INTRODUCTION — This topic review will discuss the basic physiology and interpretation of capnography and its use in emergency settings.
DEFINITION AND BACKGROUND — The term capnography refers to the noninvasive measurement of the partial pressure of carbon dioxide (CO2) in exhaled breath expressed as the CO2 concentration over time. The relationship of CO2 concentration to time is graphically represented by the CO2 waveform, or capnogram (figure 1). Changes in the shape of the capnogram are diagnostic of disease conditions, while changes in end-tidal CO2 (EtCO2), the maximum CO2 concentration at the end of each tidal breath, can be used to assess disease severity and response to treatment. Capnography is also the most reliable indicator that an endotracheal tube is placed in the trachea after intubation.
Oxygenation and ventilation are distinct physiologic functions that must be assessed in both intubated and spontaneously breathing patients. Pulse oximetry provides instantaneous feedback about oxygenation (see "Pulse oximetry"). Capnography provides instantaneous information about ventilation (how effectively CO2 is being eliminated by the pulmonary system), perfusion (how effectively CO2 is being transported through the vascular system), and metabolism (how effectively CO2 is being produced by cellular metabolism).
Capnography became a routine part of anesthesia practice in Europe in the 1970s and in the United States in the 1980s. It is now part of the standard of care for all patients receiving general anesthesia and is part of routine monitoring in the pre-hospital and acute care settings. (see "Basic patient monitoring during anesthesia", section on 'Capnography')
PRINCIPLES OF OPERATION — Carbon dioxide (CO2) monitors measure gas concentration, or partial pressure, using one of two configurations: mainstream or sidestream. Mainstream devices measure respiratory gas (in this case CO2) directly from the airway, with the sensor located on the airway adapter at the hub of the endotracheal tube (ETT). Sidestream devices measure respiratory gas via nasal or nasal-oral cannula by aspirating a small sample from the exhaled breath through the cannula tubing to a sensor located inside the monitor (picture 1).
Mainstream systems are configured for intubated patients. Sidestream systems are configured for both intubated and nonintubated patients.
Sidestream systems are configured to use high flow rates (around 150 cc/min) or low flow rates (around 50 cc/min). Flow rates vary according to the amount of CO2 needed in the breath sample to obtain an accurate reading. Low-flow systems have a lower occlusion rate (from moisture or patient secretions) and are accurate in patients with low tidal volumes (eg, neonates, infants, and adult patients with hypoventilation and low tidal volume breathing). Low-flow systems are also resistant to dilution from supplemental oxygen. High-flow systems sampling at ≥100 cc/min have been shown to be inaccurate in neonates, infants, young children, and in hypoventilating adult patients [1-3].
CO2 monitors are either quantitative or qualitative. Quantitative devices measure the precise end-tidal CO2 (EtCO2) either as a number (capnometry) or a number and a waveform (capnography). Qualitative devices (eg, colorimetric detectors) report the range in which the EtCO2 falls (eg, 0 to 10 mmHg or >35 mmHg) as opposed to a precise value (eg, 38 mmHg).
The colorimetric EtCO2 detector, which consists of a piece of specially treated litmus paper that changes color when exposed to CO2 (purple for EtCO2 <3 mmHg; tan for 3 to 15 mmHg; and yellow for >15 mmHg). Its primary use is for verification of ETT placement. Exhalation of CO2 from an ETT placed in the trachea will change the color of the litmus paper from purple to yellow. An improperly placed ETT in the esophagus will not conduct CO2 and no change will occur in the color of the litmus paper, which will remain purple. When evaluating studies of EtCO2, it is essential to distinguish those involving qualitative measurements (colorimetric) from those describing quantitative methods with a graphic waveform display (capnography). (See "Confirmation of correct endotracheal tube placement in adults".)
Capnography uses infrared (IR) radiation to make measurements. Molecules of CO2 absorb IR radiation at a very specific wavelength (4.26 µm), with the amount of radiation absorbed having a nearly exponential relation to the CO2 concentration present in the breath sample. Detecting these changes in IR radiation levels, using appropriate photo-detectors sensitive in this spectral region, allows for the calculation of the CO2 concentration in the gas sample.
