2001 OPEN FORUM Abstracts
PflexCAN BE RELIABLY ESTIMATED FROM STATIC PRESSURE-VOLUME CURVES GENERATED WITHPEEP SET AT 5 cmH2O
Dean Holland RRT, RobertEstetter RRT, John Boynton RRT, Kenneth Hawkins RRT, Grant O?Keefe MD MPH,Departments of Respiratory Care and Surgery Parkland Health and Hospital System,Dallas, TX
Introduction: Itis possible that the determination of the lower ?inflection point? (Pflex)on a static pressure-volume (static P-V) curve can be used to optimize positiveend-expiratory pressure (PEEP) in patients suffering from acute lung injury(ALI/ARDS). The clinical utility of pressure-volume curve measurements depends,in part on the safety of the technique. Equally important is the eliminationof inter-reader variability in the interpretation of Pflex. The purposeof this work is to determine whether Pflex estimation, based uponobjective interpretation of static P-V curve measurement can be done with PEEPset at 5 cmH2O rather than 0 cmH2O. We hypothesized that the Pflexwould not be different with PEEP set at 5 cmH2O compared with 0 cmH2O.
Methods: Eightpatients who presented to our SICU with PaO2/FiO2 ratio <300, mean lung injuryscore of 2.91, identified risk factors for ARDS, and the absence of exclusioncriteria were identified as study participants. Exclusion criteria include anyof the following factors: a history of chronic obstructive pulmonary disease,heart failure, or persistent chest tube leak. Following sedation patients wereparalyzed and static P-V curves performed with PEEP set at 0 and 5 cmH2O, appliedin random order. Static P-V curves were obtained using two commercially availableventilators and the valve method we have previously reported (respiratory rate60, tidal volume 50cc, I:E ratio 1:5, and FIO2 80-100%). Pflex wascalculated for each curve using a commercial computer program (STATA) and applyingthe formula published by Venegas. Data points were limited to those associatedwith zero flow. Inflection point estimates at the two PEEP levels for each patientwere compared using the paired t-test.
Results: Allpatients that demonstrated a Pflex >5 cmH2O on 0 cmH2O of PEEPdemonstrated a Pflex on 5cmH2O. STATA objectively predicted Pflexon 5 cmH2O within + 2.68 cmH2O of Pflex predicted on 0 cmH20.Variability did not change whether the maneuver was performed on a PEEP levelof 0 cmH2O or 5 cmH2O first. There was no statistical difference between theinflection point estimates at the two PEEP settings (p = 0.66).
Discussion: Wehave applied the work of Venegas to a baseline of 5 cmH2O in the ALI/ARDS populationand have noted no significant difference compared with the results of Pflexinitiated at 0 cmH2O. In a sub-group of the ALI/ARDS population, ?charging?the patient circuit with PEEP prevents the flooding of flow sensors during themaneuver. In addition, any PEEP level that predisposes this patient populationto alveolar collapse or flooding should be avoided in the interest of patientwell being. With the exception of circumstances where Pflex is <5cmH20, obtaining static P-V curves with PEEP set at 5 cmH20 appears to resultin reliable Pflexestimates.