Abstract Rationale While global estimates of mechanical energy (ME) are done using ventilator parameters, the regional distribution of ME during invasive mechanical ventilation is poorly understood. Objectives 1) To assess regional distribution of ME using electrical impedance tomography (EIT). 2) To compare ME distribution during pressure control (PCV) and volume control ventilation (VCV) in a porcine lung injury model. Methods Yorkshire pigs (N = 6, weight = 40.8 ± 2.4 kg) were utilized. Under general anesthesia in the supine position, placement of tracheostomy, Swan-Ganz catheter, arterial line, and esophageal balloon was performed. An EIT belt was fitted. Pre-injury PCV and/or VCV decremental positive end expiratory pressure (PEEP) trials were performed. Repeated lung lavages (until PaO2 100 at FiO2 1) and 1.5 hours of high-stretch ventilation were performed, creating a two-hit ARDS model. Decremental PEEP trials were repeated. Pressure-volume loops using ΔZ (EIT impedance) were created from raw EIT data, and the ME at each PEEP was measured via the trapezoidal rule. ME (anterior, posterior, and whole lung) was normalized to pre-injury maximum. Pre- and post-injury analyses of ME, compliance, collapse, and hyperdistention were performed for whole, anterior, and posterior lung regions. Results As PEEP decreased in both PCV (constant driving pressure, constant stress) and VCV (constant tidal volume, stress determined by respiratory system), compliance increased before declining, hyperdistention decreased and collapse increased. In PCV, ME paralleled compliance, while in VCV, ME decayed exponentially, independent of compliance (Figure 1). Post-injury, peak ME decreased in PCV but was unchanged in VCV. During pre-injury PCV, a two-way repeated-measures ANOVA showed significant effects of anterior-posterior ME distribution (p = 0.0065), PEEP (p = 1.3 × 10−7), and their interaction (p = 0.00016). In the anterior and posterior regions, ME distribution was significantly affected by injury status (anterior: p = 0.027; posterior: p = 0.31), PEEP (p = 0.00021; p = 0.0014), and their interaction (p = 0.013; p = 0.000010). Conclusions In VCV, peak ME occurs at high PEEP levels where compliance is reduced, creating conditions that favor injury. In PCV, peak ME occurs where compliance is greatest, indicating tolerated stress. Constant stress may be better suited to reduce lung injury, while VCV may risk VILI based on the PEEP. Anterior-posterior differences in ME distribution suggest that ME may be concentrated in regions of collapse or hyperdistention, predisposing these areas to injury. Further work will include pixel-by-pixel analysis of ME and compliance. This abstract is funded by: WashU Departmental Funding
Stanford-Hill et al. (Fri,) studied this question.