Progressive Decline in Plethysmographic Lung Volumes in Infants

Abstract

During the last 30 years, there has been an unexplained trend toward declining values for plethysmographic assessments of lung volume at functional residual capacity (FRC) in infants. The aim of this study was to compare data collected from healthy infants using contemporary equipment with published reference data and to explore reasons for discrepancies. Lung volumes were measured in 32 healthy infants (age, 4-93 weeks; weight, 3.9-12.4 kg) using a new, commercially available infant plethysmograph. Mean (SD) FRC was 19.6 (3.4) ml/kg (within subject coefficient of variation 3.4 [2.3%]), which was on average 7.0 [3.5] ml/kg and 2.3 [1.2] SD (Z) scores lower than the recently collated reference data from an American Thoracic Society task force. A total of 66% of these healthy infants had a FRC that was below the predicted normal range. Comparison of equipment, software, and protocols with those from previous reports revealed the importance of minimization of dead space and of adequate subtraction of all compressible occluded volume when calculating FRC in infants. These findings emphasize the need to establish reference data for lung function tests in infants that are appropriate for the equipment and protocols in current use.

New reference equations to improve interpretation of infant lung function

“…9,11,12 Had we relied on expressing data as weight-corrected values, the current results would be similar to those previously reported for FRC p , 26 and C rs 7,8 but higher than those reported for V T by Fuchs et al 15 when using an ultrasonic flowmeter in a large population of unsedated newborns studied during the first 8 weeks of life. In contrast to previous observations, 7,8,15 after adjustment for body size, significant sex differences were only noted for FRC p , with boys tending to have slightly higher values, a difference that increased with age, and which was therefore adjusted for by including an interaction term in the equation.…”

“…2,25 Plethysmographic Functional Residual Capacity (FRC p ) FRC p was measured as described previously. 1,26,27 Up to five technically satisfactory measurements were obtained and the mean of 3-5 valid measurements reported. In exceptional cases, the mean of two good quality, repeatable (within 5% of each other) measurements was used.…”

“…Measurements of tidal breathing, passive respiratory mechanics and FRC p were undertaken according to international guidelines. [1][2][3][24][25][26] Not all tests were attempted on every occasion in each infant, depending on the precise research protocol. The withinsubject within-occasion coefficient of variability (CV) was calculated as [SD/mean] Â 100.…”

Lung Function Tests in Neonates and Infants with Chronic Lung Disease of Infancy: Functional Residual Capacity

“…29 During recent years, guidelines for methodology were published, 5 and systems that are licensed for research and clinical use in infants are commercially available. The equipment is relatively expensive, bulky, and unsuitable for bedside measurements or assessments in very small or sick infants.…”

“…The observation of lower FRC pleth values in infants during recent years has been accompanied by the realization that previous assumptions regarding the ''physiological'' presence and amount of trapped gas in healthy infants may need to be reconsidered, since systematic differences between FRC pleth and FRC gas in such infants are small 9,16 or no longer evident. 15,29,39 The improved accuracy of contemporary techniques has reopened dis- Validation of equipment and techniques for infant lung function testing is notoriously difficult due to the lack of recognized ''gold standards,'' the limits imposed regarding infant sedation for purely ''methodological'' studies in some countries, and the limited duration of sleep epochs, even after sedation, for data collection. These difficulties emphasize the advantages of adhering to published guidelines developed following intensive international collaboration.…”