The levels of evidence (A-D) and the strength of the recommendations (1-2) are defined at the end of the "Major Recommendations" field. The words "recommended" and "suggested" are used to reflect the strength of the recommendation, as level 1 and level 2, respectively.
Capnography is the continuous analysis and recording of the carbon dioxide (CO2) concentration in respiratory gas. Although the terms capnography and capnometry are sometimes considered synonymous, capnometry means only the measurement of CO2 in respiratory gas (i.e., analysis alone), without a continuous written record or waveform. Capnographic waveforms may be time-based or volume-based (Hess & Branson, 1994).
Capnography can be performed by trained healthcare personnel in any setting in which mechanically ventilated patients are found.
There are 3 broad categories of indications for capnography/capnometry: verification of artificial airway placement; assessment of pulmonary circulation and respiratory status; and optimization of mechanical ventilation.
- Verification of Artificial Airway Placement. Even when the endotracheal tube is seen to pass through the vocal cords and tube position is verified by chest expansion and auscultation during mechanical ventilation, providers should obtain additional confirmation of airway placement with waveform capnography or an exhaled CO2 or esophageal detector device (Neumar et al., 2010).
- Exhaled CO2 detectors, including colorimetric and non-waveform, reliably detect intratracheal placement in patients whose cardiac out-put is not exceedingly low or who have not had prolonged circulatory failure. Their use in prolonged cardiac arrest merits further study (Neumar et al., 2010; Goldberg et al., 1990).
- When waveform capnography is not available, these methods can be used in addition to clinical assessment as the initial method for confirming correct tube placement in a patient in cardiac arrest.
- Capnography may be used as an adjunct to determine that tracheal rather than esophageal intubation has occurred (Hess & Branson, 1994; Wenzel et al., 2001; Rudraraju & Eisen, 2009).
- All intubations must be confirmed by some form of end-tidal CO2 (PETCO2) measurement (Neumar et al., 2010; Field et al., 2010).
- Effective ventilation through a supraglottic airway device such as the laryngeal mask airway (LMA) should result in a capnograph waveform during cardiopulmonary resuscitation (CPR), and after return of spontaneous circulation (Neumar et al., 2010).
- When feasible, monitoring PETCO2 during chest compressions is encouraged (Neumar et al., 2010).
- If the PETCO2 is <10 mm Hg during CPR, the clinician should attempt to improve the quality of compressions.
- An abrupt and sustained increase in PETCO2 is a sensitive indicator of return of spontaneous circulation.
- PETCO2 monitoring is one of the objective standards required for monitoring patients in transport, to ensure integrity of the airway (Goldberg et al., 1990; Braman et al., 1987; Singh et al., 2006).
- Providers should observe a consistent capnographic waveform with ventilation to confirm and monitor endotracheal tube placement in the field, in the transport vehicle, on arrival at the hospital, and after any patient transfer, to reduce the risk of unrecognized tube misplacement or displacement (Neumar et al., 2010; Silvestri et al., 2005).
- Capnography can be used to detect inadvertent airway intubation during gastric tube insertion (Howes, Shelley, & Pickett, 2005).
- Life-threatening airway disasters and ventilator disconnection can be averted with continuous capnography (Poirier et al., 1998; Ahrens & Sona, 2003; Joint Commission, 2002).
- Assessment of Pulmonary Circulation and Respiratory Status. Capnography assists in:
- Determining changes in pulmonary circulation and respiratory status sooner than pulse oximetry. In patients without lung disease, substantial hypercarbia may present before pulse oximetry notifies the clinician of a change in ventilation (Poirier et al., 1998; Roberts & Maniscalco, 1995; Hall et al., 1993; Roberts et al., 1995; Shibutani et al., 1994).
- Monitoring the adequacy of pulmonary, systemic, and coronary blood flow (Shibutani et al., 1994; Levine, Wayne, & Miller, 1997), as well as estimation of the effective (non-shunted) pulmonary capillary blood flow by a partial rebreathing method (de Abreu et al., 2002; de Abreu et al., 1997; vanHeerden et al., 2000).
- Evaluating the partial pressure of exhaled CO2, especially PETCO2.
- Screening for pulmonary embolism (Rodger & Wells 2001; Rodger et al., 2001; Bolyard et al., 1998; Rumpf, Krizmaric, & Grmec, 2009).