CO2 WAVEFORM — The capnogram consists of four phases (figure 1).
●Phase 1 (dead space ventilation, A-B) represents the beginning of exhalation where the dead space is cleared from the upper airway and the CO2 concentration approaches zero.
●Phase 2 (ascending phase, B-C) represents the rapid rise in carbon dioxide (CO2) concentration in the breath stream as the CO2 from the alveoli reaches the upper airway.
●Phase 3 (alveolar plateau, C-D) represents the CO2 concentration reaching a uniform level in the entire breath stream from alveolus to nose. Point D, occurring at the end of the alveolar plateau, represents the maximum CO2 concentration at the end of the tidal breath and is appropriately named the end-tidal CO2 (EtCO2). This is the number that appears on the monitor display.
●Phase 4 (D-E) represents the inspiratory cycle where the CO2 falls back to zero.
A normal capnogram (ie, a valid breath), for patients of all ages, is characterized by a set of specific elements: the CO2 concentration starts at zero and returns to zero, a maximum CO2 concentration is reached with each breath (ie, EtCO2), the amplitude is a function of the EtCO2 concentration, the width is a function of the expiratory time, and there is a characteristic shape with normal lung function.
●Patients with normal lung function have characteristic trapezoidal capnograms and narrow gradients between their alveolar CO2 (ie, EtCO2) and arterial CO2 concentration (PaCO2) of 0 to 5 mmHg (figure 1). Gas in the physiologic dead space accounts for this normal gradient.
●Patients with obstructive lung disease have impaired expiratory flow and uneven emptying of alveoli due to ventilation-perfusion mismatch, and demonstrate a more rounded ascending phase and an upward slope in the alveolar plateau (figure 2). In patients with abnormal lung function and ventilation-perfusion mismatch, the EtCO2-PaCO2 gradient widens depending on the severity of the lung disease [4,5]. The EtCO2 in patients with lung disease is only useful for assessing trends in ventilatory status over time; isolated EtCO2 values may or may not correlate with the PaCO2.
CLINICAL APPLICATIONS FOR INTUBATED PATIENTS — Capnography can be used in intubated patients for:
●Verification of endotracheal tube (ETT) placement
●Continuous monitoring of tube location during transport
●Gauging effectiveness of resuscitation and prognosis during cardiac arrest
●Indicator of ROSC during chest compressions
●Titrating end-tidal carbon dioxide (EtCO2) levels in patients with suspected increases in intracranial pressure
●Determining prognosis in trauma
●Determining adequacy of ventilation
A table summarizing capnographic findings in different clinical scenarios is provided (figure 3A-B).
Verification of ETT placement — Correct ETT placement must be verified immediately following every tracheal intubation, preferably with waveform capnography. The role of EtCO2 in confirming ETT placement is discussed separately. (See "Confirmation of correct endotracheal tube placement in adults".)
In addition to verification and monitoring of ETT placement, capnography provides similar information about the placement of alternate advanced airway devices (laryngeal tube airway, laryngeal mask airway, esophageal-tracheal combitube).
The accuracy of EtCO2 for confirming ETT location can help clinicians faced with difficult airway management decisions. As an example, in a case involving one of the authors, a patient resuscitated following a submersion injury became difficult to ventilate and hypotensive following RSI, but the presence of a waveform reassured clinicians about ETT placement, allowing them to consider other reasons for decompensation. Decompression of the patient’s distended stomach with an orogastric tube resolved the difficulties with ventilation and blood pressure.
Monitoring ETT location during transport — UMI has catastrophic consequences and can occur when an ETT is dislodged during patient transport. Continuous monitoring of ETT location during transport prevents UMI. EtCO2 confirmation of initial ETT placement and continuous capnographic monitoring of ETT location is an accepted standard of care by the American Society of Anesthesiologists [6] and is recommended as the most reliable method by other national organizations [7-9].
A prospective, observational study of 153 prehospital intubations found a 23 percent UMI rate among patients without continuous EtCO2 monitoring, while patients with continuously monitored EtCO2 sustained the desired 0 percent UMI rate [10]. The attached figures depict one airway confirmation algorithm using colorimetric CO2 detectors and another using capnography. They can be used for both prehospital and in-hospital intubated patients.
