Respiratory Gas Monitoring

Overview

The gases of interest to the anesthesia caregiver include oxygen (O ₂ ), carbon dioxide (CO ₂ ), nitrous oxide (N ₂ O), and the potent volatile inhaled anesthetic agents of which desflurane, sevoflurane, and isoflurane are in common use. Other gases that may be relevant in certain situations are nitrogen (N ₂ ), helium (He), nitric oxide (NO), and xenon (Xe). Although gas monitors from different manufacturers may appear to offer various options to the user, ultimately, these monitors use one or more of a limited number of technologies to make the analysis and present the data.

The monitoring of respired gases has evolved considerably over the past few decades. Contemporary systems are reliable, accurate, have rapid response times, and are becoming less expensive as a result of competition among manufacturers. Early gas monitoring systems were large, stand-alone units that usually were placed on a shelf on the anesthesia machine or centralized and shared among operating rooms (ORs). As a result of advances in technology and miniaturization, on many contemporary anesthesia workstations, gas analysis is performed in one component module of a modular physiologic monitoring system ( Table 8.1 ) or integrated into the anesthesia workstation. This chapter is intended to provide a framework for understanding the methods by which respiratory gases are analyzed, as well as clinical applications, limitations, and pertinent standards of care.

Table 8.1

Selected Commercially Available Respiratory Gas Monitors

Manufacturer	MaxTec	Medtronic	Philips-Respironics		Masimo	Masimo		Masimo

Model	Max 10	microMEDICO ₂	Capnostat 5	LoFlo	IRMA AX+	ISA capno	ISA OR+	EMMA
Gases Measured	O ₂	CO ₂	CO ₂	CO ₂	CO ₂ , N ₂ O, agents	CO ₂ ,	O ₂ , CO ₂ N ₂ O, agents	CO ₂
Application(s)	FiO ₂	Monitoring	Monitoring	Monitoring	Anesthesia	Anesthesia	Anesthesia	Monitoring
Configuration(s) (sample flow rate)	Mainstream	Sidestream (50 mL/min)	Mainstream	Sidestream (50 mL/min)	Mainstream	Sidestream (50 mL/min)	Sidestream (50 mL/min)	Mainstream
Technology/ Optical design	Fuel cell	Infrared Solid-state	Solid-state	Solid-state	Filter wheel	Solid-state	Filter wheel	Filter wheel
Distinguishing features or specs of interest	0%–90% of change < 15 seconds	Discrete emission line source	Coaxial design, Capno2mask	Removable sample cell	Micro-filter wheel	10 second warmup	Micro-filter wheel	Micro-filter wheel
Water handling	N/A	In-line filter	N/A	In-line filter	N/A	In-line filter	In-line filter	N/A

CO ₂ , Carbon dioxide; FiO ₂ , fraction of inspired oxygen; N ₂ O, nitrous oxide; O ₂ , oxygen.

Images used with permission from the respective listed trademark companies.

Gas Sampling Systems

For a respired gas mixture to be analyzed, either the gas must be brought to the analyzer, or the analyzer must be brought to the gas in the airway. A fuel cell oxygen analyzer located in the breathing system near the inspiratory unidirectional valve is one example of bringing the analyzer to the gas in the circuit. Fig. 8.1A and B shows two mainstream analyzer modules. Because gas is not removed from the circuit for analysis elsewhere, this is termed a nondiverting or mainstream analyzer. Alternatively, and more commonly, the gas to be analyzed is continuously sampled from the vicinity of the patient’s airway and is conducted via fine-bore tubing to the analyzer unit ( Fig. 8.1C and D ). This arrangement is termed a sidestream or diverting system because the gas is diverted from the airway for analysis elsewhere.

Fig. 8.1, Mainstream and sidestream gas modules. (A) Mainstream IRMA multigas analyzer module that measures CO 2 , N 2 O, and anesthetic agents. (B) Mainstream lightweight Capnostat 5 CO 2 sensor. (C) Sidestream-sampling multigas analyzer modules. (D) LoFlo CO 2 module.

