Accuracy of oxygen delivery through bubble humidifiers and nasal catheters.

OBJECTIVE: To evaluate the equipment used for nasal insufflation of oxygen and determine its accuracy. STUDY DESIGN: Original study. METHODS: Oxygen delivery assemblies consisting of a flowmeter, bubble humidifier, oxygen delivery tubing and nasal insufflation catheters were assembled. Single and double catheter assemblies were made for four sizes of nasogastric feeding tubes (3.5 Fr, 5.0 Fr, 8.0 Fr and 10.0 Fr) resulting in 64 individual assemblies. A gas flow analyzer measured oxygen flow at the tip of the nasal catheter assemblies and from the pressure relief valve (PRV) of the bubble humidifiers. Statistical analyses were conducted to assess the functionality of assemblies. For functional assemblies, the accuracy of oxygen flow relative to the prescribed flow settings was determined. RESULTS: Catheter size was significantly associated with the functionality of assemblies. Probability (95% confidence interval) of 3.5 Fr, 5.0 Fr and 8.0 Fr assemblies being functional was estimated at 0.53 (0.14, 0.89), 0.83 (0.36, 0.98) and 0.98 (0.76, 0.99), respectively. All 10.0 Fr assemblies were functional. Functional assemblies, in general, consistently under-delivered the prescribed flow because a large portion of set flow was diverted through the bubble humidifier PRV. CONCLUSIONS: Leaks through the PRV cause significant diversion of oxygen prior to it reaching the catheter tips. Smaller patients are particularly susceptible, as small catheters limit oxygen delivery creating proportionally greater leaks through the PRV. CLINICAL RELEVANCE: It was not possible to accurately deliver oxygen because of leaks through the PRV. Targeting a specific outcome (e.g., oxyhemoglobin saturation > 94%, PaO2 80-120 mmHg; 11-16 kPa) and avoiding unnecessarily high fractions of inspired oxygen cannot be done if flow delivery cannot be accurately assured. One possible solution would be to use a bubble humidifier with a 6 psi PRV that does not leak prior to reaching the opening pressure.

On accuracy and precision of flowmeters used for oxygen therapy in a veterinary teaching hospital.

OBJECTIVE: To determine the accuracy and precision of flowmeters used for oxygen therapy in a veterinary teaching hospital. STUDY DESIGN: An observational study. METHODS: A total of 50 flowmeters used for oxygen therapy were evaluated using Defender 530 gas flow analyzers to measure flow. For each flowmeter, a minimum of seven flow settings were tested in random order and in triplicate. Flow measured at ambient conditions was converted to standardized flow specifications (21.1 °C and 760 mmHg) and analyzed using general linear mixed models. Flowmeters were considered accurate at a given flow setting when the targeted mean flow was within the corresponding 95% confidence interval. Precision of flow was characterized based on the magnitude of variance component estimates. RESULTS: Flowmeters of 1.0, 3.5 and 8.0 L minute-1 were considered accurate across flow settings corresponding to their capacity range. Flowmeters of 7.0 and 15.0 L minute-1 were accurate at flow settings ≤2.0 L minute-1. For flow settings ≥3.0 L minute-1, average oxygen flow was consistently below reference values. Precision varied with the capacity of the flowmeter, ranked by decreasing precision as 1.0 > 3.5 > 8.0 > 7.0 > 15.0 L minute-1. CONCLUSIONS AND CLINICAL RELEVANCE: A flowmeter of the smallest maximum capacity within the desired flow range is more appropriate for smaller patients where accurate, precise flow delivery is needed. Although 15.0 L minute-1 flowmeters were accurate at flow settings ≤2.0 L minute-1, the graduated increments do not allow exact flow settings <0.5 L minute-1. Flowmeters of 15 L minute-1 capacity should be useful for high-flow oxygen delivery for which accuracy and precision are not critical.

Effect of cleaning status on accuracy and precision of oxygen flowmeters of various ages.

