Development of Protective Artificial Respiration Strategies

Acute lung injury (ALI) and the more progressed acute respiratory distress syndrome (ARDS) are characterized by a massive inflammation of the lung tissue that results in a severe pulmonary dysfunction. Therapies of ALI and ARDS consist of ventilation of the injured lung. The understanding of the complex flow field inside the human airways is of high relevance for the optimization of artificial respiration. The German Research Foundation DFG launched the priority program "Protective Artificial Respiration" (PAR) to gain deeper knowledge about the fundamental fluid mechanical and medical processes of respiration and to develop protective ventilation strategies.The primary aims of the presented study, which was part of the PAR program, were the investigation of the tracheobronchial flow field at steady and oscillatory respiration, the in-vivo investigation of the intrapulmonary pressures and the associated thoracic volumes using porcine lung models, and the experimental and numerical analysis of the fluid mechanical equilibrium processes during simulated alveolar/airway recruitments.The flow field inside the tracheobronchial tree was determined by 2C/2D, 2C/3D, and 3C/3D particle-image velocimetry measurements using three different silicone casts, which were based on computer tomographic scans of two different human lungs. The airflow was analyzed for physiological and fluid-mechanical fundamental ventilation parameters. The results show a homogenization of the inhaled air for increased respiration rates and evidence a natural washout of the airborne particles at expiration. The in-vivo measurements were performed for a set of healthy and injured lungs to simulate ALI/ARDS conditions, whereby correlations evidence a criterion for an optimized exhalation time. The analysis of the fluid mechanical equilibrium processes supports the fundamental understanding and suggests recruitment methods for collapsed airways or alveoli. The experimental investigations used a lung model with a generic geometry and elastic vessels modeling the nonlinear properties of an injured lung. The numerical simulations, which were verified by the experimental in-vitro and in-vivo results, solved one-dimensional mass and energy equations of the flow inside the generic, multiple-bifurcating lung model. The spatial pressure drop distribution and the frequency spectra were analyzed resulting in the description of the Pendelluft phenomenon and its spatial influence. Inaddition, analysescomparing sinusoidal and artificial ventilation suggest an improved inspiration pattern.