- Optimization of Mechanical Ventilation. Capnography during mechanical ventilation allows:
- Continuous monitoring of the integrity of the ventilator circuit, including the artificial airway (Spahr-Schopfer, Bissonnette, & Hartley, 1993) or bag mask ventilation, in addition to potentially detecting mechanical ventilation malfunctions (Muniz, 2008; Hardman, Mahajan, & Curran, 1999; Kumar et al., 1992)
- Decreasing the duration of ventilatory support (Cheifetz & Myers, 2007)
- Adjustment of the trigger sensitivity (Thompson & Jaffe, 2005)
- Evaluation of the efficiency of mechanical ventilation, by the difference between PaCO2 and the PETCO2 (Kerr et al., 1996)
- Monitoring of the severity of pulmonary disease (Bedforth & Hardman, 1999; Ghamra & Arroliga, 2005) and evaluating the response to therapy, especially therapies intended to improve the ratio of dead space to tidal volume (VD/VT) and ventilation-perfusion matching (V/Q) (de Abreu et al., 1997; Bolyard et al., 1998; Engoren, 1993; Hardman & Aitkenhead, 1999; Hubble et al., 2000; Jellinek et al., 1993; Kallet et al., 2005; Russell & Graybeal, 1994; Szaflarski & Cohen, 1991; Taskar et al., 1995; McSwain et al., 2010).
- Monitoring of V/Q during independent lung ventilation (Cinnella et al., 2001; Colman & Krauss, 1999).
- Monitoring of inspired CO2 when it is being therapeutically administered (Fatigante et al., 1994).
- Graphic evaluation of the ventilator-patient interface. Evaluation of the capnogram may be useful in detecting rebreathing of CO2, obstructive pulmonary disease, the presence of inspiratory effort during neuromuscular blockade (curare cleft), cardiogenic oscillations, esophageal intubation, and cardiac arrest (Bhavani-Shankar et al., 1992).
- Measurement of the volume of CO2 elimination to assess metabolic rate and/or alveolar ventilation (Russell & Graybeal, 1994; Brandi et al., "Effects of ventilator resetting," 1999; Brandi et al., "Energy expenditure," 1999; Sullivan, Kissoon, & Goodwin, 2005)
- Monitoring of VD/VT to determine eligibility for extubation in children (Hubble et al., 2000; Wratney & Cheifetz, 2006)
- There is a relationship between VD/VT and survival in patients with the acute respiratory distress syndrome (Nuckton et al., 2002; Lucangelo et al., 2008; Raurich et al., 2010).
There are no absolute contraindications to capnography in mechanically ventilated patients, provided that the data obtained are evaluated with consideration given to the patient's clinical condition.
Capnography with a clinically approved device is a safe, noninvasive test, associated with few hazards in most populations. Hazards/complications are different for the 2 types of capnographic device.
- Dead Space. Adapters inserted into the airway between the airway and the ventilator circuit should have a minimal amount of dead space. This effect is inversely proportional to the size of the patient being monitored (Szaflarski & Cohen, 1991; Jacobus, 2009).
- The addition of the weight of a mainstream adapter can increase the risk of accidental extubation in neonates and small children (Jacobus, 2009).
- The gas sampling rate from some sidestream analyzers may be high enough to cause autotriggering when flow-triggering of mechanical breath is used. This effect is also inversely proportional to the size of the patient (Jacobus, 2009).
- The gas sampling rate can diminish delivered tidal volume (VT) in neonates and small patients while using volume targeted or volume controlled ventilation modes (Jacobus, 2009).
Limitations of Procedure or Device
Capnography, when performed using a device calibrated and operated as recommended by the manufacturer, has few limitations. It is important to note that although the capnograph provides valuable information about the efficiency of ventilation (as well as perfusion), it is not a replacement or substitute for assessing the PaCO2 (Hess & Branson, 1994; Jellinek et al., 1993; Hess, 1998; Isert, 1994; Laffon et al., 1998). The difference between PETCO2 and PaCO2 increases as dead-space volume increases. (Russell & Graybeal, 1992). In fact, the difference between the PaCO2 and PETCO2varies in the same patient over time (Russell & Graybeal, 1994, 1995; Seguin et al., 2001; Grenier et al., 1999). Alterations in breathing pattern and VT may introduce error into measurements designed to be made during stable, steady-state conditions. (Brandi et al., "Effects of ventilator resetting," 1999; Brandi et al., "Energy expenditure," 1999; Gamma de Abreu, Melo, & Giannella-Neto, 2000). Interpretation of results must take into account the stability of physiologic variables such as minute ventilation, VT, cardiac output, V/Q, and CO2 body stores. Certain situations may affect the reliability of the capnogram. The extent to which the reliability is affected varies somewhat among types of devices. Limitations include:
- The composition of the respiratory gas may affect the capnogram (depending on the measurement technology incorporated).
