Correcting static intrinsic positive end-expiratory pressure for expiratory muscle contraction - Validation of a new method

We have recently shown (Eur. Respir. J. 1997;10:522-529) that in spontaneou sly breathing and actively expiring patients, static intrinsic positive end -expiratory pressure (PEEPi,st) can be corrected for expiratory muscle cont raction by subtracting the average expiratory rise in gastric pressure (Pga ,exp rise), calculated from three breaths just prior to an airway occlusion , from the end-expiratory airway pressure (Paw) of the first occluded inspi ratory effort (PEEPi,st avg). However, since in some patients there is subs tantial variability in the intensity of expiratory muscle activity and henc e in Pga,exp rise, this method may be inaccurate because the Pga,exp rise o f breaths preceding airway occlusion may differ from that of the first post occlusion breath. In the present study, we introduced a new method consisti ng of synchronous subtraction of Pga,exp rise from Paw, both occurring duri ng airway occlusion (PEEPi,st sub). PEEPi,st sub and PEEPi,st avg were each compared with the reference PEEPi,st (PEEPi,st ref), which was obtained du ring muscular paralysis and simulation of the spontaneous breathing pattern by the ventilator. We found that, in 25 critically ill patients, PEEPi,st sub (mean +/- SD, 5.3 +/- 2.6 cm H2O) was nearly identical to PEEPi,st ref (5.4 +/- 2.4 cm H2O). Their mean difference was -0.06 cm H2O with limits of agreement -0.96 to 0.84 cm H2O, indicating a strong agreement between thes e methods. In contrast, mean difference of PEEPi,st avg and PEEPi,st ref wa s 0.73 cm H2O with limits of agreement -3.97 to 5.43 cm H2O, indicating lac k of agreement. Coefficient of variation of Pga,exp rise was 14.3 +/- 7.2% (range, 5.2 to 28.3%). There was a good correlation between the coefficient of variation of Pga,exp rise and the difference between PEEPi,st avg and P EEPi,st ref (r = 0.909; p < 0.001). We conclude that PEEPi,st can be accura tely measured in spontaneously breathing patients by synchronous subtractio n of Pga,exp rise from Paw during airway occlusion.