Acute lung injury from mechanical ventilation at moderately high airway pressures

We have explored adverse pulmonary effects of mechanical ventilation at a peak inspiratory pressure of 30 cmH2O in paralyzed and anesthetized healthy sheep. A control group of eight sheep (group A) was mechanically ventilated with 40% oxygen at a tidal volume of 10 ml/kg, a frequency of 15 breaths/min, a peak inspiratory pressure less than 18 cmH2O, and a positive end-expiratory pressure of 3-5 cmH2O. During the ensuing 48 h, there were no measurable deleterious changes in lung function or arterial blood gases. Another 19 sheep were ventilated with 40% oxygen at a peak inspiratory pressure of 30 cmH2O under a different set of conditions and were randomly assigned to two groups. In group B, the respiratory rate was kept near 4 breaths/min to keep arterial PCO2 in the normal range; in group C, the frequency was kept near 15 breaths/min by including a variable dead space in the ventilator circuit to keep arterial PCO2 near baseline values. There was a progressive deterioration in total static lung compliance, functional residual capacity, and arterial blood gases. After some hours, there were abnormal chest roentgenographic changes. At time of death we found severe pulmonary atelectasis, increased wet lung weight, and an increase in the minimum surface tension of saline lung lavage fluid.