Gas analysis is performed in real time so that changes can be rapidly appreciated by the anesthesia caregiver. It is therefore important to know the total response time of the complete system. The total response time is composed of two components: the transit time and the rise time. The transit time is the time lag for the gas sample to reach the analyzer; this term applies only to systems that use diverting gas sampling. The rise time is the time taken by the analyzer to react to the change in gas concentration. An analyzer’s response to a sudden (square wave) change in gas concentration is generally sigmoid in shape, so that rise time is specified as the time to change from 10% to 90% of the sudden total change in gas concentration at the analyzer inlet ( Fig. 8.2 ). The responses of sidestream systems are strongly dependent on the sampling tube inner diameter, length, and gas sampling rate. As an example, the GE Compact Airway module (GE Healthcare, Waukesha, WI) with a 3-meter sampling line and 200 ± 20 mL/min gas sampling rate typically has a sampling delay time of 2.5 seconds and a total response time of 2.9 seconds, which includes the sampling delay and rise time. For comparison, the Capnostat 5 mainstream carbon dioxide sensor (Philips Respironics, Cambridge, MA) is specified to have a rise time of less than 60 milliseconds. A rapid total system response time is essential for accurate concentration readings and high-fidelity waveforms.

Fig. 8.2, Delay time (lag time) is the time from a step function change in CO 2 concentration at the sampling site to the achievement of 10% of the final CO 2 value in the capnometer. Rise time is the time required to achieve a rise from 10% to 90% of the final CO 2 value in the capnometer when a step function change in CO 2 concentration occurs at the sampling site.

Mainstream (Nondiverting) Systems

In mainstream (nondiverting) systems, gas flows past the analyzer interface placed in the main gas stream. Until recently, mainstream analysis at the patient’s airway was possible only for carbon dioxide using infrared (IR) technology (e.g., Capnostat 5, Philips Respironics, Cambridge, MA) and for oxygen using a fuel cell. As a result of advances in miniaturization, mainstream multigas analysis at the patient’s airway is now available (IRMA, Masimo, Irvine, CA); this permits rapid acquisition of breath-by-breath data for carbon dioxide, nitrous oxide, oxygen, and the five potent inhaled anesthetics. Although mainstream analyzers overcome the gas sampling problem, they require a special airway adapter and analysis module to be placed in the breathing system near the patient’s airway. From this location, the analyzers produce a sharp concentration-versus-time waveform in real time, but they may be bulky and vulnerable to damage with repeated falls onto hard surfaces. Current designs are lightweight (e.g., Masimo IRMA sensor head), which uses a miniaturized, spinning filter wheel that weighs only 1 oz. Such designs add only a small amount of dead space, and some, such as the Capnostat 5, use solid-state technology. In addition, waste gas scavenging is not necessary with a nondiverting system.

Mainstream analyzer modules are subject to interference by water vapor, secretions, and blood. Because condensed water blocks all IR wavelengths, leaving too low a source intensity to make a measurement, spurious carbon dioxide readings may result. The cuvette’s window may be heated (usually to 41°C), or it may be coated with water-repellant material to prevent such condensation and interference. Adding the mainstream analyzer to the breathing system creates two additional interfaces for a potential breathing circuit disconnection. Disposable breathing system adapters are now available, whereas previously, the nondisposable adapters required cleaning between patient uses. Battery powered mainstream devices with integrated displays are finding use in the field and in the hospital as backup devices on crash carts (e.g., EMMA, Masimo, Irvine, CA; Fig. 8.3 ). In addition, a mainstream carbon dioxide analyzer is available for patients receiving oxygen via nasal cannula (Cap-ONE, Nihon Kohden America, Foothill Ranch, CA). Mainstream sensors generally read slightly lower because they do not incorporate a drying system to remove the water vapor that dilutes the gas measurement.

Fig. 8.3, EMMA Capnocheck mainstream capnometer.

Sidestream (Diverting) Systems

Compared with mainstream analyzers, the advantages of diverting analyzers are that, because they are remote from the patient, they can be of any size and therefore offer more versatility in terms of monitoring capabilities. They can be used when the monitor must be remote from the patient, such as in magnetic resonance imaging (MRI) or radiation therapy. The sampled gas is continuously drawn from the breathing circuit via an adapter placed between the circuit and the patient’s airway (the Y-piece in a circle breathing system); it passes through a filter or water trap before entering the analyzer. The gas sampling flow rate is usually about 200 mL/min, with a range of 50 to 250 mL/min. Disadvantages include problems with the catheter sampling system, such as clogging with secretions or water, kinking, failure of the sampling pump, slower total system response time (although usually <3 seconds), and artifacts when the gas sampling rate is poorly matched to the patient’s inspiratory and expiratory gas flow rates.

