As a result, a variable extrathoracic obstruction primarily affects the inspiratory portion of the flow volume loop, viewed as a flattening of the usual deep inspiratory curve. The expiratory portion of the loop appears relatively normal. Conversely, a variable intrathoracic obstruction mainly affects the expiratory limb, again giving a flattened appearance to that aspect of the loop. This can be difficult to distinguish from the more common small to medium sized airways obstruction that characterizes bronchitis, asthma, and emphysema. Finally, a fixed intrathoracic or extrathoracic obstruction affects both inspiration and expiration, giving a flow volume loop that has an overall box-like shape as both inspiratory and expiratory limbs flatten.

Variable extrathoracic obstructions may be caused by vocal cord paralysis, thyromegaly, tracheomalacia or neoplasm while large airways variable intrathoracic obstructions can also result from tracheomalacia or neoplasm. Examples of fixed obstruction include tracheal stenosis, foreign body or neoplasm.