Lung Function Testing in Infants

Abstract

The neonatal period and the first year of life are times in which a significant amount of disease is caused by lung disorders related to a number of age-related factors. These include the lack of lung development due to preterm birth, the effects of intensive care treatment on the immature lung and early life infections with highly pathogenic agents including respiratory syncytial virus. Prenatal factors such as impaired foetal growth and maternal smoking in pregnancy can also have significant effects on lung function both in the immediate neonatal period but also throughout the first year of life and beyond. It has become increasingly recognised that lung pathology in infancy and early childhood is an important predisposing factor in the aetiology of long-term lung diseases in later childhood and possibly into adult life. The measurement of lung function in the neonatal period and during infancy has required the development of equipment specially adapted to the physical size of the patient and the unique physiology of breathing throughout this early phase of life. During the latter part of the 20th century specific tests to measure respiratory mechanics and to enable forced expiratory manoeuvres using the forced expiratory technique were developed. In more recent times forced inspiration to maximal lung capacity has also been developed. Some lung function tests can be performed reliably and reproducibly in unsedated preterm neonates. An important factor in measuring lung function after this period and especially during the first year of life is the need to sedate the patient in order to obtain cooperation during the specific respiratory manoeuvres mentioned above. Currently, chloral hydrate is the most frequently used agent. It has been shown to be safe and effective and also not to affect the results obtained as compared to unsedated infants. Standardisation of techniques and the establishment of normal values has been another major area of work in this field. The establishment several years ago of a joint European Respiratory Society/American Thoracic Society (ERS/ATS) task force on infant lung function has been responsible for many publications in this field and also the setting of standards for manufacturers of lung function equipment for this age group. Anatomical aspects are also important in this age group. During infancy, nasal breathing is predominant and can account for as much as 50% of total airway resistance, this may be especially important if measuring lung function 3–4 weeks after an upper respiratory tract infection. Another important anatomical aspect in infancy is that the chest wall is much more compliant than in later life. This means that during normal passive expiration the inward pull of the intrinsic elastic properties of the lung parenchyma will result in a lower level of overall lung capacity as compared to that of older children and adults. The result of this is that in some infants small airway closure occurs very close to the end of normal tidal breathing.