Are Low Tidal Volumes Safe?
TTigh airway pressure may have adverse effects on pulmonary parenchyma. Greenfield et al demonstrated that severe bilateral pulmonary atelectasis occurs when healthy dogs are mechanically ventilated for 2 h with peak airway pressures equal to 26 to 32 cm H20. Barsch and coworkers showed that longer exposure (22 to 70 h) to elevated airway pressures leads to alveolar edema, hyaline membrane formation, and interstitial hemorrhage. Recently, Kolobow et aldemonstrated that respiratory function deteriorates and lung compliance falls when previously healthy anesthetized sheep are subjected to prolonged ventilation with high airway pressures (50 cm H20) using tidal volumes (Vt) of 50 to 70 ml/kg.
Conventional wisdom dictates that relatively large (ie> supraphysiologic) Vt are necessary to prevent atelectasis in intubated, mechanically ventilated patients. This view derives from an often-cited report by Bendixen et al. In a study of 18 patients under general anesthesia, these investigators documented a progressive deterioration in oxygen tension (PaOJ while the patients were mechanically ventilated at 25 breaths/min with maximum airway pressures of 15 to 20 cm H20. Arterial oxygenation was improved by passive hyperinflation of the lungs with airway pressures sustained at 30 to 40 cm H20 for 15 s; therefore, these workers attributed the arterial hypoxemia to atelectasis.
Not all authorities, however, advocate the routine use of high Vt. Hall and Wood suggested that physiologic Vt (5 to 6 ml/kg) is beneficial in “liberating” (weaning) certain patients with low compliance from mechanical ventilation. Clearly, if minute ventilation is unchanged, lower Vt will result in lower mean and peak airway pressures. Since high airway pressures may be intrinsically deleterious to lung parenchyma, we wonder whether it is truly necessary to routinely prescribe supraphysiologic Vt for mechanically ventilated patients. Encouraged by the views expressed by Hall and Wood, we attempted to answer this question by conducting a randomized prospective trial comparing low (6 ml/kg) vs high (12 ml/kg) Vt mechanical ventilation.