Anesth Analg 2000;91:973-977
© 2000 International Anesthesia Research Society
GENERAL ARTICLES
Defining Segments and Phases of a Time Capnogram
Kodali Bhavani-Shankar, MD, and
James H. Philip, ME(E), MD
Department of Anesthesiology, Perioperative, and Pain Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts
Address correspondence and reprint requests to Kodali Bhavani-Shankar, MD, Department of Anesthesiology, Brigham and Women’s Hospital, Harvard Medical School, 75 Francis St., Boston, MA 02115. Address e-mail to Bhavani{at}capnography.com
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Abstract
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The division of a time capnogram into inspiratory and expiratory segments is arbitrary and results in the inability of a time capnogram to detect rebreathing instantaneously. Demarcation of a time capnogram into inspiratory and expiratory components using gas flow signals will not only facilitate prompt detection of rebreathing, but will also allow application of standardized and physiologically appropriate nomenclature for better understanding and interpretation of time capnograms. A Novametrix® CO2-SMO plus respiratory profile monitor (Novametrix Medical Systems, Wallingford, CT) was used to obtain a simultaneous display of CO2 and respiratory flow waveforms on a computer screen during spontaneous and controlled ventilation using a circle system with the inspiratory valve competent (no rebreathing) and with the valve displaced (rebreathing). Because the response time of the CO2 analyzer was similar to the response time of the flow sensor, a comparison was made between the two waveforms to determine the inspiratory segment (Phase 0) and the expiratory segment of the time capnogram and its subdivisions (Phases I, II, and III). The end of expiration almost coincides with the downslope of the CO2 waveform in the capnograms when there is no rebreathing. However, in the presence of rebreathing, the alveolar plateau is prolonged and includes a part of inspiration (Phase 0), in addition to the expiratory alveolar plateau (Phase III).
Implications: Presently, the division of a time capnogram into inspiratory and expiratory segments is arbitrary. Demarcation of a time capnogram into various components using the gas flow signals facilitates prompt detection of the cause of abnormal capnograms that can widen the scope of future clinical applications of time capnography.
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Introduction
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Time capnography is the convenient method often used in anesthetic practice to monitor ventilation. A limitation of the time capnogram is its inability to expeditiously detect rebreathing caused by an incompetent inspiratory valve in the standard anesthesia circle circuit (1,2). This is because the division of a time capnogram into inspiratory and expiratory segments is arbitrary (3,4). Inspiration is generally assumed to occur when the CO2 concentration decreases rapidly to the baseline near the end of the expiratory alveolar plateau, and expiration to occur a few moments before the beginning of the expiratory upstroke. An incompetent inspiratory valve will allow exhaled CO2-containing gas to enter the inspiratory limb of the circuit during expiration (Fig. 1, P). During the next inspiration, the CO2-containing gas in the inspiratory limb enters the patient (Fig. 1, P), extending the expiratory alveolar plateau (Phase III) of the time capnogram (Fig. 2B). A decrease in CO2 follows the extended alveolar plateau and represents the appearance of the CO2-free gas from the machine end of the inspiratory limb (Fig. 1, Q). During this latter phase of inhalation, CO2 concentration may reach zero. The capnogram thus created may be indistinguishable from the normal capnogram, at least during the initial phase of rebreathing (Fig. 2). The time capnograph is unable to reveal rebreathing because it is unaware of the beginning of actual inspiration (1). It assumes that inspiration commenced at the start of the downstroke of the CO2 waveform. We suggested earlier in Bhavani-Shankar et al. (4) that marking the end of expiration and the beginning of inspiration on a time capnogram should enable sensitive detection of rebreathing and identify a defective circle system. One of the methods suggested was to superimpose inspiratory and expiratory flow rates on the capnogram (1).
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Figure 1. Diagrammatic representation of rebreathing pattern in circle system at the beginning of inspiration during incompetent inspiratory valve. P = CO2-containing expired gases, Q = fresh gas inflow.
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Figure 2. Respiratory flow rate waveforms and capnograms recorded during spontaneous ventilation with A, inspiratory valve competent and B, valve incompetent in the circle system. ab = inspiration; ba = expiration; XY = inspiration; YX = expiration; 0, I, II, and III = phases.
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The purpose of our current study is to demonstrate the advantage of demarcating the inspiratory and expiratory segments of a time capnogram using the data from inspiratory and expiratory flow rate waveforms. Furthermore, delineating various components of a time capnogram would enable us to better understand the components of time capnography. It would also allow us to apply a standardized and physiologically appropriate nomenclature to the various phases of the time capnogram in a more meaningful manner than has been done until now.
