Methane influences infrared technique anesthetic agent monitors

Analysis of oxygen, anaesthesia agent and flows in anaesthesia machine

The technical advancement in the anaesthesia workstations has made the peri-operative anaesthesia more safer. Apart from other monitoring options, respiratory gas analysis has become an integral part of the modern anaesthesia workstations. Monitoring devices, such as an oxygen analyser with an audible alarm, carbon dioxide analyser, a vapour analyser, whenever a volatile anaesthetic is delivered have also been recommended by various anaesthesia societies. This review article discusses various techniques for analysis of flow, volumes and concentration of various anaesthetic agents including oxygen, nitrous oxide and volatile anaesthetic agents.

Brief review: theory and practice of minimal fresh gas flow anesthesia

PURPOSE

The aim of this brief review is to provide an update on the theory regarding minimal fresh gas flow techniques for inhaled general anesthesia. The article also includes an update and discussion of the practical aspects associated with minimal-flow anesthesia, including the advantages, potential limitations, and safety considerations of this important anesthetic technique.

PRINCIPAL FINDINGS

Reducing the fresh gas flow to < 1 L·min(-1) during maintenance of anesthesia is associated with several benefits. Enhanced preservation of temperature and humidity, cost savings through more efficient utilization of inhaled anesthetics, and environmental considerations are three key reasons to implement minimal-flow and closed-circuit anesthesia, although potential risks are hypoxic gas mixtures and inadequate depth of anesthesia. The basic elements of the related pharmacology need to be considered, especially pharmacokinetics of the inhaled anesthetics. The third-generation inhaled anesthetics, sevoflurane and desflurane, have low blood and low tissue solubility, which facilitates rapid equilibration between the alveolar and effect site (brain) concentrations and makes them ideally suited for low-flow techniques. The use of modern anesthetic machines designed for minimal-flow techniques, leak-free circle systems, highly efficient CO(2) absorbers, and the common practice of utilizing on-line real-time multi-gas monitor, including essential alarm systems, allow for safe and cost-effective minimal-flow techniques during maintenance of anesthesia. The introduction of new anesthetic machines with built-in closed-loop algorithms for the automatic control of inspired oxygen and end-tidal anesthetic concentration will further enhance the feasibility of minimal-flow techniques.

CONCLUSIONS

With our modern anesthesia machines, reducing the fresh gas flow of oxygen to 0.3-0.5 L·min(-1) and using third-generation inhaled anesthetics provide a reassuringly safe anesthetic technique. This environmentally friendly practice can easily be implemented for elective anesthesia; furthermore, it will facilitate cost savings and improve temperature homeostasis.

Isoflurane measurement error using short wavelength infrared techniques in horses: influence of fresh gas flow and pre-anaesthetic food deprivation

OBJECTIVE

To quantify the isoflurane measurement error arising from the use of short wavelength infrared (IR) anaesthetic gas analysis during low flow anaesthesia in horses.

STUDY DESIGN

Prospective clinical study.

ANIMAL POPULATION

Sixty-four client-owned horses referred for elective or emergency surgery (age 1-16 years, body mass 400-650 kg).

MATERIALS AND METHODS

Horses were divided into four groups based on duration of pre-anaesthetic food deprivation period (FDP) and fresh gas flow during anaesthesia: a high flow group with normal FDP (n = 16) and three groups with low flow and normal (n = 29), long (n = 5) or no (n = 14) FDP, respectively. Circuit isoflurane concentrations were measured simultaneously using a short wavelength (methane-sensitive) analyser (Datex Capnomac Ultima) and a long wavelength (methane-insensitive) analyser (Hewlett Packard M 1025 B) for at least 60 minutes. The difference between the readings of both analysers gave the isoflurane measurement error of short wavelength IR analysis, from which the circuit methane concentration was calculated.

RESULTS

In the low flow groups, isoflurane measurement error increased over time, whereas in the high flow group, error remained constant after an initial rise in the first 15 minutes. The isoflurane measurement error was significantly lower (p < 0.005) in the high flow group compared with the low flow-normal FDP group from 15 to 60 minutes. Compared to the low flow - normal FDP group, isoflurane measurement error was significantly smaller (p < 0.001, from 15 to 60 minutes) in the low flow-long FDP group and significantly larger (p = 0.016, at 60 minutes) in the low flow-no FDP group. Within the low flow-no FDP group, values in colic cases did not differ from those in noncolic cases (p > 0.7).

CONCLUSIONS

Isoflurane measurement using short wavelength IR absorption is inaccurate. The fresh gas flow and duration of pre-anaesthetic food deprivation influence the isoflurane measurement error during anaesthesia in horses.

CLINICAL RELEVANCE

Short wavelength IR analysers are not reliable for isoflurane measurement during (low flow) anaesthesia in horses.