Do heat and moisture exchangers in the anaesthesia breathing circuit preserve body temperature in dogs undergoing anaesthesia for magnetic resonance imaging?

OBJECTIVE: To investigate whether the use of a heat and moisture exchanger (HME) preserves body temperature in dogs weighing <10 kg anaesthetised for magnetic resonance imaging (MRI). STUDY DESIGN: Prospective, randomised, clinical trial. ANIMALS: Thirty-one client-owned dogs. METHODS: Dogs were assigned randomly to a treatment group [HME (n = 16) or no HME (n = 15)]. Dogs were pseudorandomised according to the premedication they were administered, either dexmedetomidine or no dexmedetomidine. Induction agents were not standardised. General anaesthesia was maintained with isoflurane vaporised in 100% oxygen delivered using a T-piece and a fresh gas flow of 600 mL kg-1 minute-1. Rectal temperature was measured before premedication (T1), after induction (T2), before moving to the MRI unit (T3) and at the end of the MRI scan (T4). Ambient temperatures were measured in the induction room, outside and inside the MRI unit. Data were analysed using a general linear model with T4 as the outcome variable. Linear correlations were performed between T1, T2, T3 and T4, and variables that predicted T4 were investigated. RESULTS: Sex, age and body mass were not significantly different between groups. There were no significant differences in rectal temperature between groups at any time point (group with HME at the end of MRI = 36.3 ± 1.1 °C; group with no HME at the end of MRI = 36.2 ± 1.4 °C) but at the end of the MRI, dogs administered dexmedetomidine (36.6 ± 0.7 °C) had a higher rectal temperature compared with dogs not administered dexmedetomidine (35.9 ± 1.6 °C) for premedication. Rectal temperature varied directly with ambient temperature in MRI scanning room and inversely with anaesthetic duration. CONCLUSIONS AND CLINICAL RELEVANCE: Using an HME did not alter body temperature in dogs weighing <10 kg undergoing an MRI, but including dexmedetomidine in the premedication regimen seemed to preserve the body temperature during anaesthesia.

Performance of a new carbon dioxide absorbent, Yabashi lime® as compared to conventional carbon dioxide absorbent during sevoflurane anesthesia in dogs.

In the present study, we compare a new carbon dioxide (CO2) absorbent, Yabashi lime(®) with a conventional CO2 absorbent, Sodasorb(®) as a control CO2 absorbent for Compound A (CA) and Carbon monoxide (CO) productions. Four dogs were anesthetized with sevoflurane. Each dog was anesthetized with four preparations, Yabashi lime(®) with high or low-flow rate of oxygen and control CO2 absorbent with high or low-flow rate. CA and CO concentrations in the anesthetic circuit, canister temperature and carbooxyhemoglobin (COHb) concentration in the blood were measured. Yabashi lime(®) did not produce CA. Control CO2 absorbent generated CA, and its concentration was significantly higher in low-flow rate than a high-flow rate. CO was generated only in low-flow rate groups, but there was no significance between Yabashi lime(®) groups and control CO2 absorbent groups. However, the CO concentration in the circuit could not be detected (≤5ppm), and no change was found in COHb level. Canister temperature was significantly higher in low-flow rate groups than high-flow rate groups. Furthermore, in low-flow rate groups, the lower layer of canister temperature in control CO2 absorbent group was significantly higher than Yabashi lime(®) group. CA and CO productions are thought to be related to the composition of CO2 absorbent, flow rate and canister temperature. Though CO concentration is equal, it might be safer to use Yabashi lime(®) with sevoflurane anesthesia in dogs than conventional CO2 absorbent at the point of CA production.

Proper theoretical analysis of the oxygen wash-in kinetics of circle breathing systems.

