Optimization of lung ventilation and perfusion in anesthetized horses using a ventilation mode with flow-limited expiration.

OBJECTIVE: To investigate the mechanisms underlying the improved arterial oxygenation described with flow-limited expiration (FLEX) ventilation in anesthetized horses. ANIMALS: 5 healthy adult research horses. METHODS: Horses underwent volume-controlled ventilation for 60 minutes (VCV1), followed by 60 minutes of FLEX, and 30 minutes of VCV (VCV2). Main outcomes included the arterial partial pressure of oxygen-to-Fio2 (PF) ratio and electrical impedance tomography (EIT)-derived functional indices at the end of each phase. The EIT data were used to create regional maps of relative lung ventilation and perfusion as well as regional maps of ventilation/perfusion (V/Q) ratios. Ventilation indices derived from EIT included the fraction of expired volume in 1 second (FEV1; %) and the time it took for the EIT signal to drop to 50% of the peak signal at end-inspiration (TClose50; seconds). Data were analyzed with 2-way ANOVA for repeated measures. P < .05 was considered significant. RESULTS: The PF ratio increased significantly with FLEX compared to both VCV1 and VCV2 (P < .01). There were no differences in the relative distribution of ventilation nor perfusion between ventilation strategies. However, when ventilation and perfusion were superimposed and V/Q ratio maps were constructed, FLEX had a homogenizing effect toward values of 1.0. The FEV1 was shorter (P < .01) and the TClose50 was longer (P < .001) in all regions during FLEX compared to both VCV1 and VCV2. CLINICAL RELEVANCE: Our findings suggest that FLEX ventilation in anesthetized horses enhances regional V/Q matching, likely by prolonging expiratory aeration and reducing airway closure.

Evaluation of Three Methods of Sensory Function Testing for the Assessment of Successful Maxillary Nerve Blockade in Horses.

Maxillary nerve blocks (MNBs) commonly facilitate dental surgeries in standing horses. The goal of this prospective, blinded, cross-over design trial including 15 client-owned horses was to evaluate 3 methods of sensory function testing for confirming a successful MNB. Testing was performed bilaterally before sedation, 5 min after sedation, and 15 and 30 min after MNB with 0.5% bupivacaine and involved a needle prick dorsal to each naris, hemostat clamping of each nostril, and gingival algometry (measuring sensitivity to pain). Responses to stimulation were numerically scored and scores were summed up to a total score. Total score increases on the blocked side by ≥ 2 between baseline and 30 min Post MNB recordings signified a successful MNB. Sedation in the preceding 6 h, presence of sino-nasal disease, side of dental pathology, age, butorphanol administration, and detomidine dosing (µg/kg/min) throughout the tooth extraction procedure were recorded. In 73% of horses, MNB was successful. Sedation in the preceding 6 h (P = .732), age (P = .936), side of pathology (P = .516), and sino-nasal disease (P = .769) were not associated with total scores. Detomidine dosage and butorphanol use did not differ between horses in which the MNB was considered successful and for those in which it was not (P = .967 and P = .538, respectively). Scores obtained with gingival algometry were less closely associated with total scores (rho = .649) than those obtained with needle prick and nostril clamping (rho = .819 and .892, respectively). Therefore, needle prick and nostril clamping are considered the more reliable methods for use in clinical practice to determine the success of an MNB.

Caudal epidural catheterization for pain management in 48 hospitalized horses: A descriptive study of demographics, complications, and outcomes.

