Some investigators are concerned that CO2 should not be removed from EBC because CO2 (and resulting carbonic acid) is a relevant acid in the airway and should not be ignored. However, when collecting EBC for pH assay, we are primarily interested in identifying the presence of acids that are volatile specifically from an acidic source fluid. The volatility of CO2 is not appreciably dependent on the pH of the source fluid within the range relevant to the airway, and therefore the levels of CO2 in EBC tell us little about the pH of the source fluid. In regards to EBC pH, CO2 represents an unwelcome noise that gas standardization well addresses. Gas standardization is a key component that allows EBC collection for pH assay to be particularly useful and easy for clinical studies.
Data have been published specifically testing and refuting speculations that EBC pH may be an artifact of hyperventilation or oral ammonia. Data supporting that EBC pH well represents airway lining fluid pH are forthcoming. The airway must be considered to include the laryngopharynx as a potential source of acids in EBC. Indeed, any cause of airway acidification at any level can affect EBC pH values. Thus laryngopharyngeal acid reflux—with or without acid aspiration into the lower airway—may contribute to EBC acidification in samples collected with oral exhalation. Given that patients with obstructive lung disease have a high incidence of acid reflux, this pathway for EBC acidification needs to be kept in mind. It is important to note, however, that measurement of EBC pH from isolated lower airway samples have revealed specific lower airway acidification, supporting the ability of the airway to acidify in the absence of laryngopharyngeal acid reflux. read more
It is likely that there was some inclusion of subjects with unidentified respiratory disease or laryngopharyngeal acid reflux, and these factors may provide an explanation for why EBC pH is occasionally low in seemingly healthy subjects. If the saliva or hypopharynx is sufficiently acidified by drinking an acidic beverage, then EBC pH will be low until this acidic challenge is neutralized. In this, and all of our studies, we collect EBC sample only if the subjects state they have not consumed anything in the previous 60 min; however, subjects may not always accurately reveal recent ingestion, and thus acidic ingestion artifact may account for some low EBC pH values. As a noninvasive biomarker relevant to respiratory disease, EBC pH has many positive attributes. Intrasample reproducibility is excellent. As we have reported previously, reproducibility of the measurement within day and between days in healthy subjects is high and is not affected by hyperventilation, sample storage duration, or collection orally or by endotracheal tube.