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Capnographic Monitoring in Respiratory Emergencies
Capnographic Monitoring in Respiratory Emergencies Joshua Nagler, MD, ⁎ Baruch Krauss, MD, EdM Children with respiratory diseases present commonly to the emergency department. As a continuous, dynamic measure of the ventilatory status, capnography can provide valuable information in the assessment and management of these patients. After a review of the relevant physiology and technology of carbon dioxide monitoring, clinical applications for the use of capnography in patients with respiratory illnesses are discussed. Characteristic waveforms are provided, and their interpretation and clinical significance are discussed. A focus on the current literature investigating the noninvasive monitoring of patients with obstructive lung disease is included. Practical tips for successfully using capnography are also presented. Clin Ped Emerg Med 10:82-89 © 2009 Elsevier Inc. All rights reserved. KEYWORDS capnography, capnometry, ventilation, end-tidal carbon dioxide, respiratory illness, asthma
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espiratory diseases leading to abnormalities in oxygenation and ventilation are common in the emergency department (ED). Although oxygenation monitoring with pulse oximetry is routine for children in respiratory distress, noninvasive ventilation monitoring is not. Although capnography is a standard of care for verification of endotracheal tube placement and monitoring of tube position, it is just beginning to be used to assess ventilatory status in spontaneously breathing patients. We discuss the use of capnography for noninvasive monitoring children with respiratory illnesses.
Definitions Capnography is the noninvasive measurement of the partial pressure of carbon dioxide (CO2) in exhaled breath displayed as a numerical value and a waveform. The numerical value is the end-tidal CO2 (EtCO2), or the maximum CO2 concentration at the end of each tidal breath [1,2]. The CO2 waveform, or capnogram, displays changes in the CO2 concentration over the respiratory cycle (Figure 1). Trends in EtCO2 values can be used to assess disease severity and response to treatment, whereas changes in the shape of the capnogram are diagnostic of disease conditions, much like in electrocardiography. 82
Technology Capnography became a routine part of anesthesia practice in the United States in the 1980s. Modern capnography technology is built on infrared (IR) radiation techniques. This is based on the principle that CO2 absorbs a unique wavelength of IR radiation, with the amount of radiation absorbed dependent on the amount of CO2 present in the breath sample. Detecting changes in IR radiation levels with photoelectric detectors allows for calculation of the CO2 concentration in the gas sample [1-3]. Capnography measures gas concentration using 1 of 2 configurations: mainstream or sidestream. Mainstream devices measure CO2 directly from the airway, with the sensor located directly within the airway circuit. Sidestream devices measure CO2 by aspirating a small sample from the exhaled breath through tubing leading to a remote sensor located inside the monitor. Because of the location of the sensor, mainstream systems are configured for intubated patients, whereas sidestream systems can be used in both intubated and nonintubated patients. The Division of Emergency Medicine, Children's Hospital Boston, Department of Pediatrics, Harvard Medical School, Boston, MA. Reprint requests and correspondence: Joshua Nagler, MD, Division of Emergency Medicine, Children's Hospital Boston, Department of Pediatrics, Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115. (E-mail: [email protected]) 1522-8401/$ - see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.cpem.2009.03.004
Capnographic monitoring in respiratory emergencies
Figure 1 The normal capnogram. A normal capnogram is characterized by four features: a characteristic rectangular shape, 4 distinct phases, a CO2 concentration that starts at zero and returns to zero (inhalation), and a maximum CO2 concentration (EtCO2) that is reached with each breath. Phase I (A-B) is the beginning of exhalation where the dead space is cleared. Phase II (B-C) represents the rapid rise in CO2 concentration as alveolar CO2 reaches the sensor. Phase III (CD) represents continuous alveolar gas, the concluding, maximum CO2 concentration known as the end-tidal carbon dioxide level. Phase IV (D-E) is the beginning of inhalation during which atmospheric air replaces alveolar CO2-rich air.
airway interface for intubated patients is an endotracheal tube airway adapter. For nonintubated patients, a nasaloral cannula is used, which allows CO2 sampling with concomitant low-flow oxygen delivery. The accuracy of sidestream devices depends on the flow rate, that is, the size of the breath sample required to obtain an accurate reading. Low-flow sidestream systems (50 mL/min) have a lower occlusion rate (from moisture or patient secretions), and because they require relatively smaller sampling, they are accurate in patients with low tidal volumes (eg, neonates and infants). In high-flow systems, when the tidal volume of the patient drops below the required 150 mL/min, the monitor will entrain room air to compensate, falsely diluting the EtCO2.
Physiology The capnogram corresponds to a single tidal breath and consists of 4 distinct phases (Figure 1). Phase I (A-B) is the beginning of exhalation where the dead space is cleared from the upper airway. Phase II (B-C) represents the rapid rise in CO2 concentration as the CO2 from the alveoli reaches the upper airway. Phase III (C-D) occurs when the entire breath stream is alveolar gas and concludes when the CO2 concentration reaches a maximum value for that tidal breath (ie, the EtCO2). Phase IV (D-E) is the beginning of inhalation, where the CO2 concentration drops to zero as atmospheric air enters the airway [1,4]. A normal capnogram is characterized by 4 features: a characteristic rectangular shape, 4 distinct phases, a CO2 concentration that starts at zero (dead-space ventilation) and returns to zero (inhalation), and a maximum CO2 concentration (EtCO2) that is reached with each breath.
83 The amplitude of the capnogram is determined by the EtCO2 value and the width is determined by the expiratory time. Hyperventilation (increased respiratory rate, decreased EtCO2) results in a low amplitude and narrow capnogram, whereas bradypneic hypoventilation (decreased respiratory rate, increased EtCO2) results in a high amplitude and wide waveform (Table 1). Patients with normal lung function will have an EtCO2 between 35 and 45 mm Hg and a narrow EtCO2-PaCO2 gradient (0-5 mm Hg). In the normal state, the EtCO2 is slightly lower than the PaCO2 because of alveolar dead space [5]. In patients with abnormal lung function secondary to ventilation-perfusion (V/Q) mismatch, the gradient will widen with the extent of the gradient dependent on the severity of the lung disease and the relative contributions of various V/Q units of the lungs (Figure 2). In these situations, the EtCO2 may be useful for trending ventilatory status over time but not as a single number spot check that may or may not correlate well with the PaCO2 [6,7]. Increased EtCO2 readings equate with increased PaCO2; therefore, in patients without chronic hypoventilation, EtCO2 greater than 70 mm Hg indicates respiratory failure.
