Lung function in paediatrics. Lung function testing in children.

Lung function testing in children is an important part of assessment, diagnosis and monitoring of children with lung disease. Regular testing can also be used to evaluate lung growth, review the effects of treatment and assess the risks and benefits of specific clinical interventions. Such tests can also be useful in the pre-operative assessment of high risk patients undergoing surgical procedures. Children aged 5 or 6 are usually able to perform standard lung function tests such as spirometry, peak expiratory flow rate (PEFR) measurements, gas dilution techniques and plethysmography. In recent years similar techniques have been successfully developed for preschool children and infants. These will be discussed in later articles of this series. Spirometry and PEFR measurements are the easiest for children to understand and to perform, both in clinic and in the respiratory function laboratory. Spirometry primarily measures dynamic lung volumes such as forced vital capacity (FVC), forced expiratory volume in one second (FEV1) and maximal flow rates during expiration (PEFR). It also generates a diagram of flow against volume (flow-volume curve) which is particularly useful for the measurement of small airway obstruction. The spirometric forced expiratory manoeuvre begins at full inspiration—total lung capacity (TLC). At this point the airways are maintained fully open by the bony structure of the thoracic cage and the negative intrathoracic pressure generated during inspiration. During forced expiration the airways gradually close as overall chest volume decreases along with a reduction in the negative intrathoracic pressure. These changes result in closure of the small airways throughout the lung until at full expiration airflow no longer occurs. In the healthy child this does not significantly affect airflow during normal tidal breathing. The PEFR occurs normally very early during the expiratory cycle after about 12% of the expired breath has occurred. Factors which accelerate early airway closure include airway inflammation (for example asthma or cystic fibrosis), increased airway wall thickness, as in bronchopulmonary dysplasia) and increased airway muscle tone, as in asthma. Towards the end of expiration the flow-volume curve becomes effort independent. In lungs with increased airway resistance airway closure occurs before maximal contraction of the thoracic musculature. The amount of air left in the lungs at the end of a maximal expiration is the residual lung volume (RV) which can be measured by body plethysmography. The volume of air remaining in the lungs at the end of a normal tidal breath is the functional residual capacity (FRC). Total lung capacity (TLC) is the sum of vital capacity (VC), the volume of air in the lungs from residual volume to maximal inspiration plus RV. RV and FRC cannot be measured by spirometry. RV is calculated by measuring FRC and subtracting the expiratory reserve volume (ERV) which is the amount of air which can be expired from the end of a tidal breath down to maximal expiration. FRC is measured by various techniques including gas dilution (using helium), gas washout (using nitrogen) or plethysmography. The helium gas dilution method requires the subject to breathe into a closed system of known helium volume and helium concentration. Rebreathing occurs until a second stable concentration of helium is reached. During measurement, carbon dioxide is absorbed from the circuit and oxygen is added. FRC can then be calculated using a simple equation involving the original system volume, the initial and final concentration of helium. The nitrogen washout ARTICLE IN PRESS