CPAP (Continuous Positive Airway Pressure) is a valuable tool for a wide variety of patients with acute shortness of breath. There are very few reasons why any acutely dyspneic patient should not have at least a trial of CPAP. In some situations, such as acute pulmonary edema, the beneficial effects of CPAP become apparent within seconds to minutes. In others (probably comprising the majority of BLS and ALS CPAP applications), it can be challenging to determine whether CPAP is helping, not making a difference or potentially harming your patient. A very valuable, but often underused tool for evaluating the effectiveness of CPAP is capnography. This column will explain how you can use end tidal CO2 measurements to titrate and evaluate the effectiveness of CPAP.
Related: The Argument for BLS CPAP
Capnography is the non-invasive measurement of exhaled carbon dioxide (CO2) displayed as a CO2 waveform concentration over time. During the 1980s, capnography became a standard for anesthesia care in the U.S.,1 virtually eliminating misplaced invasive and supraglottic airways. Capnography reflects ventilation, perfusion, and metabolism; alternatively, pulse oximetry mirrors oxygenation status. In steady cardiac output and metabolic states, CO2 production remains relatively constant, hence measuring exhaled CO2 directly reflects ventilation. Capnography is available for both spontaneously breathing patients and patients who are receiving positive pressure ventilation.
For patients who are breathing spontaneously, exhaled CO2 can be measured using a nasal cannula type device where one of the cannula prongs (and sometimes an additional collector positioned over the mouth, referred to as an “oral pillow,”) samples respiratory gases. For patients receiving positive pressure ventilation, CO2 is measured using an adapter placed at the connection between the ventilation device or circuit and the advanced airway or mask being used on the patient. In BVM ventilation with a continuous mask seal maintained during exhalations, accurate CO2 values and waveforms will be displayed when using an adapter connected between the bag and mask.