OBJECTIVE: To evaluate oxygen flowmeters for accuracy and precision, assess the effects of cleaning and assess conformity to the American Society for Testing Materials (ASTM) standards. STUDY DESIGN: Experimental study. METHODS: The flow of oxygen flowmeters from 31 anesthesia machines aged 1-45 years was measured before and after cleaning using a volumetric flow analyzer set at 0.5, 1.0, 2.0, 3.0, and 4.0 L minute-1. A general linear mixed models approach was used to assess flow accuracy and precision. RESULTS: Flowmeters 1 year of age delivered accurate mean oxygen flows at all settings regardless of cleaning status. Flowmeters ≥5 years of age underdelivered at flows of 3.0 and 4.0 L minute-1. Flowmeters ≥12 years underdelivered at flows of 2.0, 3.0 and 4.0 L minute-1 prior to cleaning. There was no evidence of any beneficial effect of cleaning on accuracy of flowmeters 5-12 years of age (p > 0.22), but the accuracy of flowmeters ≥15 years of age was improved by cleaning (p < 0.05). Regardless of age, cleaning increased precision, decreasing flow variability by approximately 17%. Nine of 31 uncleaned flowmeters did not meet ASTM standards. After cleaning, a different set of nine flowmeters did not meet standards, including three that had met standards prior to cleaning. CONCLUSIONS: Older flowmeters were more likely to underdeliver oxygen, especially at higher flows. Regardless of age, cleaning decreased flow variability, improving precision. However, flowmeters still may fail to meet ASTM standards, regardless of cleaning status. CLINICAL RELEVANCE: Cleaning anesthesia machine oxygen flowmeters improved precision for all tested machines and partially corrected inaccuracies in flowmeters ≥15 years old. A notable proportion of flowmeters did not meet ASTM standards. Cleaning did not ensure that they subsequently conformed to ASTM standards. We recommend annual flow output validation to identify whether flowmeters are acceptable for continued clinical use.

Apneustic anesthesia ventilation improves pulmonary function in anesthetized bottlenose dolphins (Tursiops truncatus).

Introduction: Use of mechanical ventilation during general anesthesia is a necessary practice in the anesthetization of small cetaceans as spontaneous ventilation fails to provide adequate gas exchange. Currently available methods of ventilation do not account for the intermittent breathing strategy of representative species within this infraorder of fully aquatic mammals and may have a significant effect on cardiac and respiratory physiology. Methods: To understand the impact of mechanical ventilation on cardiopulmonary function in one small species of cetacean, the bottlenose dolphin (Tursiops truncatus), we compared controlled mechanical ventilation (CMV) to a novel ventilation method known as apneustic anesthesia ventilation (AAV). AAV simulates the normal inspiratory breath-hold pattern of dolphins. Ten anesthetic procedures (dental procedure, n = 9; bronchoscopy, n = 2) were performed on nine dolphins (age range: 10-42 years; mean = 32 years; median = 37 years; female = 3, 40%; male = 6, 60%). In a cross-over study design, dolphins were instrumented and randomly assigned to AAV or CMV as the initial mode of ventilation, then switched to the alternate mode. Baseline cardiopulmonary data were collected and again after 30 min on each mode of ventilation. Cardiac index, stroke volume index, systemic vascular resistance, alveolar dead space, alveolar-arterial oxygen tension gradient, arterial oxygen content, oxygen delivery index, and dynamic respiratory system compliance index were calculated at each of the four time points. Results: During AAV, dolphins had higher arterial oxygen tension, higher mean airway pressure, reduced alveolar dead space ventilation and lower alveolar-arterial oxygen difference. Cardiovascular performance was not statistically different between the two modes. Discussion: Our study suggests AAV, which more closely resembles the conscious intermittent respiratory pattern phenotype of dolphins, improves ventilation and pulmonary function in the anesthetized dolphin. Future studies should evaluate the cardiopulmonary effects of neutral buoyancy and cardiopulmonary sparing drug protocols to reduce the need for hemodynamic support of current protocols.

Cardiopulmonary effects of apneustic anesthesia ventilation in anesthetized pigs: a new mode of ventilation for anesthetized veterinary species.

