To the Editor:
We appreciate Dr. Kodali’s recent review of capnography outside the operating room environment.1
Two small yet important details would benefit from further clarification. Figure 2C is purported to display the malfunction of an inspiratory valve in an anesthesia breathing circuit. Unlike a stuck, open expiratory valve, an inspiratory valve that remains open during expiration shows the end-tidal carbon dioxide baseline returning to zero.
The best way to understand this is to consider taking the circle system breathing circuit and removing the inspiratory valve entirely from the circuit. During expiration, one half of the exhaled carbon dioxide-rich gas will “exhale” into the inspiratory limb of the breathing circuit. With the next inhalation, roughly the first half of the inspired breath will contain this exhaled gas, and the last part of inspiration will be fresh gas from the absorber and the fresh gas flow, i.e., carbon dioxide-free gas, which allows the capnogram to return to the zero baseline. Compared with the normal capnogram, with a competent inspiratory valve, the inspiratory downstroke will sluggishly return to a zero baseline (or a B angle greater than 90 degrees) as appropriately depicted in the cartoon in figure 2C. This subtle difference that occurs with the baseline returning to zero helps elucidate the difference between malfunctioning inspiratory and expiratory valves or exhausted carbon dioxide absorbent.
The second point is that in figure 3 A–D, the apparent presence of inspired carbon dioxide is an abnormal finding, and suggests that in this sedation case, there is evidence of rebreathing in the microenvironment around the face, which may occur as a result of draping. The normal inspired carbon dioxide during sedation is expected to be zero.