Monday, January 17, 2011

Identifying Optimal PEEP with the PB 840 Ventilator: the "Constant Low Flow Method"

Evaluating the Pressure/Volume curve for the lower inflection point, provides the operator a idea of where to appropriately set the level of PEEP. The curve identifies where lung recruitment begins and at what pressure/volume creates over distention and/or injury.

In newer generation ventilators (Draeger's Evita XL or Hamilton's Galileo or G5 platforms) obtaining the static pressure/volume curve at a low flow state is simple via the machines automated tools.

Conversely, in other ventilators obtaining a static low flow P/V curve can be complicated if not impossible to perform.

The PB 840 does not have a automated tool for performing a "Low Flow" maneuver, however the practitioner can do the maneuver manually using the following steps (1):

  1. The patient must be deeply sedated (e.g. RASS -3 or deeper) or a short-term paralytic can be used. This step is crucial, with the PB840 I have never been able to perform the procedure correctly unless the patient is very deeply sedated.
  2. Set up the ventilator to show a Pressure/Volume Curve (x = pressure, y = volume).
  3.  Place the patient on 100% FiO2.
  4. If the patient is not in VC-CMV (volume control) place patient in VC-CMV at current tidal & PEEP, make sure that the Flow is set to a "Square" pattern.
  5. Set the High Pressure alarm to 45 cmH2O or as needed.
In Rapid Sequence
  1. Turn PEEP to zero (0) & at the same time increase the tidal volume to obtain peak inspiratory pressures of 35-to-40 cmH2O.
  2. Decrease the set flow rate to 10 lpm, to do this you must decrease the frequency to ~ 5.
  3. Press the manual breath key & freeze the screen to obtain the pressure volume waveform.
  4. Return the patient to previous settings.
Evaluating the P/V curve
  1. Assess the initial slope of inspiratory compliance.
  2. Evaluate if there is a dramatic change in slope of inspiraotry compliance.
  3. The lower inflection point is the point of change from the initial slope to the change in slope (image 1).
  4. Set PEEP 1-2 cmH2O above the lower infliction point.

Image 1 PB 840 ventilator screen: P/V curve following a constant low flow maneuver in a patient with ARDS. Lower inflection point ~ 20 cmH2O.

1. Garbarini, P. (2003). P-Flex PEEP Determination: Constant Low Flow Method. Sentara Health Care. Norfolk, VA.

Additional Reading:
The Acute Effectiveness & Safety of the Constant-Flow, P/V Curve to Improve Hypoxemia in ALI.