Objective: To compare the cardiopulmonary effects of apneustic anesthesia ventilation (AAV) and conventional mechanical ventilation (CMV) in anesthetized pigs and to describe a new mode of ventilation for anesthetized veterinary species. Study design: Randomized, crossover design without washout. Animals: Twelve healthy, female white Landrace pigs. Methods: Following ketamine-midazolam premedication and anesthetic induction with propofol, the trachea was intubated, and each pig was positioned in dorsal recumbency. Anesthesia was maintained with propofol and sufentanil infusions. Pigs were instrumented and their lungs were sequentially ventilated with each mode, in random order, for 1 h according to predefined criteria [fraction of inspired oxygen (FiO2) = 0.21, 10 mL kg-1 tidal volume (VT), and arterial carbon dioxide tension (PaCO2) within 40-45 mmHg]. Cardiopulmonary data were collected at baseline, 30 and 60 min. In 8 pigs, thoracic computed tomography (CT) was performed following the 60 min time point for each mode of ventilation and images were analyzed to quantify lung aeration. The effects of ventilation mode, time, and order were analyzed using repeated measures ANOVA. Paired t-tests were used to compare lung aeration between modes. Significance was defined as p < 0.05. Results: Data from 12 pigs were analyzed. A significant effect of mode was found for heart rate, mean arterial pressure (MAP), pulmonary artery occlusion pressure, cardiac index (CI), stroke volume index, systemic vascular resistance, pulmonary vascular resistance, oxygen delivery index (DO2I), oxygen extraction ratio (O2ER), VT, arterial oxygen tension, arterial hemoglobin saturation, PaCO2, end-tidal carbon dioxide tension, alveolar dead space (VDalv/VTalv), venous admixture ( Q . s / Q . t ), mean airway pressure, and dynamic compliance index (CRSI). Order effects were also observed for some cardiovascular and respiratory variables. For the eight pigs that underwent thoracic CT, AAV resulted in significantly larger proportions of normally and hyperaerated lung while CMV resulted in larger proportions of hypoaerated and atelectatic lung. Conclusions: In dorsally recumbent anesthetized pigs, ventilated with FiO2 = 0.21, both modes of ventilation supported adequate oxygenation while AAV resulted in higher CRSI, and lower VDalv/VTalv and Q . s / Q . t , compared with CMV. AAV was also associated with lower MAP, CI, and DO2I and higher O2ER compared with CMV. Further investigation of AAV in anesthetized animals is warranted.

Effect of position on transdiaphragmatic pressure and hemodynamic variables in anesthetized horses.

Recumbency affects respiratory mechanics and oxygenation in anesthetized horses. Changes in pleural and abdominal pressures that can impair ventilation have not been described in all recumbencies. The objective of this study was to determine the effects of patient positioning on transdiaphragmatic pressure and selected hemodynamic variables. Horses were maintained under total intravenous general anesthesia with nasal oxygen supplementation. Transnasal balloon catheters in the stomach and thoracic esophagus were used to measure intrathoracic and gastric pressures in standing horses and in anesthetized horses positioned in right and left lateral recumbency, dorsal recumbency, reverse Trendelenburg position, and Trendelenburg position. Transdiaphragmatic pressure was calculated as the difference between gastric and intrathoracic pressures. Measurements of oxygen saturation (SpO2), heart rate, systolic, diastolic and mean arterial pressures, and respiratory rate were obtained every 5 minutes. When compared to dorsal recumbency, gastric expiratory pressure is decreased in the standing position. Thoracic expiratory pressure is decreased in standing and reverse Trendelenburg. Transdiaphragmatic expiratory pressure and SpO2 are decreased in Trendelenburg. Heart rate is increased in reverse Trendelenburg. Systolic, diastolic, and mean arterial pressures are decreased in reverse Trendelenburg and increased in left lateral and right lateral recumbency. We found that there is wide variation in respiratory pressures between horses and positions and they are not predictive of associated changes in hemodynamic variables.

