To the Editor
I am writing in response to the letter of Jurgen Link which appeared in Volume 7, No. 2, Summer 1992 Newsletter, page, 19. A properly functioning, correctly calibrated oxygen (FiO2) monitor is the single most important monitor in preventing hypoxia. When placed in the inspiratory limb, the oxygen analyzer can detect potential defects before they can affect a patient’s oxygen saturation. Among the conditions that can be detected by FiO2 monitoring are:
* Reduction or cessation of oxygen pipeline or cylinder pressure.
* Wrong gas supplied to DISS inlet or cylinder yoke inlet.
* Hypoxic oxygen/nitrous oxide mixture set at the flowmeters.
* Oxygen flowmeter inadvertently turned down or off.
* Leak at the oxygen flowmeter. Leak in the fresh gas line. Fresh gas disconnect.
* Nitrogen accumulation.
In answer to the first question, relying solely on a mechanical/optical device to mix fresh gas concentrations that are not monitored presupposes that the device is incapable of error or breakdown. I know of no such medical device which can make these claims.
In answer, to the second question, monitoring FE02 can provide useful information when compared to FiO2 readings. Use of FE02 readings alone leaves you to assume accuracy of delivered FiO2. In addition, placement of an oxygen sensor in the expiratory limb introduces potential error due to moisture condensation on the sensor. This would be particularly true for long procedures or when used with heated humidification systems.
Another option to the oxygen monitor vs. pulse oximeter question would be the use of paramagnetic sensors for the FiO2 monitor. After the initial purchase, maintenance costs would be minimal or non existent.
Wes Simpson 11
Lead Anesthesia Technician, Surgical Services Division, Sharp Memorial Hospital, San Diego, CA