EFFECT OF CAPILLARY-PRESSURE AND LUNG DISTENSION ON CAPILLARY RECRUITMENT

To investigate the effect of capillary pressure and alveolar distensio n on capillary recruitment, we used videomicroscopy to quantify capill ary 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 flo w rates minimized the arteriovenous pressure gradient (<5 mmHg), permi tting capillary pressure estimation by averaging these pressures. Capi llary pressure was varied stepwise from airway pressure to 30 mmHg abo ve airway pressure. Capillary recruitment always began as capillary pr essure exceeded airway pressure. At low positive airway pressures, the capillaries of the excised lobes opened suddenly over a narrow pressu re range. At higher airway pressures and in the intact thorax, recruit ment occurred over a wide range of capillary pressures. We conclude th at capillary perfusion begins when intracapillary pressure just exceed s alveolar pressure but that further increases in capillary pressure r ecruit capillaries depending on tension in the alveolar wall, whether imposed by positive airway pressure or by gravity when the lung is sus pended in an intact thorax.