Rapid respiratory rates and long sampling catheter lines may decrease the accuracy of the readings and the fidelity of the tracings. This is because there would be samples from many breaths stored in the catheter, and the breaths could “smudge” into one another, thereby “damping” the tracing with loss of clear peaks and troughs. If a diverting system is used with a very small patient, such as a neonate, and the gas sampling rate exceeds the patient’s expiratory gas flow rate, spurious readings may result because the expired gas will be contaminated by fresh gas. Similarly, if an uncuffed tracheal tube is used and a leak develops between the tube and the trachea, the gas sampling pump may draw room air into the tracheal tube and into the analyzer.

Ideally, the gas sampling flow rate should be appropriate for the patient and for the breathing circuit used. Thus the sampling flow rate may limit the use of low-flow or closed-circuit anesthesia techniques. If the gas sampling rate exceeds the fresh gas inflow rate, the potential exists for negative pressures to be created in the breathing system. Once the sampled gas has been analyzed, it should be directed to the waste gas scavenging system or returned to the patient’s breathing system (see Chapter 5 ).

Leaks in the gas sampling line, both inside the monitor and between the patient’s airway and the monitor inlet, will result in erroneous readings that may not be obvious. Also, these monitors require calibration using a certified standard gas mixture that is directed into the monitor’s gas inlet connection. A leak inside the monitor that allows room air to contaminate the gas sample would result in miscalibration. Commercial test systems are available that assess response time and check for analyzer leaks (e.g., https://smarttanktester.com ; Accessed December 16, 2018).

In certain multigas analyzers that incorporate a paramagnetic oxygen sensor, simultaneous room air sampling (10 mL/min) may be required to provide a reference. This air is therefore added to the waste gas exiting the monitor at a rate of 10 mL/min; it may then be returned to the patient circuit. This might create a problem during closed-circuit anesthesia because nitrogen, albeit at a rate of about 8 mL/min, would be added to the breathing circuit (see Paramagnetic Oxygen Analyzers).

Units of Measurement

The respiratory tract and the anesthesia delivery system contain respired gases in the form of molecules that are in constant motion. When the molecules strike the walls of their container, they give rise to pressure, defined as force per unit of area; the greater the number of gas molecules present, the greater the pressure exerted for any given temperature. Dalton’s law of partial pressures states that the total pressure exerted by a mixture of gases is equal to the arithmetic sum of the partial pressures exerted by each gas in the mixture. The total pressure of all gases in the anesthesia system at sea level is equivalent to approximately 760 mm Hg. Although anesthetic gas monitors may display data expressed in millimeters of mercury (mm Hg), kilopascals (1 kPa = 7.5 mm Hg), or as volumes percent (vol%), it is important to understand in principle how the measurement was made. The reader should understand the difference between partial pressure , an absolute term, and volumes percent, an expression of a proportion, or ratio.

If the partial pressure of one component of a gas mixture is known, a reading in volumes percent can be computed as follows:

\frac{Partial pressure of gas (mm Hg)}{Total pressure of all gases (mm Hg)} \times 100%

Number Of Molecules (Partial Pressure)

An analytic method based on quantifying a specific property of a gas molecule determines in absolute terms—that is, in millimeters of mercury or kilopascals—the number of molecules of that gas that are present. Gas molecules composed of two or more dissimilar atoms—such as carbon dioxide, nitrous oxide, and the potent inhaled anesthetics—have bonds between their component atoms. Certain wavelengths of IR radiation excite these molecules, stretching or distorting the bonds, and the molecules also absorb the radiation. Carbon dioxide molecules absorb IR radiation at a wavelength of approximately 4.3 μm. The greater the number of molecules of carbon dioxide present, the more radiation at 4.3 μm is absorbed. This property of the carbon dioxide molecule is applied in the IR carbon dioxide analyzer. Because the total amount of IR radiation absorbed at a specific wavelength is determined by the number of molecules present, and the motion of each molecule contributes to the total pressure, the amount of radiation absorbed is a function of partial pressure; thus, an IR analyzer measures partial pressure.

