Methacholine Challenge Test (MCT) in Healthy Adults

Asthma is a common chronic disease that is characterized by a history of variable respiratory symptoms and variable expiratory airflow limitation, and usually associated with airway hyperresponsiveness.

In general, FEV1 is used to test for airway hyperresponsiveness (AHR), and a 20% decrease after methacholine challenge test (MCT) is considered a positive test.

There is increasing recognition that the small airways are involved in 40-80% of patients with asthma, and the function of these airways is overlooked (the "silent zone") when measuring FEV1, which mainly reflects the function of the central airways.

Small airway dysfunction (SAD) can be assessed by IOS where pressure applied to the airways at a range of frequencies, and components of respiratory resistance and reactance are measured. Resistance at 5 Hz (R5) and 20 Hz (R20), respectively, represent total airway resistance and proximal airway resistance. The difference between these two values can be calculated (R5-R20). High R5-R20 and low reactance at 5 Hz (X5) indicate the presence of SAD.

Knowledge of MCT and IOS is limited. Only three studies have included adult patients, and none of these with healthy controls. These studies have combined IOS and spirometry in the same test, although it is not recommended to perform IOS after a test that requires deep breathing, e.g. FEV1 measurement. Deep breathing is considered to affect the impedance via reduced tone in the smooth muscle.

The investigators have the following objectives for these studies:

1. To determine the IOS response to MCT in healthy adults.

2. To investigate whether spirometry (FEV1) carried out in relation to MCT influences IOS by comparing the IOS response to MCT performed with and without spirometry in healthy adults.