OBJECTIVE: To correctly define the theoretical treatment of oxygen wash-in kinetics in circle breathing systems and reevaluate previously published results for the rate of change of oxygen concentration in a large animal circle breathing system. STUDY DESIGN: Theoretical analysis of previously published data. ANIMALS: None. METHODS: Previously published data for the rate of change of oxygen concentration in a large animal circle breathing system with two reservoir bag sizes (40 and 20 L) at three different flow rates (3, 6, and 10 L minute(-1)) were obtained from the original publication, via digital extraction, and analyzed to determine system time constants. The results of this analysis were compared to those originally reported and the level of mathematical agreement between experiment and theory was quantified. RESULTS: Theoretical time constants for the system with 40 L reservoir bag with flow rates of 3, 6, and 10 L minute(-1) were 16.4, 8.2, and 4.9 minutes, respectively. Experimentally derived time constants for this system with flow rates of 3, 6, and 10 L minute(-1) were 18.1, 9.2, and 5.4 minutes, respectively. Percent differences between experimental and theoretical time constants for this system with flow rates of 3, 6, and 10 L minute(-1) were 10.4, 12.2, and 10.2%, respectively. For the system with a 20 L reservoir bag and 6 L minute(-1) flow rate, the theoretical and experimentally derived time constants were 5.5 and 5.6 minutes, respectively, with a 1.8% difference. The average relative deviations between theory and experiment for the system at 6 L minute(-1) flow with a 40 or 20 L reservoir bag were 1.3% and 3.0%, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: Proper theoretical analysis of experimentally obtained data for the wash-in kinetics of oxygen into a large animal circle breathing system leads to improved mathematical agreement between theory and experiment when compared to the originally published results. Application of this method should allow more accurate prediction of the rate of change of oxygen concentration in anesthetic circuits.

Clinical evaluation of an end-tidal target-controlled infusion closed-loop system for isoflurane administration in horses undergoing surgical procedures.

A new volatile anaesthetic agent delivery system was tested in 15 horses undergoing scheduled surgical procedures. The delivery system consisted of a laptop computer (with dedicated software), a computer-controlled syringe driver (loaded with liquid isoflurane) connected to the inspiratory arm of a large-animal circle breathing system and a respiratory gas monitor, providing isoflurane end-tidal concentrations (ET(measured)) every 20 s to the computer. Following induction and connection to the breathing system, mechanical ventilation was started. The bodyweight (BW), fresh gas flow, breathing system and ventilator volume, and end-tidal isoflurane target (ET(target)) were entered into the computer. Using Lowe's equation, the software calculated the prime dose to be delivered by the syringe driver over 2 min. After this, the system delivered each minute the amount of isoflurane as determined by the following equation: Isoflurane (mL) = {2 × λ(B/G) × (200 × BW(0.75)) × (ET(target) - ET(measured)) + (fresh gas flow - (BW(0.75) × 0.07)) × (ET(measured))}/206. A fresh gas flow of 4 L oxygen min(-1) was administered until the inspired fraction of oxygen reached 0.7, and was then decreased. A target of 1.5% end-tidal isoflurane was initially used and subsequently adjusted to the clinical requirements. The system performance was evaluated using the median prediction error (MDPE) and the median absolute performance error (MDAPE), which were -3.6% and 5.29%, respectively. It was concluded that this system was useful to achieve end-tidal target-controlled infusion of isoflurane during equine anaesthesia.

Mechanical ventilation of six dogs anaesthetised with isoflurane or sevoflurane delivered by a Komesaroff anaesthetic machine.

After intravenous induction, six beagles were connected to a Komesaroff machine provided with a single in-circuit vaporiser and ventilated mechanically at either nine or 14 breaths/minute while anaesthetised with either isoflurane or sevoflurane. The vaporiser was initially set at position 4/4 (fully open) and the anaesthetic concentrations were measured after one and five minutes; the vaporiser was then set at the lowest setting able to maintain anaesthesia. Cardiorespiratory variables were measured throughout the study. In most cases anaesthesia was maintained at setting 1/4 with isoflurane and at setting 1.5/4 or 2/4 with sevoflurane.

Cardiac output measurement by partial carbon dioxide rebreathing, 2-dimensional echocardiography, and lithium-dilution method in anesthetized neonatal foals.

The objective of this study was to assess 2 noninvasive methods of measuring cardiac output (CO) in neonatal foals by comparing results to that of the lithium-dilution method. Ten neonatal foals were anesthetized and CO was manipulated by varying the depth of anesthesia and infusion of dobutamine. Concurrent CO measurements were obtained by lithium dilution (reference method), partial carbon dioxide (CO2) rebreathing, volumetric echocardiography (cubic, Teichholz, Bullet, area-length, and single and biplane modified Simpson formulas), and transthoracic Doppler echocardiography. Thirty pairs of lithium-dilution/noninvasive CO measurements were taken from the 10 foals. For each method, relative bias was calculated as a percentage of the average CO. Lithium determinations of CO ranged between 3.09 and 1 1.1 L/min (mean +/- SD = 6.39 +/- 2.1 L/min), resulting in cardiac indices ranging between 79.0 and 209 mL/kg/min (mean +/- SD = 131 +/- 35.9 mL/kg/min). Relative bias of Doppler echocardiography significantly increased (P < .05), whereas that of partial CO2 rebreathing significantly decreased (P = .03) with increasing CO. Other methods were not influenced by the level of CO. Among methods not influenced by the level of CO, relative bias of the Bullet method (-4.2 +/- 20.9%; limits of agreement -45.2 to 36.7%) was significantly lower (P < .05) than that of each of the other noninvasive methods evaluated. Volumetric echocardiography using the Bullet method provides an accurate and noninvasive estimate of CO in anesthetized neonatal foals and warrants investigation in critically ill conscious foals.