Effectiveness of CPR — In the 1980s, studies using animal models showed that EtCO2 levels reflect cardiac output during cardiopulmonary resuscitation (CPR) and can be used as a noninvasive measure of cardiac output. A landmark study in 1988 demonstrated this principle in humans [11]. During cardiac arrest, when alveolar ventilation and metabolism are essentially constant, EtCO2 reflects pulmonary blood flow. Therefore, EtCO2 can be used as a gauge of the effectiveness of cardiac compressions. As effective CPR leads to a higher cardiac output, EtCO2 will rise, reflecting the increase in perfusion.
The measurement of EtCO2 varies directly with the cardiac output produced by chest compression and has been described in both EMS [12] and ICU patients [11]. Both of these prospective, observational studies found an EtCO2 level <3 mmHg immediately after cardiac arrest, with a higher level generated during cardiac compressions and a mean peak >7.5 mmHg just before return of spontaneous circulation (ROSC) [11,12]. This peak in EtCO2 level is the earliest sign of ROSC and may occur before return of a palpable pulse or blood pressure. Another observational study of data collected during human CPR found a positive correlation between the depth of chest compressions and EtCO2 (10 mm increase in compression depth increased EtCO2 by 1.4 mmHg), as well as ventilation rate and EtCO2 (increase of 10 breaths/minute decreased ETCO2 by 3 mmHg), and confirmed that higher EtCO2 values during CPR correlate with increased ROSC and survival [13].
Return of spontaneous circulation — During cardiac arrest, EtCO2 is the earliest indicator of the return of spontaneous circulation (ROSC) [11,12]. When the heart restarts, the dramatic increase in cardiac output, and resulting increase in perfusion, leads to a rapid increase in EtCO2 as the CO2 that has accumulated during cardiac arrest is effectively transported to the lungs and exhaled. This process manifests as a sudden rise in EtCO2. (See 'Effectiveness of CPR' above.)
American Heart Association (AHA) guidelines for cardiac resuscitation emphasize the importance of continuing chest compressions without interruption until a perfusing rhythm is reestablished. Experimental evidence indicates that interruptions in chest compressions are followed by sustained periods of reduced blood flow, which only gradually return to pre-interruption levels. Capnogram monitoring virtually eliminates the need to stop chest compressions to check for pulses. Reestablishment of a perfusing rhythm is accompanied immediately by a dramatic increase in EtCO2. Once this rise in EtCO2 is noted, chest compressions can safely be stopped while cardiac rhythm and blood pressure are assessed [9,14].
According to an observational study of 145 patients with out of hospital cardiac arrest (OHCA), several seconds may be required after electrical conversion to a potentially perfusing rhythm before effective mechanical contractions and a subsequent rise in EtCO2 occur [15]. Accordingly, stopping compressions in response to a rhythm change in order to check for a pulse may result in degradation of the rhythm, as coronary perfusion had not yet been restored. EtCO2 is a more reliable harbinger of reperfusion than a pulse check. (See "Adult basic life support (BLS) for health care providers".)
Specific EtCO2 levels have been shown to correlate with ROSC. In a systematic review of 17 observational studies including over 6100 adults with cardiac arrest, which included metaanalyses of data from five studies, EtCO2 ≥10 to 20 mmHg during CPR was strongly associated with ROSC while persistent EtCO2 below 10 mmHg after 20 minutes of CPR had a 0.5 percent likelihood of ROSC [16]. Subsequent retrospective studies support this general finding [17,18]. A retrospective, observational study of 526 OHCA patients with a nonshockable rhythm found that the first ETCO2 measured after placement of an airway device predicted ROSC and survival after hospital admission [17]. Specifically, patients with an ETCO2 >45 mmHg had an increased probability of sustained ROSC and 30-day survival. A retrospective study of 324 OHCA patients found that higher EtCO2 levels were associated with higher rates of successful defibrillation [18].
Confounding factors in resuscitation — Drugs used in resuscitation may affect EtCO2 values. EtCO2 often decreases rapidly moments after administration of epinephrine. Sodium bicarbonate may produce a transient increase in EtCO2, but the rise in EtCO2 levels after ROSC is greater and longer lasting than after a sodium bicarbonate bolus [19,20].
