Physics and physiology of ventilation-perfusion coupling: substantiation and initial experience of end-expiratory pressure choice during mechanical ventilation

Alveolar gas flow competition with pulmonary capillary blood flow for restricted space within lungs during mechanical ventilation, which is well known in respiratory physiology but neglected in everyday clinical practice, is thoroughly discussed in the paper. Anatomical and physiological conditions of oxygen and carbon dioxide counter-diffusion through alveolar-capillary barrier and the role of ventilation-perfusion relationship are described, and the possibility of blood flow displacement from opened alveoli with high pressure inside to poorly ventilated compartments of low intra-alveolar pressures is demonstrated. Commercially available metabolographic and real-time gas analysis monitors from different manufacturers were used for optimal PEEP adjustment by means of pulmonary VO$_2$ and VCO$_2$ shifts comparison, influence of patient volaemic status on the physiologic possibility of alveolar recruitment is discussed. Limitations of modern approach to the mechanical lung ventilation aimed at maximal lung opening in underlined and refuted with consideration on effective diffusion surface changes. Inefficacy of mechanical ventilation is especially obvious when acute respiratory failure follows ventilation-perfusion mismatch, like it appears in COVID-19. Partially it can explain discouraging mechanical ventilation outcomes during current pandemic.