© 2005 American Thoracic Society
Progressive Decline in FRC in Infants: Physiology or Technology?To the Editor:Hülskamp and colleagues (1), using a Jaeger plethysmograph (Jaeger Masterscreen BabyBody plethysmograph; Jaeger, Hoechberg, Germany), reported the mean functional residual capacity (FRC) in healthy infants to be 19.6 (SD 3.4) ml/kg, that is, 2.3 standard deviation scores lower than the predicted normal range of collated reference data from the ATS-ERS task force (2). Huskamp and coworkers' values (1) were also below data reported for gas dilution techniques (3), which is surprising. As a consequence, we were concerned that their lower lung volumes reported using the Jaeger plethysmograph (1) might reflect the fact that equipment was underrecording. This hypothesis is important to test, because if correct, it would mean that an inappropriate reference range is produced using such equipment. To test our hypothesis, we compared FRC results obtained using the Jaeger plethysmograph, a conventional plethysmograph (Department of Medical Engineering, Hammersmith Hospital, London, UK) (4), and a helium gas dilution system (EBS 2615, Equilibrated Bio Systems, Smithtown, NY). Measurements were performed in vitro using a lung model filled with copper wool to minimize the adiabatic effect and in vivo on 10 sedated infants, median age of 13 (range 6–14) mo. Seven infants were born prematurely (median gestational age 29 wk, range 24–31 wk), four had had bronchopulmonary dysplasia, and three were born at term (one had unilateral pulmonary aplasia and two had had surgical repair of congenital anomalies). In vitro, the volumes measured by the Jaeger plethysmograph were lower than those measured by the Hammersmith plethysmograph (p < 0.001) and the helium gas dilution system (p < 0.001). The measured volumes by the Jaeger plethysmograph were between 87% and 89% of the actual volume, by the Hammersmith plethysmograph between 99% and 103% of the actual volume, and by the helium gas dilution system between 99% and 101% of the actual volume. In vivo, the median FRC measured using the Jaeger plethysmograph (19.6, range 13.9–36.2 ml/kg) was lower than that using the Hammersmith system (26.5, range 21.7–37.8 ml/kg) (p = 0.02) and the helium gas dilution system (21.9, range 15.2–30.6 ml/kg) (p = 0.09). Our in vivo results using the Jaeger plethysmograph were similar to those obtained by Hulskamp and coworkers (1) and were on average 25–30% less than those expected from predicted values (2). The data we report suggest that the Jaeger plethysmograph underrecords both in vivo and in vitro, possibly due to a greater sensitivity to the adiabatic effect. These results emphasize the importance of comprehensively validating all new lung function measurement systems before introducing them into clinical or research practice.
King's College London, London, United Kingdom FOOTNOTES Conflict of Interest Statement: None of the authors have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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