Prognosis in cardiac arrest — In several prospective, observational studies, EtCO2 levels of ≤10 mmHg measured 20 minutes after the initiation of advanced cardiac life support accurately predicted death in adult patients with cardiac arrest [21-24]. The prognostic value of measuring EtCO2 has been demonstrated in animal [25] and human studies [21-24,26].
Cause of cardiac arrest — EtCO2 may also be helpful in determining the etiology of cardiac arrest. Two animal studies reported higher EtCO2 levels at the onset of cardiac arrest caused by primary asphyxia than arrest caused by ventricular fibrillation [27,28]. Researchers found results consistent with these animal experiments in a prospective, observational study of prehospital victims of cardiac arrest. Patients in the asphyxia group (initial rhythm of asystole or pulseless electrical activity associated with conditions such as airway foreign body, aspiration, asthma, or drowning) had higher EtCO2 levels compared with patients in the ventricular tachycardia/fibrillation group (initial rhythm of ventricular tachycardia/fibrillation associated with acute myocardial infarction) [29]. These between group differences disappear within minutes if return of spontaneous circulation has not occurred.
Increased ICP and trauma prognosis — EtCO2 monitoring can help clinicians avoid inadvertent hyperventilation of patients with traumatic brain injury and suspected increased intracranial pressure (ICP). It may also help determine the prognosis of trauma victims.
Arterial CO2 tension affects blood flow to the brain. High CO2 levels result in cerebral vasodilation, while low CO2 levels result in cerebral vasoconstriction. Sustained hypoventilation (defined as PaCO2 levels ≥50 mmHg) results in increased cerebral blood flow and increased ICP, which can harm head-injured patients. Sustained hyperventilation (defined as PaCO2 ≤30 mmHg) is also detrimental and is associated with worse neurologic outcome in severely brain-injured patients. Consequently, ventilation rates to achieve eucapnia are recommended by the brain trauma foundation [30].
Use of continuous EtCO2 monitoring in intubated trauma patients is supported by several studies:
●A prospective observational study found a lower incidence of inadvertent hyperventilation among intubated patients with severe head injury who underwent continuous EtCO2 monitoring compared with those without EtCO2 monitoring (5.6 versus 13.4 percent; odds ratio [OR] 2.64; 95% CI 1.12-6.20) [31].
●In a controlled study of intubated blunt trauma victims, patients assigned to capnography-guided ventilation during transport were significantly more likely to arrive at the emergency department appropriately ventilated, based on PaCO2 levels obtained by arterial blood gas [32].
Studies have found that a low EtCO2 in trauma patients is associated with mortality and hemorrhagic shock:
●In a prospective observational study of blunt trauma patients requiring prehospital intubation, EtCO2 values obtained 20 minutes after intubation distinguished the great majority of survivors from non-survivors [33]. Median EtCO2 among survivors was 30.8 mmHg and among non-survivors 26.3 mmHg (95 percent CI of difference between medians 3 to 6.75 mmHg).
●In a retrospective observational study of 135 trauma patients transported by emergency medical services to a level one trauma center, the mean prehospital EtCO2 level distinguished survivors (34 mmHg, 95% CI 32-35) from non-survivors (18 mmHg, 95% CI 9-28) and had better discrimination regarding prognosis than vital signs or pulse oximetry [34].
●In a retrospective review of 373 trauma patients receiving a massive transfusion protocol and undergoing damage control surgery, operative ETCO2 ≤23mmHg was a predictor of mortality [35].
●In two retrospective studies of non-intubated adult trauma patients, initial side-stream ETCO2 ≤28.5 to 29.5 mmHg was associated with higher in-hospital mortality, blood transfusion requirement, and complication rates [36,37].
CLINICAL APPLICATIONS FOR SPONTANEOUSLY BREATHING PATIENTS — In a spontaneously breathing, nonintubated patient, capnography can be used for the following:
●Performing rapid assessment of critically ill or seizing patients
●Determining response to treatment in acute respiratory distress
●Determining adequacy of ventilation in obtunded or unconscious patients, or in patients undergoing procedural sedation
●Detecting metabolic acidosis in diabetic patients and in children with gastroenteritis
●Providing prognostic indicators in patients with sepsis or septic shock
A table summarizing capnographic findings in different clinical scenarios is provided (figure 3A-B).