- The infrared spectrum of CO2 has some similarities to the spectra of both oxygen and nitrous oxide (Bhavani-Shankar et al., 1992). A high concentration of either oxygen or nitrous oxide, or both, may affect the capnogram, so a correction factor should be incorporated into the calibration of any capnograph used in such a setting (Hess, 1998).
- The reporting algorithm of some devices (primarily mass spectrometers) assumes that the only gases present in the sample are those that the device is capable of measuring. When a gas that cannot be detected by the mass spectrometer (such as helium) is present, the reported CO2 values are incorrectly elevated in proportion to the concentration of the gas present (Hess & Branson, 1994; Graybeal, 1994).
- The breathing frequency may affect the capnograph. A high breathing frequency may exceed the capnograph's response capabilities. The presence of high airway resistance, respiratory rate, or inspiratory-to-expiratory ratio may decrease the accuracy of the measurement obtained from a sidestream capnograph, compared to a mainstream capnograph (McEvedy et al., 1990; Tingay, Stewart, & Morley, 2005). In addition, a breathing frequency >10 breaths/min affects different capnographs differently (Graybeal, 1994).
- Contamination of the monitor or sampling system by secretions or condensate, a sample tube of excessive length, too high a sampling rate, or obstruction of the sampling chamber can lead to unreliable results.
- Use of filters between the patient airway and the capnograph's sampling line may lead to artificially low PETCO2 readings (Hardman, Mahajan, & Curran, 1999; Hardman, Curran, & Mahajan, 1997).
- The sensitivity for confirmation of endotracheal intubation by color change could range from 67% to 72% (Keller et al., 2009).
- Clinical conditions associated with false negative readings include:
- Low cardiac output may cause a false negative result when attempting to verify endotracheal tube (ETT) position in the trachea (Li, 2001).
- During CPR a positive test confirms placement of the ETT within the airway, whereas a negative test indicates either esophageal intubation or airway intubation with poor or absent pulmonary blood flow and requires an alternate means of confirmation of tube position (American Heart Association [AHA], 2005; "The International Liaison Committee on Resuscitation [ILCOR]," 2006; American College of Emergency Physicians [ACEP], 2002).
- When the endotracheal tube is in the pharynx and when antacids and/or carbonated liquids are present in the stomach, a false negative reading may be present. However, the waveform does not continue during subsequent breaths (Sum Ping, Mehta, & Symreng, 1992).
- Elimination and detection of CO2 can be dramatically reduced in patients with severe airway obstruction and pulmonary edema (Ward & Yealy, 1998).
- Clinical conditions associated with false positive readings include:
- Colorimetric CO2 detectors may give a false positive if contaminated with acidic or CO2-filled gastric content, intratracheal medications such as epinephrine, extreme humidity, or the presence of trichloroethylene or chloroform anesthetics. Most require at least 6 breaths before a decision can be made (Goldberg et al., 1990; Kamlin et al., 2005).
- Detection of CO2 in expired gas after esophageal intubation as a result of prior bystander mouth-to-mouth ventilation may result in a false positive reading (Kramer-Johansen, Dorph, & Steen, 2008).
- A transient rise in PETCO2 after sodium bi-carbonate administration is expected, but should not be misinterpreted as an improvement in quality of CPR or a sign of return of spontaneous circulation (Neumar et al., 2010).
- Inaccurate measurement of expired CO2 may be caused by leaks or other clinical circumstances preventing collection of expired gases (Kallet, 2008), including:
- Leaks in the ventilator circuit
- Leaks around the tracheal tube cuff, an uncuffed tube, or the mask, including LMA
- Bronchopleural fistula
- Dialysis or extracorporeal life support
Assessment of Need
Capnography is considered a standard of care during general anesthesia. The American Society of Anesthesiologists has suggested that capnography be available for patients with acute ventilatory failure on mechanical ventilatory support (American Society of Anesthesiologists [ASA], 2011). The American College of Emergency Physicians recommends capnography as an adjunctive method to ensure proper endotracheal tube position (ACEP, 2002). The 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care recommend capnography to verify endotracheal tube placement in all age groups (Goldberg et al., 1990). Assessment of the need to use capnography with a specific patient should be guided by the clinical situation. The patient's primary cause of respiratory failure and the severity of his or her condition should be considered.