Le décubitus affecte la mécanique respiratoire et l'oxygénation chez les chevaux anesthésiés. Les changements dans les pressions pleurales et abdominales qui peuvent affecter la ventilation n'ont pas été décrites dans tous les décubitus. L'objectif de la présente étude était de déterminer les effets du positionnement du patient sur la pression trans-diaphragmatique et une sélection de variables hémodynamiques. Des chevaux furent maintenus sous anesthésie intraveineuse générale totale avec supplémentation en oxygène par voie nasale. Des cathéters à ballon intra-nasal placés dans l'estomac et l'oesophage thoracique furent utilisés pour mesurer les pressions intrathoracique et gastrique chez des chevaux en position debout et des chevaux anesthésiés positionnés en décubitus latéral droit et gauche, en décubitus dorsal, en position renversée de Trendelenburg et en position de Trendelenburg. La pression trans-diaphragmatique fut calculée comme étant la différence entre les pressions gastrique et intrathoracique. Les mesures de saturation en oxygène (SpO2), du rythme cardiaque, des pressions artérielles systolique, diastolique et moyenne, ainsi que le rythme respiratoire furent obtenues à toutes les 5 minutes. Lors de la comparaison avec le décubitus dorsal, la pression expiratoire gastrique est diminuée dans la position debout. La pression thoracique expiratoire est diminuée en position debout et en position renversée de Trendelenburg. La pression expiratoire trans-diaphragmatique et la SpO2 sont diminuées en position Trendelenburg. Le rythme cardiaque est augmenté en position renversée de Trendelenburg. Les pressions artérielles systolique, diastolique et moyenne sont diminuées en position renversée de Trendelenburg et augmentées en décubitus latéral gauche et droit. Nous avons trouvé qu'il y avait de grandes variations dans les pressions respiratoires entre les chevaux et les positions et qu'elles ne sont pas prédictives de changements associés dans les variables hémodynamiques.(Traduit par Docteur Serge Messier).

Bronchoscopically delivered microwave ablation in an in vivo porcine lung model.

BACKGROUND: Percutaneous microwave ablation is clinically used for inoperable lung tumour treatment. Delivery of microwave ablation applicators to tumour sites within lung parenchyma under virtual bronchoscopy guidance may enable ablation with reduced risk of pneumothorax, providing a minimally invasive treatment of early-stage tumours, which are increasingly detected with computed tomography (CT) screening. The objective of this study was to integrate a custom microwave ablation platform, incorporating a flexible applicator, with a clinically established virtual bronchoscopy guidance system, and to assess technical feasibility for safely creating localised thermal ablations in porcine lungs in vivo. METHODS: Pre-ablation CTs of normal pigs were acquired to create a virtual model of the lungs, including airways and significant blood vessels. Virtual bronchoscopy-guided microwave ablation procedures were performed with 24-32 W power (at the applicator distal tip) delivered for 5-10 mins. A total of eight ablations were performed in three pigs. Post-treatment CT images were acquired to assess the extent of damage and ablation zones were further evaluated with viability stains and histopathologic analysis. RESULTS: The flexible microwave applicators were delivered to ablation sites within lung parenchyma 5-24 mm from the airway wall via a tunnel created under virtual bronchoscopy guidance. No pneumothorax or significant airway bleeding was observed. The ablation short axis observed on gross pathology ranged 16.5-23.5 mm and 14-26 mm on CT imaging. CONCLUSION: We have demonstrated the technical feasibility for safely delivering microwave ablation in the lung parenchyma under virtual bronchoscopic guidance in an in vivo porcine lung model.

Comparison of anesthetic induction in cats by use of isoflurane in an anesthetic chamber with a conventional vapor or liquid injection technique.

OBJECTIVE: To compare 2 techniques for induction of cats by use of isoflurane in an anesthetic chamber. DESIGN: Prospective, randomized study. ANIMALS: 51 healthy cats. PROCEDURES: Cats were randomly allocated to 2 induction techniques. Cats were premedicated with acepromazine (0.1 mg/kg [0.045 mg/lb], SC) and buprenorphine (0.01 mg/kg [0.0045 mg/lb], SC) 30 minutes before induction. Cats were then placed into an induction chamber, and anesthetic induction was initiated. One technique involved a conventional flow-through system that used an oxygen flowmeter and an isoflurane vaporizer to flow vapors into the induction chamber. Alternatively, liquid isoflurane was injected into a vaporization tray that was mounted to the interior surface of the chamber lid. Inductions were videotaped for analysis. Five variables (head bobbing, head swinging side to side, paddling, rotating 180 degrees to 360 degrees, and rolling over or flipping) were scored to assess induction quality. Time variables recorded during induction corresponded to the interval until onset of excitatory motion, duration of excitatory motion, interval until recumbency, and interval until complete induction. RESULTS: Compared with cats anesthetized by use of a conventional vapor chamber technique, cats anesthetized by use of the liquid injection technique had a significantly shorter interval until recumbency and interval until complete induction and lower scores for quality of induction, indicating a smoother induction. CONCLUSIONS AND CLINICAL RELEVANCE: Anesthetic induction in cats by use of a liquid injection technique was more rapid and provided a better quality of induction, compared with results for cats induced by use of a conventional vapor technique.

