Capnography – Not Just for Children
At AAST’s 39th Annual Meeting, Julie DeWitte provided an excellent lecture on capnography. She reviewed how the two main methods work (end-tidal and transcutaneous technologies), when they should be used and what it means when there are discrepancies between the two methods. She presented several illustrative cases for discussion. Unfortunately, due to a severe case of laryngitis, she didn’t really present the talk. Joel Porquez did an admirable job of filling in for Julie, but we didn’t have the opportunity to hear it directly from her. Now, thanks to the magic of a recording studio and some video editing, the lecture is available at the AAST website.
Capnography, the measurement of CO2 in the respiratory gases, is recommended during a diagnostic sleep study in children. This means that in order to maintain AASM center accreditation, centers that study children must have CO2 monitoring equipment available. The gold standard is arterial PCO2, but this is rarely obtained in association with a sleep study. The end-tidal and transcutaneous measures are recommended surrogates1. Monitoring is optional in children undergoing PAP titration and in adults. If you have the equipment and you are not using it for adults, you may be missing out on important information. And if you don’t have CO2 monitoring equipment, you should be considering adding it to routine sleep studies.
The popularity of CO2 monitoring has increased over the years due to two significant advances. First, the technology for measurement, especially for transcutaneous monitoring, has improved dramatically. The size of the sensors and stability of output has improved. The transcutaneous sensors used to be the size of a quarter, and required a drop of fluid to be applied and a delicate membrane to be attached. The probe was heated to the point where red spots were noted unless the probe was relocated every hour or two. Newer systems require significantly lower temperatures, and risks of burns are low. The second advance has been affordability, with newer machines costing one-half to one-quarter of the previous cost.
In her talk, DeWitte noted the changing mix of patients in her sleep laboratory. This is primarily due to the use of home sleep tests for “routine” obstructive apnea patients. The patients referred for laboratory sleep studies are increasingly complex and often have multiple comorbidities. Although we rely on pulse oximetry, there are benefits to also measuring CO2, especially in these complex patients. For example, distinguishing among COPD, obesity hypoventilation and overlap syndrome is critical to finding the most effective treatment platforms.
DeWitte provides an excellent review of the methods for monitoring CO2, including differences between the two noninvasive techniques and troubleshooting tips. To my mind, the most interesting part of the presentation is her review of several patients where CO2 monitoring was essential in determining the proper diagnosis and treatment.
And, like a fine wine, this educational offering pairs nicely with the presentation by Dr. Frances Chung at the Kentucky Sleep Society meeting last fall. Dr. Chung includes CO2 analysis in her evaluation of patients prior to surgery and provides an anesthesiologist’s view of its usefulness. You can view the modules here and here. If your center already has capnography equipment for pediatric use and has not begun to measure CO2 in adults, I strongly recommend taking the time to listen to DeWitte and Dr. Chung.
- Berry RB, Brooks R, Gamaldo CE, et al. AASM Scoring Manual, v. 2.4. AASM: Darien, IL. 2017