In the analysis of gases by Raman spectroscopy, as was used in the Datex-Ohmeda Rascal II analyzer (GE Healthcare), a helium-neon laser emits monochromatic light at a wavelength of 633 nm. When this light interacts with the intramolecular bonds of specific gas molecules, it is scattered and reemitted at wavelengths different from that of the incident monochromatic light. Each reemission wavelength is characteristic of a specific gas molecule present in the gas mixture and therefore is a function of its partial pressure. Thus Raman spectroscopy also measures partial pressures.

A sufficient number of molecules of any gas to be analyzed—that is, adequate partial pressures—must be present to facilitate gas analysis by the IR and Raman technologies. These systems also must be pressure compensated if analyses are being made at ambient pressures other than those used for the original calibration of the systems.

Measurement Of Proportion (Volumes Percent)

Another approach to gas analysis is to separate the molecular component species of a gas mixture and determine what proportion (percentage) each gas contributes to the total (100%). This approach is applied in mass spectrometry. Thus if 21 molecules of oxygen were in a sample of gas containing 100 molecules, oxygen would represent 21% of the gas sample and therefore might reasonably be assumed to represent 21% of the original gas mixture. The result is expressed as 21 vol% or as a fractional concentration (0.21). Such a monitor does not measure partial pressures; it measures only proportions. If the system is provided with an absolute pressure reading that is equivalent to 100%, the basic measured proportions can be converted to readings in millimeters of mercury. In the above example, if 100% were made equivalent to 760 mm Hg, oxygen would have a calculated partial pressure of 159 mm Hg (760 × 21%).

These fundamental differences ( Table 8.2 ) in the approaches to gas analysis and their basic units of measurement are important, particularly when the data presented by these monitors are interpreted in a clinical setting and may affect patient management.

Table 8.2

Gas Monitoring Technologies

Measurement Method	Relevant Property	Representative Gas(es)	Representative Technology / Notes
Physical
Mass spectrometry	Mass/charge ratio	O ₂ , N ₂ , N ₂ O, AA, He, Ar, CO ₂	E.g., quadripole/Perkin-Elmer
Raman spectroscopy	Raman scattering	O ₂ , CO ₂ , N ₂ O, AA, N ₂	Not for use with He, Xe, Ar e.g., Ohmeda Rascal II (off-market)
Infrared spectroscopy	Vibrational frequencies	CO ₂ , N ₂ O, AA	Limited to polar gas species Widely available Not O ₂
Laser absorption spectroscopy	Vibrational frequencies	O ₂	760.26 nm e.g., Oxigraf
Ultrasound absorption	Vibrational frequencies	CO ₂	Research
Paramagnetism	Magnetic field attraction	O ₂	E.g., Servomex
Chemical
Fuel cell	Consumption of O ₂	O ₂	Used widely for FiO ₂ e.g., Maxtec
Colorimetric (qualitative, quantitative)	pH sensitive dye	CO ₂	E.g., Covidien, Mercury Medical, Respirion
Luminescence quenching	Decreased fluorescence activity with certain chemistries	O ₂	Research (e.g., Ocean Optics)
Electrochemical reaction	Conductivity of a zirconia ceramic cell	O ₂	Nernst equation e.g., Datex-Ohmeda

AA, Anesthetic agent; Ar, argon; CO ₂ , carbon dioxide; He, helium; N ₂ , nitrogen; N ₂ O, nitrous oxide; O ₂ , oxygen.

Gas Analysis Technologies

One of the earliest IR measurements of a respiratory gas in exhaled human breath was of carbon dioxide in physicist John Tyndall’s laboratory in the 1860s. Contemporary respiratory multigas analyzers use some form of IR spectroscopy to measure a number of different gases such as carbon dioxide, nitrous oxide, and the potent inhaled anesthetic agents. The same multigas analyzers measure oxygen by a paramagnetic, rapid-responding analyzer or by a fuel cell (slow or rapid responding). Although some technologies are no longer in general clinical use, it is worthwhile to review them briefly to appreciate their principles of operation and how monitoring has evolved.

Mass Spectrometry

For many anesthesia caregivers, the term mass spec was used as if it were synonymous with respiratory gas analysis. Indeed, many anesthetic record forms still incorrectly include this term, but this technology is no longer in routine clinical use. It was, however, the first multigas monitoring system in widespread clinical use, following a description by Ozanne and colleagues in 1981.

