CHAPTER 30 Gas Analyzers

Matthew S. Davis

Gas Analyzers

Gas analyzers are devices that determine the type and concentration of gases being delivered to or exhaled from a patient. Gas analyzers are an essential part of the American Society of Anesthesiologists (ASA) standards for basic anesthesia monitoring; however, not all gas analyzers truly “monitor” gases. The primary difference between devices labeled “analyzer” or “monitor” is that a monitor includes adjustable alarm settings. In other words, an analyzer will measure whatever substances it is designed for, but a monitor will take that data and signal an alert if and when it falls outside a preset range. Gas monitors can measure a single gas or multiple gases depending on the design and technology used. For this chapter, we will use the generic term Anesthetic Gas Monitor (AGM) to refer to all types of technology.

Types

AGMs can generally be grouped into two types: sidestream (or diverting) and mainstream (or nondiverting). These two types are so named because sidestream monitors divert some of the gas they are analyzing to an analyzer that is separate from the circuit. Mainstream monitors use a sampling cuvette that is placed in-line with the circuit and do not divert gas.

Mainstream

Mainstream monitors are only capable of analyzing oxygen (O₂) and carbon dioxide (CO₂). Many anesthesia machines still use a nondiverting O₂ sensor cell that is in-line with the breathing circuit. This technology will be described later in the chapter. Since mainstream monitors must be directly in-line with gas flows and must be placed close to the patient, they can suffer from interference from condensation and secretions from the patient. The advantage of mainstream monitors is that because they are close to the gas source, they are quick to respond to changes in gas concentrations.

Sidestream

Sidestream monitors are by far the most common gas analyzers used today. Sidestream monitors use a length of small-bore tubing to connect the circuit to a separate analyzing device. A small pump aspirates gases into the analyzing device. One advantage here is that bulky analyzing equipment does not need to be directly attached to whatever tubing is delivering gas to the patient (see Figs. 30.1 and 30.2). The gas analyzer may be separate from the anesthesia machine, often placed on top of the machine and connected via software to the monitor displays; newer machine designs now integrate the gas analyzer. Sidestream monitors have improved over the years and, like other technologies, have become more efficient and portable. There are now sidestream monitors available as part of transport monitors or as optional attachments to popular IV pumps. This makes them nearly ubiquitous, so that capnography is available (Figs. 30.3 and 30.4) in nearly all settings, even when an anesthesia machine is not present.

Guimaraes2e-ch030-image001 — FIGURE 30.1. Modern sidestream anesthesia gas monitor built into an anesthesia machine.

Guimaraes2e-ch030-image002 — FIGURE 30.2. Stand-alone gas monitor mounted to the top of an anesthesia machine. (Courtesy of Philips Healthcare.)

Guimaraes2e-ch030-image003 — FIGURE 30.3. A small transport monitor with ETCO₂ capability. Notice the small proprietary connection for the CO₂ sample line. (Courtesy of Philips Healthcare.)

Guimaraes2e-ch030-image004 — FIGURE 30.4. ETCO₂ channel popular infusion pump. Proprietary sample line connection hidden behind the circular flap bottom right. (Courtesy and © Becton, Dickinson and Company.)

Technology

As stated earlier, some AGMs are only capable of analyzing one gas at a time; this is due to the technology they use. There are several prominent technologies used to determine the type and concentration of gas. For the sake of brevity, we will discuss four of the commonly encountered technologies: electrochemical, paramagnetic, infrared, and Raman spectroscopy. These technologies all have different uses and can be used separately or in combination with one or all of the others to give a comprehensive analysis of an anesthetic gas.

Electrochemical

There are two types of electrochemical cells used for oxygen analysis: polarographic cells (sometimes called Clark cells) and galvanic sensors (Fig. 30.5).

Guimaraes2e-ch030-image005 — FIGURE 30.5. An electrochemical O₂ cell. Notice the permeable membrane set into the sensor. This should not be touched and kept clean to avoid disrupting its permeability.

Polarographic 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 cathode is proportional to the concentration of oxygen in the gas mixture that is 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. Disposable cells that have reached the end of their useful life can be placed in regular trash. Reusable sensors should be serviced whenever they fail to calibrate properly: emptying the used electrolyte, cleaning the electrodes, and refilling with electrolyte gel according to manufacturer guidelines.