Detection of Adverse Airway or Respiratory Events Capnography provides not only a continuous, real-time assessment of CO2 exchange but also of respiratory rate. Because CO2 measurements are taken directly from the airway (via oral-nasal cannula), capnographic respiratory rate is more accurate than impedance-based respiratory monitoring and even clinical assessment. In patients with obstructive apnea, capnography will show a flatline waveform indicating no ventilation, whereas impedancebased monitoring will interpret the chest wall movement as a valid breath despite the absence of ventilation. Similarly, capnography has been found to be more sensitive than physician detection of apnea. In a recent study, 26% (10/ 39) of patients experienced 20-second periods of apnea during monitored anesthesia. All episodes of apnea were detected by capnography but not by the anesthesia providers [8]. Both central and obstructive apnea can be rapidly detected by capnography. Loss of the capnogram, in conjunction with absence of breath sounds and no chest wall movement or breath sounds, confirms the diagnosis of central apnea. This can be particularly helpful in patients with respiratory illnesses that predispose to central apnea, such as infants with RSV bronchiolitis and pertussis. Obstructive apnea is similarly characterized by loss of the capnogram and breath sounds, although chest wall movement is preserved. To further differentiate laryngospasm from upper airway obstruction as the etiology of obstructive apnea, airway alignment maneuvers can be performed (Table 1). Laryngospasm will not
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84 Table 1
Waveform analysis.
RR indicates respiratory rate. Sp02 indicates oxygen saturation measured by pulse oximetry. *Varying waveform amplitude and width. **Depending on duration and severity of bronchospasm. ***Depending on duration of episode.
respond to jaw thrust or repositioning, whereas anatomic soft tissue obstruction, usually caused by the tongue or soft palate, will be corrected.
Recognition of airway or respiratory compromise using capnography is not limited to central and obstructive apneic events. It can also rapidly identify
Capnographic monitoring in respiratory emergencies
85
Figure 2 Effect of ventilation/perfusion (V/Q) mismatching on EtCO2. The end-tidal partial pressure of carbon dioxide (PETCO2) is determined by the relative contribution of many alveoli, and the PaCO2 of individual alveoli is determined by its V/Q. Partial pressure of carbon dioxide in the alveoli decreases as V/Q increases. At the extremes, a very high (∞) or very low (0) V/Q will result in a PaCO2 of zero. Larger proportions of V/Q mismatched space will result in a lower PETCO2 and a larger gradient between measured PETCO2 and PaCO2. With relatively little contribution from those alveoli demonstrated here, the gradient (40 to 36 mm Hg = 4 mm Hg) is not significant. Data from Hess (Principles and practice of intensive care monitoring. New York, McGraw-Hill; 1998).
bronchospasm and respiratory failure (Table 1) and has been demonstrated to detect a deterioration in ventilation well before pulse oximetry will demonstrate a falling oxyhemoglobin saturation [9,10]. This is especially important in infants and toddlers with a smaller functional residual capacity and greater oxygen consumption, who may rapidly deteriorate once hypoxemia occurs.
Verification of Endotracheal Tube Placement and Monitoring of Tube Position Patients with significant respiratory distress or respiratory failure may require intubation to support oxygenation and/or ventilation. Capnography is considered standard of care in confirming endotracheal tube placement because it is rapid, practical, and reliable [11,12]. In patients with a pulse, it has been shown to be more accurate than traditional confirmation methods, including auscultation, chest rise, and tube condensation, and more rapid than oximetry in identifying esophageal intubations [13-18]. In addition, it can be used for continuous monitoring of tube position, which has been shown to be particularly important during transport in the prehospital setting [19-21]. Capnography has additional utility in the management of respiratory diseases beyond the prominent role in intubation. As a continuous, dynamic measure of ventilatory status, capnography may predict severity of disease,
facilitate therapeutic decisions, evaluate response to therapy, or guide patient disposition.
Obstructive Lung Disease: Utility of EtCO2 Values Traditional measures to establish the severity of acute asthma exacerbations have included clinical scoring systems, spirometry, blood gas analysis, and oxygen saturation. Although these offer valuable information, each has limitations. Asthma scoring systems can be useful, although many are based on subjective measures, may be limited to select age groups, and often have limited validity testing [22-24]. Spirometry provides an objective, quantitative measure of the degree of airway obstruction but is effort dependent and requires coordination and cooperation. This may not be feasible in younger children and patients in acute respiratory distress and may not be accurate in some circumstances [25,26]. Blood gas sampling is painful and can be difficult to perform in infants. Furthermore, results from a single point in time have limited value over the course of an ED visit. Finally, pulse oximetry provides a measure of oxygenation but does not assess ventilation and is not a consistent predictor of need for hospitalization [27,28]. Capnography, on the other hand, is effort independent and noninvasive and can be trended over time. To be an effective tool in assessment of patients with acute asthma, EtCO2 must accurately reflect ventilation.