Closed system anaesthesia in dogs using liquid sevoflurane injection; evaluation of the square-root-of-time model and the influence of CO2 absorbent.

OBJECTIVE: To determine whether predictable alveolar concentrations of sevoflurane are reliably produced in dogs when liquid sevoflurane is injected into closed circuit breathing systems, as calculated by Lowe's square-root-of-time anaesthetic uptake model, and to confirm the validity of the model using soda lime and calcium hydroxide lime. STUDY DESIGN: Prospective clinical study. ANIMALS: Eleven healthy dogs with a mean body mass of 34 +/- 9 kg scheduled for pelvic limb orthopaedic surgery. MATERIALS AND METHODS: Following pre-anaesthetic medication, anaesthesia was induced with propofol and maintained with sevoflurane in a closed circle system. Epidural anaesthesia was performed with morphine and bupivacaine. Liquid sevoflurane was injected into the circuit by syringe, using dosages and time intervals derived from Lowe's square-root-of-time anaesthetic uptake model. The target alveolar concentration chosen was 1.1 x MAC (2.6% end-tidal sevoflurane). Either soda lime (group S; n = 6) or calcium hydroxide lime (Amsorb; group A; n = 5) were used for CO(2) absorption. Sevoflurane concentration and the respiratory gas composition were measured with an infrared gas analyser. RESULTS: End-tidal sevoflurane concentrations were close to the predicted value of 2.6% at 9 minutes (2.53 +/- 0.1% group S; 2.60 +/- 0.26% group A) and 16 minutes (2.55 +/- 0.30 group S; 2.52 +/- 0.28% group A) but declined thereafter to reach 50% (group S) and 64% (group A) of the predicted value at 121 minutes. There was a constant trend towards higher end-tidal sevoflurane concentrations in group A but the difference was not statistically significant. CONCLUSIONS: The square-root-of-time model leads to significantly lower alveolar concentrations than expected, suggesting that the rate of sevoflurane uptake in dogs declines less rapidly than predicted. The use of Amsorb tends to reduce the deviation from predicted concentrations. CLINICAL RELEVANCE: The model used in this study provided only an approximate guide to the volume of liquid sevoflurane required. Consequently, the definitive dose schedule must be based on measured anaesthetic concentrations and clinical monitoring.

Inhaled carbon monoxide concentration during halothane or isoflurane anesthesia in horses.

OBJECTIVE: The purpose of this study was to assess carbon monoxide (CO) exposure during equine anesthesia with either halothane (H) or isoflurane (I) delivered in a circle rebreathing system. STUDY DESIGN: Prospective clinical investigation. ANIMALS: Fifty client-owned horses. METHODS: Horses were randomly assigned for anesthetic maintenance with H (n = 26) or I (n = 24). Two large animal anesthetic machines were used and assigned to a single agent for 2-4 weeks at a time. Machines were disassembled and soda lime changed prior to switching anesthetic agents. Inhalant anesthetic concentration and CO concentration were measured in gas samples obtained from the inspiratory limb of the anesthetic circuit. Values were recorded at 15 minute intervals for 90 minutes. Soda lime status (new or used) and mode of ventilation (spontaneous or mechanical) were also recorded. Data were analyzed using a five-factor ANCOVA with repeated measures. RESULTS: Inspired CO concentration for H and I increased from 1 +/- 3 and 6 +/- 11 ppm at baseline to 54 +/- 33 and 21 +/- 18 ppm at 90 min, respectively (mean +/- sd). H was associated with significantly greater CO concentrations than I at 30 to 90 min, although baseline CO was significantly greater in the I group than the H group. Oxygen flow rates were 9.9 +/- 0.5 L/min at baseline for H and I, and 5.0 +/- 0.4 and 5.0 +/- 0.7 L/min at 90 min for H and I, respectively. There were no significant differences between groups for O2 flow at any time point. Neither mechanical ventilation nor new versus used soda lime affected CO concentration. CONCLUSIONS: Significantly higher concentrations of CO were recorded during the administration of H than I. CLINICAL RELEVANCE: Levels of CO observed during the administration of either H or I for 90 minutes to horses were not clinically significant.

