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End-tidal carbon dioxide detectors--are they useful in children? MS BhendeChildren's Hospital of Pittsburgh, PA 15213-2583, USA., USA
Correspondence Address: Source of Support: None, Conflict of Interest: None PMID: 0008737558 Keywords: Adult, Animal, Carbon Dioxide, metabolism,Child, Emergency Medical Services, Equipment Design, Heart Arrest, physiopathology,Human, Respiratory Function Tests, instrumentation,Sensitivity and Specificity, Swine,
Carbon dioxide (CO2), which is produced in the body during cellular metabolism, is transferred to the heart via the venous system and then into the lungs from where it diffuses out into the exhaled air[1],[2],[3]. Endtidal CO2 (ETCO2 measured at end exhalation) therefore reflects metabolism, circulation and ventilation, and atlows the clinician to evaluate these systems in a noninvasive fashion[1],[2],[3]. If any two systems are held relatively constant, ETCO2 changes will reflect changes in the third system[1]. Increased ETCO2 can reflect hypermetabolic states such as sepsis and malignant hyperthermia, and decreased ETCO2 can be caused by shock, pulmonary embolism, decreased cardiac output and decreased pulmonary blood flow during cardiopulmonary resuscitation (CPR) [1],[2],[3]. Capnometers which measure expired CO2 use infrared absorption or mass spectroscopy[1],[2],[3]. They are used to monitor patients in the operating rooms and intensive care units. However, this technology is usually not available in the emergency departments or in the prehospital setting. ETCO2 monitoring is rapidly becoming a standard of care in anesthesia practice[4]-[5]. The normal ETCO 2 is approximately 38 mm Hg (a 5% concentration at 760 mm of atmospheric pressure)[1]-[2]. In patients with normal perfusion and ventilation, ETCO2 measurement closely arterial CO2 partial pressures[1]. Endotracheal intubation is a commonly performed procedure in life-threatening situations in the operating room, intensive care unit, emergency department and in the prehospital setting. Inadvertent, undetected esophageal intubation is catastrophic and can occur in the hands of the most experienced people[6],[7],[8],[9]. The usual clinical methods of confirming endotracheal tube (ETT) position, such as bilateral breath sound auscultation, chest movement visualization, clouding of the ETT, auscultation over the stomach, etc., occasionally fail[6]. Besides visualization of the ETT going through the vocal cords, ETCO2 measurement has been found to be the most reliable method of confirming ETT position[1],[2],[3],[6]. Because CO2 is exhaled through the trachea and is not usually detected in the esophagus, capnometry can distinguish between endotracheal and esophageal intubation[1],[2],[3],[6]. This has been studied in animals[10],[11],[12] and humans[10],[13] in both the non-arrest[10],[11],[13] and arrest[12] settings. Measurement of ETCO2 has been shown to be superior to pulse oximetry in the early detection of esophageal intubation, especially in patients preoxygenated with 100% oxygen[14]-[15]. During cardiac arrest, ETCO2 has been shown to fall abruptly to low levels at the onset of arrest because of the sudden decrease in cardiac output and pulmonary perfusion. This is followed by an increase in ETCO2 after the onset of effective CPT, and then returning to normal or higher-than-normal levels at return of spontaneous circulation (ROSC)[16],[17],[18]. During CPR. ETCO, has been shown to correlate with cardiac output[16],[17],[18],[19],[20], coronary perfusion pressure[21], efficacy of cardiac compression[22], ROSC[16],[17],[18] and even surviva1[23],[24],[25],[26]. Thus, capnometry has been shown to be a useful non-invasive monitoring tool during CIPT[1],[2],[12],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26]. Expired CO2 has been shown to be higher in animals and humans ultimately resuscitated and therefore have potential prognostic value[23],[24],[25],[26].
