The mechanism of lung volume change during mechanical ventilation

To understand ventilator-induced lung injury (VILI) during positive pressure ventilation, mechanisms of normal alveolar mechanics must first be established. Isotropic "baIloonlike" alveolar volume (V-A) change has been viewed as the prevailing mechanism of normal lung volume (V-L) changes. We hypothesized that change in V-L is predominantly caused by alveolar recruitment-derecruitment (R/D). Fifteen mongrel dogs were anesthetized and intubated with a tracheal divider. Through a thoracotomy incision, in vivo microscopy of subpleural alveoli was performed as the degassed lung was inflated to 80% TLC, and then deflated to residual volume (RV). Still photomicrographs were evaluated to determine if change in V-L is due to change in VA or R/D Of alveoli. We noted a steady, significant increase in alveolar recruitment as V-L increased to 80% TLC (p < 0.05). However, V-A increased significantly, but only to 20% TLC (p < 0.05). Once recruited, alveoli did not demonstrate any further volume change, whereas the lung as a whole maintained a normal pressure/volume relationship. In our model, changes in V-L predominantly are caused by R/D.