The placement of caudal epidural catheters in horses has become more frequent as a multi-modal analgesic strategy. Despite its integration into clinical practice, there are limited reports describing the use of caudal epidural catheterization for prolonged use in horses. The purpose of this study was to characterize the hospitalized caseload undergoing epidural catheterization for long-term epidural analgesic administration, to report the response to epidural therapy and observed complications, and to describe patient outcomes. Medical records of hospitalized equine patients that underwent placement of a caudal epidural catheter for analgesic management between 2017 and 2021 were analyzed retrospectively. For the 62 catheters placed in the 48 cases, the most frequent diagnosis category prompting epidural analgesia was orthopedic (43/48, 89.6%). Synovial sepsis was the most frequent specific diagnosis prompting epidural catheter placement (11/48, 22.9%). The initial response to epidural therapy was characterized as positive for 37/62 (59.7%) catheters. Complications were documented for 46/62 (74.2%) catheters. However, most of these complications were classified as mild (51.6%) or moderate (14.5%), and exaggerated physiologic responses were observed most frequently. Of the horses studied, 52.1% survived to be discharged from the hospital. With awareness of potential complications and vigilant monitoring, caudal epidural catheters should be considered for equine patients as an analgesic strategy.

Flow-controlled expiration improves respiratory mechanics, ventilation, and gas exchange in anesthetized horses.

OBJECTIVE: Mechanical ventilation is usually achieved by active lung inflation during inspiration and passive lung emptying during expiration. By contrast, flow-controlled expiration (FLEX) ventilation actively reduces the rate of lung emptying by causing linear gas flow throughout the expiratory phase. Our aim was to evaluate the effects of FLEX on lung compliance and gas exchange in anesthetized horses in dorsal recumbency. ANIMALS: 8 healthy horses. PROCEDURES: All animals were anesthetized twice and either ventilated beginning with FLEX or conventional volume-controlled ventilation in a randomized, crossover design. Total anesthesia time was 3 hours, with the ventilatory mode being changed after 1.5 hours. During anesthesia, cardiac output (thermodilution), mean arterial blood pressures, central venous pressure, and pulmonary arterial pressure were recorded. Further, peak, plateau, and mean airway pressures and dynamic lung compliance (Cdyn) were measured. Arterial blood gases were analyzed every 15 minutes. Data were analyzed using ANOVA (P < 0.05). RESULTS: FLEX ventilation resulted in significantly higher arterial oxygen partial pressures (521 vs 227 mm Hg) and Cdyn (564 vs 431 mL/cm H2O) values compared to volume-controlled ventilation. The peak and plateau airway pressure were lower, but mean airway pressure was significantly higher (4.8 vs 9.2 cm H2O) in FLEX ventilated horses. No difference for cardiovascular parameters were detected. CLINICAL RELEVANCE: The results of this study showed a significant improvement of the Pao2 and Cdyn without compromising the cardiovascular system when horses were ventilated by use of FLEX compared to conventional ventilation.

Pharmacology of the Equine Foot: Medical Pain Management for Laminitis.

One of the biggest challenges in managing laminitis in horses remains the control of pain. The best analgesic approach is a multimodal approach, including nonsteroidal anti-inflammatory drugs, opioids, and/or constant rate infusions of α-2 agonists, ketamine, and lidocaine. Recent literature indicates that amitriptyline and soluble epoxide hydrolase inhibitor might be beneficial. Clinically oriented studies will be needed if they have a place in laminitis pain management. The systemic pain control can be combined with local techniques such as long-acting local anesthetics or epidural catheterization that allows for administration of potent analgesic therapy with a lower risk of negative side effects.

A critical evaluation for validation of composite and unidimensional postoperative pain scales in horses.

