Closed-circuit halothane and enflurane using an in-circle Goldman vaporizer

Delivery of sevoflurane to dogs using a Stephens anaesthetic machine

OBJECTIVE

To investigate the sevoflurane concentrations produced within the Stephens anaesthetic machine circuit (vaporizer in-circle system) at different fresh gas flow rates (FGFRs), temperatures, vaporizer settings and vaporizer sleeve positions when used to anaesthetize dogs of different body sizes.

STUDY DESIGN

Experimental non-blinded studies.

ANIMALS

Eighteen mixed breed dogs, weights 4-39 kg.

METHODS

Anaesthetic induction with propofol was followed by maintenance with sevoflurane in oxygen via the Stephens anaesthetic machine. In study 1, the vaporizer setting, temperature and circuit FGFRs were altered with the vaporizer sleeve down (n = 3), or in separate experiments, up (n = 3). Delivered (Fi'SEVO) and expired sevoflurane concentrations were recorded. Study 2 determined the vaporizer settings (sleeve up) required to achieve predetermined multiples of minimal alveolar concentration (MAC) of Fi'SEVO when sevoflurane was delivered to dogs (n = 12) of different bodyweights and at different FGFRs.

RESULTS

Delivered concentrations of sevoflurane were sufficient to maintain anaesthesia in all dogs, regardless of bodyweight, FGFR, vaporizer temperature and sleeve position. Fi'SEVO increased with increasing temperature, when the vaporizer sleeve was down, when vaporizer setting was increased and when FGFR was decreased. As the FGFR increased or the dog's bodyweight decreased, higher vaporizer settings were required to produce the same Fi'SEVO. The median Stephens vaporizer settings to achieve an Fi'SEVO of 1.3 MAC ranged from 4.3 to 5.0 for a small dog (1-10 kg), 2.5 to 5.6 for a medium dog (15-25 kg) and 2.5 to 3.5 for a large dog (30-40 kg), depending on the FGFR.

CONCLUSION AND CLINICAL RELEVANCE

The Stephens anaesthetic machine can deliver to dogs, weighing 4 kg and above, concentrations of sevoflurane sufficient or in excess of that required to maintain anaesthesia, at temperatures from 10 to 35 °C, FGFRs of 1 to 5 times the patient's estimated metabolic oxygen requirement and at any vaporizer sleeve position.

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.

The Komesaroff anaesthetic machine for delivering sevoflurane to dogs

Objective To quantify the vapour output of the Komesaroff machine when using sevoflurane and to determine its performance for inducing and maintaining sevoflurane anaesthesia in dogs. Study design Prospective experimental study. Animals Six clinically normal beagles, aged 3-6 years and weighing 20 ± 1.65 kg (mean ± SEM). Methods The first study was performed using five Komesaroff vaporizers to measure the sevoflurane concentration delivered at each tap setting (I to IV) at 5, 10, 15, 20, 25, 30 and 35 minutes. For this study a ventilator was connected to the Komesaroff machine and set to deliver a tidal volume of 250 mL at 10 cycles minute^-1; oxygen flow was 100 mL minute^-1. A three-litre reservoir bag was attached to the Y-piece connector to act as a lung model. In the second study anaesthesia was induced in dogs with sevoflurane delivered by face-mask mask and carried in 2 L minute^-1 100% oxygen and with the vaporizer set at the fully open position. The quality and speed of induction were recorded. After orotracheal intubation, anaesthesia was maintained for 60 minutes with sevoflurane using an oxygen flow of 100 mL minute^-1. The dogs were allowed to breathe spontaneously. The respiratory rate (RR), heart rate (HR), oesophageal temperature, systolic (SAP) mean (MAP) and diastolic (DAP) arterial pressure, end-tidal CO₂ concentration (Fe'_CO2) end-tidal (Fe'_SEVO) and peak-inspired (Fi_SEVO) percentages of sevoflurane, and vaporizer tap setting were recorded every 5 minutes during anaesthesia. Results The delivery of sevoflurane was constant for each vaporizer setting. The mean output of sevoflurane was 0.44 ± 0.01% for setting I, 2.59 ± 0.18% for setting II, 3.28 ± 0.22% for setting III and 3.1 ± 0.5% for setting IV. In the second study, the mean induction time was 7.72 ± 0.60 minutes and the quality of the induction was good in all dogs. The mean vaporizer tap setting for the maintenance of anaesthesia was 3.48 ± 0.12 and the mean values for Fe'_SEVO and Fi_SEVO were 2.42 ± 0.04% and 2.87 ± 0.06%, respectively. The pedal withdrawal reflex persisted throughout anaesthesia. Conclusions It proved impossible to produce surgical anaesthesia with sevoflurane delivered by the Komesaroff machine despite the highest possible sevoflurane concentration being delivered. Clinical relevance Sevoflurane delivered from the Komesaroff machine cannot be relied upon to maintain surgical anaesthesia in spontaneously breathing dogs.

