Abstract

Most existing lung function tests require a substantial degree of interactive co-operation between patient and clinician. This reduces the clinical usefulness of such tests in patients with cognitive impairment (mainly elderly people), a reduced level of consciousness (for example in patients with a head injury or a severe metabolic disturbance) and children too young to perform the tests. There is a need for measures of airways status that are easier to apply. If the airflow is briefly occluded during inspiration the pressure fall in the airway is proportional to the force of contraction of inspiratory muscles and, because it occurs before flow begins, it is not mechanically altered by airflow resistance. The maximum rate of pressure fall is referred to as dP/dtmax with the units kPa s−1. After the brief occlusion, the air column accelerates at a rate that is dependent on both dP/dtmax and the resistance to airflow. The maximum rate of change of airflow is referred to as dV/dtmax, with the units L s−1 s−1. This paper posits an hypothesis that the ratio dV/dtmax/dP/dtmax provides an index with the units L s−1 kPa−1 that is a measure of the transformation of inspiratory effort into airflow, and can be referred to as inspiratory transductance. This index is predicted to rise when airflow resistance falls, such as would occur when an attack of asthma responds to bronchodilator drugs. The index is expected to continue to retain directional integrity even when inspiratory muscles become fatigued. Measuring the index would require little co-operation from a conscious patient and none from an unconscious patient.