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 in the gas being monitored. Galvanic analyzers may be more reliable than polarographic analyzers. Galvanic sensors are also slow to respond to changes in gas composition, but some newer models have dual cells to increase response time and provide greater stability. When galvanic cells are beyond their useful life, they must be disposed off according to local regulations similar to automotive batteries.

Paramagnetic

Paramagnetic sensors expose the gas sample to an uneven magnetic field. Some gases, like oxygen, will orient themselves with a magnetic field. Oxygen has a relatively high magnetic susceptibility as compared to nitrogen, carbon dioxide, nitrous oxide, and inhaled anesthetic agents. As the gas being sampled is sent through the sensor, oxygen molecules are attracted to the stronger magnetic fields. The resultant shifts measured in the sensor are proportional to the percentage of oxygen in the sample.

Paramagnetic sensors react rapidly to changes in gas composition and have a long life span but are motion sensitive. As a result, they are not utilized within a breathing system because of the potential for movement during ventilation but are instead built into multiple-gas monitors. They are not calibrated separately but in concert with the other gases the monitors are designed to detect. This may be accomplished through an internal calibration standard or by using an external calibration gas.

Infrared

Infrared monitoring uses infrared light and specially calibrated sensors to determine the type and concentration of a preset spectrum of gases. Infrared analysis is only capable of monitoring gases whose molecules contain more than one element. Examples of these gases include carbon dioxide, nitrous oxide, and inhaled anesthetic agents. This means nonpolar, elemental gases like oxygen, nitrogen, and helium cannot be monitored with infrared analysis.

Infrared monitoring works on the concept that most gases absorb infrared light. The amount of light absorbed by the gas is in direct proportion to the concentration of that gas in the sample (Beer-Lambert law). The specific wavelength of infrared light is unique to each gas, thus allowing for analysis of multiple gases with the same analyzer. The analyzer draws gas into its sample chamber where it is exposed to infrared light. The light is broken into specific wavelengths by specialized filters that correspond to the gases that need to be analyzed. When compared to a reference (reference gas or stored calibration), the concentration of the desired gas can be determined.

Infrared analysis is fast, reliable, and capable of monitoring multiple gases at one time. The technology has improved and become more reliable over the years. Infrared analysis of CO₂ is now available in a wide variety of monitors, ranging from portable infusion pump modules and transport monitors to larger stand-alone monitors designed to sit on anesthesia machines; many modern anesthesia machines have their own built-in analyzers as optional add-ons.

Raman Spectroscopy

When light interacts with matter, it reacts in different ways; it may be absorbed, transmitted, reflected, or scattered. Raman spectroscopy looks at the scattered light to determine the makeup of a gas sample. When using light of a single specific wavelength (usually from a laser), the majority of the light reflected and scattered off of a sample will maintain the original wavelength. However, a very small amount of that light will change the wavelength; this is called Raman scattering. The change in wavelength can be accurately predicted for every gas molecule and with very sensitive equipment detected from a gas sample. When a single wavelength light is aimed at a gas sample, the amount of photons scattered at the new wavelength is proportional to the concentration of that gas in the sample. Because the wavelength scattered by each molecule is unique, we can determine the makeup of mixed gas samples with a high degree of accuracy and speed.

Calibrations and Troubleshooting

Regardless of the technology used, correct calibration of the monitor is the only way to ensure that accurate measurements are obtained. Manufacturer instructions must be followed exactly in order for accurate calibration to be ensured. Although the details may vary from brand to brand, the following general principles should always be observed.

1. When using electrochemical sensors, the cell must “warm up” and reach a stable temperature in order for the calibration to be accurate. In some types of monitors, this can be as short as 2-3 minutes; some older models can take up to 45 minutes.

2. If a calibration gas is being used, it must be specific to the monitor being calibrated.

3. Calibrations should be performed, at minimum, at the intervals specified by the manufacturer.

4. Any monitor that fails manufacturer calibration standards should be removed from service immediately or as soon as reasonably possible.