A portable, disposable, colorimetric ETCO2 detector (FEPtm End-tidal CO2 detector, Fenem, Ic., New York, NY; now Easy Cap, Nellcor, Inc., Hayward CA) is a device that weighs 19.5 gms and has 15-mm inlet and outlet ports that allow it to be attached between the ETT and ventilation bag [Figure - 1]. A non-toxic, pH-sensitive chemical indicator (metacresol purple)[27], visible through a clear dome, detects CO2 in gas mixtures flowing through it. Concentrations of CO2 are indicated by reversible color changes. Colour ranges are marked on a reference chart and indicate approximate CO2 concentrations [A (purple), 0.03% -< 0.5% (< 4 mm Hg); B (tan), 0.5% -<2% (4-<15 mm Hg): C (yellow), 2%-5% (15-38 mm Hg). The device responds to breath-by-breath CO2 changes and works for about 2 hours. Humidity decreases the clinical lifespan of the detector[28]. When the detector is attached to the ETT of a correctly intubated patient it is yellow during expiration and purple during inspiration; when attached to an ETT placed in the esophagus, it remains purple. Readings are obtained after six breaths, as per the manufacturer's recommendation, in order to avoid false positive readings (i.e. yellow color despite esophageal ETT position) caused by the presence of CO2 in the esophagus immediately after ingestion of carbonated beverages or after bag-valve-mask ventilation[13],[29],[30]. Indicator color is to be interpreted only if there is reversible color change, as permanent yellow discoloration can occur in gastric juice or when drugs such as epinephrine come in direct contact with the indicator membrane[31],[32],[33]. Mean minimal CO2 concentration needed for color change has been shown to be 0.54% (4.1 mm Hg)[34]. The device has a dead space of 38 cc and a flow resistance of 3 cm/H2O ± 1 cm at 60 L/min. The use of this detector in children weighing < 15 kg has not been recommended by the manufacturer because of the concern that ETCO, in a child's small tidal volume may be diluted in the large dead space of the detector, thereby producing a false negative result (i.e. remaining purple and indicating esophageal placement despite correct intratracheal position of the ETT)[35].
The detector has been studied in non-arrest[27],[35],[39],[40],[42],[43],[44],[45],[46],[47],[48] and arrest setting[36],[37],[38],[41],[46],[49] in animals[35],[36],[37],[38] children[27],[39],[40],[41],[42] and adults.
The ETCO2 detector was tested in 11 newborn piglets weighing 2.11-3.0 kg[35]. Detector readings were obtained after six breaths and a reading was considered positive if there was any color change from purple (A, < 0.5% CO2) to Yellow (B or C, > 0.5% CO2). The detector correctly identified the 21 tracheal and 33 esophageal intubations. Prior instillation of carbonated beverage or prior bag-valve-mask ventilation did not cause false positive readings in this and another animal study using the detector[35],[36],[37],[38]. The device was 100% sensitive and specific in piglets weighing > 2 kg with spontaneous circulation[35]. In children, the detector was tested in the OR[39], and during transport[27],[40]. It was tested in a total of 237 intubations in children with spontaneous circulation with at least 128 children weighing < 15 kg (63 of whom weighed < 5 kg)[27],[39],[40],[42]. There was one false negative result[40] (i.e. the detector remained purple despite correct ETT position), and there were no false positive results. The false negative result was reported in a 900-grm premature infant who was severely hypocarbic with a PCO2 of 16 mm Hg. In that case, the ETCO2 was approximately 14 mm Hg, and CO2 in the expired gas was about 2% (assuming atmospheric pressure of 760 mm Hg). If tidal volume is assumed to be 8 cc/kg, this patient's tidal volume was about 7 cc, and CO2 in the detector was approximately 0.37% [(7 cc (tidal volume) X 2 (% expired CO2)/38 cc (dead space of detector)40]. This is below the sensitivity of the detector which has been shown to be 0.54% CO2[34]. In a 2-kg infant with comparable hypocarbia, the detector would have correctly identified the ETT' position[40]. Had this premature infant been normocarbic (i.e., PCO2 of 35-40 mm Hg), the detector would be 0.9% [(7x5)/38])[35],[40]. Removal of the detector from the circuit once readings are obtained has been recommended in small children to avoid the risk of rebreathing because of the large dead space[39],[40]. The 38 ml dead space would preclude continuous use in a spontaneously breathing patient but is unlikely to pose problems during controlled ventilation. Continuous use of the detector during in-hospital transfer of intubated children weighing > 15 kg was also studied and found to pose no problems[27]. In that study, one patient became severely hypotensive en route resulting in decreased ETCO2, which was demonstrated by a change in detector color from yellow (C) to tan (B)[27]. Once the pressure was normalized by using pressor agents, the detector color returned to the C range. Therefore, the detector has potential for use in monitoring the cardiovascular status of a patient as long as ventilation is kept constant. Studies in adult patients in the non-arrest have also shown that the detector is extremely sensitive and specific in identifying ETT position[43],[44],[45],[46],[47],[48]. Two false positive results were noted in patients with oropharyngeal intubations who were still having spontaneous respirations[46],[47]. The detector, Iike the capnometer, does not indicate where in the respiratory tree the tube is placed, so it cannot, for example, differentiate right main stem bronchus or oropharygeal intubation from tracheal ETT position[3],[11],[40],[46],[47].
