Chapter Outline
SYSTEMATIC INTERPRETATION OF TIME CAPNOGRAPHY
Overview
The capnograph functions as an “electronic stethoscope” that shows the cyclic appearance and disappearance of carbon dioxide (CO 2 ): it appears when the lungs are being ventilated, and it disappears when they are not. The American Society of Anesthesiologists (ASA) updated their standards for basic anesthetic monitoring in July 1989, recommending that not only should carbon dioxide be identified in the expired gas to confirm correct placement of an endotracheal tube or laryngeal mask airway, it should also be a standard monitor for assessing ventilation for “every patient receiving general anesthesia.” The standards related to carbon dioxide monitoring were again updated effective July 2011 to include carbon dioxide monitoring for any patient undergoing moderate or deep sedation. Carbon dioxide homeostasis involves many organ systems. Most importantly, the clinician skilled in capnography can interpret the capnogram to gain information about a patient’s adequacy of ventilation as well as metabolism and the cardiovascular system.
Terms and Definitions
The Greek root kapnos, meaning “smoke,” is used to form the word capnometry, the practice of measuring the carbon dioxide in respiratory gas, and capnometer, the instrument used for this purpose. Carbon dioxide can be thought of as the “smoke” of cellular metabolism. The term capnometer is used to identify an instrument that provides only digital data—specifically, the minimum and maximum values of carbon dioxide during each respiratory cycle. This is in contrast to a capnograph, an instrument that displays, in addition to digital data, a capnogram, the graphic representation of the CO 2 concentration, or partial pressure, over time ( Fig. 10-1 ).
Carbon dioxide levels are most commonly represented as a pressure-versus-time plot, or time capnography. An alternative and newer representation is as a pressure-versus-volume plot, or volumetric capnography ( Fig. 10-2 ). The physiologic mechanisms responsible for the different phases in time capnography can be translated to the phases represented in volumetric capnography. The tracing of carbon dioxide concentration versus expired volume allows for a real-time continuous calculation and display of anatomic and physiologic dead space ventilation. The alterations of physiologic dead space measured in real time can be used to indicate changes in the alveolar dead space component because the anatomic component usually remains static. In fact, the volumetric capnogram and its derivative calculations may give a more accurate picture of the ventilation/perfusion (V/Q) ratio than the corresponding CO 2 -versus-time trace ( Table 10-1 ).
| Time Capnography | Volume Capnography |
|---|---|
| Familiar, commonplace | Informs about lung V/Q status |
| Intubation not required | Provides Vd/Vt composition |
| Expiratory phase obvious | Provides airway gas flow data |
Both capnometers and capnographs digitally report carbon dioxide concentrations as “inspired” and “end tidal.” Actually, these instruments do not and cannot determine the different phases of respiration; they simply report the minimum and maximum CO 2 values detected during each CO 2 (respiratory) cycle. In certain instances—such as with an incompetent inspiratory valve, a Mapleson-type breathing system, or an erratic breathing pattern—the minimum CO 2 concentration may not always equal the inspired CO 2 concentration. Similarly, the maximum concentration measured may not always be the end-tidal concentration. Thus the terms minimum inspired (P I CO 2 min) and maximum expired (P ET CO 2 max) partial pressure of CO 2 (PaCO 2 ) are actually more correct. Most time-based capnograph devices can be configured to display the carbon dioxide recorded at two speeds: a high speed allows the user to interpret information about each breath, and a slow speed enables appreciation of the CO 2 trend.
