Effect of capillary pressure and lung distension on capillary recruitment

To investigate the effect of capillary pressure and alveolar distension on capillary recruitment, we used video-microscopy to quantify capillary recruitment in individual subpleural alveolar walls. Canine lobes were perfused with autologous blood either while inflated by positive airway pressure or while inflated by negative intrapleural pressure in the intact thorax with airway pressure remaining atmospheric. Low flow rates minimized the arteriovenous pressure gradient (< 5 mmHg), permitting capillary pressure estimation by averaging these pressures. Capillary pressure was varied stepwise from airway pressure to 30 mmHg above airway pressure. Capillary recruitment always began as capillary pressure exceeded airway pressure. At low positive airway pressures, the capillaries of the excised lobes opened suddenly over a narrow pressure range. AT higher airway pressures and in the intact thorax, recruitment occurred over a wide range of capillary pressures. We conclude that capillary perfusion begins when intracapillary pressure just exceeds alveolar pressure but that further increases in capillary pressure recruit capillaries depending on tension in the alveolar wall, whether imposed by positive airway pressure or by gravity when the lung is suspended in an intact thorax.