Assessing airway, breathing, and circulation — The airway, breathing, and circulation (ABCs) of patients can be rapidly assessed using the capnography waveform and end-tidal carbon dioxide (EtCO2) values [38]. The presence of a normal waveform denotes a patent airway and spontaneous breathing [39]. Normal EtCO2 levels (35 to 45 mmHg) signify adequate ventilation and perfusion [11,40]. Capnography can be used to assess unresponsive patients ranging from those who are actively seizing to victims of chemical terrorism (table 1) [41]. Unlike pulse oximetry, capnography does not misinterpret motion artifact and provides reliable readings in low perfusion states. Studies have found that an EtCO2 ≤29.5 mmHg was independently predictive of the need for massive transfusion [36,42].
Seizure — Capnography is the only monitoring modality that is accurate and reliable in actively seizing patients because the capnogram is determined entirely by respiratory activity and is not confounded by muscle activity or movement artifact. Capnographic data (respiratory rate, EtCO2, and capnogram) can be used to distinguish among:
●Seizing patients with apnea (flatline waveform, no EtCO2 reading, and no chest wall movement)
●Seizing patients with ineffective ventilation (small waveforms, low EtCO2 values), and
●Seizing patients with effective ventilation (normal CO2 waveform, normal EtCO2 values). (See "Convulsive status epilepticus in adults: Classification, clinical features, and diagnosis" and "Convulsive status epilepticus in adults: Management".)
Acute respiratory distress — Capnography provides dynamic monitoring of ventilatory status in patients with acute respiratory distress from any cause, including: bronchiolitis, asthma, cystic fibrosis, heart failure, and chronic obstructive pulmonary disease (COPD). Several studies have found that quantitative waveform analysis can discriminate between heart failure and COPD [43-45].
By measuring EtCO2 and respiratory rate with each breath, capnography provides instantaneous feedback on the clinical status of the patient. Respiratory rate is measured directly from the airway (nose and mouth) with oral or nasal cannula, providing a more reliable reading than impedance respiratory monitoring. In upper airway obstruction and laryngospasm, impedance monitoring detects chest wall movement, interprets this as a valid breath, and displays a respiratory rate, even though the patient is not ventilating. In contrast, capnography detects no ventilation and shows a flatline waveform.
Clinicians can rapidly assess EtCO2 trends. A patient with a respiratory rate of 30 will generate 150 EtCO2 readings in five minutes. This provides sufficient information to determine whether the patient's ventilation is worsening despite treatment (increasing EtCO2), stabilizing (stable EtCO2), or improving (decreasing EtCO2). As an example, increasing EtCO2 would reflect the worsening ventilation of an acutely tachypneic patient with obstructive lung disease who is developing respiratory muscle fatigue or a more severe lower airway obstruction.
Procedural sedation — We recommend continuous capnographic monitoring in all patients undergoing procedural sedation and analgesia (PSA). Capnography can rapidly detect the common adverse airway and respiratory events associated with PSA, including respiratory depression, apnea, upper airway obstruction, laryngospasm, and bronchospasm. Patients monitored with capnography experience significantly fewer episodes of hypoxia than those monitored in standard fashion, especially since the administration of supplemental oxygen renders pulse oximetry less effective as an early warning device for respiratory depression during PSA. (See "Procedural sedation in adults in the emergency department: General considerations, preparation, monitoring, and mitigating complications", section on 'Monitoring' and "Procedural sedation in children: Approach", section on 'Monitoring'.)
Respiratory depression caused by oversedation will manifest an abnormally high or low EtCO2 before pulse oximetry detects a falling oxyhemoglobin saturation, especially in patients receiving supplemental oxygen. In one trial, the median time between the onset of respiratory depression, as determined by EtCO2, and hypoxia was 60 seconds (range 5 to 240 seconds) [46].