Comparison of recoveries from anesthesia of horses placed on a rapidly inflating-deflating air pillow or the floor of a padded stall.

OBJECTIVE: To compare recoveries from anesthesia of horses placed on a conventional padded stall floor or on a specially designed air pillow. DESIGN: Prospective study. ANIMALS: 409 horses (> 1 year old) that were anesthetized for surgical procedures during a 37-month period. PROCEDURES: By random allocation, horses were allowed to recover from anesthesia in either a foammat-padded recovery stall or an identical recovery stall equipped with a rapidly inflating-deflating air pillow. All recoveries were videotaped for subsequent analysis by an independent evaluator. Times to first movement, first attempt to attain sternal recumbency, attainment of sternal recumbency, first attempt to stand, and successful standing were recorded. The numbers of attempts before achieving sternal recumbency and standing were counted, and scores for quality of standing and overall recovery were assigned. Recovery-related variables were compared between groups. RESULTS: Compared with horses allowed to recover in a conventional manner, horses that recovered from anesthesia on the air pillow had a significantly longer rest period before attempting to attain sternal recumbency and rise to standing. Once the pillow was deflated, horses were able to stand after significantly fewer attempts and the quality of their standing was significantly better. Between the 2 groups of horses, there was no significant difference in overall recovery quality scores. The air pillow and padded floor systems were equally safe. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that use of a rapidly inflating-deflating air pillow promotes a longer period of recumbency and a better quality of standing after anesthesia in horses.

Effects of atracurium on intraocular pressure, eye position, and blood pressure in eucapnic and hypocapnic isoflurane-anesthetized dogs.

OBJECTIVE: To determine effects of atracurium on intraocular pressure (IOP), eye position, and arterial blood pressure in eucapnic and hypocapnic dogs anesthetized with isoflurane. ANIMALS: 16 dogs. PROCEDURE: Ventilation during anesthesia was controlled to maintain Paco2 at 38 to 44 mm Hg in group- I dogs (n = 8) and 26 to 32 mm Hg in group-II dogs (8). Baseline measurements for IOP, systolic, diastolic, and mean arterial blood pressure, central venous pressure (CVP), and heart rate (HR) were recorded. Responses to peroneal nerve stimulation were monitored by use of a force-displacement transducer. Atracurium (0.2 mg/kg) was administered i.v. and measurements were repeated at 1, 2, 3, and 5 minutes and at 5-minute intervals thereafter for 60 minutes. RESULTS: Atracurium did not affect IOP, HR, or CVP Group II had higher CVP than group I, but IOP was not different. There was no immediate effect of atracurium on arterial blood pressure. Arterial blood pressure increased gradually over time in both groups. Thirty seconds after administration of atracurium, the eye rotated from a ventromedial position to a central position and remained centrally positioned until 100% recovery of a train-of-four twitch response. The time to 100% recovery was 53.1 +/- 5.3 minutes for group I and 46.3 +/- 9.2 minutes for group II. CONCLUSIONS AND CLINICAL RELEVANCE: Atracurium did not affect IOP or arterial blood pressure in isoflurane-anesthetized dogs. Hyperventilation did not affect IOP or the duration of effect of atracurium.

Cardiopulmonary effects of xylazine and acepromazine in pregnant cows in late gestation.

OBJECTIVE: To determine effects of sedation achieved by xylazine (XYL) or acepromazine (ACE) on cardiopulmonary function and uterine blood flow in cows in late gestation. ANIMALS: 8 cows between 219 and 241 days of gestation. PROCEDURE: Doses of ACE (0.02 mg/kg) or XYL (0.04 mg/kg) were administered IV. Measurements were obtained to determine cardiopulmonary effects and oxygen delivery to the uterus. RESULTS: Heart rate was not significantly affected by administration of ACE, but it decreased markedly after administration of XYL. Uterine artery flow was decreased at all times by XYL and was always less than for ACE. Xylazine increased uterine vascular resistance through 30 minutes and caused reduced PaO2 and increased PaCO2 at all time periods. Acepromazine caused a 5% decrease in PaO2 only at 5 minutes. Xylazine reduced oxygen delivery by 59% at 5 minutes and 32% at 45 minutes. In contrast, ACE caused a nonsignificant reduction of oxygen delivery by 16% at 15 minutes and a return to baseline values by 45 minutes CONCLUSIONS AND CLINICAL RELEVANCE: Xylazine markedly reduces flow and availability of oxygenated blood to the uterus, which may critically impair delivery of oxygen to the fetus at a stressful and important time of development or delivery. Acepromazine was associated with slight reductions of much shorter duration. When XYL is used to sedate pregnant cows, it could impose physiologic distress on the fetus and potentially increase fetal morbidity and mortality. When sedation of the dam is desirable, ACE could be an alternative to XYL.

