COMPARISON OF STATIC AND DYNAMIC INTRINSIC POSITIVE END-EXPIRATORY PRESSURE USING THE CAMPBELL DIAGRAM

Intrinsic positive end-expiratory pressure (PEEPi) due to dynamic hype rinflation has been measured as a plateau airway opening pressure duri ng airway occlusion (PEEPi,sbt). PEEPi has also been dynamically deter mined as a fall in esophageal pressure (Pes) before the inspiratory fl ow starts (PEEPi,dyn). The aims of the current study were to systemati cally compare PEEPi,stat and PEEPi,dyn and to explain the underlying m echanisms of their difference. The study was performed in healthy subj ects with dynamic hyperinflation induced by expiration through a Starl ing resistor. The Campbell diagram was constructed for each subject by determining the static pressure-volume curves of the lung (Pst,[I]) a nd chest wall (Pst,[w]). For a given end-expiratory volume, PEEPi,stat was measured on the Campbell diagram as the pressure difference betwe en Pst(w) and -Pst(I). PEEPi,dyn was measured as mentioned above. The effects of respiratory muscle recruitment on PEEPi,dyn were estimated by the Pes values when Pes started to fall relative to Pst(w). We foun d that: (1) there was a great variability of the PEEPi,dyn/PEEPi,stat ratio among and within subjects; (2) expiratory muscle recruitment was evident on most occasions; (3) persistent inspiratory muscle activity during expiration was present in some subjects; (4) the Pes values at the start of inspiratory flow were frequently on the left of -Pst(I), which contributed to the difference between PEEPi,stat and PEEPi,dyn and implied a greater dynamic than static elastance presumably due to viscoelastic properties; (5) chest wall distortions characterized by i nflation of the abdomen with deflation of the rib cage during the init ial inspiratory efforts were observed in three subjects. In conclusion , interpretation of PEEPi,dyn needs to be cautious because both expira tory and tonic inspiratory muscle activities that lead to significant over- or underestimation of PEEPi by PEEPi,dyn, respectively, are asso ciated with acute dynamic hyperinflation. In addition, the effects of viscoelastic properties and chest wall distortions on PEEPi,dyn need t o be further investigated.