Inspiratory and expiratory flow rates are a function of driving pressure (i.e. the pressure difference between the alveoli and the atmosphere) and airway resistance. For this reason it would seem that airway resistance should be one of the most commonly performed pulmonary function tests but instead it is the outcome of airway resistance and driving pressure, i.e. the expiratory and inspiratory flow rates that are measured almost exclusively. One reason for this is that resistance measurements requires relatively expensive equipment such as a body plethysmograph or an impulse oscillometer as well as a fair amount of technical expertise.
The airflow perturbation device (APD) is a potentially inexpensive system for measuring respiratory resistance during tidal breathing. The device itself is mechanically simple, the concepts and mathematics that permit it to work are, however, a bit more complicated.
The APD consists of a mouth pressure transducer and a pneumotach whose end is attached to a rotating wheel. The wheel has open segments and segments with a mesh that partially obstructs airflow through the pneumotach. The rotation of the wheel causes a series of perturbations to the airflow through the pneumotach.