Common capnographic findings during PSA include the following and are summarized in the figures (figure 3A and figure 3B):
●Flatline waveform – This can occur from monitor calibration (will indicate "calibrating" on the monitor), cannula occlusion (will indicate "occlusion" on the monitor), or apnea (central or obstructive). The combination of absent chest wall movement and a flatline waveform differentiates central apnea from obstructive apnea (upper airway obstruction or laryngospasm), which manifests chest wall movement. Response to airway alignment maneuvers (eg, chin lift, jaw thrust) can distinguish upper airway obstruction from laryngospasm.
●Oscillating waveform progressively decreasing in height – This occurs with oversedation and is a sign of impending apnea.
●Small waveforms interspersed with flatline waveforms - This irregular waveform pattern occurs with hypopneic hypoventilation and indicates periodic breathing.
●Increasing waveform height – This occurs with bradypneic hypoventilation and reflects increasing EtCO2. An EtCO2 >70 mmHg in a patient without COPD indicates respiratory failure.
●Decreasing waveform height – This occurs with hypopneic hypoventilation (ie, low tidal volume breathing) and reflects decreasing EtCO2.
●Increasing waveform width – This occurs with hypoventilation and increasing expiratory time.
●Decreasing waveform width – This occurs with hyperventilation and decreasing expiratory time.
●Waveform does not return to zero – An off-baseline waveform can occur with rebreathing.
●Curved/shark-fin appearing waveform – The characteristic curved capnogram (figure 2) can be seen in a patient with obstructive lung disease (eg, asthma, COPD, cystic fibrosis) and indicates bronchospasm if it occurs in a patient without chronic lung disease.
Ventilation in obtunded or unconscious patients — Patients with altered mental status, including those with alcohol intoxication, intentional or unintentional drug overdose, and postictal patients (especially those treated with benzodiazepines), may have impaired ventilatory function. Capnography can differentiate between patients with effective ventilation and those with ineffective ventilation [47].
Detecting metabolic acidosis — There is a positive linear correlation between serum bicarbonate (HCO3) and EtCO2 in patients with diabetic ketoacidosis and children with gastroenteritis [48-52]. As the patient becomes acidotic, HCO3 decreases, causing an increase in minute ventilation, which results in a compensatory respiratory alkalosis. This process results in a decrease in EtCO2.
By increasing minute ventilation, these patients are able to lower arterial CO2 tension to help correct their underlying acidemia. The more acidotic the patient, the lower the HCO3, the higher the respiratory rate, and the lower the EtCO2. Capnography can be used as an indicator of metabolic acidosis in these patients. In addition, EtCO2 can be used to distinguish diabetics in ketoacidosis (metabolic acidosis, compensatory tachypnea, and low EtCO2) from those who are not (nonacidotic, normal respiratory rate, and normal EtCO2) [53,54]. (See "Approach to the adult with metabolic acidosis" and "Approach to the child with metabolic acidosis".)
Prognosis in sepsis — There is an inverse relationship between EtCO2 and lactate levels in sepsis, severe sepsis, and septic shock. EtCO2 performs similarly to lactate as a predictor for mortality in patients with suspected sepsis [55,56]. In a prospective study of 183 patients performed in the prehospital setting, patients with suspected sepsis and an EtCO2 value ≤25 mmHg were more frequently diagnosed with sepsis (78 versus 43 percent) and severe sepsis (47 versus 7 percent), and had higher mortality (11 versus 5 percent) [57]. The use of EtCO2 to predict mortality, sepsis, and severe sepsis in prehospital patients outperformed another well-accepted screening tool, the quick sequential organ failure assessment score (qSOFA) [58]. Patients with severely compromised respiratory reserve who cannot appropriately compensate for the lactic acidosis resulting from severe sepsis or septic shock have the worst prognosis despite ETCO2 values that are not low. (See "Evaluation and management of suspected sepsis and septic shock in adults".)
LIMITATIONS — Capnography is most effective when assessing a pure ventilation, perfusion, or metabolism problem. Capnographic findings in mixed ventilation, perfusion, or metabolism problems are difficult to interpret. In patients with complex pathophysiology, a ventilation problem may elevate end-tidal carbon dioxide (EtCO2), while a perfusion problem may simultaneously lower EtCO2. As an example, capnography may not provide useful information in a patient with acute pulmonary edema (ventilation problem) from an underlying acute myocardial infarction with decreased cardiac output (perfusion problem).
