The diagnosis and monitoring of pulmonary disease in young children are more difficult than in older age, because children under 6 years of age can rarely perform the manoeuvres needed for lung function measurements, such as forced expiration. A number of techniques applicable to lung function measurement in preschool children have been introduced. As these techniques require tidal breathing only and measurements can be performed in unsedated children down to the age of 2 years, these methods became more widely used in the 1990s.
1) Interrupter technique is based on the assumption that when the airway is suddenly closed, pressure in the mouth equilibrates with alveolar pressure. The ratio of this pressure to the airflow measured immediately before valve closure provides an estimate of interrupter resistance (Rint).
2) Forced oscillation technique (FOT) does not derive pulmonary resistance from respiration signals but from the pressure-flow-relationship of artificial oscillatory signals which are produced by an external generator. The use of multi-frequent impulses applied byimpulse oscillometry system (IOS) is a relatively new development. Regardless of the precise method of applying forced oscillation, primary measurements are made of the pressure and flow produced at the opening of the airway at a given frequency by the external applied forces. The instantaneous pressure-flow relationship (impedance of the respiratory system) can be described by its two components, resistance and reactance, and using these parameters the level of obstruction and differentiation between central obstruction and peripheral obstruction can be estimated.