To the Editor
I am pleased to offer the following answers to Dr. Link’s stimulating educational questions, published in APSF Newsletter, Summer 1992, page 19, concerning the benefit of the 02 analyser in patients monitored with a pulse oximeter.
The so-called 02/N20 ‘safety mixers’ do not provide the patient “under all conditions’ with a gas mixture containing at least 21% 02. Their only benefits are to protect against errors in flow control settings and to act as a N2O cut-off device in case of failure of 02-supply to the machine. However, (1) a mixer does not protect against faulty interconnections, crossed pipelines, pollution of the 02 pipeline with N20 or medical air, (2) it can itself be defective and allow the development of either a hypoxic (hypoxia!) or a hyperoxic (awareness!!) gas mixture, (3) the 02 bypass linked with the mixer can be defective and allow a preferential 02 leak, i.e. the build up of a hypoxic mixture, and (4) it cannot prevent the development of a hypoxic mixture downstream in the anaesthesia breathing system, especially in case of low flow anaesthesia (consumption of 02 and/or accumulation of other gases).
The 02 analyser is essential for a swift and accurate interpretation of a hypoxic event detected by the pulse oximeter. There is indeed a long delay between the onset of a problem, such as a failure of 02 supply to the machine, and its recognition by the pulse oximeter (1) delay required for washing out the anaesthesia breathing system and the FRC (time constant), delay of lung-to-finger circulation time and the monitor response time. Therefore it may be nearly eight minutes before the pulse oximeter reading falls. Owing to the exponential type of the oxyhaemoglobin dissociation curve, the pulse oximeter provides no warning until the PaO2 has fallen below 90 mmHg. At this time the hypoxic event is already in progress and every anaesthetist would be happy to be informed without delay by the 02 analyser whether the cause is located in the anesthetic machine or in the patient. If the cause of the hypoxic event is the administration of a hypoxic gas mixture (hypoxic meaning in some cases an insufficient 02 concentration for a given patient), the 02 analyser will detect it a long time before the pulse oximeter. This is of major importance, as in the case Of 02 supply failure to the machine, an additional delay (to the still ‘lost’ eight minutes) is also required until enough 02 reaches again the alveoli… the brain and the finger tip!
When the 02 sensor is located in the expiratory tubing of the circle system, the analyser provides (with an additional short delay !) the same information as when located in the inspiratory tubing. Incidentally it could also act as a disconnection monitor. However in this position it is maximally exposed to water vapor with its negative effects on the sensor. Moreover 02 analysers with slow response time (> 10 s) such as polarographic or galvanic (fuel cell) techniques, monitor only a trend, which is appropriate to the inspiratory tubing location where the concentration of gases remains relatively constant. When located in the expiratory tubing, where the concentration of gases changes continuously, these devices cannot provide accurate information on the N20 washout at the end of anaesthesia. Such information is only obtained with short response time (< I s) analysers, such as paramagnetic or mass spectrometric methods. However, for these devices the optimal location is the patient limb of the Y-connection piece and not the expiratory tubing.
It can be concluded that (1) the pulse monitor is an unsurpassed monitoring tool for 02 transport to and within the patient, (2) the 02 analyser is its essential acolyte, as it increases by at least 50% the informative power of the former and eventually the patients safety, (3) one of the most noble goals of an anesthesiologist is to provide his patient with 02. The 02 analyser is the only instrument able to recognize oxygen and to measure its concentration; therefore, it is an essential device.
J.C. Otteni, M.D., FRCAnaes
Hopitaux Universitaires de Strasbourg France
Reference
- Sykes MK, Vickers MD, Hull CJ. Principles of measurement and monitoring in anaesthesia and intensive care 3rd Ed., London: Blackwell Scientific Publications Ed., 1991, 294-295.