Effects of oxygen insufflation rate, respiratory rate, and tidal volume on fraction of inspired oxygen in cadaveric canine heads attached to a lung model.

OBJECTIVE: To assess the effects of oxygen insufflation rate, respiratory rate, and tidal volume on fraction of inspired oxygen (Fio2) in cadaveric canine heads attached to a lung model. SAMPLE: 16 heads of canine cadavers. PROCEDURES: Each cadaver head was instrumented with a nasal insufflation catheter through which oxygen was delivered. The trachea was attached to a sample collection port connected by means of corrugated tubing to a lung model. Eight treatment combinations that varied in respiratory rate (10 or 20 breaths/min), tidal volume (10 or 15 mL/kg), and oxygen insufflation rate (50 or 100 mL/kg/min) were applied to each head in a replicated Latin square design. Gas samples were manually collected, and inspired oxygen concentrations were analyzed. The Fio2 and end-tidal CO2 concentration were determined and compared among sample groups. RESULTS: Estimated least squares mean Fio2 for various treatment combinations ranged from 32.2% to 60.6%. The Fio2 was significantly increased at the higher insufflation rate (estimated marginal least squares mean, 48.7% vs 38.6% for 100 and 50 mL/kg/min, respectively), lower respiratory rate (48.9% vs 38.3% for 10 and 20 breaths/min, respectively), and smaller tidal volume (46.8% vs 40.0% for 10 and 15 mL/kg, respectively). CONCLUSIONS AND CLINICAL RELEVANCE: -Fio2 in the model was affected by oxygen insufflation rate, respiratory rate, and tidal volume. This information may potentially help clinicians interpret results of blood gas analysis and manage canine patients receiving oxygen insufflation via a nasal catheter.

Effects of inspired oxygen concentration on ventilation, ventilatory rhythm, and gas exchange in isoflurane-anesthetized horses.

OBJECTIVE: To compare the effects of 2 fractions of inspired oxygen, 50% and > 95%, on ventilation, ventilatory rhythm, and gas exchange in isoflurane-anesthetized horses. ANIMALS: 8 healthy adult horses. PROCEDURES: In a crossover study design, horses were assigned to undergo each of 2 anesthetic sessions in random order, with 1 week separating the sessions. In each session, horses were sedated with xylazine hydrochloride (1.0 mg/kg, IV) and anesthesia was induced via IV administration of diazepam (0.05 mg/kg) and ketamine (2.2 mg/kg) Anesthesia was subsequently maintained with isoflurane in 50% or > 95% oxygen for 90 minutes. Measurements obtained during anesthesia included inspiratory and expiratory peak flow and duration, tidal volume, respiratory frequency, end-tidal CO(2) concentration, mixed expired partial pressures of CO(2) and O(2), PaO(2), PaCO(2), blood pH, arterial O(2) saturation, heart rate, and arterial blood pressure. Calculated values included the alveolar partial pressure of oxygen, alveolar-to-arterial oxygen tension gradient (PaO(2) - PaO(2)), rate of change of PAO(2) - PaO(2), and physiologic dead space ratio. Ventilatory rhythm, based on respiratory rate and duration of apnea, was continuously observed and recorded. RESULTS: Use of the lower inspired oxygen fraction of 50% resulted in a lower arterial oxygen saturation and PaO(2) than did use of the higher fraction. No significant difference in PaCO(2), rate of change of PAO(2) - PaO(2), ventilatory rhythm, or other measured variables was observed between the 2 sessions. CONCLUSION AND CLINICAL RELEVANCE: Use of 50% inspired oxygen did not improve the ventilatory rhythm or gas exchange and increased the risk of hypoxemia in spontaneously breathing horses during isoflurane anesthesia. Use of both inspired oxygen fractions requires adequate monitoring and the capacity for mechanical ventilation.