86 Studies in the 1990s first established the correlation between EtCO2 and PaCO2 in otherwise healthy patients of varying ages [29-31]. Subsequently, studies in both children and adults found that a strong correlation was maintained in patients with respiratory diseases and that in most circumstances, EtCO2 values were 2 to 5 mm Hg less than simultaneous arterial samples [32-34]. Further investigation demonstrated that this correlation was not affected by respiratory rate, although it can be affected by disease severity [30-35]. Concurrent changes in capnogram shape will alert the provider to cases where the recorded EtCO2 value may be inaccurate because of severe disease. Given the established correlation between EtCO2 and PaCO2, recent research has focused on identifying the clinical utility of capnography in patients with respiratory disease such as asthma, in particular. Langhan and colleagues [36] performed a prospective, observational study using EtCO2 values (without waveform) to assess the severity of asthma exacerbations. They used vital signs and a clinical asthma score as markers for severity and found that patients with asthma exacerbations had lower EtCO2 values than age-matched controls. Furthermore, lower EtCO2 values were associated with lower oxygen saturation levels, higher respiratory rates, and higher severity scores, corroborating a relationship between EtCO2 and traditional markers for disease. In this study, initial EtCO2 values were not found to be helpful in predicting disposition, although EtCO2 values obtained after the first and last treatment were associated with hospital admission and number of bronchodilator treatments required. The authors conclude that “EtCO2 may prove to be a useful adjunct in management of patients presenting to ED” [36]. Guthrie et al [37] prospectively investigated the relationship between EtCO2 values (again without waveform) and 3 measures of disease severity: a pediatric asthma severity score, peak expiratory flow rate (PEFR), and patient disposition. End-tidal CO2 at presentation was similar in groups of patients with mild-moderate vs severe disease as categorized by pediatric asthma severity score and PEFR. Similarly, EtCO2 values were only slightly different in patients who were admitted compared with those who were discharged. There was a trend toward lower EtCO2 values after bronchodilator therapy, although this did not achieve statistical significance or have practical utility. Therefore, capnometry (EtCO2 values without waveform) alone did not seem to have clinical value in distinguishing severity of disease and response to therapy, or in predicting disposition in this population [37]. This limited clinical utility of EtCO2 values alone likely results from 2 factors. First, these values need to be interpreted in a clinical context, not dissimilar to data from blood gas analysis. An arterial blood gas with a PaCO2 of 45 mm Hg would be interpreted differently in a comfortable patient with asthma with a respiratory rate of
J. Nagler, B. Krauss 20 breaths per minute than if the same patient were breathing at 60 breaths per minute with severe retractions and showing signs of tiring. Likewise, capnometry cannot by itself determine severity without being interpreted in the context of the patient's clinical status, unless values are significantly increased suggesting respiratory failure. Second, increased V/Q mismatching can result in a falsely low EtCO2, which can affect the utility of these values in some cases [5,38,39]. To address these limitations, recent work has focused on studying the shape of the capnogram as a more precise means of estimating severity of airway obstruction than EtCO2 values alone.
Obstructive Lung Disease: Utility of the Capnogram Patients with obstructive lung disease have V/Q mismatching that results in a characteristic capnogram with a curved ascending phase (phase II) and an upsloping alveolar plateau (phase III) (Figure 1). The curved ascending phase results from slowed passage of air devoid of CO2 from the anatomic dead space, before detecting CO2-rich gas from the alveoli, a process that occurs very quickly in a healthy lung. The characteristic sloped appearance of the alveolar plateau in obstructive disease corresponds to uneven emptying of alveoli as unobstructed alveoli empty ahead of obstructed and hypoventilated ones, leading to a progressive increase in CO2 concentration throughout phase III. Systematic identification of these characteristic changes can be used to estimate severity of disease and potentially influence management. The first studies investigating capnogram shape in persons with asthma were performed nearly 50 years ago [1,40,41]. However, with the recent widespread availability of EtCO2 monitors, there has been a renewed interest in the clinical application of capnographic assessment. In a European study, significant differences in 8 indices related to capnogram shape were found between a group of known persons with asthma and control subjects. In particular, an increase in slope and length of the alveolar plateau (phase III), as well as a widening of the Q angle (Figure 1), was identified in those with asthma exacerbations. Subanalysis showed greater differences in those with more severe disease as measured by forced expiratory volume in 1 second (FEV1), suggesting that these findings were accurately reflecting the degree of airway obstruction [42]. A similar study from the United States also compared capnographic indices with spirometric evaluation. Again, the alveolar plateau slope was demonstrated to be steeper in persons with asthma versus normal controls. This change in slope correlated with PEFR, and both showed corresponding improvement after bronchodilator therapy [43]. Kunkov and colleagues [44] prospectively evaluated quantitative indices of the capnogram in pediatric patients with asthma to determine their utility in predicting
Capnographic monitoring in respiratory emergencies admission. They investigated the Q angle (see Fig 1) as well as a novel measurement called the EtCO2 ratio. This ratio is the length of the alveolar plateau divided by the respiratory rate, a means to account for any influence of tachypnea on capnogram shape. Significant differences were found between admitted and discharged patients for both the EtCO2 ratio and the Q angle. A sensitivity of an EtCO2 ratio of less than 0.15 for admission was 83%, and the specificity was 68%. Even after controlling for asthma severity, as determined by the same severity score used by Langhan et al, the adjusted odds of being hospitalized if baseline EtCO2 ratio was less than 0.15 was 18.8 (95% confidence interval, 1.9-184.7) [44]. Unlike capnometry, where EtCO2 values alone were not associated with hospitalization, specific quantitative capnogram parameters seem to be more promising. The difficulty with these various capnographic assessments is that they require measurements and calculations that are not easily performed in the ED setting. To address this challenge, Krauss and colleagues developed a computer program to identify and measure key quantitative features of the capnogram. Using capnograms from adult patients with known chronic obstructive pulmonary disease (COPD) and control subjects, they were able to differentiate those with obstructive disease from those with restrictive or no disease. Quantitative capnogram parameters were significantly lower in patients with obstructive disease, and the magnitude of the differences corresponded to severity of obstruction as measured by FEV1 [45]. The application of capnography to the management of patients with obstructive lung disease is relatively recent, and its potential utility requires further elucidation. With further research on quantitative methodology, one could envision a patient with asthma or other obstructive lung disease being evaluated over a period of several breaths with capnographic monitoring. The resultant waveforms could then be analyzed by a program embedded within Table 2
Etiologies of falsely low EtCO2 recordings.
Hypopneic hypoventilation
Entrainment of room air Disrupted airway circuit
Severe V/Q mismatch “shark fin” appearance in tachypneic patients
87 the capnograph, and a waveform with preliminary interpretation could be generated, similar to the model of patients with chest pain and electrocardiographic evaluation. This information could become part of an asthma severity score or other clinical decision rule that might influence further management.
