Physiologically, the shape and flow magnitude of the inspiratory curve is limited by three main factors:
- Towards residual volume where the airway diameter is reduced or even collapsed the resistance is high, so air flows inwards more slowly
- As the lungs inflate the airway radial traction is increased by the surrounding negative pressures
- Airway dilatation reduces airway resistance significantly, which facilitates airflow into the lung
- At higher lung volume, the lung parenchyma, resists stretch
- Towards total lung capacity, it takes more energy to stretch the lung tissue, therefore airflow slows
- At residual volume, the diaphragm is stretched so the cross-bridge formation of actin and myosin cross-bridges deep within the muscle is sub-optimal. At total lung capacity, the diaphragm is short and formation of the muscle’s microfibres are again sub-optimal
- In both instances, when the cross-bridge formation is sub-optimal, the diaphragm muscle generates less force
- At approximately 50% total lung capacity the diaphragm is at it’s optimal length and so it generates its greatest force. This is reflected by the peak inspiratory flow
So, towards residual volume the resistance of the smaller airway diameter impedes airflow, whereas towards total lung capacity the elastic recoil of the lungs and the mechanical disadvantage of the inspiratory muscles limit their ability to further stretch the lungs.
Comparing the mid expiratory flow rates to the mid inspiratory flow rates (MEF50 to MIF50) will help to identify inspiratory flow limitation. This ratio in normal subjects should be less than one; the mid inspiratory flow rate should exceed the mid expiratory flow rate.
In subjects with inspiratory muscle weakness, the inspiratory flow rates will remain convex and symmetrical in shape but the magnitude of inspiratory flow will be reduced. On the other hand, an inspiratory curve which appears to be flattened or dissected, reflects increased resistance of the upper airway.
For the majority of the inspiratory manoeuvre performed by a subject with upper airway obstruction, air is flowing at a constant speed despite their maximal effort
This pattern of inspiratory flow limitation should be highly reproducible and is often accompanied by an inspiratory stridor. Correctly performed inspiratory manoeuvres provide vital information about the strength of the inspiratory muscles and patency of the upper airway.
Similarly to the upstroke, the inspiratory limb is effort dependent; the patient must be trained to perform this manoeuvre correctly to prevent misinterpretation.