A computer-controlled closed anaesthetic breathing system

Use of an Integrated Low-Flow Anesthetic Vaporizer, Ventilator, and Physiological Monitoring System for Rodents

Low-flow digital vaporizers commonly utilize a syringe pump to directly administer volatile anesthetics into a stream of carrier gas. Per animal welfare recommendations, animals are warmed and monitored during procedures requiring anesthesia. Common anesthesia and physiological monitoring equipment include gas tanks, anesthetic vaporizers and stands, warming controllers and pads, mechanical ventilators, and pulse oximeters. A computer is also necessary for data collection and to run equipment software. In smaller spaces or when performing field work, it can be challenging to configure all this equipment in limited space. The goal of this protocol is to demonstrate best practices for use of a low-flow digital vaporizer using both compressed oxygen and room air, along with an integrated mechanical ventilator, pulse oximeter, and far infrared warming as an all-inclusive anesthesia and physiological monitoring suite ideal for rodents.

Use of a Low-flow Digital Anesthesia System for Mice and Rats

A traditional vaporizer depends on flowing gas and atmospheric pressure for passive anesthetic vaporization. Newly developed direct injection vaporizers utilize a syringe pump to directly administer volatile anesthetics into a gas stream. Unlike a traditional vaporizer, it can be used at very low flow rates, making it ideal for use on mice and rats. The equipment's capability to use low flow rates could result in a substantial cost savings due to the reduced need for anesthetic agents, compressed gas, and charcoal scavenging filters(1). A lower flow rate means less waste of anesthetic gas and likely reduces the risk of anesthetic exposure to laboratory personnel. Thus, the high levels of precision and safety associated with direct injection vaporizers, along with a reduced need for anesthetic agents, compressed gas, and charcoal filters are beneficial for research requiring small animal anesthesia. The goal of this protocol is to demonstrate the use of a syringe-driven direct injection vaporizer as part of a digital, low-flow anesthesia system. The direct injection vaporizer is capable of accurately delivering anesthesia at very low flow rates compared to a traditional vaporizer, making it a promising alternative for controlled gas anesthetic delivery to rodents.

The predictive performance of a pharmacokinetic model for manually adjusted infusion of liquid sevofluorane for use with the Anesthetic-Conserving Device (AnaConDa): a clinical study

BACKGROUND

The Anesthetic-Conserving Device (AnaConDa) can be used to administer inhaled anesthetics using an intensive care unit (ICU) ventilator. We evaluated the predictive performance of a simple manually adjusted pump infusion scheme, for infusion of liquid sevoflurane to the AnaConDa.

METHODS

We studied 50 ICU patients who received sevoflurane via the AnaConDa. They were randomly divided into three groups. A 6-h infusion of liquid anesthetic was adjusted according to the infusion scheme to a target end-tidal sevoflurane concentration of 1% (Group 1%, n = 15) and 1.5% (Group 1.5%, n = 15). The initial rate was adjusted to reach the target concentration in 10 min and then the infusion was reduced to the first hour maintenance rate and readjusted once each hour afterwards. The actual concentrations were measured in the breathing circuit and compared with the target values. In the third group (n = 20) we used the model to increase and decrease the target concentration (+/-0.3%) for 3 h and evaluated the actual change in concentration achieved. The ability of the infusion scheme to provide the target concentration was quantified by calculating the performance error (PE). Infusion scheme performance was evaluated in terms of accuracy (median absolute PE, MDAPE) and bias (median PE, MDPE).

RESULTS

Performance parameters (mean +/- SD, %) were for 1%, 1.5%, increase of concentration by 0.3% and decrease of concentration by 0.3% groups, respectively: MDAPE 5.3 +/- 5.5, 2.6 +/- 4.0, 5.0 +/- 5.6, 5.5 +/- 5.4; MDPE -5.3 +/- 5.5, -2.3 +/- 4.1, -0.1 +/- 7.1, 0.2 +/- 5.4. No significant differences were found between means of all performance parameters when the 1% and 1.5% groups were compared.

CONCLUSIONS

There is an excellent 6-h predictive performance of a simplified pharmacokinetic model for manually adjusted infusion of liquid sevoflurane when using the AnaConDa to deliver sevoflurane to ICU patients.

Measuring the costs of inhaled anaesthetics

The cost of inhalation anaesthesia has received considerable study and is undoubtedly reduced by the use of low fresh gas flows. However, comparison between anaesthetics of the economies achievable has only been made by computer modelling. We have computed anaesthetic usage for MAC-equivalent anaesthesia with isoflurane, desflurane, and sevoflurane in closed and open breathing systems. We have compared these data with those derived during clinical anaesthesia administered using a computer-controlled closed system that measures anaesthetic usage and inspired concentrations. The inspired concentrations allow the usage that would have occurred in an open system to be calculated. Our computed predictions lie within the 95% confidence intervals of the measured data. Using prices current in our institution, sevoflurane and desflurane would cost approximately twice as much as isoflurane in open systems but only about 50% more than isoflurane in closed systems. Thus computer predictions have been validated by patient measurements and the cost saving achieved by reducing the fresh gas flow is greater with less soluble anaesthetics.

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.

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.

Closed carbon dioxide filtration revisited

There are compelling reasons why the closed carbon dioxide filtration method for inhalation anaesthesia deserves serious reconsideration. Use of the closed absorption system today can provide all the benefits recognised by those who introduced it seventy to eighty years ago. A most important benefit is the increased opportunity of learning afforded the user, which leads either neophyte or senior clinician to improvement of both concept and clinical skills. The current resurgence of interest is fully appropriate for all physicians who aspire to be true specialists in the care of patients during clinical anaesthesia.

Vaporization of isoflurane by liquid infusion

We investigated the vaporization of liquid isoflurane when infused directly into a circuit. Pooling of isoflurane occurred within the circuit tubing at infusion rates used during clinical practice when constant gas flows were used. Despite pooling, the concentration of isoflurane was linearly related to infusion rate. Cyclical gas flow, such as that seen in a circle system, increased vaporization so that pooling occurred only at the higher infusion rates used during the first five minutes of totally closed circuit anaesthesia. There were no major differences in pooling or the maximum concentration of isoflurane reached between 26 gauge needle and droplet administration of isoflurane: however the maximum concentration was reached more quickly by droplet administration. We conclude that direct infusion of liquid isoflurane into an anaesthetic circuit will result in complete vaporization during maintenance anaesthesia.

The Ohmeda Rascal II. A new gas analyser for anaesthetic use

The Ohmeda Rascal II is a multigas analyser and pulse oximeter for dedicated anaesthetic.use. It uses the Raman scattering of laser light to identify and quantify oxygen, nitrogen, carbon dioxide, nitrous oxide and three volatile anaesthetic agents. Its response times equal or better the published response times of infrared or photo-acoustic devices. It is linear within the clinical ranges of all gases and vapours, simple to use, requires no maintenance, holds its calibration well, and is a suitable monitor for clinical and research use.

The uptake of isoflurane during anaesthesia

The uptake of isoflurane at a constant end-expired concentration of 1.5% in oxygen was studied in 15 women, ASA 1 or 2, undergoing elective total abdominal hysterectomy. The anaesthetic was administered by a simple computer-controlled to-and-fro closed system. After an initial period of wash-in to the system, the rate of uptake of isoflurane decreased bi-exponentially with a rapid reduction during the first 15 min. Perturbations from this bi-exponential decline reflect changes in cardiac output. The mean (SD) cumulative use of isoflurane was 4.5 (0.43) ml after 30 min and 7.3 (0.79) ml after 60 min.