Plethysmographic measurements of lung volume and airway resistance

Abstract

Functional residual capacity (FRC) is the only static lung volume that can be measured routinely in infants. It is important for interpreting volume-dependent pulmonary mechanics such as airway resistance or forced expiratory flows, and for defining normal lung growth. Despite requiring complex equipment, the plethysmographic method for measuring FRC is very simple to apply and, unlike the gas dilution techniques, enables repeat measures of lung volume to be obtained within a few minutes. This method has the further advantage that with suitable adaptations to the equipment, simultaneous measurements of airway resistance can also be obtained.The aim of this paper is to provide recommendations pertaining to equipment requirements, study procedures and reporting of data for plethysmographic measurements in infants. Implementation of these recommendations should help to ensure that such measurements are as accurate as possible and that meaningful comparisons can be made between data collected in different centres or with different equipment. These guidelines cover numerous aspects including terminology and definitions, equipment, data acquisition and analysis and reporting of results and also highlight areas where further research is needed before consensus can be reached.

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

“…FRC pleth is derived from the occluded volume by subtracting the dead space of the apparatus and any tidal volume above end-expiration at the moment of occlusion. 5 FRC pleth includes the volume of any gas trapped behind obstructed airways. Plethysmography has been used by numerous research groups since the 1960s, [6][7][8][9][10][11][12][13] and guidelinesfor standardizingequipment and techniqueswere publishedrecently.…”

“…Plethysmography has been used by numerous research groups since the 1960s, [6][7][8][9][10][11][12][13] and guidelinesfor standardizingequipment and techniqueswere publishedrecently. 5 Commerciallyproducedinfantplethysmographs that adhere to most of these specifications, and which have FDA and the Communauté Européene (CE) certificat, are now available.…”

“…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.…”

“…Accuracy of plethysmographic measurements is dependent on rapid equilibration of pressures during respiratory efforts against occlusions, so that pressure changes at the airway opening reflect those in the alveoli. 5,13,30 In the presence of severe airway obstruction, this may not occur, resulting in a phase lag (looping) between airway pressure and box volume, usually resulting in overestimation of FRC. 5,31 Conversely, in the presence of airway obstruction and uneven ventilation, FRC may be seriously underestimated when using the gas dilution or washout techniques if insufficient time is allowed for complete equilibration of gas concentrations.…”

“…5,13,30 In the presence of severe airway obstruction, this may not occur, resulting in a phase lag (looping) between airway pressure and box volume, usually resulting in overestimation of FRC. 5,31 Conversely, in the presence of airway obstruction and uneven ventilation, FRC may be seriously underestimated when using the gas dilution or washout techniques if insufficient time is allowed for complete equilibration of gas concentrations. 17,31,32 Assessments of FRC by gas dilution or washout are similarly noninvasive and, in contrast to plethysmography, can be applied in sick and ventilated infants.…”