Nondiverting (Mainstream) Systems

In 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 (Mainstream, Philips Respironics) and by the inspiratory unidirectional valve for oxygen using a fuel cell. As a result of advances in miniaturization, mainstream multigas analysis at the patient’s airway is now available (Phasein, Danderyd, Sweden); 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 vulnerable to damage with repeated drops onto hard surfaces. New designs are lightweight, such as the Phasein 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 nondiverting systems.

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 might 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. In addition, a mainstream carbon dioxide analyzer is now available for patients receiving oxygen via nasal cannula (Cap-ONE, Nihon Kohden America, Foothill Ranch, CA).

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 continually 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 ( Fig. 8-1 , arrows ) 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 “dampening” 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.

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 or may not be obvious. 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.

In multigas analyzers that incorporate a paramagnetic oxygen sensor, simultaneous room air sampling (10 mL/min) is 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 later in this chapter).

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 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). This 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 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.

Mass Spectrometry

For many anesthesia caregivers, the term mass spec is 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. 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. This arrangement allowed one centrally located analyzer to function as part of a computerized, multiplexed system that could serve up to 31 patient sampling locations (ORs and recovery room or intensive care unit [ICU] beds) on a time-share basis. The two multiplexed systems that became widely used were the Perkin Elmer system, which later became the Marquette Advantage system, and the System for Anesthetic and Respiratory Analysis (SARA).

Principles of Operation

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.

The most common design of mass spectrometer was the magnetic sector analyzer, so called because it uses a permanent magnet to separate the ion beam into its component ion spectra ( Fig. 8-6 ). A stream of gas (250 mL/min) to be analyzed is continuously drawn by a sampling pump from an airway connector via a long nylon catheter, an example of a sidestream-sampling system. During transit through the sampling catheter, the pressure decreases from atmospheric pressure, usually 760 mm Hg, in the patient circuit to approximately 40 mm Hg by the inlet of the analyzer unit. A very small amount of the gas actually sampled from the circuit, approximately 10 ⁻⁶ mL/sec, enters the analyzer unit’s high-vacuum chamber through the molecular inlet leak. The gas molecules are then bombarded by an electron beam, which causes some of the molecules to lose one or more electrons and become positively charged ions. Thus an oxygen molecule (O ₂ ) might lose one electron and become an oxygen ion (O ₂ ⁺ ) with one positive charge. The m/z ratio would therefore be 32/1, or 32. If the oxygen molecule lost two electrons, it would gain two positive charges, and the resulting ion (O ₂ ²⁺ ) would have an m/z ratio of 32/2, or 16. The process of electron bombardment also causes large molecules—such as halothane, enflurane, and isoflurane—to become fragmented, or “cracked,” into smaller, positively charged ions.

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.

The positive ions created in the analyzer are then focused into a beam by the electrostatic fields in the ion source, directed through a slit to define an exact shape for the beam, and accelerated and directed into the field of the permanent magnet. The magnetic field influences the direction of the ions and causes each ion species to curve in a trajectory whose arc is related to its m/z ratio. The effect is to create several separate ion beams exiting the magnetic field. The separated beams are directed to individual collectors, which detect the ion current and transmit it to amplifiers that create output voltages in relation to the abundance of the ion species detected. The collector plates are positioned so that an ion with a specific m/z ratio strikes a specific collector. The heaviest ions are deflected the least and travel the furthest before striking a collector (see Fig. 8-6 ). Collectors for these heavy ions are therefore located farthest from the ion source. Summing and other computer software measure the total voltage from all the collector circuits as well as measuring the individual voltages from each collector. Total voltage is considered equivalent to 100% of the analyzed gas mixture.

Individual gas collector circuit voltages are expressed as percentages of the total voltage and are displayed as percentages of the sampled gas mixture. Thus if the voltage from the oxygen collector circuit (m/z ratio = 32) represented 30% of the total voltage from all the collector circuits, oxygen would be read as constituting 30% of the total gas mixture analyzed. The Marquette Advantage and SARA systems used magnetic sector analyzers that had up to eight collectors and therefore were able to detect and analyze up to eight different ion types and their parent gases.

The mass spectrometer functions as a proportioning system for the components of a gas mixture. When it displays each of the components of the mixture as a percentage of the total, it makes the assumption that all the gases present have been detected. If ambient (atmospheric) pressure information is entered into the software, the measured percentages or proportions can be converted to readings in millimeters of mercury (i.e., partial pressures). It must be remembered that the mass spectrometer does not measure partial pressures; it calculates them from the measured proportions and the atmospheric pressure information that must be supplied to it. Thus

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Partialpressure(mmHg)=Fractionalconcentration×Totalpressure(mmHg)’>Partialpressure(mmHg)=Fractionalconcentration×Totalpressure(mmHg)Partialpressure(mmHg)=Fractionalconcentration×Totalpressure(mmHg)
Partial pressure ( mm Hg ) = Fractional concentration × Total pressure ( mm Hg )

