Positive- and Negative-Pressure Ventilation Characterized by Local and Global Pulmonary Mechanics

Rationale: There is continued debate regarding the equivalency of positive-pressure ventilation (PPV) and negative-pressure ventilation (NPV). Resolving this question is important because of the different practical ramifications of the two paradigms. Objectives: We sought to investigate the parallel between PPV and NPV and determine whether or not these two paradigms cause identical ventilation profiles by analyzing the local strain mechanics when the global tidal volume (Vt) and inflation pressure was matched. Methods: A custom-designed electromechanical apparatus was used to impose equal global loads and displacements on the same ex vivo healthy porcine lung using PPV and NPV. High-speed high-resolution cameras recorded local lung surface deformations and strains in real time, and differences between PPV and NPV global energetics, viscoelasticity, as well as local tissue distortion were assessed. Measurements and Main Results: During initial inflation, NPV exhibited significantly more bulk pressure-volume compliance than PPV, suggestive of earlier lung recruitment. NPV settings also showed reduced relaxation, hysteresis, and energy loss compared with PPV. Local strain trends were also decreased in NPV, with reduced tissue distortion trends compared with PPV, as revealed through analysis of tissue anisotropy. Conclusions: Apparently, contradictory previous studies are not mutually exclusive. Equivalent changes in transpulmonary pressures in PPV and NPV lead to the same changes in lung volume and pressures, yet local tissue strains differ between PPV and NPV. Although limited to healthy specimens and ex vivo experiments in the absence of a chest cavity, these results may explain previous reports of better oxygenation and less lung injury in NPV.