Contribution of Expiratory Muscle Pressure to Dynamic Intrinsic Positive End-Expiratory Pressure

In spontaneously breathing (SB) patients expiratory muscle contraction leads to an overestimation of dynamic intrinsic PEEP (PEEPi,dyn). To quantify this overestimation, PEEPi,dyn measured with the esophageal balloon technique was corrected for the increase in Pga over the course of expiration (Pga,exp rise), for the whole decay of Pga during inspiration (Pga,total decay) or for the part of Pga decay restricted between the onset of inspiratory effort and the point of zero flow (Pga,zf decay). Corrections were compared with the reference PEEPi,dyn (PEEPi,dyn ref ), calculated by using the Campbell diagram. In 15 ventilator-dependent, SB, and actively expiring patients, we found that the difference PEEPi,dyn − Pga,total decay (mean ± SD, 5.7 ± 1.9 cm H2O) was quite similar to PEEPi,dyn ref (5.3 ± 1.9 cm H2O). Their mean difference was 0.37 cm H2O with limits of agreement − 0.09 to 0.83 cm H2O, indicating strong agreement between these methods. PEEPi,dyn − Pga,exp rise (6.0 ± 2.1 cm H2O) was also similar to PEEPi,dyn ref. Their mean difference was 0.72 cm H2O with limits of agreement − 1.69 to 3.13 cm H2O, indicating good agreement. In contrast, mean difference of PEEPi,dyn − Pga,zf decay and PEEPi,dyn ref was 3.14 cm H2O with limits of agreement − 0.46 to 6.74 cm H2O, indicating lack of agreement. The error in measurement due to the subtraction of Pga,zf decay from PEEPi,dyn (i.e., [PEEPi,dyn − Pga,zf decay] − PEEPi,dyn ref ) was proportional to the intensity of expiratory muscle contraction, as expressed by the Pga,exp rise (r = 0.903, p < 0.001). We conclude that in actively expiring patients an adequate correction of PEEPi,dyn for the overestimation caused by expiratory muscle contraction can be made by subtracting either Pga,total decay or Pga,exp rise from PEEPi,dyn, the former achieving the best performance.