Use of Capnography in Emergency Medicine and Prehospital Critical Care
Samuel M. Galvagno Jr., D.O.
Bhavani shankar Kodali MD
Samuel M. Galvagno Jr., D.O. Chief Resident, Department of Anesthesiology, Perioperative, and Pain Medicine, Brigham and Woman’s Hospital, Harvard Medical School, Boston, MA The measurement of carbon dioxide (CO2) in expired air directly indicates changes in the elimination of C02 from the lungs. Indirectly, it indicates changes in the production of C02 at the tissue level and in the delivery of C02 to the lungs by the circulatory system. Capnography is a non-invasive monitoring technique that allows fast and reliable insight into ventilation, circulation, and metabolism (1). In the prehospital environment, it is used primarily for confirmation of successful endotracheal intubation, but it may also be a useful indicator of efficient ongoing cardiopulmonary resuscitation (CPR). In patients with hemorrhage, capnography provides information regarding tissue perfusion and may help guide fluid resuscitation (1).
Numerous national organizations, including the American Heart Association, now endorse capnography and capnographic methods for confirming endotracheal tube placement (2). Despite these recommendations, capnography is not always widely available nor consistently applied (3).
Application of Capnography in the Prehospital Arena
When CO2 is absent as measured by end-tidal capnography, it means either the endotracheal tube is in the wrong position (esophageal) or there is an absent or decreased presentation of CO2 to the lungs.
The most catastrophic complication of endotracheal intubation is inadvertent esophageal intubation. Correct placement of the endotracheal tube is suggested by, but not confirmed by the following methods:
• Bilateral, equal breath sounds
• Bilateral chest wall movements
• Absence of breath sounds / gurgling over the stomach / epigastrium
• Fogging of the endotracheal tube
• Tube / cuff palpation in the neck or suprasternal notch
• Disappearance of cyanosis
Capnography can be used to confirm successful endotracheal intubation. A normal-appearing waveform and a digital numeric display will confirm that the endotracheal tube is in trachea. In low cardiac output states such as shock, cardiac arrest, or cardiopulmonary resuscitation (CPR) with inadequate chest compressions, end-tidal CO2 may not be detected.
As explained in the physiology section, the CO2 concentration reaches a maximal level at the end of exhalation. This maximum concentration is called end-tidal carbon dioxide concentration or tension depending on whether it is expressed in fractional concentration or mm Hg. End-tidal carbon dioxide reflects CO2 concentration of alveoli emptying last. The normal values of end-tidal CO2 is around 5% or 35-37 mm Hg. The gradient between the blood CO2 (PaCO2) and exhaled CO2 (end-tidal CO2 or PetCO2) is usually 5-6 mm Hg. PetCO2 can be used to estimate PaCO2 in patients with essentially normal lungs.
The end-tidal CO2 is detected and measured by colorimetry, capnometry, and capnography. When CO2 is absent as measured by devices used to detect these parameters, either the endotracheal tube is not correctly placed (i.e. esophageal intubation) or there is an absent or decreased presentation of CO2 to the lungs (i.e. cardiac arrest). When cardiac output increases, end-tidal CO2 provides information about adequacy of ventilation and circulation.
Colorimetric devices provide continuous, semiquantitative end-tidal CO2 monitoring. A typical device has 3 color ranges:
• Purple — EtCO2 < 0.5%
• Tan — EtCO2 0.5-2%
• Yellow – EtCO2 >2%
Normal end-tidal CO2 is >4%; hence, the device should turn yellow when the endotracheal tube is inserted in patients with intact circulation.
Limitations of colorimetric devices include:
• Confirmation of endotracheal tube placement in non-cardiac arrest patients is not always reliable
• The membrane can turn ‘yellow’ (implies end-tidal CO2 > 2%) when the device is contaminated with acidic substances such as gastric acid, lidocaine, or epinephrine
• The device will not provide a reading if it is clogged with secretions or broken
Capnographs provide both a waveform and digital reading of end-tidal CO2. The digital reading for end-tidal CO2 is often displayed as mm Hg (partial pressure of CO2 in exhaled gas) or as % in exhaled gas. Most of the commonly used devices use infrared absorption of CO2 as principle of operation.
