoflurane-anesthetized horses

The magnitude of respiratory depression in equidae anesthetized with isoflurane has not been thoroughly described. The aim of this study was to quantify changes in ventilation as a function of isoflurane (ISO) dose in the horse. Anesthesia was induced and maintained in five healthy adult horses weighing 564 ± 52 kg (mean ± SD) in left lateral recumbency using only ISO in O2. During spontaneous ventilation, four doses of constant end-tidal ISO measured by a calibrated infra-red analyzer were studied in randomized order: 1.31, 1.57, 1.83, and 2.10%; corresponding to the minimum alveolar concentration (MAC), 1.2 MAC, 1.4 MAC, and 1.6 MAC, respectively. The PaO2 and PaCO2 were measured using an automated blood gas analyzer and corrected to tonometrically derived standard curves. Respiratory rate (f) and tidal volume (VT) were measured via calibrated signal integration from a Silverman-style pneumotachometer; volumes were corrected to BTPS. Minute ventilation (VE) was the calculated product of VT and f. Data were analyzed using a repeated measures anova; Tukey's test was used for pairwise comparisons (p < 0.05). The following results are listed in order of ascending MAC multiples: PaCO2 (mm Hg) = 68 ± 14, 68 ± 9, 72 ± 9, 78 ± 5; f (min−1) = 4.3 ± 2.4, 3.5 ± 1.6, 2.5 ± 1.0, 2.0 ± 0.7; VT (mL kg−1) = 23 ± 11, 22 ± 11, 22 ± 12, 21 ± 7; (mL minute−1 kg−1) = 84 ± 41, 67 ± 27, 50 ± 23, 39 ± 11. Significant differences were observed between the highest and lowest MAC multiples for PaCO2, f, and VE, but not for VT. The PaO2 was always >80 mm Hg. The equations given below express median minute ventilation and median PaCO2 as a function of ISOMAC. Median values at each dose were used instead of mean values owing to the departure from sample variance uniformity. The y-intercepts coincide with published reports of and PaCO2 for awake, unmedicated horses (0 MAC). The x-intercept of Equation 1 predicts respiratory arrest at 2.4 MAC, near the apneic threshold for ISO. Future work directed towards increasing sample size in this study may remedy heterogeneous treatment variances present within the data and permit further refinement of dose–ventilation relationships in the horse. (1) PaCO2 = 22 × MAC + 39(2)

Bispectral index in dogs anesthetized with isoflurane: comparison with sevoflurane

Bispectral index (BIS) is a numerical value derived from the EEG that is used clinically by anesthesiologists to assess CNS depression in human beings. In a previous study, canine BIS values were inversely related to end-tidal sevoflurane (SEVO) concentration: BIS = 102.7 − (14.2 × [SEVO]); R2 = 0.754. The purpose of this study was to determine the relationship between BIS and end-tidal isoflurane (ISO) in the same dogs used in the SEVO study. ISO minimum alveolar concentration (MAC) was determined using the tail-clamp technique for six English pointer dogs (three males and three females) with a mean ± SD age and weight of 3.7 ± 2.0 years and 20.2 ± 4.8 kg, respectively. One week after MAC determination, BIS was measured at 0.8, 1.0, 1.5, and 2.0 MAC multiples of ISO. Ventilation was controlled and atracurium (0.2 mg kg−1 followed by 6 µg kg−1 minute−1 IV) was administered to eliminate EMG artefact from the EEG. The BIS was determined using an A-2000 monitor (Aspect Medical Systems Inc.). After a 15-minute equilibration period, BIS data were collected for 5 minutes and median BIS values were calculated. Heart rate, direct mean arterial blood pressure (MAP), esophageal temperature, and arterial blood gases were measured at each BIS collection period. End-tidal CO2 and percentage [ISO] were monitored using a calibrated infra-red gas analyzer. Data were analyzed using RM anova (p < 0.05). The mean ISOMAC value was 1.3 ± 0.2%. The BIS values were inversely related to [ISO]: BIS = 93.9 − (25.2 × [ISO]); R2 = 0.614. Mean ± SD BIS values at 0.8, 1.0, 1.5, and 2.0 MAC were 65 ± 8, 60 ± 7, 52 ± 3, and 15 ± 27, respectively. At 2 MAC, three dogs had burst suppression that prevented reliable BIS determination. The BIS values were significantly higher during SEVO at 0.8 and 1.0 MAC compared to ISO. Minor, yet statistically significant, differences between and within anesthetic groups were observed for cardiovascular and arterial blood gas data. We conclude that a predictive relationship exists between BIS and end-tidal [ISO]. However, BIS correlated with end-tidal [SEVO] better than with [ISO]. The BIS appears to be somewhat agent dependent in this group of six dogs. Clinical use of BIS monitoring in dogs may require evaluation of specific anesthetic drug combinations.