Two types of capnographs are in use; each has its advantages and disadvantages ( Table 10-2 ). Sidestream, or sampling, capnographs aspirate respiratory gas from an airway sampling site at a rate of 50 to 400 mL/min. The sampled gas is transported through a tube to a nearby CO 2 analyzer. Mainstream, or in-line, capnographs position the actual CO 2 analyzer on the airway, and respiratory gas is analyzed in situ as it passes through a special adapter. No gas is removed from the airway with a mainstream analyzer.
| Sidestream (Picture) | Mainstream (Picture) | |
|---|---|---|
| Small size | Yes | No, but improving |
| Airway weight | No | Yes |
| Sampling tubing | Yes | No |
| Scavenging system | Yes | No |
| Electrical cord | No | Yes |
| Response delay | Yes | No |
| Humidity vulnerability | Minimal | Yes |
| Neonatal ease | No | Yes |
| Sterilization need | No | Yes |
Measurement Techniques
Several analytical techniques can be used to measure respiratory CO 2 . Carbon dioxide strongly absorbs infrared (IR) light, particularly at a wavelength of 4.3 μm. Thus most stand-alone capnometers and capnographs use IR light absorption, a relatively inexpensive technique, to measure respiratory carbon dioxide. The IR light absorbed is proportional to the concentration of the absorbing molecules, such as CO 2 , and the concentration of the gas can be determined by comparing the measured absorbance against a known standard. IR light absorbs all polyatomic gases; therefore CO 2 concentration may be slightly influenced by the presence of water vapor or nitrous oxide (N 2 O). Mass spectrography, molecular correlation spectrography, Raman spectrography, and photoacoustic spectrography also can be used to measure CO 2 concentration but are expensive and rarely used, or they are no longer available for clinical use. A chemical carbon dioxide indicator, the FEF end-tidal detector (Fenem, New York, NY) is a pH-sensitive chemical paper that changes color on exposure to CO 2 . The indicator paper changes color from purple (non–CO 2 -containing gas) to yellow (CO 2 -containing gas) in a semiquantitative manner ( Fig. 10-3 ). This response is evident with the respiratory cycle as the carbon dioxide appears and disappears. This indicator is very sensitive to even low levels of CO 2 , such as may be present in the esophagus ; therefore proper intubation must still be recertified by the usual clinical techniques.
Systematic Interpretation of Time Capnography
Carbon dioxide analysis can be broken down into three individual components: numbers (capnometry), curves (capnography), and gradients (arterial end-tidal CO 2 ). When interpreting carbon dioxide values, presumptions of blood CO 2 levels, pulmonary blood flow, and alveolar ventilation can be made; however, assessment of the numbers in conjunction with CO 2 curves assists in correctly identifying clinical situations while taking into proper consideration the adequacy of gas sampling, presence of leaks in the system, and possible malfunction of the CO 2 measuring equipment.
Is there exhaled CO 2 ? This is the fundamental question for managing every airway. When the capnograph does not register exhaled CO 2 following the patient’s exhalation, failure to ventilate the patient’s lungs is the most likely explanation, although other etiologies also should be entertained. This differential diagnosis includes esophageal intubation, accidental extubation, disconnection or failure of the sampling line or device, apnea, or cardiac arrest. The importance of capnography in helping to detect esophageal intubation cannot be overemphasized. To avoid misinterpretation of esophageal intubation as endotracheal, clinicians should confirm tracheal intubation not only by the presence, but also the persistent reappearance, of the CO 2 waveform with each respiratory cycle. This repetitive efflux of carbon dioxide from the trachea easily distinguishes itself from CO 2 trapped in the stomach or esophagus because esophageal intubation shows a progressive stepwise decrease in CO 2 with successive ventilation ( Fig. 10-4 ).
Interpreting CO 2 in the midst of a cardiac arrest requires additional evaluation to affirm correct airway management. Lack of end-tidal CO 2 or absence of color change on the CO 2 indicator device following intubation alerts the practitioner to a likely incorrect placement of the airway device. However, in the event of a cardiac arrest, little or no end-tidal carbon dioxide (ETCO 2 ) will be detected because no pulmonary blood flow is present to permit gas exchange. Direct visualization of the endotracheal tube passing through the vocal cords or the detection of bilateral breath sounds will aid further decision making.