Troubleshooting

Taking time to make sure that AGMs are properly calibrated and functional prior to use will minimize potential problems during an anesthetic administration. There will be times, however, when you will be asked to troubleshoot and/or repair a monitor while it is in use. Many times you will be called to troubleshoot a monitor because what the clinicians are seeing on the monitor does not correspond with their expectations, whether that means the readings are out of range or there has been an unexpected change. There will also be times when these unexpected readings accurately reflect what is going on with the patient. When asked to evaluate a gas monitor problem, the following steps help in doing so quickly and efficiently:

1. Be prepared and take everything you may reasonably need with you. For example, if a problem occurs with an FIO₂ monitor, bring a fresh sensor cell, batteries, etc. For a sidestream monitor, you may need to replace the sample “T,” sample line, or water trap (Fig. 30.6).

2. When you arrive, take a moment to make your own assessment of the problem. The most important tool you bring to the situation is a fresh perspective. The clinician may be involved with multiple tasks or problems and may not have been able to do a thorough analysis of the monitor problem. In fact, sometimes the clinician (whether they state this explicitly or not) may call you because they do not have the attentional resources to devote to troubleshooting a potential monitor problem, if the case is complex or a patient is in crisis. They need you to deal with the monitor so they can take care of the patient.

3. If there are multiple faults or alarm conditions, you will need to prioritize. If the patient is being adequately ventilated by a clinical evaluation, gas monitor alarms may be annoying but may not be the highest priority.

4. Use a standard approach to troubleshooting so that you do not miss any steps. For example, check connections at the monitor first and then work your way one connection at a time until you reach the final connection to the breathing system. Alternatively, start with the connection to the breathing circuit and work your way back to the machine.

5. When you remove the sample line or the water trap, cap off the sample port on the breathing circuit so that the resulting leak does not set off volume or pressure alarms.

6. Have a backup plan in mind. If you cannot make an immediate repair, be prepared to give the anesthesia provider an accurate time estimate for the replacement. Know where spare monitors are and how long it would take to get them ready for use. Recall that both ETCO₂ waveform capnography and inspired FIO₂ are required by the ASA to be continually monitored during a general anesthetic administration and must be restored as quickly as possible (see Chapter 31, ASA Standard Monitors).

7. If you do remove a monitor from service, follow your facility’s defective equipment policy to facilitate repairs and minimize downtime.

Guimaraes2e-ch030-image006 — FIGURE 30.6. Supplies that should be on hand when troubleshooting an AGM.

Most gas monitor alarms fall into one of three categories: the reading is lower than expected, the reading is higher than expected, or there is no reading. The following are common problems you may encounter in each category:

If all or most of the readings are lower than expected:

Cracked sample line: This most commonly occurs where the sample line connects to the port on the breathing circuit. Even if you cannot see the crack, tightening the connection will not solve the problem and may actually make it worse. The crack or leak may also occur where the sample line connects to the monitor.
Leaking water trap or filter: This is easy to check, even if you cannot see the defect. Remove the sample line and cover the inlet of the trap/filter with your finger. If this does not create an occlusion alarm, replace the trap/filter (Figs. 30.7 and 30.8). Replacing the water trap with a new one to see if that solves the problem is an effective, but unnecessarily expensive, way to check this possibility.
If you cannot find a leak in the sampling path, consider the possibility of a circuit leak (see Chapter 29, Preventing and Solving Anesthesia Machine Problems). This is most commonly located in the airway device, perhaps at the cuff balloon on the endotracheal tube or at the seal of the supraglottic airway. The clinician can check these as well as the breathing circuit.
The monitor may not be properly calibrated, or the calibration may have failed.

Guimaraes2e-ch030-image007 — FIGURE 30.7. Kinked sample tubing can cause both low and absent AGM readings. The tubing can also kink at the circuit end.

Guimaraes2e-ch030-image008 — FIGURE 30.8. Broken sample line at the circuit elbow. Strain on the sample line can cause it to break.

If only one gas reading is low:

Suspect that the monitor is not properly calibrated, or the calibration may have failed.
If only the inhaled anesthetic reading is low, the vaporizer may be leaking or empty. Check the “sight glass” on the vaporizer. If the vaporizer is not empty, make sure it is securely mounted and locked, listen for an audible leak, and smell for the presence of gas vapors.