Respiratory Illnesses Outside of the ED Capnography has demonstrated utility in respiratory diseases that are likely to be treated outside of the ED as well. For example, the American Thoracic Society has endorsed the use of continuous capnography during pneumograms for inpatient sleep studies and with home monitoring devices in the diagnosis and management of obstructive sleep apnea [46-48]. In the intensive care unit setting, volumetric capnography has become integrated into ventilatory management. By adding flow measurement to EtCO2 monitoring, dead-space ventilation can be discerned from alveolar ventilation. This has been shown to have clinical benefit in mechanically ventilated patients [49]. In particular, it can be used to optimize tidal volumes while minimizing ventilator-induced lung injury, particularly in patients with acute respiratory distress syndrome (ARDS) [50]. Similarly, by providing information on functional residual capacity, it can help guide ventilator recruitment strategies. Finally, for those patients with reversible lung disease who are weaning from ventilatory support, changes in the EtCO2-PaCO2 gradient can be used to help determine the optimal time for extubation [51]. Although largely used in intubated patients, volumetric capnography has recently been shown to be applicable to spontaneously breathing patients using sidestream technology as well [52]. Although there are little data on its use in nonintubated patients with respiratory illness, initial ED studies have shown clinical applicability with other disease processes. In particular, patients with profound V/Q
J. Nagler, B. Krauss
88 mismatch from pulmonary embolism have been diagnosed using the area under the curve of the spirogram (volumetric capnography waveform) [53]. It is therefore conceivable that in the future, this technique of assessing dead-space ventilation and degree of V/Q mismatching may prove useful in managing a spectrum of patients in the ED.
Practical Tips Using EtCO2 monitoring devices is usually straightforward. However, in select circumstances, the quality of the readings may be compromised. In this section, we discuss a few practical tips for reliable acquisition and interpretation of EtCO2 data. The airway interface for spontaneously breathing patients (oral-nasal cannula) is generally well tolerated. Prior studies have shown that only 3% to 4% of children are unable to tolerate the cannula during study protocols [36,54]. However, because capnography provides an instantaneous reflection of ventilation status on a breathby-breath basis, it is often necessary to allow a child to “get used to” the cannula before interpreting results. That is, anxiety or discomfort when the cannula is initially placed may cause crying or hyperventilation, which can give falsely low values. Waiting 10 to 15 seconds after return to a baseline respiratory pattern is important in these patients to establish accurate EtCO2 values. For patients who require continuous positive airway pressure or bag-mask ventilation, running a cannula beneath the mask will occasionally interrupt the seal. To avoid such disruption, an inline probe can be connected between the mask and the elbow (similar to the attachment to an endotracheal tube), which allows continuous readings from within the closed airway circuit. This method can also be helpful in mask-ventilated patients who have nasopharyngeal airways in place, in whom nasal cannula cannot be appropriately positioned. Finally, interpretation of EtCO2 values is most valid when simultaneous waveforms are available to ensure an accurate tracing, as is true with pulse oximetry. In addition, the shape of the capnogram can help discriminate if a low EtCO2 value is accurately representing a low PaCO2 value, or if it is falsely low because of an increased EtCO2-PaCO2 gradient. The 3 most common causes for an EtCO2-PaCO2 discordance can be identified by its unique waveforms, as shown in Table 2. First, hypopneic hypoventilation describes ineffective shallow breathing of dead space, which occurs most commonly in sedated, obtunded, or seizing patients. Second, any disruption of the airway circuit or insufficient gas flow can allow entrainment of CO2-poor room air, which can dilute the detected CO2 concentration at the sensor. If positive pressure ventilation is being provided, one can see the abrupt upstroke just at the end of the alveolar plateau, which corresponds to the entrained room air being pushed out of circuit immediately before the next inspiratory cycle. Finally, V/Q mismatch itself will create a gradient between PaCO2 and EtCO2. In addition,
obstructive disease can result in a delay in the highest alveolar CO2 concentration reaching the cannula. Because these patients are often tachypneic, they will frequently inspire while the expired CO2 concentration is still rising, therefore never reaching the maximum alveolar CO2. This gives a described “shark fin” appearance on the capnogram, whereby the highest obtained value during expiration (EtCO2) is still lower than the simultaneous PaCO2; that is, the PaCO2-EtCO2 gradient is high. These predictable changes in the waveform can alert the practitioner to the probability and cause of falsely low EtCO2 readings.
Summary Capnography provides a noninvasive, real-time means for assessing ventilatory status, just as pulse oximetry reflects a patient's oxygenation. The widespread availability of portable and bedside end-tidal monitors has allowed for increased use in EDs. Although capnography is best known as a rapid and accurate means for confirming endotracheal intubation, recent studies have supported its utility in managing nonintubated pediatric patients with respiratory illnesses. In particular, waveform analysis in patients with obstructive lung disease may offer valuable information regarding disease severity and disposition. In addition, the introduction of computerized analysis programs has paved the way for the development of increased systematic and automated interpretation of capnographic data. In the future, we are likely to see growing clinical application of this technology as an integral component in the evaluation of patients presenting to the ED with significant respiratory illness.
Acknowledgments Dr Krauss is a consultant for Oridion Medical, a capnography company, and holds 2 patents in the area of capnography.
