Monday, January 24, 2011

APRV: Setting P-High based on the Static Pressure Volume Curve

Some newer mechanical ventilators provide the operator with automated tools to obtain a static Pressure Volume (P/V) Curve in the ventilated patient. These tools provide the clinician a simple, safe, and reproducible method to assess the P/V curve for various pulmonary conditions. 

Photo 1: Hamilton G5 ventilator screen, showing the "P/V tool" software to obtain a static P/V curve.

For example many Respiratory care practitioners utilize the P/V to identify the lower and upper inflection points. “The lower section of the P/V curve, where the compliance is less favorable, corresponds to a condition in which a given number of alveolar units are collapsed” [1].

 It is common for the practitioner to set Positive End-Expiratory Pressure (PEEP) 1-to-2 cmH2O above the lower inflection point to prevent de-recruitment and minimize injury related to shear stress from the repetitive opening and closing of the alveoli.

The upper inflection point indicates a decrease in lung compliance and may specify over-distention of the lung units.

The assessment of the static P/V curve can provide insight on how well a patient will respond to the application of Airway Pressure Release Ventilation. Evaluation of the “hysteresis” (difference between the inflation and the deflation limb) can indicate the extent of potential lung units that may be recruited. If the patient has very little probability for alveolar recruitment then APRV is less likely to be beneficial and unjustified.

Photo 2: Post static pressure/volume maneuver using a Draeger Evita XL ventilator. Notice the LIP, UIP, & descending limb.

If the patient meets indications for utilizing APRV, the operator can use points on the P/V to safely set P-High.

  1. P-High should be set always below the pressure which generated the “Upper Inflection Point”.
  2.  P-High can be set 1-to-2 cmH2o above the “Lower Inflection Point”.
  3. The practitioner can calculate “Best Compliance” [1, 2] and set P-High according to the pressure that generated the best compliance (some P/V tools automatically calculate this).

The static P/V curve provides a more diagnostic and patient tailored approach to setting P-High.

  1. Lotti, G. Brashi, A. Measurements of Respiratory Mechanics During Mechanical Ventilation. Rhuzuns, Switzerland. 1999: 103-113.
  2. The “Best Compliance” measurement provides information on the amount of normally aerated lung tissue after maximal alveolar recruitment has been accomplished.