The mass spectrometer is an instrument that allows the identification and quantification, on a breath-by-breath basis, of up to eight of the gases commonly encountered during the administration of an inhalational anesthetic. These gases include oxygen, nitrogen, nitrous oxide, halothane, enflurane, and isoflurane. Other agents—such as helium, sevoflurane, argon, and desflurane—could sometimes be added or substituted if desired.

The mass spectrometer analyzer unit separates the components of a stream of charged particles (ions) into a spectrum according to their mass/charge (m/z) ratios. The relative abundance of ions at certain specific m/z ratios is determined and is related to the fractional composition of the original gas mixture. The creation and manipulation of ions is carried out in a high vacuum (10 ⁻⁵ mm Hg) to avoid interference by outside air and to minimize random collisions among the ions and residual gases ( Fig. 8.4 ). Although the technology of mass spectrometry had been available for many years, analyzer units dedicated to a single patient were too expensive for routine use in each operating room (OR). In 1981, the concept of a shared, or multiplexed, system was introduced.

Fig. 8.4, Schematic of a magnetic sector respiratory mass spectrometer. The respiratory gas is sampled and drawn over a molecular inlet leak. Gas molecules enter a vacuum chamber through the leak, where they are ionized and electrically accelerated. A magnetic field deflects the ions. The mass and charge of the ions determine their trajectory, and metal dish collectors are placed to detect them. The electrical currents produced by the ions impacting the collectors are processed, the composition is computed, and the results are displayed. CPU, Central processing unit; ENFL, enflurane; frag., fragment; HALO, halothane; ISO, isoflurane.

In October 1986, the American Society of Anesthesiologists (ASA) first approved standards for basic anesthetic monitoring. As the standards evolved, the requirement for continuous capnometry was not met by a shared mass spectrometry system. The systems were expensive to install and maintain, and they were not always easily upgradable when the new agents desflurane and sevoflurane were introduced. In addition, the long sampling catheters combined with rapid respiratory rates led to significant artifact; the sampling pump could cause considerable negative pressure in the breathing system, and when the system failed, all the monitored sites were affected. ^, Practitioners demanded continuous gas monitoring. Other dedicated gas monitors could serve the most important functions of a multiplexed mass spectrometry system, and the multiplexed mass spectrometer–based systems became obsolete.

Infrared Analysis

The IR spectrum ranges between wavelengths of 0.40 μm and 40 μm. Measurement of the energy absorbed from a narrow band of wavelengths of IR radiation as it passes through a gas sample can be used to measure the concentrations of certain gases. Asymmetric, polyatomic molecules—such as carbon dioxide, nitrous oxide, water, and the potent volatile anesthetic agents—absorb IR energy when their atoms rotate or vibrate asymmetrically; this results in a change in dipole moment, the charge distribution within the molecule. The symmetric molecules argon, nitrogen, helium, xenon, and oxygen do not absorb IR energy. Because the number of gas molecules in the path of the IR energy beam determines the total absorption, IR analyzers measure the partial pressure.

IR analyzers are classified as dispersive or nondispersive. In a dispersive analyzer, after passing through the gas sample, the radiation emitted by an IR source is separated, or dispersed, into the component wavelengths and arranged sequentially. When gas to be analyzed is put in the path of the spectrum of radiation, it absorbs radiation in one or more parts of the spectrum. By examining the entire spectrum, a plot of absorbance versus wavelength is obtained ( Fig. 8.5 ), from which the gas composition can be analyzed and quantified, provided the gases in the mixture have characteristic absorption peaks.

Fig. 8.5, Absorption bands of respiratory gases in the infrared spectrum.

In the nondispersive analyzer, radiation from the IR source is filtered to allow passage of only the specific wavelength bands, for which the gases of interest have distinct absorption peaks. The gas sample is placed between the filter and the IR detector ( Fig. 8.6 ) or between the IR source and the filter (see Fig. 8.6 ). The IR analyzers used clinically are predominantly of the nondispersive type.

Fig. 8.6, Block diagram of a simple, single-wavelength, infrared (IR) respiratory gas analyzer. An IR source emits a beam that passes through a filter, which passes only the wavelength absorbed by the gas of interest. The respiratory gas from the patient is sampled and passes through the gas cell in the optical path. An IR detector measures intensity of the IR wavelength that has passed through the gas sample; intensity is inversely related to the partial pressure of that gas in the sample cell. The electrical signal from the detector is processed to report the gas composition (in mm Hg); this value can be automatically converted to a reading in volumes percent if the ambient pressure is known.