Effects of tidal volume, ventilatory frequency, and oxygen insufflation flow on the fraction of inspired oxygen in cadaveric horse heads attached to a lung model.

OBJECTIVE: To measure the effects of tidal volume, ventilatory frequency, and oxygen insufflation flow on the fraction of inspired oxygen in cadaveric horse heads attached to a lung model. SAMPLE: 8 heads of equine cadavers. PROCEDURES: Each cadaveric horse head was intubated with a nasotracheal tube that extended into the proximal portion of the trachea. Oxygen was delivered through an oxygen catheter contained within and extending to the tip of the nasotracheal tube. The trachea was connected to the lung model by use of a spiral-wound hose with a sampling adaptor. Eight treatment combinations involving 2 tidal volumes (5 and 8 L), 2 ventilatory frequencies (6 and 12 mechanical breathes/min), and 2 insufflation rates (10 and 15 L/min) were applied to each head. Hand-drawn inspired gas samples were collected and analyzed for oxygen concentrations. RESULTS: The fraction of inspired oxygen (measured at mid trachea) ranged from 26.8% to 39.4%. Fraction of inspired oxygen was significantly higher with a smaller tidal volume, lower ventilatory frequency, and higher insufflation rate. CONCLUSIONS AND CLINICAL RELEVANCE: In the study model, measured fraction of inspired oxygen varied with ventilatory pattern as well as oxygen insufflation rate. Clinically, this information could be beneficial for interpretation of data regarding arterial blood gases and hemoglobin saturation and in making appropriate oxygen insufflation decisions for anesthetized horses that are breathing room air.

Effect of anesthetic breathing circuit type on thermal loss in cats during inhalation anesthesia for ovariohysterectomy.

OBJECTIVE: To compare the effects of a nonrebreathing circuit versus a reduced volume circle anesthetic breathing circuit on body temperature change in cats during inhalation anesthesia for ovariohysterectomy. DESIGN: Randomized, controlled clinical trial. ANIMALS: 141 female domestic cats hospitalized for routine ovariohysterectomy. PROCEDURES: Cats were randomly assigned to receive inhalation anesthetics from either a nonrebreathing circuit or a reduced volume circle system with oxygen flow rates of 200 and 30 mL/kg/min (90.9 and 13.6 mL/lb/min), respectively. Body temperatures were monitored throughout the anesthetic period via an intrathoracic esophageal probe placed orally into the esophagus to the level of the heart base. RESULTS: No difference in body temperature was found between the 2 treatment groups at any measurement time. The duration of procedure had a significant effect on body temperature regardless of the type of anesthetic circuit used. CONCLUSIONS AND CLINICAL RELEVANCE: Duration of the procedure rather than the type of anesthetic circuit used for inhalation anesthesia was more influential on thermal loss in cats undergoing ovariohysterectomy.

Anesthetic concentrations in enclosed chambers using an innovative delivery device.

OBJECTIVE: To quantify factors influencing anesthetic concentration when an innovative anesthetic delivery device (vapor wand) was used with enclosed chambers. STUDY DESIGN: Randomized study. METHODS: Two experimental chambers (57.4 and 171 L) were constructed. Anesthetic volumes necessary to reach a target concentration of 3% or 5% isoflurane with complete vaporization were calculated for each chamber. After centering the distal end of the vapor wand and multi-orifice sampler, each chamber was sealed. Air (450 mL) was cycled through the vapor wand in a to-and-fro fashion with an electric, modified air pump at either 20 or 40 cycles minute(-1). Samples taken at 30-second intervals were analyzed for isoflurane concentration. Times to reach 2.8% isoflurane concentration were compared for eight treatment combinations replicated three times. Curves were constructed to display the rate of rise to endpoint concentration. Analysis of variance was applied to the data. RESULTS: Chamber size, pump stroke rate, and target isoflurane concentration all affected time to reach 2.8%, and their three-way interaction was statistically significant (p < 0.05). Generally time to 2.8% was less with small chambers, more rapid pumping and a target concentration of 5%. CONCLUSIONS AND CLINICAL RELEVANCE: When a wild or aggressive animal is presented for clinical veterinary care, introduction of a vapor wand into its cage offers a safer, more convenient, and less stressful alternative for anesthesia than transfer to an induction chamber. By quantifying factors affecting the rate of rise of anesthetic concentration with its use in experimental chambers, this study should promote greater safety and predictability when used clinically. This information will be useful when anesthetic induction needs to be hastened or delayed depending on the responses of the patient and the clinical judgment of the anesthetist.

