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 of breathing (WI) are important factors in the management of severe obstructive respiratory disease. We used a computer model of spontaneously breathing patients with chronic obstructive pulmonary disease to assess the sensitivity of measurement techniques for dynamic PEEP_i (PEEP_{i dyn}) and WI to expiratory muscle activity (EMA) and cardiogenic oscillations (CGO) on esophageal pressure. Without EMA and CGO, both PEEP_{i dyn} and WI were accurately estimated (r = 0.999 and 0.95, respectively). Addition of moderate EMA caused PEEP_{i dyn} and WI to be systematically overestimated by 141 and 52%, respectively. Furthermore, CGO introduced large random errors, obliterating the correlation between the true and estimated values for both PEEP_{i dyn} (r = 0.29) and WI (r = 0.38). Thus the accurate estimation of PEEP_{i dyn} and WI requires steps to be taken to ameliorate the adverse effects of both EMA and CGO. Taking advantage of our simulations, we also investigated the relationship between PEEP_{i dyn} and static PEEP_i (PEEP_{i stat}). The PEEP_{i dyn}/PEEP_{i stat} ratio decreased as stress adaptation in the lung was increased, suggesting that heterogeneity of expiratory flow limitation is responsible for the discrepancies between PEEP_{i dyn} and PEEP_{i stat} that have been reported in patients with severe airway obstruction.