Carbon dioxide, nitrous oxide, and anesthetic gases absorb radiation at unique bands in the IR spectrum. Carbon dioxide molecules absorb strongly between 4.2 and 4.4 μm, whereas nitrous oxide molecules absorb strongly between 4.4 and 4.6 μm and less strongly at 3.9 μm ( Fig. 8.7 ). The potent volatile anesthetic agents have strong absorption bands at 3.3 μm and throughout the range 8 to 12 μm.

Fig. 8.7, Absorbance bands for CO 2 and N 2 O.

The close proximity of the nitrous oxide and carbon dioxide absorption bands may cause some analyzers to be affected by high concentrations of nitrous oxide. ^, The impact of this cross-interference—that is, the overlapping of absorption bands of other gases—can vary significantly among devices. The use of narrow-band sources or narrow-band filters with sufficiently small bandwidths can effectively reduce the impact of cross-interference. The presence of other gases, which may or may not have overlapping absorption bands, can also affect the measurement. This phenomenon is called collision broadening or pressure broadening because molecular collisions result in a change in the dipole moment of the gas being analyzed; thus the IR absorption band is broadened, and the apparent absorption at the measurement wavelength may be altered. In a typical IR carbon dioxide analyzer, 95% oxygen causes a 0.5% decrease in the measured carbon dioxide. Nitrous oxide causes a more substantial increase of approximately 0.1% carbon dioxide per 10% nitrous oxide because of collision broadening. Contemporary multigas analyzers can automatically compensate for the effect of collision broadening if they measure the concentrations of interfering gases.

A simple nondispersive IR analyzer (see Fig. 8.6 ) consists of the following basic elements :

1.
A source of IR radiation, typically a heated black body. A black body radiator is a theoretical object that is totally absorbent to all thermal energy that falls on it; it does not reflect any light and therefore appears black. As it absorbs energy, it heats up and reradiates the energy as electromagnetic radiation. Heating the black body causes emission of IR radiation.
2.
A sample cell, or cuvette. The gas to be analyzed is drawn through the cuvette by a sampling pump.
3.
A detector that generates an output signal. The signal is related to the intensity of the IR radiation that falls on it.
4.
A narrow-band pass filter. This filter allows only radiation at the wavelength bands of interest to pass through; it is interposed between the IR source and the cuvette (see Fig. 8.6 ) or between the cuvette and the detector ( Fig. 8.8 ). The intensity of radiation reaching the detector is inversely related to the concentration of the specific gas being measured.

A number of sources of IR radiation can be used to produce a broad spectrum of IR radiation. Light sources made of tungsten wires or ceramic resistive materials heated from 1500 to 4000 K emit energy over a broad wavelength range that includes the absorption spectrum of the respiratory gases. The radiation may be pulsed electronically or, if constant, may be made intermittent by being interrupted mechanically, or “chopped,” such as with a filter wheel ( Fig. 8.9 ). Because energy output of IR light sources tends to drift, optical systems have been designed to stabilize the analyzers. Three common designs are distinguished by their use of single or dual IR beams and by their use of positive or negative filtering. This material is beyond the scope of this chapter and the interested reader should refer to references on nondispersive gas analysis.

Fig. 8.9, Diagram of an infrared analyzer with multiple filters on a spinning chopper wheel.

Detectors Of Infrared Radiation

To measure carbon dioxide, nitrous oxide, and sometimes anesthetic agents, a radiation-sensitive solid-state material, lead selenide, is commonly used. Lead selenide is quite sensitive to changes in temperature; it is therefore usually thermostatically regulated (cooled or heated) or temperature compensated.

Anesthetic agents, carbon dioxide, and nitrous oxide are sometimes measured with another detector called a Luft cell. This detector uses a chamber filled with gas that expands as IR radiation enters the chamber and is absorbed. A flexible wall of the chamber acts as a diaphragm that moves as the gas expands, and a crystal converts the motion to an electrical signal.

The signal processor converts the measured electrical currents to display gas partial pressure. First, the ratios of detector currents at various points in the spinning wheel’s progress, or from multiple detectors, are computed. Next, electronic scaling and filtering are applied. Finally, linearization according to a reference table for the point-by-point conversion from electrical voltage to gas partial pressure is accomplished by a microprocessor. Compensation for cross-sensitivity or interference between gases can be accomplished by the microprocessor after linearization.

