ARDS:

• There people have reduced aerated lung volume
• Yes, it helps to set a high PEEP to increase the aerated lung volume
• Yes, very high PEEP can cause VILI
• No, nobody can agree on how high the PEEP should be

The ALVEOLI Trial: by ARDS network, (2004) - randomized 549 patients to high PEEP vs low PEEP.

• Same volumes (around 6ml/kg) and plateau pressures; No survival benefit.
• The investigators recommend you have a try of high PEEP ventilation, and if it improves the oxygenation of your ARDS patient, then you are a winner; and if it doesn’t work, don’t feel too bad, because there is no evidence it makes any difference in the long run anyway.

ASTHMA:

• These people have an increased intrinsic PEEP and higher airway resistance. The main problem is high expiratory resistance, so it takes a while to empty the lungs.
• Less responsive to PEEP than COPD
• This is probably because there is too much intrinsic PEEP; plus mucus plugs might be blocking some of the airways, making them immune to the benefits of PEEP.
• The result is that PEEP might actually cause worsening hyperinflation and increased gas trapping.
• Some data suggests that low level PEEP is beneficial.

COPD:

• These people have an increased intrinsic PEEP and higher airway resistance.
• PEEP reduces the workload of respiratory muscles by counteracting both intrinsic PEEP and airway resistance
• Also, it may actually splint the airways, resulting in improved emptying of the trapped gas.
• No trials to test whether PEEP in COPD is of any use whatsoever. 

PULMONARY OEDEMA:

• The main problem is a negative intrathoracic pressure generated due to decreased lung volume and decreased lung compliance
• PEEP works directly on this problem.
• Most people would agree that about 10cmH2O of PEEP is the standard of care for cardiogenic pulmonary oedema.

Contraindications to PEEP

• Tension Pneumothorax  - it will get worse
• Hypovolemic shock – cardiac output will decrease
• Bronchopleural fistula -  it wont heal
• High intracranial pressure - it will get higher
• Right ventricular failure - the failing right ventricle may fail more with the addition of increased afterload