Optimizing PEEP

Optimizing PEEP

• PEEP & CPAP are functionally the same, but PEEP is used during AC & SIMV where as CPAP is used for NIPPV & PSV

• PEEP & CPAP assist LV function by increasing intra-thoracic pressure thereby reducing preload

  • they also decrease LV afterload somehow by increasing intrathoracic pressure thereby increasing the pleural pressure

• PEEP should be used to correct hypoxemia in the setting of airspace consolidation or infiltration (i.e., when the CXR has white stuff)

• The major complication of PEEP is overdistention of alveoli, leading to impairment either/or

  • venous return & hypotension

  • gas exchange by compressing pulmonary capillary beds (increases dead space)

  • these usually don't happen w/a PEEP < 10-12

• All that said, the major cause of ventilator-induced lung injury (VILI) is not PEEP but excessive tidal volumes leading to volutrauma

• The different methods of titrating an optimal PEEP have been compared, and the findings demonstrate only the ARDSNet tables correlate w/the degree of whole-lung recruitability & ARDS severity

  • other methods were associated w/more hyperexpansion w/out a commensurate benefit in recruitment

• For now, the prudent approach is to simply achieve a "good enough" PEEP

ARDSNet Tables

• the ARDSNet tables are advantageous for simplicity & titratability to paO2 or SpO2

• there are two tables, one with a higher and one w/a lower PEEP strategy

• to use the tables

  • go up or down the table to maintain a paO2 55-80 or SpO2 88-94%

  • select whichever PEEP strategy appears best for your patient

    • the ALVEOLI study demonstrated no benefit of one table over the other, so pick a table according the clinical context

    • consider a lower PEEP strategy for

      • unstable hemodynamics

      • PTX

      • bronchopleural fistula

    • consider a higher PEEP strategy for

      • blunt chest trauma

      • abdominal wall trauma

      • obesity

Decremental PEEP Trial

• A decremental PEEP trial recruits a patient's lungs fully at the lowest possible CPAP

• it involves reducing PEEP until a reduction in oxygenation, compliance, or both is apparent

• it can be useful for patients w/moderate-to-severe ARDS for finding an appropriate level of PEEP

• disadvantages

  • time required to properly perform

  • need for deep sedation

  • possible hemodynamic or respiratory compromise during recruitment maneuver

• clinical trials examining decremental PEEP demonstrate improved oxygenation & respiratory compliance but have not proven any survival benefit

  • a relatively large trial actually demonstrated an increase in 28-day mortality w/such a recruitment maneuver strategy

  • in other words, it may not be reasonable to perform a decremental PEEP trial on every ventilated patient

• the trial is performed at bedside as follows

  • ensure adequate sedation (neuromuscular blockade not necessary)

  • set FiO2 to 100%

  • set CPAP to 40 w/no PS & hold 40 sec ("40 for 40" recruitment maneuver)

  • change to mode to VCAC (TV of 6 mL/kg PBW) or PCAC (driving pressure of 15)

  • set PEEP to 20

  • reduce FiO2 in 10-20% increments until SpO2 is 88-94%

  • reduce PEEP in increments of 2 until SpO2 < 88% or a notable drop in compliance (this is the level of de-recruitment)

  • repeat the 40 for 40 recruitment maneuver

  • set PEEP 2 higher than where de-recruitment occurred