Infrared Wavelength And Anesthetic Agent Specificity

Infrared analyzers must use a specific wavelength of radiation according to the absorption peak of each gas to be measured. Early agent analyzers, such as the Datex Puritan-Bennett Anesthetic Agent Monitor, used a wavelength of 3.3 μm to measure the potent inhaled anesthetics. However, use of a single wavelength did not permit differentiation among these agents ( Fig. 8.10 ). When this system was used, the analyzer had to be programmed by the user for the particular agent being administered. This set the appropriate gain in the software program, and the displayed reading was then accurate for the one agent in use. Obviously, programming such an analyzer for the wrong agent, or the use of mixed agents, would lead to erroneous readings. ^, Furthermore, the use of aerosol propellants, such as those found in handheld delivery devices for bronchodilators, in the breathing circuit would appear to the analyzer as a transient peak of halogenated agent.

Fig. 8.10, Datex Puritan-Bennett anesthetic agent analyzer. This analyzer uses a single wavelength, so the agent being measured must be entered into the software by the user; failure to do this results in erroneous readings. Note the warning: “Agent in use must match agent selected below.” Keypads on the right are for selection of halothane, enflurane, methoxyflurane, or isoflurane.

Modern IR analyzers are agent specific; that is, by measuring each agent with a unique set of wavelengths, they have the capability to both identify and quantify mixed agents in the presence of one another. Contemporary analyzers that can identify and quantify anesthetic agents incorporate individual wavelength filters in the range of 8 to 12 μm. An example is the GE Airway Module (GE Healthcare), which measures the absorption of the gas sample at seven different wavelengths selected using optical narrow-band filters. In this analyzer module, the IR radiation detectors are thermopiles . Carbon dioxide and nitrous oxide are calculated from absorption measured at 3 to 5 μm. Identification and calculation of the concentrations of anesthetic agents are accomplished by measuring absorption at five wavelengths in the 8- to 9-μm band and solving for the concentrations from a set of five equations, one for each agent ( Fig. 8.11 ). A schematic of a multiwavelength analyzer in which the beam of radiation is interrupted mechanically is shown in Fig. 8.12 .

Fig. 8.11, Absorbance bands for anesthetic agents (AAs) .

Fig. 8.12, Schematic of multiwavelength infrared analyzer with mechanical interruption (“chopping”) of infrared beam.

Sampling Systems And Infrared Analysis

Sidestream-sampling analyzers continuously withdraw between 50 and 250 mL/min from the breathing circuit through narrow-gauge sample tubing to the optical system, where the measurement is made. One of the disadvantages of sidestream monitors is the need to deal with liquid water and water vapor. Water vapor from the breathing circuit condenses on its way to the sample cuvette and can interfere with optical transmission. NAFION tubing, a semipermeable polymer that selectively allows water vapor to pass from its interior to the relatively dry exterior, is commonly used to eliminate water vapor. Also, a water trap often is interposed between the patient sampling catheter and the analyzer to protect the optical system from liquid water and body fluids. Filters integrated with the sampling tubing have replaced water traps in many of the currently available systems. Three different methods are available ( Table 8.3 ).

Table 8.3

Selected Sidestream Gas Accessories

Company		Method of Water Removal
Masimo		Active water removal with a hydrophilic wick
	NomoLine (a) HH Nasal CO ₂ Cannula (b) HH Airway Adapter Set – Infant/Neonate (c) LH Nasal/Oral CO ₂ Cannula ∗
Medtronic		Blocking the water, with a hydrophobic filter with large surface area
	Microstream (a) etCO ₂ FilterLine, NIV, nasal, adult, 2m (b) etCO ₂ Filterline H, intubated, neonatal, 4m (c) etCO ₂ Filterline, Smart CapnoLine O ₂ , oral-nasal, adult, 2m †
Philips-Respironics		Absorbing and blocking, hydrophilic fibrous element followed by a hydrophobic plug
	LoFlo (a) etCO ₂ Airway Adapter Set - ET > 4.0 mm Nafion (b) etCO ₂ Airway Adapter Set - ET ≤ 4.0 mm Nafion (c) etCO ₂ / O ₂ Oral-Nasal Cannula - Adult ††

Images used with permission of listed manufacturers.