Usually, because the mass spectrometer was sampling respired gases from the patient circuit, the total pressure entered into the computer was ambient pressure minus 47 mm Hg, the latter representing the saturated vapor pressure of water at body temperature (37° C). Thus at sea level (760 mm Hg), a pressure of 713 mm Hg would be entered into the mass spectrometer software to be apportioned among the gases present. The mass spectrometer readings, when displayed in millimeters of mercury, represent somewhat of a compromise because inspired gas usually is not fully saturated with water vapor, whereas expired gas usually is. If an incorrect value for total ambient pressure were to be entered into the mass spectrometer software, all readings in millimeters of mercury would be incorrect, but the readings in volumes percent would be correct. For example, assume that end-tidal carbon dioxide is measured as 5% by the mass spectrometer and that ambient pressure is 760 mm Hg. The reading in millimeters of mercury will be 35.65 ([760 − 47] × 5%). If a value of 500 mm Hg is entered instead of 713 mm Hg, the reading will be 25 mm Hg (500 × 5%). In any case of doubt, the astute clinician would revert to the reading in volumes percent; hence the importance of understanding how the displayed value is obtained.

Shared Mass Spectrometry Systems

Multiplexing permitted the sharing of one (rather expensive) mass spectrometer analyzer unit among up to 31 sampling locations, or stations. In a shared system, the gas from each sampling location is directed in sequence by the multiplexing valve system to the mass spectrometer for analysis. In a multiplexed system, the time between analyses at any particular location therefore depended on 1) the number of breaths analyzed from each sampling location (i.e., dwell time), 2) the number of sampling locations in use, 3) the priority settings, and 4) in the case of “stat” samples, the distance between sampling locations and the analyzer, which was up to 300 feet in some installations.

Use of multiplexed mass spectrometry systems led anesthesia caregivers to appreciate the value of continual (i.e., frequently repeated), although not continuous (without interruption), respiratory gas monitoring and the limitations—the main one being that the systems were shared, which meant that data were updated only intermittently.

In October 1986, the American Society of Anesthesiologists (ASA) first approved standards for basic intraoperative 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. The most important functions of a multiplexed mass spectrometry system could be served by other dedicated gas monitors, and the multiplexed, mass spectrometer–based systems became extinct.

Dedicated (Stand-Alone) Mass Spectrometry Systems

A number of smaller stand-alone mass spectrometers were developed for dedicated single-patient use. One model, the Ohmeda 6000 Multigas Analyzer, was a quadrupole-filter mass spectrometer. It worked on the principle that, with regard to the m/z ratio of ions, a controlled electrostatic field can prevent all but a narrow range of charged particles from reaching a target. In the Ohmeda 6000 unit, the gas sampling rate was fixed at 30 mL/min. One advantage of the quadrupole system was that it could be adapted to measure new or additional agents by changes in software only. A number of other stand-alone, quadrupole-filter mass spectrometers were marketed, but their production was discontinued as a result of cost and reliability issues.

Single-Beam Positive Filter

In single-beam positive-filter designs, the IR beam may be divided in time or in space. If the IR beam is divided in time, precision optical band-pass filters mounted on a chopper wheel spinning at 40 to 250 rpm sequentially interrupt a single IR beam. The beam retains energy at a narrow wavelength band during each interruption. If the IR beam is divided in space, after passing through the cuvette, the beam is divided into separate paths using beam splitters and mirrors before passing through optical filters. For each gas of interest, a pair of band-pass filters is selected at an absorption peak and at a reference wavelength at which relatively little absorption occurs. The chopped IR beam then passes through a cuvette containing the sample gas. The ratio of intensity of the IR beam for each pair of filters is proportional to the partial pressure of the gas and is insensitive to changes in the intensity of the IR source.

Single-Beam Negative Filter

In the single-beam negative-filter design, the filters usually are gas-filled cells mounted in a spinning wheel. During each interruption, the IR beam retains energy at all wavelengths except those absorbed by the gas. The chopped IR beam then passes through a cuvette containing the sample gas. Analogous to the positive-filter design previously described, the ratio of IR beam intensity for each pair of filter cells is proportional to the partial pressure of the gas and is insensitive to changes in intensity of the IR source.

Dual-Beam Positive Filter

In the dual-beam positive-filter design, the IR energy from the source is split into two parallel beams: one passes through the sample gas, the other passes through a reference gas. A spinning blade passes through the beams and sequentially interrupts one, the other, then both. The two beams are optically focused on a single point, where a band pass optical filter selected at the absorption peak of the gas of interest is mounted over a single detector. As before, the ratios of the intensities of the sample and reference beams are proportional to the partial pressure of the gas.