For an understanding of the physics behind capnography, click here to open a new window. Close the window to return to this section.
Capnographs can be main stream or side stream (as described in the physiology section).
Use of Capnography to Optimize Prehospital Ventilation
Capnography is an efficient means of maximizing a patient’s ventilatory status during prehospital care and transport (14). Several studies by Davis, et al. have shown that inadvertent hyperventilation of head-injured patients by paramedics has been linked to poor outcomes (15-17). In one of these studies, despite use of capnography, hyperventilation was still common (15). Nevertheless, future ventilation strategies aimed at avoiding hyperventilation (i.e. targeted ventilation strategies) will likely incorporate capnography since analysis of capnographic waveforms may be an important technique for avoidance of potential therapeutic errors (18). The role of prehospital capnography as a means of guiding manual or mechanical ventilatory therapy in unstable or critically ill patients continues to evolve.
Use of Capnography in Cardiopulmonary Resuscitation (CPR)
In the 2005 emergency cardiac care (ECC) guidelines released by the American Heart Association, providers in both the Prehospital and hospital arenas are encouraged to utilize end-tidal CO2 detectors to confirm endotracheal tube placement (2). These devices are not intended to take the place of clinical assessment, and when used within the proper context, provide confirmatory data. No study to date has shown a single device to be 100% sensitive and specific for determining proper endotracheal tube placement, and all devices such as end-tidal CO2 detectors should be considered as one of many adjuncts used to confirm successful intubation (2).
Given the simplicity of use, the American Heart Association states that it is reasonable to use end-tidal CO2 detectors in CPR, even for victims of cardiac arrest (2). In several recent studies, including one metaanalysis, one prospective controlled cohort study, and several case reports, end-tidal CO2 detectors have been shown to be useful for endotracheal tube placement (4-11). The sensitivity in these studies has been described as ranging from 20-100%, but the specificity (percentage of incorrect esophageal placement detected when no CO2 is detected) has ranged from 97-100% (2, 4-11). Therefore, the positive predictive value (probability of correct endotracheal tube placement if CO2 is detected) is nearly 100% while the negative predictive value (probability of esophageal tube placement if no CO2 is detected) has a broader range of 20-100% (2, 4-11). CO2 can occasionally be detected when the tube is placed in the esophagus; false positive CO2 determinations have been found in animals that have ingested large amounts of carbonated liquids in a cardiac arrest model (12).
False negative readings, defined as a failure to detect CO2 despite confirmed endotracheal tube placement in the trachea, may still occur during cardiac arrest. In the low-flow hypodynamic state of cardiac arrest, delivery of CO2 to the lungs is decreased. Instances associated with false negative end-tidal CO2 readings after successful endotracheal intubation also include:
• Massive pulmonary embolism
• Contamination of CO2 detector with gastric contents or acidic drugs (including epinephrine administered via the endotracheal tube)
• Severe airway obstruction (status asthmaticus)
• Severe pulmonary edema
Detection of CO2 may be reduced following an intravenous bolus of epinephrine (13). Studies in animals and humans have demonstrated that end-tidal CO2 decreases when a large dose of epinephrine is used during CPR (29). Epinephrine administration may decrease end-tidal CO2 tensions in cardiac arrest, but it does so unpredictably (29). End-tidal CO2 decreases an average 0.3 Torr after epinephrine administration (29). This decrease in end-tidal CO2 has been attributed to an increase in shunt fraction and also due to a drop in cardiac output secondary to increased afterload by epinephrine (30). The duration of decrease in end-tidal CO2 has not been quantified, but this phenomenon may be clinically useful for timing subsequent epinephrine dosing (31). Hence, in cases where CO2 is not detected following what appears to be a successful endotracheal intubation, secondary confirmatory methods should be used such as direct visualization or esophageal detector devices (2).
End-tidal CO2, as measured continuously with capnography, can be used to provide feedback to optimize chest compressions during CPR (32). Monitoring end-tidal CO2 during cardiac arrest may detect unrecognized CPR provider fatigue.
