A model of the spontaneously breathing patient: applications to intrinsic PEEP and work of breathing

Schuessler, Thomas F., Stewart B. Gottfried, and Jason H. T. Bates. A model of the spontaneously breathing patient: applications to intrinsic PEEP and work of breathing.J. Appl. Physiol. 82(5):1694-1703, 1997.Intrinsic positive end-expiratory pressure(PEEP_i) and inspiratory work ofbreathing (WI) are important factors in the management of severe obstructive respiratory disease. Weused a computer model of spontaneously breathing patients with chronicobstructive pulmonary disease to assess the sensitivity of measurementtechniques for dynamic PEEP_i(PEEP_{i dyn}) andWI to expiratory muscle activity(EMA) and cardiogenic oscillations (CGO) on esophageal pressure.Without EMA and CGO, bothPEEP_{i dyn} andWI were accurately estimated(r = 0.999 and 0.95, respectively). Addition of moderate EMA causedPEEP_{i dyn} andWI to be systematically overestimated by 141 and 52%, respectively. Furthermore, CGOintroduced large random errors, obliterating the correlation betweenthe true and estimated values for bothPEEP_{i dyn}(r = 0.29) andWI (r = 0.38). Thus the accurateestimation of PEEP_{i dyn} andWI requires steps to be taken toameliorate the adverse effects of both EMA and CGO. Taking advantage ofour simulations, we also investigated the relationship betweenPEEP_{i dyn} and staticPEEP_i(PEEP_{i stat}). ThePEEP_{i dyn}/PEEP_{i stat}ratio decreased as stress adaptation in the lung was increased,suggesting that heterogeneity of expiratory flow limitation isresponsible for the discrepancies betweenPEEP_{i dyn} andPEEP_{i stat} thathave been reported in patients with severe airwayobstruction.