The primary difference between a device labeled an “analyzer” and one labeled a “monitor” is that a monitor includes adjustable low, or low and high alarm settings. In other words, a gas analyzer will measure whatever substances it is designed for but will not alert you if the values measured fall outside an acceptable range. Although the terms analyzer and monitor are frequently used interchangeably, a true analyzer should only be used in situations where continuous, uninterrupted observation is possible. Because the addition of alarm settings can contribute to vigilance, a monitor should always be used with patients undergoing an anesthetic.

Gas monitors can measure either a single gas or multiple gases and are grouped into two general classifications: mainstream and sidestream. Although anesthesia providers can use mainstream monitors, the vast majority of monitors in current use are sidestream. Sidestream monitors divert gas mixtures from the breathing circuit by means of a continuous pump. The sample gas (normally 50-250 mL/min) is either returned to the breathing circuit or sent into the gas scavenger system. The exhaust from a sidestream monitor must be connected to one of these outflow sources. With the exception of a true emergency, it should never be allowed to empty into the room air. Mainstream monitors measure the gas of interest directly in the breathing circuit, usually consisting of a disposable cuvette and a reusable sensor cell. The primary limitations are that mainstream monitors are only capable of measuring one gas and there is a potential for interference caused by moisture collecting in the cuvette.

The simplest form of gas analyzer/monitor, and the one that was used first by anesthesia providers, is the fractional inspired oxygen (FIO₂) monitor. As its name implies, the FIO₂ monitor measures the fraction (displayed as a percentage) of oxygen present in the inspiratory limb of a breathing system. Human error or anesthesia machine malfunction could cause a gas mixture with insufficient oxygen to be delivered into the breathing system; therefore, inspired oxygen monitoring is essential to prevent hypoxia in an anesthetized patient. It is also a key component of both the American Society of Anesthesiologists (ASA) Standards for Basic Monitoring and Recommendations for Pre-Anesthesia Checkout.

Anesthesia technicians have a responsibility to ensure that FIO₂ monitors are properly calibrated prior to anesthesia machine use. At minimum, calibration to room air (21% oxygen) should be performed at least every 24 hours and prior to use on the first patient. Some monitors have the capability of performing a 100% oxygen calibration as well. This should be performed in addition to room air calibration and should not be performed in place of it. Be sure to follow the manufacturers’ recommendations when performing either a room air or 100% oxygen calibration. Keep the following key points in mind with all brands of FIO₂ monitors you are working with or the type of sensor it uses:

There are three primary types of sensor (measurement) cells used in oxygen monitors: polarographic, galvanic, and paramagnetic. Each type of sensor has strengths and drawbacks as well as specific concerns for the anesthesia technologist.

Polarographic cells, sometimes referred to as Clark cells, use an anode and a cathode that are suspended in an electrolyte solution containing potassium chloride. The solution is separated from the inspiratory gases by a semipermeable membrane, usually made of Teflon or polypropylene. The strength of the current generated between the anode and the cathode is proportional to the concentration of oxygen in the gas mixture being analyzed. These sensors are highly accurate but can be slow to respond to rapid changes in gas composition. Polarographic cells are sold as either reusable or disposable. Cells that have reached the end of useful life can be placed in regular trash. Reusable sensors should be serviced whenever they fail to calibrate properly. Emptying the used electrolyte gel and using a soft cotton swab to clean the electrodes, membrane, and inner surface of the sensor capsule normally accomplish this. Fill the capsule with fresh electrolyte gel and replace the electrodes, being careful not to entrap any air bubbles.

Galvanic or fuel cell sensors also use an anode and a cathode suspended in an electrolyte solution such as potassium hydroxide. The principle of operation is similar to that of the polarographic cell, and currents generated in the cell are in proportion to the concentration of oxygen molecules in the gas being sampled. Single-cell sensors can be slow to respond to changes in gas composition and are more prone to calibration drift. Newer dual-cell designs offer increased accuracy, faster response times, and greater stability. Galvanic cells are similar in composition to your car battery. The correct method of disposal will vary with state and local laws, but they should never be placed in regular trash, sharps, biohazardous, or pharmaceutical containers.