Efficacy of the Komesaroff anaesthetic machine for delivering isoflurane to dogs.

The Komesaroff machine is a low-flow, closed-circle circuit with a low resistance vaporiser, of the Goldman type, in circuit. This trial assessed the mechanical consistency of the delivery of isoflurane by the vaporisers, and used six dogs to compare the in vivo cardiorespiratory effects of the anaesthetic agent delivered by the Komesaroff machine with the effects of a circle system with high flows in the semi-closed mode. The delivery of isoflurane was constant for each vaporiser setting and no potentially dangerous concentrations of isoflurane were observed. The mean (sem) percentages of isoflurane were 0.18 (0.019) at setting I, 1.46 (0.055) at setting II, 3.12 (0.066) at setting III and 3.01 (0.047) at setting IV. There were no significant differences between the two types of circuit in vivo, and the measured haemodynamic variables were satisfactory throughout the experiments.

Cardiorespiratory effects of low-flow and closed circuit inhalation anesthesia, using sevoflurane delivered with an in-circuit vaporizer and concentrations of compound A.

OBJECTIVES: To determine the concentrations of sevoflurane and compound A (a degradation product of sevoflurane) in the anesthetic circuit when sevoflurane was delivered with an in-circuit vaporizer, and to determine the cardiorespiratory effects of sevoflurane in dogs. ANIMALS: 6 mixed-breed dogs. PROCEDURE: In-circuit vaporizers were connected to the inspiratory limb of a circle rebreathing system connected to a ventilator. A reservoir bag was attached to the Y-piece connector to act as an artificial lung, and sevoflurane concentrations in the anesthetic circuit were measured at vaporizer settings of 1, 3, 5, 7, and 10 and oxygen flow rates of 250 and 500 ml/min. Cardiorespiratory effects of sevoflurane were determined in dogs while they were breathing spontaneously, during controlled ventilation, and during closed circuit anesthesia. Concentrations of compound A were determined by means of gas chromatography with flame ionization. RESULTS: The concentration of sevoflurane in the anesthetic circuit increased with vaporizer setting and time. For oxygen flow rates of 250 and 500 ml/min, vaporizer settings between 5 and 7 and between 7 and 10, respectively, produced sevoflurane concentrations closest to values reported to produce surgical anesthesia in dogs. Significant differences were not observed in cardiorespiratory variables with time or among anesthetic conditions. Concentrations of compound A in the anesthetic circuit were less than values reported to produce renal toxicoses and death in rats. CONCLUSION: Results suggested that sevoflurane can be administered to nonsurgically stimulated dogs, using an in-circuit vaporizer and low (< 15 ml/kg/min) oxygen flow rates, without causing significant cardiorespiratory depression or clinically important concentrations of compound A.

[Minimal-flow anesthesia in the dog]. / Minimal-Flow-Narkose beim Hund.

Many veterinary practices possess an anesthetic machine with a rebreathing system, and therefore the facility to induce anesthesia under more cost-effective reduced fresh gas flow conditions in a semi-closed system. However, as the fresh gas flow is frequently far too high, the rebreathing element is used rarely or not at all, making the anesthesia unnecessarily expensive. The relationships between the fresh gas setting and the final concentrations of expired air are discussed, and experience in 53 dogs with minimal flow anesthesia (500 ml/min), an extreme variant of anesthesia induction using a semi-closed system with minimal excess gas volume and a high proportion of rebreathed gas, is described.

[Isoflurane anesthesia in rabbits in a closed anesthetic system]. / Zur Isoflurannarkose beim Kaninchen im geschlossenen Narkosesystem.