∗ Active water removal with a hydrophilic wick

† Blocking the water, with a hydrophobic filter with large surface area

†† Absorbing and blocking, hydrophilic fibrous element followed by a hydrophobic plug

Other Technologies

Infrared Photoacoustic Spectrometer

The photoacoustic spectrometer is similar to the basic IR spectrometer ( Fig. 8.13 ). IR energy is passed through optical filters that select narrow-wavelength bands that correspond to the absorption characteristics of the respired gases.

Fig. 8.13, Schematic diagram of a photoacoustic spectrometer. An infrared (IR) source emits a beam that passes through a spinning chopper wheel that has several rows of circumferential slots. The interrupted IR beams then pass through optical filters that select specific wavelengths of light chosen to be at the absorption peaks of the gases to be measured. Each interrupted IR light beam impinges on its respective gas in the measurement chamber, causing vibration of the gas as energy is absorbed and released from the molecules. The vibration frequency of each gas is dependent on the spacing of its slots on the chopper wheel. A microphone converts the gas vibration frequencies and amplitudes into electrical signals that are converted to the gas concentrations for display.

The photoacoustic technique has the distinct advantage over other IR methods in that a simple microphone detector can be used to measure all the IR-absorbing gases. However, this device is sensitive to interference from loud noises and vibration. This technology was used in the Brüel and Kjær Anesthetic Gas Monitor 1304, and it is used currently in atmospheric trace gas monitors, such as the Innova 1412 (LumaSense Technologies, Santa Clara, CA).

Recent Technologies

Mainstream multigas IR analysis was introduced by Phasein (now Masimo, Sweden) in their IRMA series of multigas analyzers. The IRMA mainstream probe measures IR light absorption at 10 different wavelengths to determine gas concentrations in the mixture. Adult, pediatric, and infant disposable airway adapters are available. In a bench study, the monitor was found to have a response time for carbon dioxide (96 vs. 348 ms) and oxygen (108 vs. 432 ms) that was significantly less than a contemporary sidestream-sampling gas monitor. The same miniaturized technology is used in Phasein’s EMMA Emergency Capnometer, a device that displays respiratory rate and incorporates apnea, high carbon dioxide, and low carbon dioxide audible and visual alarms ( Fig. 8.14 ).

Fig. 8.14, Principles of fuel cell oxygen analyzer. Oxygen in the gas sample permeates a membrane and enters a potassium hydroxide (KOH) electrolyte solution. An electrical potential is established between a lead anode and noble metal cathode as oxygen is supplied to the anode. The measured voltage between the electrodes is proportional to the oxygen tension of the gas sample. Temperature compensation is required for accurate measurement.

Although Microstream (originally Oridion Medical, now Medtronic, Minneapolis, MN) capnography is now used as a generic term to refer to a sampling flow rate of 50 mL/min, a rate now available from several manufacturers, it also encompasses the unique IR emission source. This approach is called molecular correlation spectroscopy (MCS), and it produces selective emission of a spectrum of discrete wavelengths, approximately 100 discrete lines in the 4.2- o 4.35-μm range, which match those for carbon dioxide absorption. This is compared with the broad IR emission spectrum of black body emitters used with conventional nondispersive IR technology and permits use of a smaller sample cell (15 μL). This technology has been adapted for use in portable carbon dioxide monitors.

Raman Spectroscopy

When light strikes gas molecules, most of the energy scattered is absorbed and re-emitted in the same direction, and at the same wavelength, as the incoming beam (Rayleigh scattering). ^, At room temperature, about one millionth of the energy is scattered at a longer wavelength, producing a so-called red-shifted spectrum. Raman scattering can be used to measure the constituents of a gas mixture ( Fig. 8.15 ). Unlike IR spectroscopy, Raman scattering is not limited to gas species that are polar. Carbon dioxide, oxygen, nitrogen, water vapor, nitrous oxide, and the potent volatile anesthetic agents all exhibit Raman activity. Monatomic gases such as helium, xenon, and argon, which lack intramolecular bonds, do not exhibit Raman activity. Nitrogen and oxygen, which are not measured with IR spectroscopy, are measured with Raman spectroscopy. This monitor is no longer in production in part due to the laser breaking up the halothane molecule and contaminating the optics.

Fig. 8.15, (A) Schematic of a Raman spectrometer. (B) Selected portion of the screen of the Ohmeda Rascal II (GE Healthcare, Waukesha, WI) showing measurements of inspired and end-tidal nitrogen.

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