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89 31. Tobias JD, Flanagan JF, Wheeler TJ, et al. Noninvasive monitoring of end-tidal CO2 via nasal cannulas in spontaneously breathing children during the perioperative period. Crit Care Med 1994;22:1805-8. 32. Abramo TJ, Wiebe RA, Scott SM, et al. Noninvasive capnometry in a pediatric population with respiratory emergencies. Pediatr Emerg Care 1996;12:252-4. 33. Plewa MC, Sikora S, Engoren M, et al. Evaluation of capnography in nonintubated emergency department patients with respiratory distress. Acad Emerg Med 1995;2:901-8. 34. Corbo J, Bijur P, Lahn M, et al. Concordance between capnography and arterial blood gas measurements of carbon dioxide in acute asthma. Ann Emerg Med 2005;46:323-7. 35. Pianosi P, Hochman J. End-tidal estimates of arterial PCO2 for cardiac output measurement by CO2 rebreathing: a study in patients with cystic fibrosis and healthy controls. Pediatr Pulmonol 1996;22: 154-60. 36. Langhan ML, Zonfrillo MR, Spiro DM. Quantitative end-tidal carbon dioxide in acute exacerbations of asthma. J Pediatr 2008;152:829-32. 37. Guthrie BD, Adler MD, Powell EC. End-tidal carbon dioxide measurements in children with acute asthma. Acad Emerg Med 2007;14:1135-40. 38. Hess D. Capnometry. In: Tobin MJ, editor. Principles and practice of intensive care monitoring. New York (NY): McGraw-Hill; 1998. 39. van Meerten RJ. Expiratory gas concentration curves for examination of uneven distribution of ventilation and perfusion in the lung. First communication: theory. Respiration 1970;27:552-64. 40. Muranyi L, Osvath P, Uhl K, et al. Continuous registration of the CO2 contents in expired air (capnography) in the inhalative provocation of children. I. Acetylcholine provocation of asthmatic school-age children. Acta Paediatr Acad Sci Hung 1969;10:133-54. 41. Poppius H. Expiratory CO2 curve in pulmonary diseases. Scand J Respir Dis 1969;50:135-46. 42. You B, Peslin R, Duvivier C, et al. Expiratory capnography in asthma: evaluation of various shape indices. Eur Respir J 1994;7:318-23. 43. Yaron M, Padyk P, Hutsinpiller M, et al. Utility of the expiratory capnogram in the assessment of bronchospasm. Ann Emerg Med 1996;28:403-7. 44. Kunkov S, Pinedo V, Silver EJ, et al. Predicting the need for hospitalization in acute childhood asthma using end-tidal capnography. Pediatr Emerg Care 2005;21:574-7. 45. Krauss B, Deykin A, Lam A, et al. Capnogram shape in obstructive lung disease. Anesth Analg 2005;100:884-8. 46. Smith TC, Proops DW, Pearman K, et al. Nasal capnography in children: automated analysis provides a measure of obstruction during sleep. Clin Otolaryngol Allied Sci 1993;18:69-71. 47. Magnan A, Philip-Joet F, Rey M, et al. End-tidal CO2 analysis in sleep apnea syndrome. Conditions for use. Chest 1993;103:129-31. 48. American Thoracic Society. Standards and indications for cardiopulmonary sleep studies in children. Am J Respir Crit Care Med 1996; 153:866-78. 49. Blanch L, Romero PV, Lucangelo U. Volumetric capnography in the mechanically ventilated patient. Minerva Anestesiol 2006;72:577-85. 50. Lucangelo U, Bernabe F, Vatua S, et al. Prognostic value of different dead space indices in mechanically ventilated patients with acute lung injury and ARDS. Chest 2008;133:62-71. 51. Maslow A, Stearns G, Bert A, et al. Monitoring end-tidal carbon dioxide during weaning from cardiopulmonary bypass in patients without significant lung disease. Anesth Analg 2001;92:306-13. 52. Verschuren F, Heinonen E, Clause D, et al. Volumetric capnography: reliability and reproducibility in spontaneously breathing patients. Clin Physiol Funct Imaging 2005;25:275-80. 53. Kline JA, Arunachlam M. Preliminary study of the capnogram waveform area to screen for pulmonary embolism. Ann Emerg Med 1998;32(3 Pt 1):289-96. 54. Nagler J, Wright RO, Krauss B. End-tidal carbon dioxide as a measure of acidosis among children with gastroenteritis. Pediatrics 2006;118: 260-7.
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espiratory diseases leading to abnormalities in oxygenation and ventilation are common in the emergency department (ED). Although oxygenation monitoring with pulse oximetry is routine for children in respiratory distress, noninvasive ventilation monitoring is not. Although capnography is a standard of care for verification of endotracheal tube placement and monitoring of tube position, it is just beginning to be used to assess ventilatory status in spontaneously breathing patients. We discuss the use of capnography for noninvasive monitoring children with respiratory illnesses.
Definitions Capnography is the noninvasive measurement of the partial pressure of carbon dioxide (CO2) in exhaled breath displayed as a numerical value and a waveform. The numerical value is the end-tidal CO2 (EtCO2), or the maximum CO2 concentration at the end of each tidal breath [1,2]. The CO2 waveform, or capnogram, displays changes in the CO2 concentration over the respiratory cycle (Figure 1). Trends in EtCO2 values can be used to assess disease severity and response to treatment, whereas changes in the shape of the capnogram are diagnostic of disease conditions, much like in electrocardiography. 82
Technology Capnography became a routine part of anesthesia practice in the United States in the 1980s. Modern capnography technology is built on infrared (IR) radiation techniques. This is based on the principle that CO2 absorbs a unique wavelength of IR radiation, with the amount of radiation absorbed dependent on the amount of CO2 present in the breath sample. Detecting changes in IR radiation levels with photoelectric detectors allows for calculation of the CO2 concentration in the gas sample [1-3]. Capnography measures gas concentration using 1 of 2 configurations: mainstream or sidestream. Mainstream devices measure CO2 directly from the airway, with the sensor located directly within the airway circuit. Sidestream devices measure CO2 by aspirating a small sample from the exhaled breath through tubing leading to a remote sensor located inside the monitor. Because of the location of the sensor, mainstream systems are configured for intubated patients, whereas sidestream systems can be used in both intubated and nonintubated patients. The Division of Emergency Medicine, Children's Hospital Boston, Department of Pediatrics, Harvard Medical School, Boston, MA. Reprint requests and correspondence: Joshua Nagler, MD, Division of Emergency Medicine, Children's Hospital Boston, Department of Pediatrics, Harvard Medical School, 300 Longwood Avenue, Boston, MA 02115. (E-mail: [email protected]) 1522-8401/$ - see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.cpem.2009.03.004
Capnographic monitoring in respiratory emergencies
Figure 1 The normal capnogram. A normal capnogram is characterized by four features: a characteristic rectangular shape, 4 distinct phases, a CO2 concentration that starts at zero and returns to zero (inhalation), and a maximum CO2 concentration (EtCO2) that is reached with each breath. Phase I (A-B) is the beginning of exhalation where the dead space is cleared. Phase II (B-C) represents the rapid rise in CO2 concentration as alveolar CO2 reaches the sensor. Phase III (CD) represents continuous alveolar gas, the concluding, maximum CO2 concentration known as the end-tidal carbon dioxide level. Phase IV (D-E) is the beginning of inhalation during which atmospheric air replaces alveolar CO2-rich air.
airway interface for intubated patients is an endotracheal tube airway adapter. For nonintubated patients, a nasaloral cannula is used, which allows CO2 sampling with concomitant low-flow oxygen delivery. The accuracy of sidestream devices depends on the flow rate, that is, the size of the breath sample required to obtain an accurate reading. Low-flow sidestream systems (50 mL/min) have a lower occlusion rate (from moisture or patient secretions), and because they require relatively smaller sampling, they are accurate in patients with low tidal volumes (eg, neonates and infants). In high-flow systems, when the tidal volume of the patient drops below the required 150 mL/min, the monitor will entrain room air to compensate, falsely diluting the EtCO2.