If all or most of the readings are higher than expected:

Check the exhaust port or line to make sure it is not occluded. An exhaust occlusion changes the flow of gas through the monitoring chamber and can result in false high readings.
Suspect that the monitor is not properly calibrated, or the calibration may have failed.
If only the inhaled anesthetic is reading high, the vaporizer may have failed. If the vaporizer is easily removed, replace it with another vaporizer and see if the readings become normal.

If there is no reading:

Make sure the monitor is turned on. It happens.
The circuit may have become disconnected, the patient may have become unintentionally extubated, or the patient may not be ventilating; this condition should also cause volume and pressure alarms on the anesthesia machine to respond. (It is possible for these flow apnea alarms to be inadvertently turned off, e.g., in cardiopulmonary bypass modes or when ventilation is held for some other reason.)
There may be an occlusion. In some models of monitors, there is no occlusion alarm. Check for an occlusion at the sample port, in the sample line, and at the water trap/filter inlet.
The monitor’s calibration may have failed.
The internal pump may have stopped working. You can check this by occluding the inlet port with your finger to see if you can feel negative pressure.
The sample cell of a mainstream monitor may have failed or become disconnected. Disconnections can occur at either the breathing system connection or at the monitor.

Commonly Monitored Gases

Several gases are monitored by the anesthesia team and can vary depending on the environment, patient conditions, and procedures being performed. The most commonly monitored gases are oxygen (O₂), carbon dioxide (CO₂), and various anesthetic gases: usually isoflurane, sevoflurane, desflurane, and nitrous oxide (N₂O).

Oxygen

Oxygen is most often monitored as the fraction of inspired oxygen (FIO₂). 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. Inspired oxygen monitoring is essential to prevent hypoxia in an anesthetized patient. It is also a key component of the ASA standards for basic monitoring.

CO₂

Waveform capnography measures the continual rise and fall of carbon dioxide with each breath. This reassures the anesthesia provider that ventilation is continually taking place, allows accurate assessment of the respiratory rate, and confirms that the heart is delivering CO₂ to the lungs (it provides some information about cardiac output). ETCO₂ is used to confirm correct placement of endotracheal tubes or laryngeal mask airways in the airway. It can determine if disconnections or apneas occur. Continuous capnography to evaluate ventilation is an ASA standard both during general anesthesia and when moderate or deep sedation is used during regional and/or local anesthesia or monitored anesthesia care. The standard range for ETCO₂ (in a closed system without leaks) is 35-45 mm Hg. Waveform capnography also permits assessment of CO₂ levels during inspiration, as fractional inspired carbon dioxide (FICO₂). Room air does not contain CO₂ for inspiration. The purpose of CO₂ absorbent is to “scrub” CO₂ from the gas in the circuit, permitting the patient to rebreathe all the other gases of the circuit, conserving anesthesia gas supplies as well as heat and moisture (see Chapter 27, The Breathing Circuit). FICO₂ rises when the capacity of the absorbent to remove CO₂ from the circuit is exhausted. CO₂ breakthrough and the observation of high levels of FICO₂ (>3 mm Hg) mean that the absorbent should be changed.

Anesthetic Agents

Anesthetic agents are monitored on inspiration and expiration. The inspiratory measurement (FiAg) tells clinicians how much agent they are delivering to the patient as a percentage of the gas delivered to the patient. This allows them to titrate their anesthetic precisely, to meet the patients’ needs. The ETag allows the clinician to see how much agent the patient has circulating in the blood, as the anesthetic levels in the lung are in close equilibrium with those in the blood. The ETag can be compared with the known MAC (minimum alveolar concentration) of the agent (which varies with the patient’s age and medical conditions) as an anesthetic dosing guideline. ETag has been shown in some studies to be as reliable as processed EEG (see Chapter 32, Neurologic Monitoring) as a dosing guide to prevent awareness under anesthesia: it is a critical monitor to understand whether the patient is receiving enough anesthetic to remain asleep but is not overdosed with anesthetic. Chapters 8 (Respiratory Monitoring) and 17 (Overview of a General Anesthetic) have more detailed information about anesthetic agents and their use.