Disconnection within the breathing system is easily detected with capnography by a flat capnogram and a CO 2 reading of zero. A flat capnogram also is correctly produced by apnea, which may ensue when patients who were previously spontaneously breathing receive agents that hinder respiratory drive. Not until all the above mechanisms have been absolutely ruled out by clinical examination should failure of the capnograph be considered. If the capnograph itself is suspect, the anesthesiologist can quickly disconnect the sampling or sensing adapter and exhale into it to determine whether the capnograph is working and the sampling catheter is patent.
Inspiratory Segment
During mechanical inspiration ( phase 0 ), fresh gas with no carbon dioxide flows by the CO 2 sampling or sensing site, and the capnograph traces the inspiratory baseline (see Fig. 10-1 ). The CO 2 concentration during this phase is zero because there is no rebreathing of CO 2 with a normally functioning circle breathing system. If the inspiratory baseline is elevated and carbon dioxide is greater than zero, CO 2 is being rebreathed; the differential diagnosis includes an incompetent expiratory valve, exhausted CO 2 absorbent, gas channeling through the absorbent, or an imperfectly calibrated capnometer. Also, a rapid respiratory rate combined with a low tidal volume, as is often seen in small children, may exceed the frequency response characteristics of the monitor. The inspiratory baseline may or may not be elevated when the inspiratory valve is incompetent, depending on the size of the breath—that is, if the tidal volume is greater than the volume of the inspiratory hose, it will reach zero; if the tidal volume is less, it will not. Typically, a malfunctioning inspiratory valve and a sidestream capnograph with a high response time show an extension or decrease in the slope of the downstroke.
Expiratory Segment
This segment of the capnograph tracing is divided into phases I, II, III, and occasionally IV (see Fig. 10-1 ).
Phases and Angles
Phase I
Shortly after mechanical inspiration ends, the lungs recoil, gas quickly exits through the trachea, and CO 2 -free gas from the apparatus and anatomic dead space, roughly one third of the tidal volume, passes by the IR sensor. Phase I appears as an extension of the horizontal baseline, extending that initiated during phase 0.
Phase II
As the CO 2 -free gas from the apparatus and the anatomic dead space is washed out and replaced by CO 2 -rich alveolar gas, the expiratory upstroke appears on the capnogram. The upstroke, which should be steep, becomes slanted or S-shaped if gas flow is partially obstructed, the sidestream analyzer is sampling gas too slowly, or the response time of the capnograph is too slow for the patient’s respiratory rate. Gas flow may be obstructed in the breathing system, such as by a kinked tracheal tube, or in the patient’s airway, as with chronic obstructive pulmonary disease or acute bronchospasm.
α-Angle
Phase II and phase III are separated by the α-angle. Changes in the α-angle correlate with the sequential emptying of the alveoli and thus the overall V/Q matching of the lung. As V/Q matching becomes more heterogenous, the variations in lung time constants increase, and the α-angle increases (α >90 degrees). This is further demonstrated by an increasing slope in phase III.
Phase III
As exhalation continues, the capnogram plateaus, with a slightly increasing slope. If ventilation and perfusion were perfectly matched in all lung regions, alveolar gas would have a constant CO 2 concentration, and the expiratory plateau would be perfectly horizontal. However, ventilation and perfusion are not perfectly matched in all lung units, especially in patients who are supine and whose lungs are being mechanically ventilated with positive pressure. Therefore CO 2 typically continues to increase slowly as a result of cyclic variation in alveolar CO 2 during ventilation, which is greatest during expiration, or because of late emptying of the alveoli with the lowest V/Q ratio, which are therefore the richest in CO 2 .
β-Angle
The downstroke that follows phase III is normally 90 degrees (β-angle) and represents the inspiratory phase, during which CO 2 -free gas passes over the sampling sensor and is inhaled. Malfunctioning inspiratory valves and rebreathing and a low–tidal volume rapid respiratory rate will increase the β-angle and delay or prevent the inspiratory baseline from returning to zero. Examples of typical abnormal CO 2 capnograms are illustrated in Figures 10-5 to 10-9 .
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