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 as a detector. Lead selenide is quite sensitive to changes in temperature; it therefore is 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 microphone 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

IR analyzers must use a specific wavelength of radiation according to the absorbance peak of each gas to be measured. Early agent analyzers, such as the Datex Puritan-Bennett Anesthetic Agent Monitor ( Fig. 8-11 ), 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 (see Fig. 8-7 ). 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.

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 Datex-Ohmeda Compact Airway Module (GE Healthcare) (see Figs. 8-2 and 8-9 ), 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 ( Fig. 8-12 ). Identification and calculation of the concentrations of anesthetic agents are done 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. 8-13 ). A schematic of a multiwavelength analyzer in which the beam of radiation is interrupted mechanically is shown in Figure 8-14 .

Absorbance bands for CO ₂ and N ₂ O.

(From Datex-Ohmeda Compact Airway Modules technical reference manual. Document no. 800 1009-5. Helsinki, 2003, Datex-Ohmeda Division, Instrumentarium.)

Absorbance bands for anesthetic agents ( AAs ).

(From Datex-Ohmeda Compact Airway Modules technical reference manual. Document n. 800 1009-5. 2003, Datex-Ohmeda Division, Instrumentarium.)

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

(Courtesy Dräger Medical, Telford, PA.)

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 (see Fig. 8-1 ). Filters integrated with the sampling tubing have replaced water traps in many of the currently available systems. Three different methods are available; these include 1) blocking the water, such as with a hydrophobic filter with large surface area (Oridion Medical, Jersusalem, IL), 2) absorbing and blocking, such as with a hydrophilic fibrous element followed by a hydrophobic plug (Philips Respironics, Wallingford, CT), and 3) active water removal with a hydrophilic wick, such as an elastomer, described as being able to “sweat” water collected from the gas sample flow to the outer surface of the cover (Nomoline, PhaseIn).

Infrared Photoacoustic Spectrometer

The photoacoustic spectrometer is similar to the basic IR spectrometer ( Fig. 8-15 ). IR energy is passed through optical filters that select narrow-wavelength bands that correspond to the absorption characteristics of the respired gases. Carbon dioxide is measured at a wavelength of 4.3 μm, nitrous oxide at 3.9 μm, and the potent inhaled agents at a wavelength between 10.3 and 13.0 μm. Evenly spaced windows are located along the circumference of a rotating wheel. The optical components are located astride the wheel along one of its radii. A series of IR beams pulse on and off at particular frequencies, according to the rate of rotation of the wheel and the spacing of the windows. The gas flowing through the measurement cuvette is exposed to the pulsed IR beams. As each gas absorbs the pulsating IR energy in its absorption band, it expands and contracts at that frequency, and resulting sound waves are detected with a microphone. The partial pressure of each gas in the sample is then proportional to the amplitude, or “volume,” of the measured sound.

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.

(From Raemer DB: Monitoring respiratory function. In Rogers MC, Tinker JH, Covino BG, Longnecker DE, editors: Principles and practice of anesthesiology. St Louis, 1992, Mosby–Year Book.)

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. Also, because only one wavelength is used to measure the potent inhaled anesthetics, this monitor is unable to distinguish among the agents, which requires that it be programmed for the agent in use; also, erroneous readings might arise in the presence of mixed anesthetic agents. 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

Philips Respironics introduced a flexible monitoring interface, now known as CO ₂ nnect & Go, which allows the user to choose, based on the patient and environment, which carbon dioxide–monitoring modality should be used, sidestream or mainstream. A complete measurement system is available for both mainstream (Capnostat 5) and sidestream (LoFlo, either external or internal format) modes of gas sampling.

Mainstream multigas IR analysis has recently been introduced by Phasein 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 ( Fig. 8-16 ). 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. 8-17 ).

Mainstream multigas analyzer module that measures carbon dioxide, nitrous oxide, oxygen, and anesthetic agents. Rapid-response fuel-cell oxygen analyzer and IRMA Plug-in and Measure.

(Courtesy Phasein AB, Danderyd, Sweden.)

EMMA Capnocheck mainstream capnometer Plug-in and Measure.

(Courtesy Phasein AB, Danderyd, Sweden.)

Although Microstream (Oridion Medical) 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 vendors, 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- to 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.

Fuel Cell Oxygen Analyzer

The fuel cell ( Fig. 8-23 ), or galvanic cell, is basically an oxygen battery that consists of a diffusion barrier; a noble metal cathode, either gold mesh or platinum; and a lead or zinc anode in a basic, usually potassium hydroxide, electrolyte bath ( Fig. 8-24 ). The sensor is covered by an oxygen-permeable membrane and is exposed to the gas in the breathing circuit. Oxygen diffusing into the sensor is reduced to hydroxyl ions at the cathode in the following reaction:

Related posts:

Breathing Circuits Waste Anesthetic Gases and Scavenging Systems Pulse Oximetry Temperature Monitoring Standards and Regulatory Considerations Vigilance, Alarms, and Integrated Monitoring Systems

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