Summary

AGMs are critical to modern anesthesia care. Not only are they considered standard by ASA; they help clinicians deliver better, safer care. With modern advances and miniaturization, there should be little to no excuse to not have a monitor available when requested.

Review Questions

1. What is an advantage of mainstream monitors?

A) Bulky analyzing equipment does not need to be directly attached to whatever tubing is delivering gas to the patient.

B) They are quick to respond to changes in gas concentration.

C) They are more efficient and portable than sidestream monitors.

D) They are less likely than sidestream monitors to suffer from interference.

E) They are capable of analyzing anesthetic gases, in addition to oxygen and carbon dioxide.

Answer: B

Mainstream monitors must be directly in-line with gas flows and must be placed close to the patient, making them more susceptible to interference from condensation and secretions from the patient. Mainstream monitors are only capable of analyzing oxygen and carbon dioxide and are not more efficient and portable than sidestream monitors. However, since mainstream monitors must be placed close to the gas source, they are quick to respond to changes in gas concentration.

2. Polarographic and galvanic cells are two types of electrochemical cells used for oxygen analysis. Which of the following statements is true regarding the differences between polarographic and galvanic cells?

A) Galvanic cells are sold as either reusable or disposable.

B) Polarographic cells should be disposed of similarly to automotive batteries.

C) Polarographic sensors are highly accurate but can be slow to respond to rapid changes in gas composition.

D) Polarographic analyzers are thought to be more reliable than galvanic analyzers.

E) None of the above (statements A-D are all false).

Answer: C

Polarographic sensors are highly accurate but can be slow to respond to rapid changes in gas composition. Galvanic analyzers, however, are thought to be more reliable than polarographic analyzers. Polarographic cells are sold as either reusable or disposable, which will dictate how they should be disposed; galvanic cells should always be disposed of in a way that is similar to the disposal of automotive batteries.

3. You know that the cell for an electrochemical sensor must obtain a stable temperature in order for calibration to be accurate. At most, how long should you prepare for the cell to warm up?

A) 5 minutes

B) 15 minutes

C) 30 minutes

D) 45 minutes

E) 60 minutes

Answer: D

Although some monitors may only take a few minutes to warm up, some older models can take up to 45 minutes to obtain a stable temperature. A stable temperature must be reached for calibration to be accurate.

4. What is a reason for only one gas reading being low?

A) Suspect that the monitor is not properly calibrated.

B) There is a crack on the sample line.

C) The sample line is leaking.

D) The water trap or filter is leaking.

E) The exhaust port is calibrated.

Answer: A

If the sample line is leaking or cracked, you would expect all gas readings to be low. Likewise, if the water trap or filter is leaking, all or most of the gas readings will be low. If the exhaust port is blocked, all or most gas readings will be high. If the monitor is not properly calibrated, it can cause any number of problems to present: if only one gas reading may be low, all gas readings may be low, readings may be higher than expected, or there may be no readings.

5. You need to monitor the concentration of oxygen being delivered to the patient. Which type of monitor would not work for this purpose?

A) Electrochemical

B) Paramagnetic

C) Infrared

D) Sidestream

Answer: C

If you need to analyze the concentration of oxygen being delivered to the patient, you should not choose an infrared sensor; infrared sensors are only capable of analyzing multiple gases with a structure of more than one element. Infrared sensors are not capable of analyzing nonpolar, elemental gases like oxygen.

6. What is the difference between a gas analyzer and a gas monitor?

A) A gas analyzer measures gas concentration; a gas monitor also has adjustable alarm settings.

B) A gas analyzer is capable of measuring multiple different gases, including anesthetic agents; a gas monitor measures only oxygen in the circuit.

C) A gas analyzer uses a variety of different analysis technologies; a gas monitor is always a galvanic cell.

D) A gas analyzer is a broad term for all the different capabilities of gas analysis; a gas monitor is the specific term for ASA standard capnography and oxygen monitoring.

Answer: A

The primary difference between devices labeled “analyzer” or “monitor” is that a monitor includes adjustable alarm settings. In other words, an analyzer will measure whatever substances it is designed for, but a monitor will take that data and signal an alert if and when it falls outside a preset range. Gas monitors can measure a single gas or multiple gases depending on the design and technology used.