Proper pain therapy requires adequate pain assessment. This study evaluated the reliability and validity of the Unesp-Botucatu horse acute pain scale (UHAPS), the Orthopedic Composite Pain Scale (CPS) and unidimensional scales in horses admitted for orthopedic and soft tissue surgery. Forty-two horses were assessed and videotaped before surgery, up to 4 hours postoperatively, up to 3 hours after analgesic treatment, and 24 hours postoperatively (168 video clips). After six evaluators viewing each edited video clip twice in random order at a 20-day interval, they chose whether analgesia would be indicated and applied the Simple Descriptive, Numeric and Visual Analog scales, CPS, and UHAPS. For all evaluators, intra-observer reliability of UHAPS and CPS ranged from 0.70 to 0.97. Reproducibility was variable among the evaluators and ranged from poor to very good for all scales. Principal component analysis showed a weak association among 50% and 62% of the UHAPS and CPS items, respectively. Criterion validity based on Spearman correlation among all scales was above 0.67. Internal consistency was minimally acceptable (0.51-0.64). Item-total correlation was acceptable (0.3-0.7) for 50% and 38% of UHAPS and CPS items, respectively. UHAPS and CPS were specific (90% and 79% respectively), but both were not sensitive (43 and 38%, respectively). Construct validity (responsiveness) was confirmed for all scales because pain scores increased after surgery. The cut-off point for rescue analgesia was ≥ 5 and ≥ 7 for the UHAPS and CPS, respectively. All scales presented adequate repeatability, criterion validity, and partial responsiveness. Both composite scales showed poor association among items, minimally acceptable internal consistency, and weak sensitivity, indicating that they are suboptimal instruments for assessing postoperative pain. Both composite scales require further refinement with the exclusion of redundant or needless items and reduction of their maximum score applied to each item or should be replaced by other tools.

The effects of flumazenil on ventilatory and recovery characteristics in horses following midazolam-ketamine induction and isoflurane anaesthesia.

BACKGROUND: Flumazenil antagonises the actions of benzodiazepines. There has been no prior research specifically investigating this anaesthetic reversal agent for horses. OBJECTIVES: To determine the effects of flumazenil administration in horses on (a) ventilatory parameters after midazolam-ketamine induction and maintenance with isoflurane in oxygen and on (b) the characteristics of recovery from general anaesthesia. STUDY DESIGN: Blinded, randomised, crossover experiment. METHODS: Six horses were randomly assigned to receive high-dose flumazenil (Fhigh , 20 µg/kg), low-dose flumazenil (Flow , 10 µg/kg) and saline (control). Cardioventilatory parameters were monitored. After 90 minutes of isoflurane anaesthesia, a bolus of Fhigh , Flow or saline was administered i.v. The horses were recovered using head and tail rope assistance. The times to first movement, to achievement of sternal recumbency, to the first attempt to stand and the total recovery time were determined. The recovery quality was evaluated using a 115-point recovery scoring system. The cardioventilatory parameters and recovery times were analysed using mixed-effects regression analyses. Intraclass correlation (ICC) analysis was used to evaluate the recovery scores. A Mann-Whitney U test assessed the relationship between recovery score and flumazenil administration. RESULTS: A significant difference with flumazenil administration was found for SpO2 , mean arterial pressure, I:E ratio, minute volume of ventilation (MV) and peak inspiratory pressure. There was a significant difference with flumazenil administration for the time to sternal recumbency, the time to the first attempt to rise and the total recovery time. There was no significant difference in total recovery score with flumazenil administration. MAIN LIMITATIONS: Plasma levels of midazolam and flumazenil were not obtained. CONCLUSIONS: Flumazenil has a dose-dependent effect on MV and recovery time, which may make it useful in cases for which a prolonged anaesthetic recovery is undesirable.

Changes in Arterial Blood Pressure and Oxygen Tension as a Result of Hoisting in Isoflurane Anesthetized Healthy Adult Horses.

