Respiratory Gas Analysers
Respiratory Gas Analysers
The purpose of respiratory gas analysis during anaesthesia is to identify and measure the concentrations, on a breath by breath basis, of the individual gases and vapours in use. It may also be useful as a guide to cardiac function or to identify trace contaminant gases. Different techniques use different physicochemical properties of the gas or vapour. An understanding of the physical principle underlying each method is necessary in order to recognise the value and limitations of each. In terms of the device’s ability to respond on a breath by breath basis, there are two important components: the time taken for the gas to be sampled from the anaesthetic machine or breathing system, the delay time; then there is the time taken for the device to measure the gas concentration, the response time. This is depicted in Figure 16.1. Most of the delay occurs in the delay time or transit time and can be reduced either by analysing the gas sample close to the airway, or by using as short and thin a sampling tube and as high a sampling flow rate to the analyser as possible [Chan et al. 2003]; the sampling flow rate is usually of the order of 100 to 200 ml min−1. If minimal fresh gas flow rates are being used in a circle anaesthetic breathing system and the sampled gas is not returned to the breathing system, then a high gas sampling rate could represent a significant gas leak. Figure 16.1 shows a sigmoid curve of recorded gas concentration change in response to a square wave input change. The response of a gas analyser is often expressed as the time taken to produce a 90–95% response to a step or square wave input change. A square wave change in gas concentration can be produced by moving a gas sampling tube rapidly into and out of a gas stream, by bursting a small balloon within a sampling volume containing a gas sample, or by switching a shutter to a gas sample volume using a solenoid valve. An important part of the use of gas analysers is zeroing and calibration since they are all prone to drift in both zero and gain.
Keywords: Clarke polarographic electrode, Datex-Engstrom device, Raman spectroscopy, Rayleigh refractometer, Rayleigh scattering, anaerobic threshold, diamagnetic molecules, exercise testing, fuel cells, interference fringes, mass spectrometry, nitrogen meter, paramagnetic oxygen analyser, piezoelectric gas analysis, polarography, preoperative exercise testing, quadrupole mass spectrometer, refractometry, respiratory gas analysis, ultraviolet gas analysis
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