Using the Stephens anaesthetic apparatus-which is a closed system with an in-circuit, nonprecision vaporizer-and isoflurane as anaesthetic gas, 18 rabbits were anaesthetized and showed sufficient hypnosis, analgesia, and muscle relaxation during bone surgery. Induction of anaesthesia was achieved with intravenous propofol and all rabbits were intubated afterwards. During the following isoflurane inhalation anaesthesia the mean arterial blood pressure decreased considerably (compared to control measures before induction), the heart rate remained the same or showed a slight increase, and the respiratory rate decreased. The arterial pO2 decreased corresponding to the respiratory depression after propofol induction and increased again during spontaneous ventilation with 100% oxygen. The changes in arterial pCO2 and pH were representative for a rise in the CO2-stimulation threshold. A moderate metabolic acidosis could be seen due to preanaesthetic excitement of the animals. Recovery time was short (between one and 11 minutes) and no signs of excitation could be detected. The consumed volume of isoflurane was 0.80 ml/kg BW/h.

Sistema circular de anestesia com absorvedor de dióxido de carbono: uso em animais de pequeno porte / Circular system of anesthesia with dioxide carbonic absober: use in small animals

A utilizaçäo de sistema circular é apresentada como opçäo para ser usado como equipamento na anestesia em animais de pequeno porte. É feita a descriçäo pormenorizada dos diversos componentes do sistema circular, e sua resistência à respiraçäo.

Sistema circular de anestesia para animais de pequeno porte: estudo das resistências ao fluxo de ar / Anesthetic circle system for small animals: study of the resistance to air flow

A circle system with carbon dioxide absorber was construted to allow reinhalation of anesthetic gases by small animals and to facilitate the control of the concentration of gases and the measurement of metabolic data. The resistance to air flow was evalueted under increasing air flows (o,25 to 10l); different diameters of tracheal tubes (2.5 to 4.0 mm); two types of connector increased (3.5 and 5.00 mm) and different volumes of canisters, with and without absorber. The results showed increased in flow and to decreases in the diameter of tracheal tubes and connectors. The three different volumes of canister, with or without absorber, did not induce variation in the resistance to air flow. Based on the results, the authors conclude that the proposed circle system can be used for anesthesia of small animals

Anestesia inalatória em equinos com halotano e óxido nitroso: efeitos sobre os gases sanguíneos e equilíbrio ácido-base / Effects of halothane and nitrous oxide anesthesia on blood gases and acid-base-balance in horses

Estudaram-se os valores do pH, pCO2, HCO3, BE, pO2, concentraçäo e saturaçäo da hemoglobina do sangue arterial e venoso de 10 equinos mantidos anestesiados durante uma hora com halotano e óxido nitroso, o qual foi misturado com o oxigênio na proporçäo de 1:1, para fazer o transporte do halotano. Houve queda na frequência respiratória, provocando aumento da paCO2 e diminuiçäo do pHa, caracterizando uma acidose respiratória. Os dados sobre pHa, paCO2 e HCO3 näo mostraram diferença estatística entre os tempos. Houve uma queda significativa da concentraçäo de hemoglobina após 15 minutos de anestesia e a saturaçäo da hemoglobina encontrou-se abaixo dos limites normais. A comparaçäo dos valores do sangue arterial e venoso demonstra que näo houve diferença significativa da concentraçäo de hemoglobina, enquanto que a saturaçäo da hemoglobina do sangue arterial apresentou diferença significativa. Houve diferença significativa entre os tempos das variáveis frequência do pulso e näo houve diferença significativa na temperatura corporal. A comparaçäo dos dados da gasometria do sangue arterial e venoso revelou diferença significativa do pH, pCO2, e saturaçäo da hemoglobina. Conclui-se que é aconselhável o uso de sangue arterial na realizaçäo de gasometria em equinos

Closed-circuit liquid injection isoflurane anesthesia in the horse.

Six horses were administered isoflurane anesthesia by liquid injection into a closed breathing circuit according to the square root of time model. The unit dose (UD) was calculated using Lowe's formula to provide an end-tidal concentration of 1.3%, or the minimum alveolar concentration of isoflurane. The mean UD was 4.2 +/- 0.2 mL. The mean end-tidal isoflurane concentration (ETiso) for each interval after injection, and the peak and minimum concentrations for each injection interval, did not change beginning with the second injection, indicating that the square root of time model accurately predicted isoflurane uptake in the horse. Mean ETiso measured for the interval after the first injection was 0.68 +/- 0.06%, which was significantly (p < .05) lower than the mean concentration after all subsequent injections (1.1 +/- 0.1%). Mean peak end-tidal concentration was 1.1 +/- 0.25% after the first injection and 1.7 +/- 0.26% for all other injections. Mean minimum end-tidal concentration was 0.77 +/- 0.13% for all injection periods. This model proved to be an acceptable technique for administration of isoflurane anesthesia to horses.