Card: PEEP Trial

Although there is no consensus regarding what determines "best" PEEP, some experts define "best" PEEP as the level of PEEP at which oxygen delivery is optimal.

Others believe that the level of PEEP that maximizes lung compliance, coincides with the greatest oxygen delivery, lowest alveolar deadspace and maximal alveolar recruitment. A PEEP trial will determine the best PEEP in an individual patient.

In a PEEP trial, the level of PEEP is increased gradually in increments of 2 cm H₂O to an arbitrary upper limit (i.e. 15 to 18 cm H₂O ).

If a pulmonary artery catheter is in place, oxygen delivery ( DO₂ ) is calculated with each change in PEEP, using the formula:

DO₂ = ( Hgb x SaO₂ x 1.34 + PaO₂ x 0.003 ) x CO

where Hgb = hemoglobin concentration, SaO₂ = arterial oxygen saturation and PaO₂ = partial pressure of oxygen in arterial blood.

Oxygen delivery declines if the drop in cardiac output caused by PEEP outweighs the rise in arterial oxygen content. Therefore, the best PEEP may be less than the amount that achieves the highest oxygen saturation. If a pulmonary artery catheter is not available, the best PEEP may be approximated by determining the level which results in the highest compliance (CL) for a given tidal volume, using the formula:

CL = TV / (Ppl - PEEP)

where TV = tidal volume and Ppl = plateau pressure.

Whatever technique is chosen, it is important to remember that

a) raising PEEP and pushing oxygen delivery can lead to significantly increased peak airway pressures and risk of barotrauma, and

b) cardiac output may fall indepently of changes in thoracic compliance.

A combination of both methods is beneficial.