Variable ventilation induces endogenous surfactant release in normal guinea pigs.

Variable or noisy ventilation, which includes random breath-to-breath variations in tidal volume (Vt) and frequency, has been shown to consistently improve blood oxygenation during mechanical ventilation in various models of acute lung injury. To further understand the effects of variable ventilation on lung physiology and biology, we mechanically ventilated 11 normal guinea pigs for 3 h using constant-Vt ventilation (n = 6) or variable ventilation (n = 5). After 3 h of ventilation, each animal underwent whole lung lavage for determination of alveolar surfactant content and composition, while protein content was assayed as a possible marker of injury. Another group of animals underwent whole lung lavage in the absence of mechanical ventilation to serve as an unventilated control group (n = 5). Although lung mechanics did not vary significantly between groups, we found that variable ventilation improved oxygenation, increased surfactant levels nearly twofold, and attenuated alveolar protein content compared with animals ventilated with constant Vt. These data demonstrate that random variations in Vt promote endogenous release of biochemically intact surfactant, which improves alveolar stability, apparently reducing lung injury.