Physiology The capnogram corresponds to a single tidal breath and consists of 4 distinct phases (Figure 1). Phase I (A-B) is the beginning of exhalation where the dead space is cleared from the upper airway. Phase II (B-C) represents the rapid rise in CO2 concentration as the CO2 from the alveoli reaches the upper airway. Phase III (C-D) occurs when the entire breath stream is alveolar gas and concludes when the CO2 concentration reaches a maximum value for that tidal breath (ie, the EtCO2). Phase IV (D-E) is the beginning of inhalation, where the CO2 concentration drops to zero as atmospheric air enters the airway [1,4]. A normal capnogram is characterized by 4 features: a characteristic rectangular shape, 4 distinct phases, a CO2 concentration that starts at zero (dead-space ventilation) and returns to zero (inhalation), and a maximum CO2 concentration (EtCO2) that is reached with each breath.
83 The amplitude of the capnogram is determined by the EtCO2 value and the width is determined by the expiratory time. Hyperventilation (increased respiratory rate, decreased EtCO2) results in a low amplitude and narrow capnogram, whereas bradypneic hypoventilation (decreased respiratory rate, increased EtCO2) results in a high amplitude and wide waveform (Table 1). Patients with normal lung function will have an EtCO2 between 35 and 45 mm Hg and a narrow EtCO2-PaCO2 gradient (0-5 mm Hg). In the normal state, the EtCO2 is slightly lower than the PaCO2 because of alveolar dead space [5]. In patients with abnormal lung function secondary to ventilation-perfusion (V/Q) mismatch, the gradient will widen with the extent of the gradient dependent on the severity of the lung disease and the relative contributions of various V/Q units of the lungs (Figure 2). In these situations, the EtCO2 may be useful for trending ventilatory status over time but not as a single number spot check that may or may not correlate well with the PaCO2 [6,7]. Increased EtCO2 readings equate with increased PaCO2; therefore, in patients without chronic hypoventilation, EtCO2 greater than 70 mm Hg indicates respiratory failure.
Detection of Adverse Airway or Respiratory Events Capnography provides not only a continuous, real-time assessment of CO2 exchange but also of respiratory rate. Because CO2 measurements are taken directly from the airway (via oral-nasal cannula), capnographic respiratory rate is more accurate than impedance-based respiratory monitoring and even clinical assessment. In patients with obstructive apnea, capnography will show a flatline waveform indicating no ventilation, whereas impedancebased monitoring will interpret the chest wall movement as a valid breath despite the absence of ventilation. Similarly, capnography has been found to be more sensitive than physician detection of apnea. In a recent study, 26% (10/ 39) of patients experienced 20-second periods of apnea during monitored anesthesia. All episodes of apnea were detected by capnography but not by the anesthesia providers [8]. Both central and obstructive apnea can be rapidly detected by capnography. Loss of the capnogram, in conjunction with absence of breath sounds and no chest wall movement or breath sounds, confirms the diagnosis of central apnea. This can be particularly helpful in patients with respiratory illnesses that predispose to central apnea, such as infants with RSV bronchiolitis and pertussis. Obstructive apnea is similarly characterized by loss of the capnogram and breath sounds, although chest wall movement is preserved. To further differentiate laryngospasm from upper airway obstruction as the etiology of obstructive apnea, airway alignment maneuvers can be performed (Table 1). Laryngospasm will not
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84 Table 1
Waveform analysis.
RR indicates respiratory rate. Sp02 indicates oxygen saturation measured by pulse oximetry. *Varying waveform amplitude and width. **Depending on duration and severity of bronchospasm. ***Depending on duration of episode.
respond to jaw thrust or repositioning, whereas anatomic soft tissue obstruction, usually caused by the tongue or soft palate, will be corrected.
Recognition of airway or respiratory compromise using capnography is not limited to central and obstructive apneic events. It can also rapidly identify
Capnographic monitoring in respiratory emergencies
85
Figure 2 Effect of ventilation/perfusion (V/Q) mismatching on EtCO2. The end-tidal partial pressure of carbon dioxide (PETCO2) is determined by the relative contribution of many alveoli, and the PaCO2 of individual alveoli is determined by its V/Q. Partial pressure of carbon dioxide in the alveoli decreases as V/Q increases. At the extremes, a very high (∞) or very low (0) V/Q will result in a PaCO2 of zero. Larger proportions of V/Q mismatched space will result in a lower PETCO2 and a larger gradient between measured PETCO2 and PaCO2. With relatively little contribution from those alveoli demonstrated here, the gradient (40 to 36 mm Hg = 4 mm Hg) is not significant. Data from Hess (Principles and practice of intensive care monitoring. New York, McGraw-Hill; 1998).
bronchospasm and respiratory failure (Table 1) and has been demonstrated to detect a deterioration in ventilation well before pulse oximetry will demonstrate a falling oxyhemoglobin saturation [9,10]. This is especially important in infants and toddlers with a smaller functional residual capacity and greater oxygen consumption, who may rapidly deteriorate once hypoxemia occurs.
Verification of Endotracheal Tube Placement and Monitoring of Tube Position Patients with significant respiratory distress or respiratory failure may require intubation to support oxygenation and/or ventilation. Capnography is considered standard of care in confirming endotracheal tube placement because it is rapid, practical, and reliable [11,12]. In patients with a pulse, it has been shown to be more accurate than traditional confirmation methods, including auscultation, chest rise, and tube condensation, and more rapid than oximetry in identifying esophageal intubations [13-18]. In addition, it can be used for continuous monitoring of tube position, which has been shown to be particularly important during transport in the prehospital setting [19-21]. Capnography has additional utility in the management of respiratory diseases beyond the prominent role in intubation. As a continuous, dynamic measure of ventilatory status, capnography may predict severity of disease,
facilitate therapeutic decisions, evaluate response to therapy, or guide patient disposition.
