In order to understand PEEP let’s first understand what occurs in the alveoli when each breath is taken.

Airways

As inspiration occurs (1) the alveoli expands to allow the air in. Gas exchange can then take place as the blood supply moves past the wall of the alveoli.

During expiration the alveoli contracts down (2). It does not completely collapse, partly due to the presence of a substance called surfactant (3). This increases the surface tension within the alveoli ensuring that complete collapse cannot take place.

Unfortunately, ventilation of a patient tends to inactivate the pulmonary surfactant which then leads to collapse of the alveoli (4), making gas exchange more difficult as the surface area of the lung is now reduced. The ventilator also causes an increase in alveolar capillary permeability and causes the activation of inflammatory cells and the release of cytokines.

The consequence of this is that the alveoli are opening and collapsing much more than they would normally and will also be subject to higher pressures in order to reopen them with each breath. This combination will damage the alveoli further.

Positive end expiratory pressure (PEEP), is a pressure applied by the ventilator at the end of each breath to ensure that the alveoli are not so prone to collapse. This ‘recruits’ the closed alveoli in the sick lung and improves oxygenation.

So PEEP:

  • Reduces trauma to the alveoli
  • Improves oxygenation by ‘recruiting’ otherwise closed alveoli, thereby increasing the surface area for gas exchange.
  • Increases the functional residual capacity- the reserve in the patients lungs between breaths which will also help improve oxygenation.
  • Ventilation/perfusion mismatches are improved.
  • Increases the compliance of the lung- compliance is the relationship between the change on volume and the change in pressure in the lung. With PEEP, less pressure is needed to get the same volume of air into the lung as the alveoli are already partially inflated and therefore do not need that high initial pressure to open them. (Remember the balloon analogy- hard to blow up initially, but then much easier to inflate after the initial breath).

Problems with PEEP

PEEP can cause some problems for those patients who have some airway obstruction i.e. Asthmatics and those with COPD.

auto peepIf we look at the alveoli of a person with obstructive disease we can see the obstruction on the airway (3) and the ventilator is blowing air down into the alveoli (1).

Once the ventilator has finished putting air into the lung, expiration is then a passive process, relying on the passive recoil of the chest wall and lung (2).

But because the obstruction is there, this air takes longer to get out of the lung. The ventilator does not wait for the air to come out before it delivers the next breath. This means in the obstructed patient that not all the air will come out of the alveoli before the next breath comes in.

The air that is left over will exert a pressure on the alveolar walls, helping to keep them open (4).

As continued breaths come in the alveoli will become larger, so exerting more pressure on the internal walls of the alveoli (4). The increased force on the inside tends to then increase the recoil exerted by the lung tissue on the outside of the alveolar wall (5). This increased recoil will help push some more air out of the alveoli past the obstruction.

This process will continue until a steady state is reached, where the amount of air coming in is equal to the amount of air coming out (6).

This balancing of pressure, with the ventilators involvement, keeps the alveoli open and is referred to as Auto-PEEP and the lung volumes, which were higher than before, are referred to as Dynamic Hyperinflation.

The phenomenon of not being able to get one breath out of the lung before the next breath comes in is known as Breath Stacking.

The other problem PEEP can cause is a drop in cardiac output. Venous return to the heart is very dependent on the difference in pressure between that in the thoracic cavity (Pt), where the heart is enclosed, and that in the circulatory system (Pet).

VR = Pet – Pt

PEEP on CO1PEEP will cause a rise in the intra thoracic pressure, meaning the difference between the two pressures will fall, causing a reduction in the venous return.

The increased pressure in the thoracic cavity also increases the pressure in the pulmonary system, meaning that the right side of the heart has higher pressures to push against to get the blood through the lungs.

This in turn makes the right side become bigger, which then pushes against the left side of the heart which will then reduce cardiac output.

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[Audio_files_sig]
Intubation
6 Ways To Be Better With a BVM
Mechanical Ventilation- Physiologic Effects
Anatomy of Adult ETT
Mechanical Ventilation- Terminology
Mechanical Ventilation- Modes of Ventilation I
Mechanical Ventilation- Modes of Ventilation II
Mechanical Ventilation- Pressure/Volume/Flow Loops
Mechanical Ventilation- Peak Pressure and Plateau Pressure
PEEP (Positive End Expiratory Pressure)
Increase the rate or the tidal volume
Ventilation/Perfusion
Ventilation Screen- What do some of those numbers mean?
Phase Variables
I:E ratios and total cycle time.
Why do we ventilate?.
Volume controlled ventilation and compliance.
Lung compliance.
How do I describe how my patient is being ventilated?
Pressure/Volume loop
AC versus SIMV
A-a gradient
Pressure Support
Pressure Support Ventilation Curves
Pressure/Volume/Flow Curves
Ventilator Induced Lung Injury
Trigger, Limit and Cycle
Ventilator Associated Pnuemonia

 

Originally posted 2015-04-01 07:00:31. Republished by Blog Post Promoter

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