Capnography, often referred to by emergency clinicians as end-tidal carbon dioxide monitoring, is a noninvasive method of measuring cardiopulmonary and metabolic parameters that can be utilized in many clinical applications. Growing evidence in the literature in support of the use of capnography has led to increased clinical use of this modality in many pediatric subspecialties. Understanding capnography and the literature supporting its practice will advance its use by emergency clinicians in the pediatric emergency department, promoting improved patient care and safety. This issue reviews the technology and physiology involved in measuring exhaled carbon dioxide and the interpretation of waveforms, and it highlights uses for capnography in pediatric patients in the emergency department. Uses include confirmation of intubation, maintenance of ventilation in intubated and nonintubated children, monitoring of effectiveness of cardiopulmonary resuscitation, and as an adjunct for monitoring of sedated children and children with lower respiratory disease and metabolic derangements.
Key words: capnography, capnometry, colorimetric capnography, end-tidal carbon dioxide
On a weekday afternoon in your pediatric ED, a worried mother brings in her 4-year-old daughter, who fell from a piece of playground equipment at her daycare center. She has an obvious deformity of her left forearm, and radiographs confirm displaced fractures of the distal radius and ulna. You inform the mother that her daughter’s injury will require a closed reduction by your institution’s orthopedic team and that you would like to sedate her for the procedure. While doing your presedation assessment, you note that the patient’s pulse oximetry reading is 93%, breath sounds are decreased over both lung fields,and there is an occasional wheeze. The nurse places a nasal cannula on the child and administers supplemental oxygen at a rate of 2 L/min. Her oximetry reading rises to 98%. The mother informs you that her daughter has had 2 or 3 episodes of wheezing in the past, all related to concurrent upper respiratory infections. How will you evaluate this patient’s current respiratory status? Will you maintain the child on supplemental oxygen during the sedation? How will you monitor her respiratory parameters during the sedation?
Shortly after that, a member of your nursing staff asks you to speak with the parent of an 18-month-old boy you evaluated 30 minutes ago. The patient had a 3-day history of fever, vomiting and diarrhea, and poor fluid intake, and he had not had a wet diaper in almost 24 hours. On your exam, you noted him to be tired appearing, tachycardic for age, with dry lips, and a capillary refill of approximately 3 seconds. His abdomen was soft and nontender. You ordered a serum electrolyte panel and requested that the nurse place an IV catheter for hydration. The nurse tells you that she has been unable to obtain blood or place the IV despite multiple attempts. The mother does not want her child to have any more needle punctures or attempts at IV placement, and she asks you if the blood test and IV are absolutely necessary. How will you respond to this mother? Is there a noninvasive objective measure that can help you to determine the severity of this child’s dehydration?
As you consider your options, EMTs rush in with an intubated teenager. CPR is in progress. They tell you that they were flagged down by a group of the boy’s friends at a nearby park. The boy collapsed during a game of basketball, and he was apneic and pulseless on the scene. The EMTs tell you that automated external defibrillator pads were placed immediately, and no shock was indicated. Since the park was very near the hospital, they chose to “scoop and run," and 1 of the EMTs intubated him in the ambulance en route. The intubating EMT tells you he did appreciate breath sounds in both axillae after the endotracheal tube was placed, although it was difficult to hear in the moving ambulance with the sirens blaring. He reports a “positive color change” on the colorimetric CO2 detector, but states that the color was more beige than bright yellow. He did see the endotracheal tube go through the vocal cords and some watervapor in the tube while he was bagging, so he is sure he intubated the trachea. A resident working with you takes over the manual ventilations of the patient and directs an intern to take over chest compressions from the EMT team. A nurse places the boy on a monitor and checks the femoral pulse during compressions. She reports that she can palpate a pulse with each compression and tells the intern he is compressing effectively. The resident suggests that the intern stop chest compressions for a moment to check whether there is a cardiac rhythm on the monitor. Is the resuscitation of this patient optimal? Are there more objective indicators that can be used to confirm endotracheal intubation and maximize the quality of the CPR your team is providing?
Capnography is the measurement and monitoring of the partial pressure of carbon dioxide (CO2) in exhaled breaths, and it is often referred to as end-tidal carbon dioxide (ETCO2) monitoring. The modernday ability to measure ETCO2 and the insight it provides into human metabolism and cardiopulmonary physiology has been over a century in the making.1
Some of the earliest experiments in CO2 measurement were by John Tyndall, a professor of natural philosophy from the United Kingdom. In the mid-1860s, he experimented with carbonic acid and discovered that CO2 was superior to oxygen, nitrogen, and hydrogen in the absorption and transmission of radiant heat. In 1905, John Scott Haldane created an early spectrometer that was able to measure the volume and calculate the percentage of CO2 in a mixture of gases. In his studies of respiratory physiology (often using himself as a subject), he was one of the earliest investigators to suggest that human respiratory drive is exquisitely sensitive to the rising partial pressure of CO2 in alveolar air.2 The first modern capnograph is attributed to Karl Friedrich Luft. The “Luft cell” (1937) made use of infrared technology to measure CO2 concentration, and it continues to be the technologic basis of modern-day CO2 measurement.2
Evidence-based medicine requires a critical appraisal of the literature based upon study methodology and number of subjects. Not all references are equally robust. The findings of a large, prospective, randomized, and blinded trial should carry more weight than a case report.
To help the reader judge the strength of each reference, pertinent information about the study, such as the type of study and the number of patients in the study will be included in bold type following the references, where available. The most informative references cited in this paper, as determined by the author, will be noted by an asterisk (*) next to the number of the reference.
Heather J. Becker, MD, Melissa Langhan, MD
June 1, 2013