During CPR, ETCO2 decreases because of decreased cardiac output and pulmonary blood flow[1],[2],[16],[17],[18],[19],[20],[21],[22]. The ETCO, detector readings were found to correlate with infrared capnometry in animal CPR studies[36],[37] but direct contact of the drug can cause permanent discoloration of the indicator membrane[31],[32],[33],[47]. The detector was studied during 52 intubations in 43 children who were undergoing CPR[41]. All ten esophageal intubations were correctly identified by the detector but only 35 of the 42 endotracheal intubations were correctly identified. In this setting, the detector had a sensitivity of 83.3%, specificity of 100% PPV of 100% and NPV of 58.81%. These results are similar to those obtained in adult patients undergoing CPR[46],[47],[48],[49]. During CPR, a positive result is indicative of correct endotracheal tube position, but a negative result means something is wrong. i.e., the tube is in the esophagus or cardiac output is severely diminished. A negative result always needs an alternate means of confirmation of ETT position because of the uncertainty about tube location[41],[46],[49]. The worst-case scenario would involve removal of a correctly placed tube, but without the more serious consequences of ventilating through a tube placed in the esophagus[39]. In the above-mentioned studies, most patients were in arrest in the prehospital setting, some with long arrest times[41],[46],[47],[48]. Prolonged arrest times have also been shown to be associated with lower expired CO2[47]. In a study of adult patients who experienced in-hospital cardiac arrest and underwent immediate CPR, the detector was shown to be very sensitive in identifying ETT position[49]. No patient who was correctly intubated and had an initial reading in the A range (< 0.5% CO2) survived[41],[46],[47],[48]. Higher initial ETCO2 readings were associated with an increased rate of short-term survival in both the pediatric and adult cardiac arrest studies[41],[47],[48]. Similarly, higher (> 2% CO2) final readings (obtained at end of CPR) were also associated with increased short-term survival in adults and children[41],[48]. Further studies will be required to determine in individual cases whether end-tidal CO2 readings correlate with prognosis because of the patient's underlying state or because of suboptimal chest compressions. Administration of intravenous sodium biocarbonate has been shown to transiently increase ETCO2[18], and epinephrine has been shown to cause a dose-dependent decrease in ETCO2[50],[51]. The effects of these drugs on detector readings have not been studied. Thus, the detector may have a useful role in monitoring CPR and predicting short-term survival. Further clinical research is needed to determine whether it is appropriate to consider ETCO2 as a factor in the decision to continue or stop resuscitation efforts.
The ETCO2 detector cannot detect hyper-or hypocarbia[35],[39],[40] right main stem bronchus intubation 340 or oropharyngeal intubation in a spontaneously breathing patient[3],[11],[46],[47]. False positive readings which may occur immediately postingestion of carbonated beverages or, after bag-valve-mask ventilation[13],[29],[30] can be avoided by obtaining readings after six breaths[13],[35],[38]. False negative results can occur when the pulmonary blood flow is severely diministed, such as during CPR[41],[46],[47],[48],[49] and in hypocarbic infants weighing < 2 kg with spontaneous circulation[40]. Active humidification decreases the lifespan of the detector from 2 hours to about half an hour[28]. Permanent yellow discoloration due to direct contamination of the indicator with gastric contents, pulmonary edema fluid or intracheal drugs can occur[31],[32],[33],[47], and if it does a new ETCO2 detector should be used. The large dead space precludes continuous use of the detector in infants[35],[39],[40] but this may be overcome in the future by a smaller detector or a portable infrared capnometer.
The ETCO2 detector is portable and therefore convenient to use in the ED and the prehospital setting and during transport of intubated children. It is an inexpensive (approximately $17) single-use device, which requires no sterilization, warm-up time or calibration. The detector has a shelf life of about 1 year and its use is easily learned.
The ETCO2 detector is a useful piece of equipment in preventing the tragedy of esophageal intubation. Provided the detector is responsive to cyclical changes in CO2 concentration, it is a very useful and reliable tool for verification of ETT position in children weighing > 2 kg with spontaneous circulation. During CPR, a positive result indicates correct ETT position, but a negative result requires alternate means of confirmation. The detector can help monitor CPR and may have a role in predicting short-term survival. We recommend incorporating the detector into routine care where no other means of detecting expired CO2 is available, as a vital step in avoiding catastrophic esophageal intubation.
The author would like to thank Paediatric Emergency Care and Williams and Wilkins for permission to publish this article, as a similar article will be published by them. Sanders AB. Capnometry in emergency medicine. Ann Emerg Med 1989; 18:1287-1290.
[Figure - 1]
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