PEEP and Intrinsic PEEP

Created on Tue, 06/16/2015 - 17:00
Last updated on Tue, 06/16/2015 - 17:07

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.

 

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.