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Methods
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We used a Novametrix® CO2-SMO plus respiratory profile monitor (Novametrix Medical Systems, Wallingford, CT) which uses an infrared mainstream sensor to measure CO2 and a mainstream flow rate detector to measure inspiratory and expiratory flow rates during a respiratory cycle. However, the currently available Novametrix® monitor does not incorporate a provision to simultaneously display respiratory gas flow waveforms and CO2 waveforms on a single screen to allow a comparative study. To overcome this limitation, Novametrix® provided us with special software. This FDA-approved software enabled us to connect a IBM-compatible PC notebook computer via RS232 serial ports to the Novametrix® CO2-SMO analyzer and provide a simultaneous display of CO2 and respiratory flow waveforms on a computer screen so that respiratory gas flow rate waveforms are displayed above the CO2 waveforms. Because the response times of the CO2 analysis and display, and flow analysis and display are comparable, the beginning of expiration and inspiration on the CO2 waveforms can be determined from these points on the respiratory gas flow waveforms (Fig. 2). The response time of CO2 analysis (to register a change from 10% of the final value to 90% of the final value in response to a step change in PCO2) is <60 ms, whereas it is approximately 25 ms (to register a change from 10% to 90% of the final value in response to a step change in flow rate) for flow sensor (engineering specifications for Novametrix® CO2-SMO). This difference of <35 ms in the response time between the two sensors corresponds to a difference of 0.3 mm on a time scale between the two waveform recordings (each small division on the x axis represents 200 ms, which is 2 mm), that is considered insignificant.
Time capnograms and flow rate waveforms were obtained from a volunteer (informed consent) seated comfortably and breathing through a mouthpiece connected to an anesthesia circle system with a fresh gas flow of 2.0 L/min oxygen. The volunteer was blinded to the anesthesia circuit and the Novametrix® analyzer. When the volunteer was breathing at a constant respiratory rate, the inspiratory valve was quickly made incompetent by displacing the diaphragm while continuing to record flow rate waveforms and CO2 waveforms. Blinding the volunteer to the procedure enabled us to obtain capnograms at similar respiratory rates during various parts of our study.
We also obtained capnograms and flow rate waveforms during controlled ventilation using a circle anesthesia system and an anesthesia mannequin (Eagle Simulator®; Center for Medical Simulation, Boston, MA) programmed for controlled ventilation. The lungs of the mannequin are designed and programmed to deliver CO2 during each breath to produce a normal-appearing CO2 waveform during each respiratory cycle. Capnograms and flow rate waveforms were obtained with the inspiratory valve in a normal position (competent), as well as with the valve in the incompetent position while maintaining the same inspiration/expiration time ratio and respiratory rate.
On each of the recordings, the beginning and the end of inspiration and expiration were identified on the flow rate waveforms. Perpendicular lines were drawn through these points to intercept CO2 waveforms to determine the inspiratory and expiratory segments of each capnogram in the study.
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Results
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Figures 2A and 2B show the waveforms obtained during spontaneous ventilation through a circle system; Figures 3A and 3B display waveforms obtained during controlled ventilation. The ab segment represents inspiration, and the ba segment represents expiration on the respiratory waveforms. These designations were used to demarcate inspiratory and expiratory segments of respective time capnograms. The expiration ended at point X on the time capnogram. The shaded area under the CO2 curve beyond the extrapolated line represents the inspiratory phase of the respiratory cycle, thus constituting rebreathing.
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Figure 3. Respiratory flow rate waveforms and capnograms recorded during controlled ventilation with A, inspiratory valve competent, and B, valve incompetent in the circle system. ab = inspiration; ba = expiration; XY = inspiration; YX = expiration; 0, I, II, and III = phases.
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Discussion
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Numerous terms, such as PQRS, ABCDE, EFGHIJ, and Phases I–IV, have been used to depict the various components of a time capnogram (3–13). A standard terminology is essential for communication, teaching, understanding, and research. A terminology to represent the various phases of a time capnogram (based on logic, tradition, and convention) was suggested previously in Bhavani-Shankar et al. (Fig. 4) (4). A time capnogram can be divided into inspiratory (Phase 0) and expiratory segments. The expiratory segment, similar to a single breath nitrogen curve or single breath CO2 curve, is divided into Phases I–III, and occasionally, Phase IV, which represents the terminal increase in CO2 concentration. Phase I represents the CO2-free gas from the airways (anatomical dead space); Phase II consists of rapid S-shaped upswing on the tracing (caused by mixing of dead space gas with alveolar gas); and Phase III is the alveolar plateau representing CO2-rich gas from the alveoli. The angle between Phase II and Phase III is the  angle. Normally, the angle ranges between 100 and 110 degrees (13), and increases as the slope of Phase III increases. The slope of Phase III is dependent on the ventilation/perfusion status of the lung. Therefore, the angle is an indirect indication of ventilation/perfusion status of the lung (13). The nearly 90° angle between Phase III and the descending limb is the ß angle (1,4). This angle can be used to assess the extent of rebreathing (1). During rebreathing, this angle can increase from 90° up to 180° (Figures 2B and 3B).
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Figure 4. Time capnogram. Inspiratory segment (Phase 0) and expiratory segment (divided into Phases I, II, III). angle = angle between Phase II and Phase III, ß angle = angle between Phase III and descending limb of Phase 0 (inspiration).