Assessment of Outcomes
Results should reflect the patient's condition and should validate the basis for ordering the monitoring. Documentation of results (along with all ventilatory and hemodynamic variables available), therapeutic interventions, and/or clinical decisions made based on the capnogram should be included in the patient's chart.
- Equipment: the capnograph and accessories (e.g., airway adapter, sampling tube, depending on capnograph). The capnograph should be calibrated as recommended by the manufacturer.
- Personnel: licensed or credentialed respiratory therapists or individuals with similar credentials (e.g., medical doctor [MD], registered nurse [RN]) who have the necessary training and demonstrated skills to correctly calibrate and evaluate the capnograph, assess the patient and the patient-ventilator system, and the ability to exercise appropriate clinical judgment.
- During capnography the following should be considered and monitored:
- Ventilatory variables: VT, respiratory rate, positive end expiratory pressure (PEEP), ratio of inspiratory-to-expiratory time, peak airway pressure, and concentrations of respiratory gas mixture (Garey et al., 2008; Engoren, 1993; Szaflarski & Cohen, 1991; Li, 2001; Gentile & Cheifetz, 2004).
- Hemodynamic variables: systemic and pulmonary blood pressure, cardiac output, shunt, and V/Q imbalances (de Abreu et al., 1997; Jellinek et al., 1993; Gamma de Abreu, Melo, & Giannella-Neto, 2000).
Capnography (or, at least, capnometry) should be available during endotracheal intubation (Poirier et al., 1998; Roberts et al., 1995; Sum Ping, Mehta, & Symreng, 1991). Capnography is not indicated for every mechanically ventilated patient; however, when it is used, the measurement period should be long enough to allow determination of the PaCO2-PETCO2 difference, to note changes in the PaCO2-PETCO2 difference as a result of therapy, and to allow interpretation of observed trends.
No specific precautions are necessary, although standard precautions (as described by the Centers for Disease Control and Prevention) (Bolyard et al., 1998) and precautions designed to limit the spread of tuberculosis (Kallet, 2008; Jensen et al., 2005) should always be implemented during patient care.
- Reusable mainstream sensors should be subjected to high-level disinfection between patients, according to the manufacturer's recommendations.
- The external surface of the monitor should be cleaned as needed, according to manufacturer's recommendations.
The following recommendations are given based on the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) scoring system (Guyatt et al., 2008; Jaeschke et al., 2008):
- Continuous waveform capnography is recommended in addition to clinical assessment as the most reliable method of confirming and monitoring correct placement of an endotracheal tube (1A).
- If waveform capnography is not available, a non-waveform exhaled CO2 monitor in addition to clinical assessment is suggested as the initial method for confirming correct tube placement in a patient in cardiac arrest (2B).
- PETCO2 is suggested as a method to guide ventilator management (2B).
- Continuous capnometry during transport of a mechanically ventilated patient is suggested (2B).
- Capnography is suggested to identify abnormalities of exhaled air flow (2B).
- Volumetric capnography is suggested to assess CO2 elimination and VD/VT to optimize mechanical ventilation (2B).
- Quantitative waveform capnography is suggested in intubated patients to monitor CPR quality, optimize chest compressions, and detect return of spontaneous circulation during chest compressions or when rhythm check reveals an organized rhythm (2C).
Strength of the Recommendations and Grade of Quality of the Evidence
|Strength of the Recommendations
||Benefits clearly outweigh the risks and burdens (or vice versa) for nearly all patients.
||Risks and benefits are more closely balanced or are more uncertain.
|Quality of the Evidence
||Well-performed randomized controlled trials or overwhelming evidence of some other sort. Further research is very unlikely to change confidence in the estimate of the effect.
||Randomized controlled trials that are less consistent, have flaws, or are indirect in some way to the issue being graded, or very strong evidence of some other sort. Further research is likely to have an important impact on confidence in the estimate of effect and may change the estimate.
||Observational evidence (from observational studies, case series, or clinical experience), or evidence from controlled trials with serious flaws. Further research is very likely to have an important impact on confidence in the estimate of effect and is likely to change the estimate.
||Any estimate of effect is very uncertain.
Adapted from: Guyatt GH, Oxman AD, Vist GE, Kunz R, Falck-Ytter Y, Alonso-Coello P, Schünemann HJ; GRADE Working Group. GRADE: an emerging consensus on rating quality of evidence and strength of recommendations. BMJ 2008;336(7650):924-926.