Baseline PEEP (PEEPbaseline) was set at 5 cmH2O and titrated at 2 cmH2O increments until the partial arterial oxygen tension (PaO2) reached at least 300 mmHg, peak airway pressure was 45 cmH2O or greater, or MAP dropped by 20% or more from the baseline value. Criteria for overinflation of lung (and therefore for discontinuation of further titration of PEEP) were reduction in PaO2 of 10% or more and an increase in arterial carbon dioxide tension of 10% or more. Optimal PEEP (PEEPopt) was defined as the PEEP that achieved the best oxygenation, whereas maximum PEEP (PEEPmax) was the greatest level of PEEP achieved during titration in each patient.
http://www.medscape.com/viewarticle/459060_2

PEEP can also be detrimental. Because the tidal breath is delivered on top of the baseline PEEP, end-inspiratory pressures are raised by PEEP application, although this may be mitigated by improved compliance (as noted above). This must be considered if the lung is at risk of overstretching injury. Moreover, because alveolar injury is often quite heterogeneous, appropriate PEEP in one region may be suboptimal in another, and excessive in yet another. Thus, optimizing PEEP is a balance between recruiting the recruitable alveoli in diseased regions without overdistending already recruited alveoli in healthier regions.
http://www.medscape.com/viewarticle/558468

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Note added at 6 hrs (2009-03-07 01:33:10 GMT)
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More on "baseline PEEP"
http://ccforum.com/content/7/3/R17

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Note added at 14 hrs (2009-03-07 09:45:24 GMT)
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BTW, these are references to lung parameters, nothing to do with cardiology.

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Note added at 16 hrs (2009-03-07 11:36:40 GMT)
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There should be no confusion between the PEEP itself and its baseline pressure. Your sentence is referring to the baseline pressure OF the PEEP. In other words, the machine setting will not maintain this pressure (which is a type of mechanical pressure that is exerted in the alveoli after expiration to prevent their collapse), but will ultimately drop to 0 cm H2O, i.e., no pressure.