We appreciate the opportunity to reply to the letter by Z.L. Borrill and coworkers regarding the European Respiratory Society (ERS)/American Thoracic Society (ATS) exhaled breath condensate (EBC) Task Force report. EBC pH measurement is indeed a rapidly growing field of research with the promise of providing previously unknown information about the airways. The measurement is simple, can be performed sample by sample and there is no problem with the detection limit. EBC, however, is a diluted fluid with a low buffering capacity, which makes its pH value sensitive to changes in CO2 content. Although the CO2 concentration of fresh EBC samples is probably similar to that found in the airway lining fluid, this CO2 concentration will change spontaneously due to the rapid interaction with environmental air after sampling. That is the main reason why deaeration of EBC samples is recommended to decrease the level of uncertainty when measuring EBC pH and to obtain more standardised readings. All of this appears to be so simple that, until recently, nobody dared to measure EBC partial pressure of carbon dioxide (PCO2) or at least to publish data on it.
It is interesting to observe the indirect approach that Z.L. Borill and coworkers use to estimate the CO2 content of the EBC and its relative contribution to EBC pH by analysing the relationship between baseline pH values and pH changes caused by deaeration.
To prove the above concept (deaeration results in EBC pH change by causing a decrease in PCO2 of the samples), there is a simply direct way, i.e. by measuring EBC pH together with CO2 concentration in the samples. Therefore, we collected EBC samples from 17 healthy subjects (10 male; mean age 25 yrs; lung function values and exhaled nitric oxide concentration in normal range; all never-smokers) and performed measurements of pH and PCO2 by a blood gas analyser directly after sampling (within 30 min) and after 10 min of deaeration using argon. Data are hereby given as mean±sd. The pre-deaeration pH was 6.70±0.24 with a PCO2 of 2.11±0.92 kPa. Argon deaeration caused a decrease in EBC PCO2 to 0.33±0.09 kPa but never to 0 kPa (p<0.0001 compared with pre-argon value), and a significant increase of the pH to 7.67±0.20 (p<0.0001; figs 1⇓ and 2⇓). There was a significant relationship between the changes in EBC PCO2 and that of pH. Furthermore, in line with the observation by Z.L. Borrill and coworkers, there was a relationship between baseline EBC pH and the observed pH increase (r2 = 0.53; p = 0.001). The observed increase in pH (mean increase of 0.97) in our healthy subjects was similar to that observed by Z.L. Borrill and coworkers in asthmatic and COPD patients. Our data are also in agreement with the suggestion by Z.L. Borrill and coworkers that a deaeration-caused pH change is a surrogate for CO2 concentration to some extent; however, a simple pH measurement cannot give an estimate of the remaining CO2 content that may still influence the pH reading.
The effect of gas standardisation on exhaled breath condensate (EBC) partial pressure of carbon dioxide (PCO2).
The effect of gas standardisation on exhaled breath condensate (EBC) pH.
In summary, argon deaeration decreases the concentration and the variability of exhaled breath condensate partial pressure of CO2, but there is always some remaining CO2 that may still be a confounding factor in pH assessment. CO2 content has a marked influence on exhaled breath condensate pH and, since exhaled breath condensate partial pressure of CO2 varies even after deaeration, it leaves some uncertainty in the exhaled breath condensate pH reading even after deaeration. It seems worthwhile to carry out some more research to define other potential modes of standardisation of this measurement, to learn more about the different factors that may influence exhaled breath condensate pH and the relationship between the pH of exhaled breath condensate and that of the airways.
Acknowledgments
This study was supported by the Hungarian National Research Foundation (T43396).
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