Uptake of isoflurane during prolonged clinical anaesthesia

Recent evidence has suggested that the rate of uptake of inhalational anaesthetic is constant during maintenance of anaesthesia, contrary to the predictions of multi-compartment uptake models. We measured isoflurane uptake using a totally closed anaesthetic system during up to 10 h of stable anaesthesia for maxillo-facial surgery on 12 adult patients. Liquid isoflurane was injected into the system under computer control to produce an end tidal concentration of 1.3 MAC of isoflurane. Bench tests demonstrated that the leakage from the system was less than 8 microl min(-1), confirming that the rate of injection of isoflurane into the system was a close upper bound on the patients' uptake. Anaesthetic usage for a 70 kg patient was 0.44e(-0.51t)+0.044e(-0.013t)+0.058e(-0.00098t) ml min(-1) of liquid isoflurane, where t is duration of anaesthesia in minutes. There was a continuing reduction in anaesthetic requirement even at the end of the period of study that was statistically significant. These data do not support the notion that isoflurane uptake is constant during stable maintenance of anaesthesia but is compatible with the conventional multi-compartment model of anaesthetic uptake and distribution.

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.

Uptake of desflurane during anaesthesia

The amount of desflurane required to maintain an end-expired concentration of 8% was measured in 30 ASA 1 and 2 patients undergoing elective spinal surgery. The anaesthetic was administered using a computer-controlled closed circle system. After an initial period during which the expired concentration of desflurane was stabilised (4 min) the rate of uptake showed a bi-exponential decline. Mean cumulative usage of desflurane was 10.1 ml of liquid at 30 min, 14.8 ml at 60 min, 25.4 ml at 120 min, 35.8 at 180 min.

The relationship between anaesthetic uptake and cardiac output

The hypothesis that anaesthetic uptake during maintenance of anaesthesia is related to cardiac output was tested on 21 patients undergoing cardiac surgery. Using a computer-controlled closed breathing system, enflurane was administered to maintain an end-expired concentration of 1%. Cardiac output was measured by thermodilution using a pulmonary artery catheter. A clear qualitative but not quantitative relationship was demonstrated. Changes in anaesthetic requirements at a constant end-expired concentration are a better guide to changes in cardiac output than changes in end-expired carbon dioxide with constant ventilation in patients undergoing cardiac surgery.

The uptake of enflurane during anaesthesia

The uptake of enflurane at a constant end-expired concentration of 2% in oxygen was studied in 38 patients, ASA 1 or 2, undergoing elective orthopaedic procedures. The anaesthetic was administered using a computer-controlled closed circle system. Following an initial 4 min period during which the expired concentration of enflurane was established, the rate of uptake of enflurane showed a triexponential decline. The mean cumulative use of enflurane after 60 min was 10.7 ml, and after 120 min was 18.4 ml.

A computer-controlled closed anaesthetic breathing system

We describe the design and working of a computer-controlled, closed anaesthetic breathing system which rapidly achieves and maintains a prescribed end-tidal concentration of isoflurane in oxygen. The system is simple to set up and not expensive; the only nonstandard component is a modified glass syringe. We have demonstrated that gas analysers may contribute as much as the patient to the accumulation of nitrogen within the breathing system. Details of our clinical experience with the system are presented in an accompanying article.

An evaluation of anaesthetic loss from a closed breathing system

We present the results of a laboratory study of the loss of isoflurane from a to-and-fro system, ventilated with oxygen, using a standard ventilator connected to the system via a long corrugated tube (the trunk) in place of the reservoir bag. No conventional fresh gas supply is used and isoflurane is injected as a liquid directly into the soda lime canister. The loss of isoflurane from the system due to mixing in the trunk was generally less than that lost by absorption into rubber and plastic system components. Glass syringes used for injection of liquid isoflurane were found to be a potential source of much greater leaks. The results showed that a to-and-fro system ventilated with oxygen via an open trunk functions as a virtually completely closed anaesthetic breathing system.

Uptake of nitrous oxide by man

Because of discrepancy in reports concerning the rate of uptake of nitrous oxide by man, it seemed desirable to measure this factor when automatic control of concentrations and rapid computer measurement of breath to breath uptake became available. Six subjects inhaled 25 per cent nitrous oxide for two hours. Total uptake and alveolar uptake were determined. The total inhaled uptake, adjusted for a 70 kg subject inhaling 75 per cent nitrous oxide, corresponded well with an absorption of about 1000 ml the first minute. After a wash-in period of less than four minutes, the alveolar uptake, which at that time and subsequently equaled the total uptake, decreased approximately according to the square root of time in minutes. Considerable variation appeared between individuals, but each course approximated the same type of decrease. Absorption continued to occur during the two hour period of observation.