Obstructive Lung Disease: Utility of EtCO2 Values Traditional measures to establish the severity of acute asthma exacerbations have included clinical scoring systems, spirometry, blood gas analysis, and oxygen saturation. Although these offer valuable information, each has limitations. Asthma scoring systems can be useful, although many are based on subjective measures, may be limited to select age groups, and often have limited validity testing [22-24]. Spirometry provides an objective, quantitative measure of the degree of airway obstruction but is effort dependent and requires coordination and cooperation. This may not be feasible in younger children and patients in acute respiratory distress and may not be accurate in some circumstances [25,26]. Blood gas sampling is painful and can be difficult to perform in infants. Furthermore, results from a single point in time have limited value over the course of an ED visit. Finally, pulse oximetry provides a measure of oxygenation but does not assess ventilation and is not a consistent predictor of need for hospitalization [27,28]. Capnography, on the other hand, is effort independent and noninvasive and can be trended over time. To be an effective tool in assessment of patients with acute asthma, EtCO2 must accurately reflect ventilation.
86 Studies in the 1990s first established the correlation between EtCO2 and PaCO2 in otherwise healthy patients of varying ages [29-31]. Subsequently, studies in both children and adults found that a strong correlation was maintained in patients with respiratory diseases and that in most circumstances, EtCO2 values were 2 to 5 mm Hg less than simultaneous arterial samples [32-34]. Further investigation demonstrated that this correlation was not affected by respiratory rate, although it can be affected by disease severity [30-35]. Concurrent changes in capnogram shape will alert the provider to cases where the recorded EtCO2 value may be inaccurate because of severe disease. Given the established correlation between EtCO2 and PaCO2, recent research has focused on identifying the clinical utility of capnography in patients with respiratory disease such as asthma, in particular. Langhan and colleagues [36] performed a prospective, observational study using EtCO2 values (without waveform) to assess the severity of asthma exacerbations. They used vital signs and a clinical asthma score as markers for severity and found that patients with asthma exacerbations had lower EtCO2 values than age-matched controls. Furthermore, lower EtCO2 values were associated with lower oxygen saturation levels, higher respiratory rates, and higher severity scores, corroborating a relationship between EtCO2 and traditional markers for disease. In this study, initial EtCO2 values were not found to be helpful in predicting disposition, although EtCO2 values obtained after the first and last treatment were associated with hospital admission and number of bronchodilator treatments required. The authors conclude that “EtCO2 may prove to be a useful adjunct in management of patients presenting to ED” [36]. Guthrie et al [37] prospectively investigated the relationship between EtCO2 values (again without waveform) and 3 measures of disease severity: a pediatric asthma severity score, peak expiratory flow rate (PEFR), and patient disposition. End-tidal CO2 at presentation was similar in groups of patients with mild-moderate vs severe disease as categorized by pediatric asthma severity score and PEFR. Similarly, EtCO2 values were only slightly different in patients who were admitted compared with those who were discharged. There was a trend toward lower EtCO2 values after bronchodilator therapy, although this did not achieve statistical significance or have practical utility. Therefore, capnometry (EtCO2 values without waveform) alone did not seem to have clinical value in distinguishing severity of disease and response to therapy, or in predicting disposition in this population [37]. This limited clinical utility of EtCO2 values alone likely results from 2 factors. First, these values need to be interpreted in a clinical context, not dissimilar to data from blood gas analysis. An arterial blood gas with a PaCO2 of 45 mm Hg would be interpreted differently in a comfortable patient with asthma with a respiratory rate of
J. Nagler, B. Krauss 20 breaths per minute than if the same patient were breathing at 60 breaths per minute with severe retractions and showing signs of tiring. Likewise, capnometry cannot by itself determine severity without being interpreted in the context of the patient's clinical status, unless values are significantly increased suggesting respiratory failure. Second, increased V/Q mismatching can result in a falsely low EtCO2, which can affect the utility of these values in some cases [5,38,39]. To address these limitations, recent work has focused on studying the shape of the capnogram as a more precise means of estimating severity of airway obstruction than EtCO2 values alone.
Obstructive Lung Disease: Utility of the Capnogram Patients with obstructive lung disease have V/Q mismatching that results in a characteristic capnogram with a curved ascending phase (phase II) and an upsloping alveolar plateau (phase III) (Figure 1). The curved ascending phase results from slowed passage of air devoid of CO2 from the anatomic dead space, before detecting CO2-rich gas from the alveoli, a process that occurs very quickly in a healthy lung. The characteristic sloped appearance of the alveolar plateau in obstructive disease corresponds to uneven emptying of alveoli as unobstructed alveoli empty ahead of obstructed and hypoventilated ones, leading to a progressive increase in CO2 concentration throughout phase III. Systematic identification of these characteristic changes can be used to estimate severity of disease and potentially influence management. The first studies investigating capnogram shape in persons with asthma were performed nearly 50 years ago [1,40,41]. However, with the recent widespread availability of EtCO2 monitors, there has been a renewed interest in the clinical application of capnographic assessment. In a European study, significant differences in 8 indices related to capnogram shape were found between a group of known persons with asthma and control subjects. In particular, an increase in slope and length of the alveolar plateau (phase III), as well as a widening of the Q angle (Figure 1), was identified in those with asthma exacerbations. Subanalysis showed greater differences in those with more severe disease as measured by forced expiratory volume in 1 second (FEV1), suggesting that these findings were accurately reflecting the degree of airway obstruction [42]. A similar study from the United States also compared capnographic indices with spirometric evaluation. Again, the alveolar plateau slope was demonstrated to be steeper in persons with asthma versus normal controls. This change in slope correlated with PEFR, and both showed corresponding improvement after bronchodilator therapy [43]. Kunkov and colleagues [44] prospectively evaluated quantitative indices of the capnogram in pediatric patients with asthma to determine their utility in predicting
Capnographic monitoring in respiratory emergencies admission. They investigated the Q angle (see Fig 1) as well as a novel measurement called the EtCO2 ratio. This ratio is the length of the alveolar plateau divided by the respiratory rate, a means to account for any influence of tachypnea on capnogram shape. Significant differences were found between admitted and discharged patients for both the EtCO2 ratio and the Q angle. A sensitivity of an EtCO2 ratio of less than 0.15 for admission was 83%, and the specificity was 68%. Even after controlling for asthma severity, as determined by the same severity score used by Langhan et al, the adjusted odds of being hospitalized if baseline EtCO2 ratio was less than 0.15 was 18.8 (95% confidence interval, 1.9-184.7) [44]. Unlike capnometry, where EtCO2 values alone were not associated with hospitalization, specific quantitative capnogram parameters seem to be more promising. The difficulty with these various capnographic assessments is that they require measurements and calculations that are not easily performed in the ED setting. To address this challenge, Krauss and colleagues developed a computer program to identify and measure key quantitative features of the capnogram. Using capnograms from adult patients with known chronic obstructive pulmonary disease (COPD) and control subjects, they were able to differentiate those with obstructive disease from those with restrictive or no disease. Quantitative capnogram parameters were significantly lower in patients with obstructive disease, and the magnitude of the differences corresponded to severity of obstruction as measured by FEV1 [45]. The application of capnography to the management of patients with obstructive lung disease is relatively recent, and its potential utility requires further elucidation. With further research on quantitative methodology, one could envision a patient with asthma or other obstructive lung disease being evaluated over a period of several breaths with capnographic monitoring. The resultant waveforms could then be analyzed by a program embedded within Table 2
Etiologies of falsely low EtCO2 recordings.
