Thursday, July 26, 2012

Setting PEEP

There is many ways to set Positive End Expiratory Pressure (PEEP). Setting PEEP too low may result in under or tidal recruitment of the lung and PEEP that is too high results in over-distention, both contribute to Ventilator Induced Lung Injury (VILI).  This post provides a synopsis of the various techniques as well as potential pros & cons.

PEEP Tables
Image 1: PEEP Tables from ARDS Network

Setting PEEP based on Fio2 levels is from the ARDSnet studies/ network.

-Based on large clinical trials.
-Simple to use that helps with adherence and reduces practice variation.
-Can utilize on any ventilator.

-Does not consider trying to decrease FiO2.
-There are multiple factors which may contribute to hypoxia and the need for higher FiO2 setting. Not all of these factors respond to higher PEEP levels.
-May lead to over distention or under inflation (e.g. an obese patient or a patient with decreased extra-thoracic compliance may need more PEEP). 

Decremented PEEP Strategy
Following a lung recruitment maneuver, the operator starts at a higher level of PEEP and decreases the PEEP (~5 cmH2O/minute) to a lower level. PEEP is decreased to the lowest level that maintains the oxygenation benefit of the recruitment maneuver. Also consider CO2 clearance if monitoring is available.  
-Easy to perform.
-Can be performed on any ICU ventilator.
-May have initial hemodynamic compromise on the higher PEEP levels.
-Takes additional time.
-Many different types of recruitment maneuvers.

Static Pressure Volume Curve Analysis

Image 2: Hamilton Medical's P/V tool (David Grooms).
Performed using a minimal flow technique /modified super syringe. The curve is evaluated for either the lower inflection point, point of maximum curvature, and/or maximum hysteresis.

-If the ventilator has automation software this is a simple procedure.
-Automated software allows for high inter-user readability.

-Patient must be heavily sedated or paralyzed, no spontaneous efforts during the maneuver.
-Controversy, on what to set PEEP level on, lower inflection point, maximum hysteresis, or deflation limb?
-Multiple steps if there is no automated software on the ventilator. See example:

Esophageal Manometry

Image 3: Hamilton G5 showing esophageal monitoring "Paux" measurement (Paul Garbarini). 
The clinician can use an esophageal balloon to measure transpulmoanry/pleural pressures & adjust settings based on these measurements.

-Reflects true transpulmoanry/pleural pressures.
-Patient does not need to be sedated.
-Extra equipment & trained personal needed (if ventilator does not have software).
-Invasive procedure (esophageal ballon catheter).
-Extra training required.

Volumetric Carbon Dioxide Measurement

Image 4: Respironics Volumetric CO2 monitor (David Grooms).
Setting PEEP is based on two parameters; carbon dioxide elimination (VCO2) and alveolar minute volume (mValv).

-Patient does not need to be sedated.
-Can identify both under recruitment and over distention.
-Extra equipment needed.
-Extra time.
-Maybe inaccurate with excessive shunts and/or dead space. 

Additional link:

Stress Index

Image 5: Screen shot of Servo-i showing stress index monitoring. 

The stress index is determined by analyzing the airway opening pressure over time, during a constant flow.
-Based on MRI data.
-Value is easy to comprehend.
-Patient has to be heavily sedated or paralyzed.
-Only can be accessed using VC-CMV.
-Flow & pressure-time curves are needed.
-Calculated value is only available on the Servo-i at this time. 

Additional link: