Positive Pressure Ventilation and Increased Airway Resistance

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.

References

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (was well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.

Oakes, Dennis L. Physiological Effects of Positive Pressure Ventilation. AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH, 1992. -this is somebody's Masters of Science thesis! They received their degree in 1992, but one expects that the fundamentals of physiology have remained the same since then.

Kumar, Anil, et al. "Continuous positive-pressure ventilation in acute respiratory failure: effects on hemodynamics and lung function." New England Journal of Medicine 283.26 (1970): 1430-1436.