Partitioning of inspiratory muscle workload and pressure assistance in ventilator-dependent COPD patients

To investigate the mechanisms underlying ventilator-dependence in patients with chronic obstructive pulmonary disease (COPD), and to assess the effects of the combination of positive end-expiratory pressure (PEEP) and pressure-support ventilation (PSV) on inspiratory muscle effort, we investigated respiratory mechanics in eight ventilator-dependent COPD patients. The patients' breathing pattern, lung mechanics, diaphragmatic effort (PTPdi), diaphragmatic tension-time index (TTdi), and arterial blood gases were measured during both spontaneous breathing (SB) and ventilatory assistance consisting of PSV alone (15, 20, and 25 cm H ₂O) and PSV combined with a PEEP of 5 cm H ₂O (reducing PSV to 10, 15, and 20 cm H ₂O, respectively, to maintain equivalent inspiratory pressure). The different levels of ventilatory support were delivered in a randomized sequence. Maximal inspiratory (MIP), esophageal (Ppl(max)) and transdiaphragmatic (Pdi(max)) pressures and respiratory drive (P0 _.1) were measured at the beginning of the procedure during SB. We found a high P0 _.1 (6.1 ± 1.7 cm H ₂O), which seemed to rule out an impairment of respiratory-center output. Apparently, inspiratory muscle strength was compatible with successful weaning (38.5 ± 8.8, 50.9 ± 9.7, and 51.8 ± 9.5 cm H ₂O for MIP, PPl(max) and Pdi(max), respectively). However, abnormal respiratory mechanics (particularly an intrinsic positive end-expiratory pressure (PEEPi) of 8.3 ± 1.9 cm H ₂O and pulmonary resistance 24.7 ± 9.5 cm H ₂O/L/s imposed an excessive load on the inspiratory muscles, as indicated by a high PTPdi (499 ± 122 cm H ₂O · s). Increasing levels of PSV progressively and significantly unloaded the patients' inspiratory muscles, although at pressures above 20 cm H ₂O uncoupling occurred between patient and ventilator respiratory frequency. Application of PEEP during PSV improved ventilatory assistance by further reducing the inspiratory effort (by 17% on average) and by ameliorating patient-ventilator interaction. We conclude that the excessive mechanical load, and in particular the high PEEPi, is the major determinant of ventilator-dependence in COPD patients. Application of PEEP improves the efficiency of PSV in unloading these patients' inspiratory muscles, and can sometimes improve patient-ventilator interaction.