Accurate measurement of intrinsic positive end-expiratory pressure: how to detect and correct for expiratory muscle activity

Abstract

It has been shown that expiratory muscle contraction leads to an overestimation of intrinsic positive end-expiratory pressure (PEEPi). To quantify this overestimation, we compared PEEPi, measured during spontaneous breathing (SB) by the end-expiratory airway occlusion technique (PEEPi,occl) with static PEEPi (PEEPi,st). PEEPi,st was measured using end-expiratory airway occlusion during simulation of SB by the ventilator with the patient relaxed, and was considered to represent the "gold standard" for PEEPi,occl. Twelve ventilator-dependent patients were studied during SB (pressure support 5-7 cmH2O). Full mechanical ventilation was resumed when they were unable to sustain SB. Subsequently, by manipulating the variables of the ventilator, we simulated the pattern of SB and measured PEEPi,st, corresponding to PEEPi,occl. On the basis of the presence or absence of expiratory rise in gastric pressure (Pga) (rapid drop of end-expiratory Pga at the beginning of inspiration, Pga,exp,rise), and abdominal muscle electromyographic (EMG) activity, patients were subdivided into those either actively (Group 1) or passively expiring (Group 2). In Group 1 (8 patients), PEEPi,occl was higher than PEEPi,st (13.3+/-2.0 vs 6.8+/-1.1 cmH2O; p<0.01). PEEPi,occl-Pga,exp,rise (6.9+/-1.1 cmH2O) was quite similar to PEEPi,st; their mean difference was 0.03 cmH2O with limits of agreement -0.48 to +0.53 cmH2O. In Group 2, PEEPi,occl was similar to PEEPi,st. We conclude that, in actively expiring patients, an accurate estimation of the actual PEEPi,st can be obtained by subtracting Pga,exp,rise from PEEPi,occl.

"Intrinsic" PEEP (PEEPi): role of expiratory muscles

“…The same phenomenon can occur at the onset of inspiration if the descent of the diaphragm causes Pga to increase before this pressure has returned to its relaxation value after the derecruitment of the abdominal muscles. Figure 1, in the paper from ZAKYNTHINOS and co-workers [11] in this issue of the Journal illustrates one example in which such an obvious underestimation of the mechanical effect of the expiratory muscles occurred.…”

“…In this issue of the European Respiratory Journal, ZAKYNTHINOS and co-workers [11] report on a method for detecting expiratory muscle use and quantifying its contribution to PEEPi. The two points at issue in this and other studies [7][8][9] are: 1) How to detect expiratory muscle use?…”

“…ZAKYNTHINOS and co-workers [11], in this issue of the Journal, have used the increase in Pga observed between the end-inspiratory and peak end-expiratory values, or the total decrease in Pga at end-expiration, to correct the initial PEEPi, measured in static conditions (PEEPi,stat) [1], and during end-expiratory airway occlusion. It must be stressed, however, that these measurements do not always reflect expiratory muscle effect with accuracy because Pga tracing is also affected by the contraction of the inspiratory muscles.…”

Setting positive end-expiratory pressure in the severely obstructive patient

“…In patients with active inspiratory muscles, measurements of static PEEPi are not reliable because patients react unpredictably to end-expiratory occlusion [23,24] (Fig. 1).…”