The Anesthetic Potency of Propanol and Butanol Versus Propanethiol and Butanethiol in α1 Wild Type and α1(S267Q) Glycine Receptors

UNLABELLED Although similar in shape and size, and although differing only by substitution of a sulfur atom for an oxygen atom, propanethiol and butanethiol differ markedly from propanol and butanol in their in vivo potency and physical properties. Recent theories of narcosis suggest that anesthetics may act by enhancing the effect of inhibitory agonists, such as glycine, on their receptors. We tested whether propanol, butanol, propanethiol, and butanethiol enhance the effect of glycine on alpha1 glycine receptors expressed in Xenopus laevis oocytes in a manner that reflects the in vivo differences found for potencies. As anticipated, we found an immediate parallel between in vivo (rat minimum alveolar concentration of anesthetic required to eliminate movement in response to a noxious stimulus in 50% of subjects) and in vitro (recombinant receptor) effects. All four compounds enhanced the effect of glycine on wild type receptors, and the extent of enhancement for a given minimum alveolar concentration-multiple was approximately the same for all compounds. We also found that propanethiol, butanethiol, propanol, and butanol did not affect, or minimally affected, the action of glycine in anesthetic resistant mutants in which the amino acid serine at position 267 was replaced by glutamine [alpha1(S267Q)]. IMPLICATIONS The in vivo potencies of propanethiol, butanethiol, propanol, and butanol correlate with their capacities to enhance the effect of glycine on alpha1 glycine receptors expressed in Xenopus laevis oocytes. These results support the notion that a protein mediates anesthetic action.

Effects of halothane, enflurane, and isoflurane on supraventricular and ventricular rate in dogs with complete atrioventricular block.

Complete atrioventricular (AV) block was produced in 32 chloralose-anesthetized autonomically intact dogs to determine the effects of halothane, enflurane, and isoflurane on supraventricular and ventricular rate. Halothane (n = 17), enflurane (n = 6), and isoflurane (n = 9) were administered in three separate experiments in sequential minimum alveolar concentration (MAC) multiples of 0.5, 1.0, 1.5, 2.0, 1.5, and 1.0. Supraventricular rate, ventricular rate, and mean arterial blood pressure (MAP) were measured and recorded at baseline and after a 20-minute equilibration period of each inhalation anesthetic at each MAC multiple. Increasing concentrations of enflurane and isoflurane significantly decreased supraventricular rate (P < .05). Ventricular rate was not significantly changed by sequential MAC multiples of halothane, enflurane, and isoflurane. Increasing concentrations of halothane, enflurane, and isoflurane significantly decreased MAP with enflurane producing the most significant decrease (P < .05). Ventricular arrhythmias occurred in 5 of 17 dogs anesthetized with halothane and 1 of 9 dogs anesthetized with isoflurane. Inhalation anesthesia can significantly decrease supraventricular rate and MAP, does not alter ventricular rate, and can produce ventricular arrhythmias in dogs with complete AV block.

Clinical and laboratory evaluation of an expired anesthetic gas monitor (Narko-Test).

A halothane analyzer (Narko-Test) was shown to have a linear response to various concentrations of cyclopropane, fluroxene, Ēthrane, diethyl ether, halothane and methoxyflurane. The factors necessary to convert the analyzer scale readings to the concentrations of the agents were 13, 2.3, 1.46, 1.25, 1.0 and 0.16, respectively. Corrections for saturated water vapor at room temperature and 100 per cent N2O were both 0.3 scale units. When used as a monitor of mixed expired gas in semiclosed or closed systems, the analyzer response was a measure of the anesthetic potency of a single agent or of the combined potency of a mixture of agents. The response to each agent was proportional to its solubility in silicon rubber or brain lipids; hence, the scale units may be regarded as multiples of minimum alveolar concentration (MAC).