The application of positive pressure to a respiratory system with increased resistance to flow does two things. Firstly, the increased PEEP balances whatever intrinsic PEEP there may be, thereby reducing the work of breathing (at least the work of spontaneous breathing)- more on that later. Secondly, the application of positive pressure in inspiration increases the pressure gradient along which gas flow occurs. No longer must the struggling respiratory muscles generate all of this pressure on their own; for a given amount of respiratory muscle effort, a greater pressure gradient is achieved, and thus a greater gas flow can occur. Thus, positive inspiratory pressure decreases the work of breathing in the context of increased airway resistance.
Let us explore this concept with the aid of silly diagrams.
Work of breathing due to airway resistance
In the spontaneously breathing patient, the lungs must generate a certain negative pressure to draw air into the lungs.
Lets call it X.
If there is an airway obstruction, work of breathing increases ( because greater pressure must be generated to overcome the obstruction)
This is flow-dependent airway resistance. In order to overcome it, the diaphragm must generate additional pressure.
Lets call it Y.
…so ideally, if the positive pressure you apply exactly matches that pressure, the effort of breathing is reduced back to its pre-obstructed state.
In absence of flow limitation, PEEP pressure is transmitted directly to the alveoli.
Because airway pressure = flow x resistance +(alveolar pressure) + positive pressure
when resistance approaches zero, airway pressure = alveolar pressure + positive pressure
Thus in a normal person with normal airway resistance, the positive pressure goes straight to the alveoli.