Sunday, February 20, 2011

Using Volumetric Carbon Dioxide Measurements to Optimize “P-High” During Airway Pressure Release Ventilation

There are many concerns when utilizing Airway Pressure Release Ventilation (aka. APRV, BiLevel, BiVent) in regards to lung injury.
 Inappropriate P-High settings may lead to large release-volumes resulting in over-distention, and volume induced lung injury [1].

One tool previously mentioned to help identify the optimal P-High setting is the static pressure/volume curve. Unfortunately, this tool cannot be utilized in patients that are spontaneously breathing, and it would be unrealistic to try to perform this maneuver once APRV is initiated.

Nevertheless, there is another option. This alternative uses volumetric carbon dioxide monitoring, which is available on newer mechanical ventilators. Respironics offers a stand-alone unit the "NM3", which gives the practitioner additional flexibility.

End Tidal CO2 (EtCO2) monitoring has been available for many years in the operating room, it is valuable in identifying proper E.T. tube placement, helps with targeting minute ventilation during general anesthesia, and has become the "fifth vital sign" [2].

Monitoring Volumetric CO2 provides extra values: Cardiac output, cardiac index, stroke volume, pulmonary capillary blood flow, determining true alveolar tidal volume, and dead-space, and monitoring appropriate PEEP levels & alveolar recruitment.

To set P-High Based on Volumetric CO2 you will need to monitor two parameters:

  1. Carbon Dioxide Elimination (VCO2)
  2. Alveolar Minute Volume (MValv)

Step one
- Collect a baseline VCO2 & MValv trend of at least 10-15 minutes of data.

Step Two
-          Increase P-High by 2 cmH2O & monitor VCO2 & MValv for five minutes. VCO2 may immediately decrease after increasing P-High. MValv will rise sharply & VCO2 will return to baseline if lungs are recruited.
-          If MValv does not rise & VCO2 does not return to baseline, the alveoli are most likely over-distended & the pulmonary capillaries compressed. Return P-High to the previous setting.

Step Three (if there was a ↑ in MValv)
-          Continue to increase P-High in 2 cmH2O increments, while monitoring (5 minutes) for Rises in MValv & VCO2.
-          Stop titrating P-High when MValv plateaus & VCO2 does not return to baseline.
-          Place patient on the P-High setting which created greatest MValv with the VCO2 returning baseline, this is the point of optimal recruitment without over-stretching.

2. The "fifth Vital Sign": Respiratory Rate, Heart Rate, Blood Pressure, is the three standard vital signs, pulse-oximetry (fourth), & EtCO2 (fifth).