Background: In anesthetized adult horses, changes in recumbency can influence the cardiovascular system but how arterial blood pressures and oxygen tension change in isoflurane anesthetized animals as a direct result of hoisting has not been investigated. Objective: To evaluate effects of hoisting on hemodynamic function and pulmonary gas exchange in isoflurane-anesthetized horses. Study Design: Prospective, experimental study. Methods: Six adult horses were anesthetized three times using isoflurane in pure oxygen (inspired fraction 0.9-1.0), and allowed breathing spontaneously in lateral recumbency. After 45 min horses were hoisted using a single hoist-hobble system for 5 min and returned into left lateral recumbency. Heart rate (HR), respiratory rate (RR), and systolic (SAP), diastolic (DAP), and mean arterial blood pressures (MAP) were measured every minute starting from 5 min before to 5 min after hoisting. Arterial blood gas samples were collected before, during, and after hoisting. Results: Significant changes in hemodynamic parameters and PaO2 but not PaCO2 were found between baseline recordings and measurements obtained during and early after hoisting. The MAP decreased within the 1st min of hoisting from a mean of 74 ± 17 mmHg at baseline to 57 ± 20 mmHg (p < 0.05). Thereafter, it rapidly recovered to baseline before continuing to rise to higher than baseline values and then remaining elevated for 5 min after horses were returned into lateral recumbency. Simultaneously, the HR increased by 6-9 beats per min during the initial 3 min of hoisting before returning close to baseline values (p < 0.05). The PaO2 decreased significantly from a mean of 324.9 ±137.0 mmHg at baseline to a mean of 141.3 ± 104.2 mmHg during hoisting (p < 0.001) without recovering any more to baseline values. Clinical significance: Hoisting an adult horse during or at the end of isoflurane anesthesia carries the risk of a precipitous, though short-lived (1-2 min), drop in arterial blood pressures and a persistent decrease in arterial oxygenation. While in systemically healthy animals the observed functional impairments were not life-threatening, they may be more severe in systemically compromised horses.Therefore, arterial blood pressures and oxygenation must be carefully monitored when hoisting sick equine patients during or at the end of inhalant anesthesia.

Accuracy of tidal volume delivery by five different models of large-animal ventilators.

OBJECTIVE: To determine the accuracy of tidal volume (VT) delivery among 5 different models of large-animal ventilators when tested at various settings for VT delivery, peak inspiratory flow (PIF) rate, and fresh gas flow (FGF) rate. SAMPLE: 4 different models of pneumatically powered ventilators and 1 electrically powered piston-driven ventilator. PROCEDURES: After a leak flow check, each ventilator was tested 10 times for each experimental setting combination of 5 levels of preset VT, 3 PIF rates, and 4 FGF rates. A thermal mass flow and volume meter was used as the gold-standard method to measure delivered VT. In addition, circuit systems of rubber versus polyvinyl chloride breathing hoses were evaluated with the piston-driven ventilator. Differences between preset and delivered VT (volume error [ΔVT]) were calculated as a percentage of preset VT, and ANOVA was used to compare results across devices. Pearson correlation coefficient analyses and the coefficient of determination (r2) were used to assess potential associations between the ΔVT and the preset VT, PIF rate, and FGF rate. RESULTS: For each combination of experimental settings, ventilators had ΔVT values that ranged from 1.2% to 22.2%. Mean ± SD ΔVT was 4.8 ± 2.5% for the piston-driven ventilator, compared with 6.6 ± 3.2%, 10.6 ± 2.9%, 13.8 ± 2.97%, and 15.2 ± 2.6% for the 4 pneumatic ventilators. The ΔVT increased with higher PIF rates (r2 = 0.69), decreased with higher FGF rates (r2 = 0.62), and decreased with higher preset VT (r2 = 0.58). CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that the tested ventilators all had ΔVT but that the extent of each of ΔVT varied among ventilators. Close monitoring of delivered VT with external flow and volume meters is warranted, particularly when pneumatic ventilators are used or when very precise VT delivery is required.

Pharmacokinetics of xylazine after 2-, 4-, and 6-hr durations of continuous rate infusions in horses.

Intravenous (i.v.) bolus administration of xylazine (XYL) (0.5 mg/kg) immediately followed by a continuous rate infusion (CRI) of 1 mg kg-1  hr-1 for 2, 4, and 6 hr produced immediate sedation, which lasted throughout the duration of the CRI. Heart rate decreased and blood pressure increased significantly (p > .05) in all horses during the first 15 min of infusion, both returned to and then remained at baseline during the duration of the infusion. Compartmental models were used to investigate the pharmacokinetics of XYL administration. Plasma concentration-time curves following bolus and CRI were best described by a one-compartment model. No differences were found between pharmacokinetic estimates of the CRIs for the fractional elimination rate constant (Ke ), half-life (t1/2e ), volume of distribution (Vd ), and clearance (Cl). Median and range were 0.42 (0.15-0.97)/hr, 1.68 (0.87-4.52) hr, 5.85 (2.10-19.34) L/kg, and 28.7 (19.6-39.5) ml min-1  kg-1 , respectively. Significant differences were seen for area under the curve ( AUC 0 ∞ ) (p < .0002) and maximum concentration (Cmax ) (p < .04). This indicates that with increasing duration of infusion, XYL may not accumulate in a clinically relevant way and hence no adjustments are required in a longer XYL CRI to maintain a constant level of sedation and a rapid recovery.

Fatal Ovarian Hemorrhage Associated With Anticoagulation Therapy in a Yucatan Mini-Pig Following Venous Stent Implantation.

Swine models are commonly utilized in endovascular research for development of intravascular interventions and medical device development. As part of a pilot study for a venous vascular stent device, a 5-year-old female Yucatan mini-pig underwent bilateral external iliac vein stent placement under general anesthesia. To reduce thrombotic complications by reduction of thrombus formation on wires, sheaths, and catheters, the pig was heparinized with a total of 300 IU/kg of heparin, establishing an activated clotting time (ACT) of 436 s. The ACT had returned to below 200 s by the end of the procedure. To prevent postoperative thrombosis, the pig received an anticoagulation therapy protocol consisting of enoxaparin, clopidogrel, and aspirin. There were no complications during the immediate postoperative period. However, the pig died 4 days after surgery. Necropsy established the cause of death as abdominal exsanguination due to severe, acute, intra-ovarian hemorrhage, most likely related to ovulation. Life-threatening ovarian hemorrhage is occasionally seen in women with congenital or acquired bleeding disorders; to our knowledge this is the first report of fatal ovarian hemorrhage in an animal enrolled in a pre-clinical research trial.

Comparison of various types of inert gas components on efficacy of an alveolar recruitment maneuver in dorsally recumbent anesthetized horses.

OBJECTIVE: To assess effects of nitrogen and helium on efficacy of an alveolar recruitment maneuver (ARM) for improving pulmonary mechanics and oxygen exchange in anesthetized horses. ANIMALS: 6 healthy adult horses. PROCEDURES: Horses were anesthetized twice in a randomized crossover study. Isoflurane-anesthetized horses in dorsal recumbency were ventilated with 30% oxygen and 70% nitrogen (treatment N) or heliox (30% oxygen and 70% helium; treatment H) as carrier gas. After 60 minutes, an ARM was performed. Optimal positive end-expiratory pressure was identified and maintained for 120 minutes. Throughout the experiment, arterial blood pressures, heart rate, peak inspiratory pressure, dynamic compliance (Cdyn), and Pao2 were measured. Variables were compared with baseline values and between treatments by use of an ANOVA. RESULTS: The ARM resulted in significant increases in Pao2 and Cdyn and decreases in the alveolar-arterial gradient in the partial pressure of oxygen in all horses. After the ARM and during the subsequent 120-minute phase, mean values were significantly lower for treatment N than treatment H for Pao2 and Cdyn. Optimal positive end-expiratory pressure was consistently 15 cm H2O for treatment N, but it was 10 cm H2O (4 horses) and 15 cm H2O (2 horses) for treatment H. CONCLUSIONS AND CLINICAL RELEVANCE: An ARM in anesthetized horses might be more efficacious in improving Pao2 and Cdyn when animals breathe helium instead of nitrogen as the inert gas.

Evaluation of the effects of gas volume and composition on accuracy of volume measurement by two flow sensors and delivery by a piston-driven large-animal ventilator.