In high-flow CO2 systems, when the tidal volume of the patient drops below the flow rate of the system, the monitor will entrain room air to compensate, falsely diluting the EtCO2 reading and slurring the ascending phase of the waveform. For this and other reasons, low flow systems are preferred. (See 'Principles of operation' above.)
Capnography is highly accurate for determining tracheal placement of an endotracheal tube (ETT) in nearly all clinical circumstances. However, the test characteristics of capnography in cardiac arrest can vary, and this is discussed above. (See 'Verification of ETT placement' above.)
SUMMARY AND RECOMMENDATIONS
●Definition and background – Carbon dioxide (CO2) monitoring is a versatile noninvasive diagnostic tool for continuous assessment of the ventilatory status of both intubated and nonintubated patients. Capnography provides instantaneous information about ventilation (how effectively CO2 is being eliminated by the pulmonary system), perfusion (how effectively CO2 is being transported through the vascular system), and metabolism (how effectively CO2 is being produced by cellular metabolism). A table summarizing capnographic findings in different clinical scenarios is provided (figure 3A-B). (See 'Definition and background' above.)
●Principles of operation – Colorimetric detectors provide a qualitative measurement of end-tidal CO2 (EtCO2); capnometry provides a quantitative EtCO2 measurement; capnography provides a quantitative measure and a waveform depicting CO2 levels over time. (See 'Principles of operation' above.)
●CO2 waveform – The capnogram consists of four phases: dead space ventilation, ascending phase, alveolar plateau, and descending inspiratory phase, which are depicted in the figure (figure 1). (See 'CO2 Waveform' above.)
●Clinical applications – Capnography is of greatest value for emergency departments and emergency medical services for confirming placement of endotracheal tubes (ETTs), continuous monitoring of tube position during transport, assessing the status of patients in cardiac arrest, and monitoring the respiratory status of patients undergoing procedural sedation. (See 'Clinical applications for intubated patients' above.)
●Verification of ETT placement – Correct ETT placement must be verified immediately following every tracheal intubation, preferably with waveform capnography. The role of EtCO2 in confirming ETT placement is discussed separately. (See "Confirmation of correct endotracheal tube placement in adults".)
●Monitoring resuscitation in cardiac arrest – In a patient in cardiac arrest, we suggest that EtCO2 be used to monitor resuscitation efforts. EtCO2 monitoring can be performed on patients in cardiac arrest to determine the adequacy of chest compressions and the return of spontaneous circulation. During cardiac arrest, EtCO2 reflects pulmonary blood flow and can be used as a gauge of the effectiveness of cardiac compressions. As effective CPR leads to a higher cardiac output, EtCO2 will rise. (See 'Effectiveness of CPR' above.)
●Assessing return of spontaneous circulation – EtCO2 is also the earliest indicator of the return of spontaneous circulation. When the heart restarts, the dramatic increase in cardiac output, and resulting increase in perfusion, leads to a rapid increase in EtCO2. (See 'Return of spontaneous circulation' above.)
●Monitoring during procedural sedation – We recommend continuous capnographic monitoring in all patients undergoing procedural sedation and analgesia (PSA). Capnography can rapidly detect common adverse airway and respiratory events including central apnea, obstructive apnea (upper airway obstruction, laryngospasm), bronchospasm, and respiratory failure (algorithm 1). The evidence for capnographic monitoring is presented separately. (See 'Procedural sedation' above and "Procedural sedation in adults in the emergency department: General considerations, preparation, monitoring, and mitigating complications", section on 'Monitoring'.)
●Applications in spontaneously breathing patients – Capnography can be helpful in monitoring the status of patients with seizures, acute respiratory distress, unconsciousness, and metabolic acidosis. (See 'Clinical applications for spontaneously breathing patients' above.)
●Limitations – Capnography is most effective when assessing a pure ventilation, perfusion, or metabolism problem. Capnographic findings in mixed ventilation, perfusion, or metabolism problems are difficult to interpret. (See 'Limitations' above.)
ACKNOWLEDGMENT — The editorial staff at UpToDate acknowledge Salvatore Silvestri, MD, now deceased, who contributed to an earlier version of this topic review.
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