Waveform capnography is exciting technology that has the potential to become very useful in the prehospital setting
Capnography is a noninvasive method of monitoring carbon dioxide levels in the exhaled breath. The cells of the body produce about 200 milliliters of carbon dioxide (CO2) per minute under normal metabolic circumstances (Guyton, 1971). When illness, physical activity, or environmental conditions increase the metabolic rate, cells produce much more.
In order to keep the concentration in equilibrium, the body must eliminate the carbon dioxide (via exhalation) as fast as it is created. Eliminating it too quickly or allowing the concentration to build can produce serious consequences.
Use of pediatric capnography
Although capnography has been around for decades in the operating room, it is a relatively new addition to the EMS toolbox. Two of the most valuable uses for waveform capnography are to verify both initial endotracheal tube (ETT) placement and tube surveillance during transport (Sullivan, Kissoon, & Goodwin, 2005).
In a simulated model of ETT dislodgement in a pediatric patient, rescuers using capnography were able to correct dislodgement before the patient suffered significant reductions in oxygen saturation (Langhan et al., 2011).
EMS personnel must recognize that false positives do occur. For example, it is possible that rescuers can insufflate a child’s stomach with a sufficient quantity of exhaled air (mouth-to-mask) to initially produce a capnography waveform with inadvertent esophageal tube placement. However, subsequent assisted-ventilations should produce a smaller and smaller waveform until the waveforms eventually disappear within five or six breaths.
Therefore, rescuers must verify the presence of the capnography waveform upon initial tube placement and to assure that waveform remains throughout the subsequent resuscitation attempt.
Weak EtCO2 signal from an incomplete endotracheal seal
If the ETT is too small, rescuers may find it impossible to effectively ventilate the lower airways of an infant with significantly diminished lung compliance. Since the narrowest portion of the infant’s airway lies just below the cricoid cartilage, an inappropriately small-uncuffed ETT will not completely seal the child’s airway. This will result in air leaks around the tube, which reduces the delivered tidal volumes and weakens the capnography signal (Neema, Jayant, Sethuraman, & Rathod, 2008).
Benefits of EtCO2 monitoring in cardiac arrest
Despite these limitations, the latest American Heart Association guidelines recommend the prehospital detection of exhaled CO2 (either with capnography or colorimetry) for confirmation of ETT placement in children of all ages (Kleinman et al., 2010).
The guidelines further note that capnography may be beneficial for determining the quality of chest compressions during cardiopulmonary resuscitation (CPR) (Kleinman et al., 2010). Chest compression quality is a major determinate of resuscitation success (Ristagno et al., 2007).
Almost two decades ago, researchers using a canine model of pediatric cardiac arrest found a significant correlation between colorimetric CO2 displays and capnometry measurement (Bhende, Karasic, & Menegazzi, 1995). Further analysis in a series of 40 children between the ages of 1 week and 10 years demonstrated that return of spontaneous circulation (ROSC) resulted in appreciable increases in expired-breath CO2 concentrations (Bhende & Thompson, 1995).
Consistently low ETCO2 readings during the initial moments of the resuscitation attempt could reflect less than optimal CPR quality and suggests the rescuer should focus on efforts to improve chest compressions. Adult research indicates an abrupt and sustained increase in ETCO2 readings just before rescuers can clinically identify ROSC (Pokorna et al., 2010).
Does using EtCO2 strengthen diagnostic impression?
Recently, some researchers have examined the utility of using capnography to measure and monitor the status of the patient with respiratory distress. Researchers at a level one pediatric emergency department (ED) in Virginia found that ETCO2 readings are highly correlated with venous pCO2 across a range of respiratory illnesses in children (Moses, Alexander, & Agus, 2009).
There is little question that capnography produces a unique waveform in the presence of bronchospasm. With effective therapy, this characteristic pattern returns to normal. Rescuers trained in waveform recognition can gauge the effectiveness of their treatment based on changes in the waveform.
However, an ED evaluation of children suffering from acute exacerbations of asthma found that although feasible, capnography monitoring correlated poorly with standard measures of bronchospasm, such as pulmonary index, one-minute forced expiratory volume, and peak expiratory flow rate (Evered, Ducharme, Davis, & Pusic, 2003).
Further, researchers could not demonstrate a statistically significant correlation between initial capnography measurements and the need for hospital admission in children younger than 24 months with clinical evidence of bronchiolitis (Lashkeri, Howell, & Place, 2012).
Similarly, researchers at a freestanding tertiary children’s hospital in Alabama could not quantify ETCO2 values with asthma severity in children between the ages of 3 and 17 years (Guthrie, Adler, & Powell, 2007).
Areas of promise for EtCO2 use
On the other hand, capnography may accurately estimate the cardiovascular or metabolic status of a patient with normal pulmonary compliance (Garcia, Abramo, Okada, Guzman, Reisch, & Wiebe, 2003). Using a convenience sample of children, researchers in Rhode Island demonstrated that nasal capnography was useful as a non-invasive screening tool for identifying children with diabetic ketoacidosis (DKA) (Fearon & Steele, 2002).
A retrospective review of children admitted to the intermediate care unit for DKA at Children’s Hospital Boston demonstrated a strong correlation between ETCO2 readings and the degree of acidosis (Agus, Alexander, & Mantell, 2006). Researchers could not find a significant correlation between elevated blood glucose values and the presence of metabolic acidosis in pediatric patients with a history of Type I diabetes (Gilhotra & Porte, 2007).
However, they were able to demonstrate that no patient with an eventual diagnosis of diabetic ketoacidosis (DKA) had an ETCO2 reading greater than 30 mm Hg suggesting that, when combined with traditional assessment techniques, nasal capnography may be a more accurate predictor of the presence of DKA than blood glucose values.
In an evaluation of young children (mean age 4 years) who presented to the emergency department with symptoms consistent with gastroenteritis, ETCO2 measurement highly correlates with serum bicarbonate levels suggesting that capnography could be used as a non-invasive measure of acidosis (Nagler, Wright, & Krauss, 2006).
Waveform capnography are exciting technologies that have the potential to become very useful tools in the prehospital setting. Besides the advantages offered in verifying ETT position, capnography can have a very powerful role during CPR in the field.
Strong medical direction, strict protocols, and active continuous quality improvement programs are needed to ensure its proper use. Capnography may drastically alter the future care provided for intubated patients, patients in respiratory distress, and children who suffer out-of-hospital cardiac arrest.
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Ristagno, G., Tang, W., Chang, Y. T., Jorgenson, D. B., Russell, J. K., Huang, L., Wang, T., Sun, S., & Weil, M. H. (2007). The quality of chest compressions during cardiopulmonary resuscitation overrides importance of timing of defibrillation. Chest, 132(1), 70–75. doi:10.1378/chest.06-3065
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