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