Intrinsic PEEP and its evils
Intrinsic PEEP is still PEEP. It has all the benefits and problems of actual machine-generated PEEP. It is an end-expiratory positive pressure. A spontaneously breathing patient will have to fight hard to get air into their lungs if there is too much intrinsic PEEP; the pressures their respiratory muscles generate must turn that positive pressure into a negative pressure- and not just slightly negative, but significantly negative (to create a large enough gradient between the airway opening pressure and the alveolar pressure, to suck air past the obstructed bronchospasming airway).
In this diagram, the airway resistance is increased.
The intrinsic PEEP (Pi) is 5cmH2O
The pleural pressure is -1cmH2O
The total intrathoracic pressure is till +4 ( i.e 4 cm H2O above the atmospheric)
Thus, there will be no flow until this gradient becomes negative (i.e. the intrathoracic pressure becomes negative compared to atmospheric pressure)
The X and Y in this situation represent a normal work of inspiration, which in this situation is not enough to overcome intrinsic PEEP.
Increased work of breathing to overcome intrinsic PEEP
In the diagram, the intrapleural pressure had to be decreased to -6 cmH2O in order for flow to occur. Now, there is a pressure gradient (of -1cmH2O ) and this drives the flow.
The work of breathing to achieve this piddly gradient has been Greatly Increased, according to the diagram.
Effect of PEEP on decreasing the work of breathing due to intrinsic PEEP
PEEP supplements the additional force required to drive the pressure gradient. In essence, extrinsic PEEP increases the effective atmospheric pressure so that the intrathoracic pressure ends up being more negative, and a pressure gradient is easier to generate.
This greatly reduces the work of breathing.