Am. J. Respir. Crit. Care Med.,
Volume 160, Number 3, September 1999, 785-790
Correcting Static Intrinsic Positive End-expiratory
Pressure for Expiratory Muscle Contraction
Validation of a New Method
SPYROS G.
ZAKYNTHINOS,
THEODOROS
VASSILAKOPOULOS,
EPAMINONDAS
ZAKYNTHINOS,
CHARIS
ROUSSOS,
and
GEORGE E.
TZELEPIS
Department of Critical Care and Pulmonary Services, University of Athens, Medical School, Evangelismos Hospital, Athens, Greece;
and Wayne State University, Detroit, Michigan
We have recently shown (Eur. Respir. J. 1997;10:522-529) that in spontaneously breathing and actively expiring patients, static intrinsic positive end-expiratory pressure (PEEPi,st) can be corrected for expiratory muscle contraction 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 inspiratory effort (PEEPi,st avg). However, since in
some patients there is substantial variability in the intensity of expiratory muscle activity and hence in
Pga,exp rise, this method may be inaccurate because the Pga,exp rise of breaths preceding airway
occlusion may differ from that of the first postocclusion breath. In the present study, we introduced a
new method consisting of synchronous subtraction of Pga,exp rise from Paw, both occurring during
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 during 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 these methods. In contrast, mean difference of PEEPi,st avg and PEEPi,st ref was 0.73 cm H2O with limits of agreement 3.97 to 5.43 cm H2O, indicating lack 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
PEEPi,st ref (r = 0.909; p < 0.001). We conclude that PEEPi,st can be accurately measured in spontaneously breathing patients by synchronous subtraction of Pga,exp rise from Paw during airway occlusion.
