GAS ANALYSIS by Dr. John Patterson Ph.D

Many techniques exist which can measure clinically important gases such as oxygen and carbon dioxide to high degrees of accuracy.
Most are unsuitable for rapid monitoring as they involve chemical determinations that are very slow. Some examples of these more involved techniques include gas chromatography, mass spectrometry, emission spectroscopy and Clarke electrodes.
Most are unsuitable for breath-by-breath monitoring because of slow response times, sample preparation or inconvenience. Fortunately there are a number of methods which are compact, quick, reliable, easy to calibrate and rugged.

Gaseous Oxygen

Currently, there are only a small number of technologies used in medical gas analysers suitable for 'bedside' monitoring by being small, reliable, quiet and easy to operate. The disposable components should be minimal and the calibration procedure quick, easy and reliable. Response time needs to be very quick for breath-by-breath monitoring, but less critical for studies using mixing chambers.

Apart from solid state sensors that are in their infancy, the most mature technologies for gas analysis in clinical departments and medical laboratories are the so-called fuel cells and devices using paramagnetic properties. These units are reliable, relatively small, easily calibrated and, simple to use and maintain. Although polarographic techniques can be used, protection of the platinum electrode and the need for semi-permeable membranes reduces their effectiveness.

Fuel cell (Zirconium oxide oxygen cell)

This popular compact cell has few real disadvantages. The most obvious is the need to run the cell at very high temperature (7000C) which, unless suitably protected is a significant hazard. Servicing of any analyser is not a trivial undertaking, but the high temperatures ensure extra care must be exercised.

These cells can be made quite small and consequently have rapid response times suitable for breath-by-breath monitoring. Their relatively small dead space ensures that mixing or diffusion of the sample is minimised. Flow rates through the cell need to be high enough to ensure correct flushing, but not so high as to cool the cell and this can reduce their suitability for situations where very small tidal volumes occur. The electronics associated with fuel cells is simple and should be very reliable.

In operation, the fuel cell depends on two things: ionized oxygen can carry an electrical current; and, ionised oxygen exclusively penetrates into the matrix of the zirconium oxide ceramic. If a suitable zirconium oxide barrier is placed between two concentrations of ionised oxygen, diffusion will result in the oxygen moving from the high concentration towards the low concentration by entering the ceramic.

If a permeable film of metal is deposited on the two surfaces of the ceramic, and a very stable voltage is applied between the surfaces, then a current proportional to the number of oxygen ions will flow between the two surfaces. It is this current which is the 'output' of the oxygen cell.

Three point calibration is needed. The first point is the short-circuited cell output (with no oxygen concentration difference). The second point should be one close to the expected range of measured oxygen concentration (normally, 12% - 16%: expired alveolar oxygen concentration), and the third is usually close to ambient oxygen levels (21%: inspired oxygen levels). For situations where elevated oxygen levels are being used, it is customary (and good practice) to calibrate around the expected operational range (zero, 21% and 80%, for example). The calibration gases need to be very accurately analysed, with good analysers the concentration should be known to 3 decimal places. Response times for these analysers are generally below 90 ms with flow rates through them of around 100 ml/min.

Paramagnetic analysers

These devices come in a number of styles. Only one style at present is suitable for rapid gas analysis in clinical applications: the differential paramagnetic device. These also come in two variants: balance type and the pressure type. Each relies on the unusual magnetic properties (for a gas) of oxygen in being attracted to a magnetic field. The balance types are relatively frail and have been replaced by the pressure type.

In the differential pressure paramagnetic oxygen analyser, the oxygen containing reference gas is directed at each of the inlet ports of a differential pressure transducer and then to a common exit point, or mixing junction. At, or near, the exit point there is a magnetic field. If a third gas stream containing oxygen is introduced into the mixing point the effect of the magnetic field is to increase the density and if the field is pulsating, then the density changes are also pulsating. If these increases in density occur at the inlet of one of the reference branches, then the pressure rises in that branch as the flow is retarded. Changes in pressure are then proportional to the oxygen content of the third stream. Such analysers have response times under 200 ms and are therefore quite effective for breath-by-breath monitoring. They are simple, rugged and reliable, but must be shielded from external magnetic fields.

In an alternative design the oxygen containing mixture replaces one of the reference gases and there is no third gas entry. In this system the magnetic field is modulated in the audible frequency range. The pressure transducers are replaced by sensitive microphones that monitor the alternative expansion and contraction of the gas in the delivery tubes as the density of the oxygen fluctuates under the influence of the magnetic field. Again the output is proportional to the concentration of oxygen in the mixture. The second microphone which records the pressure waves in the reference arm balances effects of any heating on the gas components.

This latter design is stable for long periods and is sufficiently fast as to be suitable for breath-by-breath recording (250 ms response time).