Hypopneic hypoventilation
Entrainment of room air Disrupted airway circuit
Severe V/Q mismatch “shark fin” appearance in tachypneic patients
87 the capnograph, and a waveform with preliminary interpretation could be generated, similar to the model of patients with chest pain and electrocardiographic evaluation. This information could become part of an asthma severity score or other clinical decision rule that might influence further management.
Respiratory Illnesses Outside of the ED Capnography has demonstrated utility in respiratory diseases that are likely to be treated outside of the ED as well. For example, the American Thoracic Society has endorsed the use of continuous capnography during pneumograms for inpatient sleep studies and with home monitoring devices in the diagnosis and management of obstructive sleep apnea [46-48]. In the intensive care unit setting, volumetric capnography has become integrated into ventilatory management. By adding flow measurement to EtCO2 monitoring, dead-space ventilation can be discerned from alveolar ventilation. This has been shown to have clinical benefit in mechanically ventilated patients [49]. In particular, it can be used to optimize tidal volumes while minimizing ventilator-induced lung injury, particularly in patients with acute respiratory distress syndrome (ARDS) [50]. Similarly, by providing information on functional residual capacity, it can help guide ventilator recruitment strategies. Finally, for those patients with reversible lung disease who are weaning from ventilatory support, changes in the EtCO2-PaCO2 gradient can be used to help determine the optimal time for extubation [51]. Although largely used in intubated patients, volumetric capnography has recently been shown to be applicable to spontaneously breathing patients using sidestream technology as well [52]. Although there are little data on its use in nonintubated patients with respiratory illness, initial ED studies have shown clinical applicability with other disease processes. In particular, patients with profound V/Q
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88 mismatch from pulmonary embolism have been diagnosed using the area under the curve of the spirogram (volumetric capnography waveform) [53]. It is therefore conceivable that in the future, this technique of assessing dead-space ventilation and degree of V/Q mismatching may prove useful in managing a spectrum of patients in the ED.
Practical Tips Using EtCO2 monitoring devices is usually straightforward. However, in select circumstances, the quality of the readings may be compromised. In this section, we discuss a few practical tips for reliable acquisition and interpretation of EtCO2 data. The airway interface for spontaneously breathing patients (oral-nasal cannula) is generally well tolerated. Prior studies have shown that only 3% to 4% of children are unable to tolerate the cannula during study protocols [36,54]. However, because capnography provides an instantaneous reflection of ventilation status on a breathby-breath basis, it is often necessary to allow a child to “get used to” the cannula before interpreting results. That is, anxiety or discomfort when the cannula is initially placed may cause crying or hyperventilation, which can give falsely low values. Waiting 10 to 15 seconds after return to a baseline respiratory pattern is important in these patients to establish accurate EtCO2 values. For patients who require continuous positive airway pressure or bag-mask ventilation, running a cannula beneath the mask will occasionally interrupt the seal. To avoid such disruption, an inline probe can be connected between the mask and the elbow (similar to the attachment to an endotracheal tube), which allows continuous readings from within the closed airway circuit. This method can also be helpful in mask-ventilated patients who have nasopharyngeal airways in place, in whom nasal cannula cannot be appropriately positioned. Finally, interpretation of EtCO2 values is most valid when simultaneous waveforms are available to ensure an accurate tracing, as is true with pulse oximetry. In addition, the shape of the capnogram can help discriminate if a low EtCO2 value is accurately representing a low PaCO2 value, or if it is falsely low because of an increased EtCO2-PaCO2 gradient. The 3 most common causes for an EtCO2-PaCO2 discordance can be identified by its unique waveforms, as shown in Table 2. First, hypopneic hypoventilation describes ineffective shallow breathing of dead space, which occurs most commonly in sedated, obtunded, or seizing patients. Second, any disruption of the airway circuit or insufficient gas flow can allow entrainment of CO2-poor room air, which can dilute the detected CO2 concentration at the sensor. If positive pressure ventilation is being provided, one can see the abrupt upstroke just at the end of the alveolar plateau, which corresponds to the entrained room air being pushed out of circuit immediately before the next inspiratory cycle. Finally, V/Q mismatch itself will create a gradient between PaCO2 and EtCO2. In addition,
obstructive disease can result in a delay in the highest alveolar CO2 concentration reaching the cannula. Because these patients are often tachypneic, they will frequently inspire while the expired CO2 concentration is still rising, therefore never reaching the maximum alveolar CO2. This gives a described “shark fin” appearance on the capnogram, whereby the highest obtained value during expiration (EtCO2) is still lower than the simultaneous PaCO2; that is, the PaCO2-EtCO2 gradient is high. These predictable changes in the waveform can alert the practitioner to the probability and cause of falsely low EtCO2 readings.
Summary Capnography provides a noninvasive, real-time means for assessing ventilatory status, just as pulse oximetry reflects a patient's oxygenation. The widespread availability of portable and bedside end-tidal monitors has allowed for increased use in EDs. Although capnography is best known as a rapid and accurate means for confirming endotracheal intubation, recent studies have supported its utility in managing nonintubated pediatric patients with respiratory illnesses. In particular, waveform analysis in patients with obstructive lung disease may offer valuable information regarding disease severity and disposition. In addition, the introduction of computerized analysis programs has paved the way for the development of increased systematic and automated interpretation of capnographic data. In the future, we are likely to see growing clinical application of this technology as an integral component in the evaluation of patients presenting to the ED with significant respiratory illness.
Acknowledgments Dr Krauss is a consultant for Oridion Medical, a capnography company, and holds 2 patents in the area of capnography.
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