OBJECTIVE To evaluate the effects of 4 gas compositions at various volumes (simulated tidal volumes [VTs]) on accuracy of measurements obtained with 2 types of flow sensors and accuracy of gas volume delivery by a piston-driven ventilator. SAMPLE 4 gas mixtures (medical air [21% O2:79% N2], > 95% O2, O2-enriched air [30% O2:70% N2], and heliox [30% O2:70% He]). PROCEDURES For each gas mixture, reference VTs of 1 to 8 L were delivered into an anesthetic breathing circuit via calibration syringe; measurements recorded by a Pitot tube-based flow sensor (PTFS) connected to a multiparameter host anesthesia monitor and by a thermal mass flow and volume meter (TMFVM) were compared with the reference values. Following leak and compliance testing, the ventilator was preset to deliver each gas at VTs of 1 to 8 L into the calibration syringe. Effects of gas volume and composition on accuracy of VT measurement and delivery were assessed by ANOVA. Agreements between delivered and flow sensor-measured VT and preset versus ventilator-delivered VT were determined by Bland-Altman analysis. RESULTS Flow sensor measurements were accurate and not influenced by gas composition. Mean measurement error ranges for the PTFS and TMFVM were -4.99% to 4.21% and -4.50% to 0.17%, respectively. There were no significant differences between ventilator-delivered and reference VTs regardless of gas volume or composition. Bland-Altman analysis yielded biases of -0.046 L, -0.007 L, -0.002 L, and 0.031 L for medical air, > 95% O2, O2-enriched air, and heliox, respectively. CONCLUSIONS AND CLINICAL RELEVANCE The PTFS and the TMFVM measured VTs and the piston-driven ventilator delivered VTs with error rates of < 5% for all gas compositions and volumes tested.

Efficacy of an alveolar recruitment maneuver for improving gas exchange and pulmonary mechanics in anesthetized horses ventilated with oxygen or a helium-oxygen mixture.

OBJECTIVE To evaluate efficacy of an alveolar recruitment maneuver (ARM) with positive end-expiratory pressures (PEEPs) in anesthetized horses ventilated with oxygen or heliox (70% helium and 30% oxygen). ANIMALS 6 healthy adult horses. PROCEDURES In a randomized crossover study, horses were anesthetized and positioned in dorsal recumbency. Volume-controlled ventilation was performed with heliox or oxygen (fraction of inspired oxygen [Fio2] > 90%). Sixty minutes after mechanical ventilation commenced, an ARM with PEEP (0 to 30 cm H2O in steps of 5 cm H2O every 5 minutes, followed by incremental steps back to 0 cm H2O) was performed. Peak inspiratory pressure, dynamic lung compliance (Cdyn), and Pao2 were measured during each PEEP. Indices of pulmonary oxygen exchange and alveolar dead space were calculated. Variables were compared with baseline values (PEEP, 0 cm H2O) and between ventilation gases by use of repeated-measures ANOVAs. RESULTS For both ventilation gases, ARM significantly increased pulmonary oxygen exchange indices and Cdyn. Mean ± SD Cdyn (506 ± 35 mL/cm H2O) and Pao2-to-Fio2 ratio (439 ± 36) were significantly higher and alveolar-arterial difference in Pao2 (38 ± 11 mm Hg) was significantly lower for heliox, compared with values for oxygen (357 ± 50 mL/cm H2O, 380 ± 92, and 266 ± 88 mm Hg, respectively). CONCLUSIONS AND CLINICAL RELEVANCE An ARM in isoflurane-anesthetized horses ventilated with heliox significantly improved pulmonary oxygen exchange and respiratory mechanics by decreasing resistive properties of the respiratory system and reducing turbulent gas flow in small airways.

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.

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.