Comparison of isoflurane and sevoflurane for short-term anesthesia in piglets.

OBJECTIVES: To compare isoflurane (ISO) and sevoflurane (SEVO) short-term anesthesia in piglets during castration. STUDY DESIGN: Prospective, randomized study. ANIMALS: A total of 114 male piglets aged 6-10 days, body weight 1.3-5.0 kg. METHODS: Piglets were randomly selected from multiple litters and randomly assigned to being anesthetized with ISO or SEVO prior to castration. To calculate appropriate doses for induction and maintenance of anesthesia, a square root of time model was used, with calculations based on metabolic size and attainment of 1.3x minimum alveolar concentration. The equipotent target alveolar concentration of ISO was 1.82% and for SEVO 4.03%. After doses were calculated, a table listing piglet weights and agent requirements was produced. Anesthetics were delivered via liquid anesthetic injection into a previously developed rebreathing inhaler that was filled with oxygen prior to use. Piglets were anesthetized, castrated and allowed to recover prior to return to the sow. Times for induction, recovery and total time to standing were recorded, and end-tidal carbon dioxide (Pe'CO2) tensions were measured by capnography immediately after mask removal. Each response variable was analyzed in sas using the Proc Mixed procedure, with piglet weight and days of age as covariates. Castration problems and mortality were assessed relative to unanesthetized littermates. RESULTS: There were no statistically significant differences in age, weight or total anesthetic time between the anesthetics. Induction time was shorter, recovery time longer, and Pe'CO2 lower with ISO. No morbidity or mortality was associated with either group. CONCLUSION AND CLINICAL RELEVANCE: Isoflurane and SEVO, delivered in a novel inhaler, provided economical, safe, rapid anesthetic induction and maintenance. Optimal conditions were provided for castration and recoveries were brief and smooth. Statistically significant differences in times would be of minor clinical importance. The cost of anesthesia was much less with ISO than with SEVO.

An inhaler device using liquid injection of isoflurane for short term anesthesia in piglets.

OBJECTIVES: To test a novel inhaler for administering isoflurane (ISO) anesthesia to piglets during castration and other surgical procedures of short duration. STUDY DESIGN: Prospective, randomized study. Animals Fifty-seven male piglets aged 6-10 days, body weight 1.1-3.5 kg. METHODS: An inhaler was developed which consisted of a mask, center body with open-close valve, vaporization chamber with wick and injection port, and a rebreathing bag. Liquid ISO required for induction of anesthesia and surgery was calculated, based on a desired alveolar ISO concentration of 1.82%. Dose was calculated using a square root of time model and metabolic size (B.W. 0.75). For practical use the calculated dose was expressed in relation to scale weight (kg). Isoflurane was delivered into the liquid injection port, followed by oxygen to fill the rebreathing bag and initiate vaporization. After the mask was fitted over the piglet's nose, the sliding open-close valve was opened to allow respiratory flow to move gases in and out of the inhaler and rebreathing bag. Fifty-seven male piglets received anesthesia prior to castration. Morbidity and mortality were assessed relative to unanesthetized litter mates. Induction, recovery and total anesthetic times were measured. End-tidal CO2 was measured immediately after mask removal by capnography. Costs of equipment and anesthetic agent were calculated. RESULTS: Mean induction time was rapid, 47.5 +/-8.7 seconds, generally with minimal or no struggling. Surgery usually lasted less than 30 seconds and was always completed prior to the 120 seconds allotted for induction and surgery. Anesthesia was adequate and recovery time was 122 +/- 44 seconds. Total time from start to standing was 260 +/- 51 seconds. The end-tidal CO2 was 5.2 +/- 1.1 kPa (39.4 +/- 8.4 mmHg). No morbidity or mortality was associated with either group. Inhaler construction costs were below $100, and liquid ISO cost ranged between $0.02 and $0.03 per piglet. CONCLUSION AND CLINICAL RELEVANCE: Isoflurane delivered in a novel inhaler has the potential to provide economical, safe, rapid anesthetic induction and safe, smooth recovery in piglets.