Use of Capnography in Trauma Resuscitation
End-tidal CO2 is a marker of pathophysiological states encountered in trauma since it reflects cardiac output. Deakin et al. showed that end-tidal CO2 may be of value in predicting outcome from major trauma (19). In a study of 191 blunt trauma patients, only 5% of patients with an end-tidal CO2 determination of 3.25 kPa survived to discharge (19).
Despite the findings from Davis et al., other studies have shown capnography to be of value for tight control of ventilation in prehospital major trauma victims (15-17, 20). Helm et al. showed that the incidence of normoventilation was significantly higher (63.2% vs. 20%, p<0.0001) in a group of patients who were monitored with capnography in the field (20).
Use of Capnography in Pediatric Emergencies
Correct tracheal tube placement is essential in the care of all ventilated patients and the use of capnography in the pediatric intensive care unit (PICU) may be even more important than in the adult intensive care unit. A survey conducted by PICU physicians in the United Kingdom revealed up to 89% of consultants had less than 1 year of training in anesthesiology (21). While not universally available, capnography may help avoid serious morbidity and mortality in pediatric patients who require confirmation of proper endotracheal tube placement.
End-tidal CO2 monitoring for pediatric patients in the prehospital setting has been shown to be safe, effective, and potentially life-saving (22). Use of devices such as the Capno-Flo © resuscitator during transport of critically ill children has been shown to be 100% sensitive an specific for initial confirmation of correct endotracheal tube position (23). While such devices might be useful for initial confirmation, they may not be as useful as capnography for continuous evaluation of endotracheal tube position during transport.
Capnography as a Prognostic Indicator
The relationship between cardiac output and end-tidal CO2 is logarithmic (24). Decreased presentation of CO2 to the lungs is the major rate-limiting determinant of the end-tidal CO2 during periods of low blood flow. White et al. studied the utility of end-tidal CO2 measurements in out-of-hospital cardiac arrest (25). They concluded that capnography can detect the presence of pulmonary blood flow even in the absence of major pulses (pseudo-electromechanical dissociation) and also can rapidly indicate changes in pulmonary blood flow (cardiac output) caused by alterations in cardiac rhythm (25).
Data suggests that the end-tidal CO2 correlates well with coronary perfusion pressure (CPP) (26). It is also known to correlate with cerebral perfusion pressure and blood flow during CPR, but not with neurologic outcome (27). This correlation between perfusion pressure and end-tidal CO2 is likely to be secondary to the relationship of end-tidal CO2 and cardiac output.
Callaham et al. used initial end-tidal CO2 readings to predict return of spontaneous pulse during CPR (28). Patients who developed a pulse had a mean end-tidal CO2 of 19 +/- 14 Torr at the start of resuscitation as opposed to those who did not have a mean PetCO2 of 5 +/- 4 Torr (28). Using an initial end-tidal CO2 value of 15 torr, the authors were able to identify 71% of the patients who were subsequently resuscitated with a specificity of 98% (28). However, the end-tidal CO2 value was not found to be a sufficient criterion by itself for terminating resuscitation efforts.
Capnographic measurements have been evaluated as a prognostic indicator of outcome in cardiac arrest. Ahrens et al. recommended using capnography for its prognostic value (33). In a study of 127 patients, all but 1 patient with an end-tidal CO2 value less than 10 mm Hg died before discharge (33). The results of this study were confirmed with another prospective investigation involving 139 adult victims of out-of-hospital, non-traumatic cardiac arrest. None of the patients with an average, initial, and final end-tidal CO2 level of less than 10 mm Hg were successfully resuscitated (34). The authors concluded that end-tidal CO2 monitoring can be correlated with resuscitation outcome in CPR. In another study by Salen et al., end-tidal values greater than 35 torr, as measured by a mainstream capnography, were associated with improved chances of survival as compared to the median end-tidal CO2 level of 13.7 torr for nonsurvivors (35). In the same study, capnography was shown to be superior to cardiac sonography (35).
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