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

INTRODUCTION: Nitrous oxide (N2O) is an anesthetic gas with antinociceptive properties and reduces the minimum alveolar concentration (MAC) for volatile anesthetic agents, potentially through mechanisms involving central alpha2-adrenoceptors. We hypothesized that 70% N2O in the inspired gas will significantly reduce the MAC of sevoflurane (MACSEVO) in sheep, and that this effect can be reversed by systemic atipamezole. MATERIALS AND METHODS: Animals were initially anesthetized with SEVO in oxygen (O2) and exposed to an electrical current as supramaximal noxious stimulus in order to determine MACSEVO (in duplicates). Thereafter, 70% N2O was added to the inspired gas and the MAC re-determined in the presence of N2O (MACSN). A subgroup of sheep were anesthetized a second time with SEVO/N2O for re-determination of MACSN, after which atipamezole (0.2 mg kg-1, IV) was administered for MACSNA determinations. Sheep were anesthetized a third time, initially with only SEVO/O2 to re-determine MACSEVO, after which atipamezole (0.2 mg kg-1, IV) was administered for determination of MACSA. RESULTS: MACSEVO was 2.7 (0.3)% [mean (standard deviation)]. Addition of N2O resulted in a 37% reduction of MACSEVO to MACSN of 1.7 (0.2)% (p <0.0001). Atipamezole reversed this effect, producing a MACSNA of 3.1 (0.7)%, which did not differ from MACSEVO (p = 0.12). MACSEVO did not differ from MACSA (p = 0.69). Cardiorespiratory variables were not different among experimental groups except a lower ETCO2 in animals exposed to SEVO/N2O. CONCLUSIONS: N2O produces significant MACSEVO-reduction in sheep; this effect is completely reversed by IV atipamezole confirming the involvement of alpha2-adrenoreceptors in the MAC-sparing action of N2O.

Comparison of an infrared anaesthetic agent analyser (Datex-Ohmeda) with refractometry for measurement of isoflurane, sevoflurane and desflurane concentrations.

OBJECTIVE: To assess agreement between infrared (IR) analysers and a refractometer for measurements of isoflurane, sevoflurane and desflurane concentrations and to demonstrate the effect of customized calibration of IR analysers. STUDY DESIGN: In vitro experiment. SUBJECTS: Six IR anaesthetic monitors (Datex-Ohmeda) and a single portable refractometer (Riken). METHODS: Both devices were calibrated following the manufacturer's recommendations. Gas samples were collected at common gas outlets of anaesthesia machines. A range of agent concentrations was produced by stepwise changes in dial settings: isoflurane (0-5% in 0.5% increments), sevoflurane (0-8% in 1% increments), or desflurane (0-18% in 2% increments). Oxygen flow was 2 L minute(-1) . The orders of testing IR analysers, agents and dial settings were randomized. Duplicate measurements were performed at each setting. The entire procedure was repeated 24 hours later. Bland-Altman analysis was performed. Measurements on day-1 were used to yield calibration equations (IR measurements as dependent and refractometry measurements as independent variables), which were used to modify the IR measurements on day-2. RESULTS: Bias ± limits of agreement for isoflurane, sevoflurane and desflurane were 0.2 ± 0.3, 0.1 ± 0.4 and 0.7 ± 0.9 volume%, respectively. There were significant linear relationships between differences and means for all agents. The IR analysers became less accurate at higher gas concentrations. After customized calibration, the bias became almost zero and the limits of agreement became narrower. CONCLUSIONS AND CLINICAL RELEVANCE: If similar IR analysers are used in research studies, they need to be calibrated against a reference method using the agent in question at multiple calibration points overlapping the range of interest.

Critical incidence reporting systems - an option in equine anaesthesia? Results from a panel meeting.

OBJECTIVE: To provide a brief introduction into Critical Incident Reporting Systems (CIRS) as used in human medicine, and to report the discussion from a recent panel meeting discussion with 23 equine anaesthetists in preparation for a new CEPEF-4 (Confidential Enquiry into Perioperative Equine Fatalities) study. STUDY DESIGN: Moderated group discussions, and review of literature. METHODS: The first group discussion focused on the definition of 'preventable critical incidents' and/or 'near misses' in the context of equine anaesthesia. The second group discussion focused on categorizing critical incidents according to an established framework for analysing risk and safety in clinical medicine. RESULTS: While critical incidents do occur in equine anaesthesia, no critical incident reporting system including systematic collection and analysis of critical incidents is in place. CONCLUSIONS AND CLINICAL RELEVANCE: Critical incident reporting systems could be used to improve safety in equine anaesthesia - in addition to other study types such as mortality studies.