Thursday, February 10, 2011

A Problem with Plateau Pressures

When utilizing mechanical ventilation the plateau pressure is commonly evaluated to identify the potential for lung injury.  Many protocols (sepsis, ARDS) indicate targeting plateau pressures ≤ 30 cmH2O as a lung protective tactic.  

Unfortunately, in some patients the Plateau pressure may be falsely elevated and be relatively different then the transpulmonary/pleural pressure.

This disparity can be very common in the morbidly obese patient, patients with abdominal compartment syndrome, and patients with extreme fluid overload. In the obese patient, it has been well documented that as body mass index (BMI) increases, functional residual capacity (FRC) drops significantly. Compliance is falsely calculated due to the abdomen pressing down on the thorax.

Image 1: Low flow P/V curve on PB840 ventilator, showing a lower inflection point of 12 cmH2O. Patient was  post-op bowel surgey, normal lungs, no ARDS. However, patient was ~5'-3" @ 165 kg.

This misleading measurement can lead the practitioner to adjust the ventilator inappropriately. An example of this is when setting “P-High” during Airway Pressure Release Ventilation (APRV, Bi-Level, and Bi-Vent). One protocol states to set P-High at or 2 cmH2O above the measured plateau pressure. Setting P-High based on a falsely high plateau pressure may lead to under recruitment.
Another area of concern is when setting PEEP & increasing the mean airway pressure. If the plateau is already high then the practitioner is restricted in regards to increasing PEEP and/or tidal volume.

So what can be done in these patient populations in which the plateau pressure may be falsely elevated?

The clinician can use an esophageal balloon to measure transpulmoanry/pleural pressures & adjust settings based on these measurements.

I have little knowledge in this area so I contacted a colleague, who is experienced in esophageal balloon monitoring.

Commentary from an interview with Troy Whitacre RRT, University of Missouri:

1.      How many patients have you placed esophageal balloons in?

Troy- “Approximately 12 patients”.

2.      What type of patients were they?
Troy- “1 H1N1 patient, 1 patient with abdominal compartment syndrome, more than six morbidly obese patients”.

3.      How many times were measured plateau pressures different from the alveolar pressure obtained from the balloon?
Troy- “only one time was the alveolar pressure similar to the measured plateau pressure, this was in the ARDS H1N1 case”.

4.      What was the highest difference between plateau & pleural pressures do you recall?
Troy- “measured plateau pressures in the high 40’s –to-low 50’s, were as the esophageal measurement was in the high teens-to- mid 20’s (~25 cmH2O difference)”.

5.      After you obtained the balloon measurement what usually happened?
Troy- “except in the H1N1, we always increased the PEEP”.

6.      How has utilizing esophageal balloon monitoring changed your practice?
Troy- " It has allowed us to be a bit more liberal with PEEP in the setting of the super morbid obese, abdominal compartment syndrome and with extreme third spacing where the chest wall has become grossly thickened".

7.      I know your institution liberally, use higher PEEP levels, what is your starting PEEP & how do you titrate?
Troy- “we start at 8 cmH2O (same as ARDS net, @ 40% fiO2), titrate up for oxygenation, volume recruitment/compliance, or for triggering/synchrony but always attempt to limit PIP & plateau pressures.

8.      Has there been any case that you discovered that you were using too much PEEP & what was this based on?
Troy- “Yes, with H1N1 ARDS patient, although limiting tidal volume to 4-6 ml/kg, the esophageal balloon reading was still unreasonably high. Unable to titrate due to high fio2 & low spo2. The patient was placed on HFV (oscillator); however the patient status remained unchanged”.


Mechanical Ventilation in ARDS due to Sepsis


M Plataki… - Current Opinion in Critical Care, 2011
Introduction Despite advances in the understanding and treatment of acute lung injury (ALI) and
its more severe form, the acute respiratory distress syndrome (ARDS), incidence and mortality
remain high [1,2]. Evidence has been accumulating for the past 50 years that mechanical ...