PEEP is used to recruit collapsed airways or to help keep open airways that would otherwise collapse at the end of expiration. It will increase the functional residual capacity, thereby increasing the number of lung units available for gas exchange. Hence the fraction of the cardiac output passing through non-ventilated lung units (the shunt fraction) is reduced.

The price paid for this benefit is an elevation in mean intrathoracic pressure which not only increases the risk of barotrauma/volutrauma, but can also result in a diminution of cardiac output. As the amount of oxygen delivered to the tissues depends upon arterial oxygen content and cardiac output, a decreased cardiac output could result in decreased oxygen delivery even if PaO2 is improved.

PEEP may decrease the cardiac output not only by increasing the mean intrathoracic pressure (which may consequently decrease venous return) but also by pressure transmitted directly to the heart from the adjacent lung tissue. In addition, PEEP is transmitted preferentially to normal lung regions. This means that PEEP applied at an inappropriately high level will inhibit blood flow through well-ventilated lung units and will cause a redistribution of blood flow to less well-ventilated areas.

Criteria used to determine 'best PEEP' have included the following:

1. lung mechanics;

2. the PEEP setting that reduces shunt fraction to less than 15 per cent of the cardiac output;

3. the setting giving the greatest oxygen delivery for the lowest FiO 2;

4. the level showing the greatest recruitment of lung units on CT imaging.

However, the optimal method for determining 'best PEEP' is still controversial, and is usually a compromise of balancing the advantages with the disadvantages of PEEP. In most patients there is arguably little to be gained in choosing a PEEP level higher than 15 cmH 2O.

If we use a combination of limitation of airway pressure and a limited amount of PEEP, this will result in a low tidal volume in patients who have a reduced lung compliance. This accepts that the CO2 levels will be allowed to rise (permissive hypercapnia). Pressure limitation and accepting smaller tidal volumes may, in addition to the advantages mentioned above, be instrumental in reducing shear stress.

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