The inspiratory portion of the loop is a deep semi-circular curve plotted on the negative (bottom) portion of the flow axis. The maximal inspiratory curve is often ignored or performed with sub-maximal technique. However, it provides invaluable information about the inspiratory muscle function and the patency of the upper airway.
The normal shape of the inspiratory loop is semi-circular as is illustrated below.
The primary muscle of inspiration is the diaphragm. During inspiration the diaphragm contracts, shortening the muscle fibre length. The shape of the diaphragm flattens, pulling the lungs downwards, compressing the abdominal contents and pushing the ribcage up and outwards. These actions create a negative pressure within the lungs, facilitating inspiration of air.
During a maximal inspiration, the accessory muscles of inspiration are also recruited. This pulls the rib cage upwards and outwards further to fill the lungs completely.
It is important to remember is that the inspiratory portion of the flow-volume loop is effort dependent; it requires maximal contractile force of the inspiratory muscles throughout the manoeuvre. The subject should understand the urgency and effort required for this portion of the test.
Unlike the shape of the expiratory portion of the flow-volume loop (triangular shape), the distinct feature of a maximal inspiratory flow volume loop is its curved symmetry.
During expiration, the flow rates are very different towards TLC compared with those towards RV. However the flow rates of the inspiratory curve are limited to a similar extent at both the TLC and RV. During the central portion of the manoeuvre the inspiratory flow rates are notably high.