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One of the limitations of this terminology is that it is unknown precisely where in the time-capnogram, inspiration, and expiration begin. It is generally assumed that inspiration begins when the CO2 concentration decreases abruptly at the end of the alveolar plateau (Phase III) caused by inhalation of CO2-free gases. However, this is not always true, as can be seen from our observations. The alveolar plateau is prolonged during rebreathing because of inspiratory valve dysfunction in a semiclosed circuit (Figures 2 and 3) and the time capnograph cannot help us identify the inspiratory prolongation of the alveolar plateau occurring because of rebreathing of CO2. Thus, during rebreathing, the plateau may indeed include a part of inspiration (Phase 0) in addition to the expiratory alveolar plateau.
Breen and Bradley (2) suggested that CO2 spirography (CO2 concentrations versus inspired and expired volume during respiratory cycle) should be used to detect rebreathing in situations in which rebreathing is not detected by time capnography. By multiplying and integrating airway-measured flow and PCO2, they computed overall expired and inspired  CO2 (CO2 volume) during intermittent positive pressure ventilation in a circle anesthesia circuit. They also recorded time capnograms. The time capnogram did not show appreciable changes when the inspiratory valve was made incompetent. They found, however, a significant increase in inspired CO2 when the inspiratory valve was compromised, suggesting rebreathing. They concluded that CO2 spirography (in contrast to time capnography) is required to detect inspiratory valve incompetence during mechanical ventilation. However, the method suggested by Breen and Bradley (2) is complex and may not be suitable for routine clinical use.
Comparison between respiratory flow rate waveforms and CO2 waveforms to determine inspiratory and expiratory segments of the time capnogram, can be easily facilitated in the monitors that use mainstream CO2 measurement technology. In the Novametrix® CO2-SMO analyzer, the response time of the CO2 analyzer was similar to the response time of the flow sensor, thereby enabling us to make a comparison between the two waveforms to determine the inspiratory and expiratory segments of the time capnogram. Graphic extrapolation from the respiratory flow waveform to the time capnogram clearly demarcates the inspiratory segment (Phase 0) and the beginning of Phase I of the expiratory segment in the time capnogram. The end of expiration almost coincides with the downslope of the CO2 waveform in the capnograms, if there is no rebreathing or minimal rebreathing because of the mask and ‘Y’ piece of the anesthesia circuit (ß angle is approximately 90°). However, in capnograms recorded with an incompetent inspiratory valve, there is substantial rebreathing, as shown by the shaded area beyond Phase III with the ß angle increasing to 180° (Figures 2B and 3B). We believe that incorporation of flow direction information into mainstream capnography would allow a more physiologically meaningful interpretation of time capnograms. Capnographs using side-stream sensor technology may not allow comparison between CO2 waveforms and flow rate waveforms because of the longer response time of side-stream CO2 analyzers, which occurs because of the 6-ft sampling tube. It may be, however, that an algorithm can be built into the software of side stream capnographs to allow correction for a delayed CO2 response and thus, facilitate the comparison between the two waveforms.
We conclude that the simultaneous display of flow waveforms and time capnograms allows easy delineation of the inspiratory and the expiratory portions of time capnograms. This facilitates expeditious detection of rebreathing, even before the increase of the baseline (Phase 0) or a subsequent increase in end-tidal PCO2 because of rebreathing. Moreover, delineation of segments also helps in the differential diagnosis of abnormal CO2 waveforms. For example, in a recent case report (14), the authors were unable to promptly diagnose rebreathing (produced because of failure of CO2 absorption by the soda lime during closed-circuit anesthesia) because the resulting abnormal capnogram had two curves (a second peak after the primary peak). The differential diagnosis of such a capnogram could be a curare cleft (if the second peak occurred during expiration), or a signature capnogram (if the second peak occurred during inspiration) (3). The authors finally localized the fault to a totally exhausted soda lime whose change of color was concealed by the colored tint of the plastic container (14). By this time there was substantial rebreathing resulting in an increase in end-tidal PCO2 and possibly hypercarbia.
In addition, delineating various components of a time capnogram would widen the scope for future applications of time capnography. For example, it would permit the identification of any differences between end-tidal CO2 (at the end of expiratory flow) and end-expiratory pause CO2 (at the beginning of inspiratory flow) particularly during low frequency ventilation. Normally, both these values are similar when Phase III is flat. However, the difference between the two assumes importance in time capnograms where the slope of Phase III is steep (chronic obstructive pulmonary diseases). This distinction may result in a more accurate prediction of arterial PCO2 using time capnography. Furthermore, the demarcation of the expiratory segment may allow the exploration of estimating physiological dead space and its components by using a time capnogram as is currently possible in a volume capnogram (single breath test-CO2 curve). However, it is premature to predict whether the clinical benefits of including flow data into present capnographic technology would outweigh the economic cost involved, and further studies are required.
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Accepted for publication June 15, 2000.
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Abstract
Introduction
