Plethysmography and Gas Dilution TechniquesMarchal F. · Schweitzer C.
Service d’Explorations Fonctionnelles Pédiatriques, Hôpital d’Enfants,Vandoeuvre, France
Measurement of static lung volumes - i.e., total lung capacity (TLC), functional residual capacity, and residual volume (RV) - has undergone significant progress in its routine application in paediatrics over the last decade. In addition to providing an index of lung size and growth, many techniques are able to explore uneven ventilation and air trapping. Body plethysmography - by either variable pressure or volume displacement - offers the possibility to measure both thoracic gas volume and airway resistance, and is the gold standard to study these two parameters. Specific airway resistance - the product of airway resistance and thoracic gas volume - is increasingly measured in preschool children. It appears to be useful in the assessment of responses to bronchodilators but requires careful and critical methodological evaluation when performed during tidal breathing. Gas dilution techniques are widely used in children employing multiple-breath or singlebreath techniques. The wash-in helium dilution technique is easy to perform even in preschool children, and results may be compared to thoracic gas volume determined by body plethysmography to estimate the volume of trapped air in the lungs. Nitrogen wash-out techniques may also provide indices of ventilation inhomogeneity. Asthma, cystic fibrosis, and chronic lung disease of prematurity represent the main chronic obstructive disorders in childhood.The measurement of static lung volumes is of help in interpreting spirometry, and may document hyperinflation as well as provide indices of uneven ventilation. The simple index RV/TLC is a robust indicator of lung hyperinflation and trapped air. In addition, body plethysmography offers the unique opportunity to assess the effects of volume history on lung function, which deserves further evaluation in childhood. In the next decade, optical reflectance motion analysis and magnetic resonance functional imaging may provide further insight into the physiological mechanisms that control lung volume and a more precise assessment of uneven ventilation.
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