LAUSR

The amount of energy delivered to the respiratory system is recognized as a cause of Ventilator Induced Lung Injury (VILI). How energy dissipation within the lung causes damage is still a matter of debate. Expiratory flow control has been proposed as a strategy to reduce the energy dissipated into the respiratory system during expiration and, possibly, VILI. We studied 22 healthy pigs (29±2 kg), which were randomized into a control (n=11) and a valve group (n=11), where the expiratory flow was controlled through a variable resistor. Both groups were ventilated with the same tidal volume, PEEP and inspiratory flow. Electric impedance tomography was continuously acquired. At completion, lung weight, wet to dry ratios and histology were evaluated. The total mechanical power was similar in the control and valve groups (8.54±0.83 J min-1 and 8.42±0.54 J min-1 respectively, p=0.552). The total energy dissipated within the whole system (circuit + respiratory system) was remarkably different (4.34±0.66 vs 2.62±0.31 J/min, p<0.001). However, most of this energy was dissipated across the endotracheal tube (2.87±0.3 vs 1.88±0.2 J/min, p<0.001). The amount dissipated into the respiratory system averaged 1.45±0.5 in controls vs 0.73±0.16 J min-1 in the valve group, p<0.001. Although respiratory mechanics, gas exchange, hemodynamics, wet to dry ratios and histology were similar in the two groups, the decrease of end-expiratory lung impedance was significantly greater in the control group (p=0.02). We conclude that with our experimental conditions, the reduction of energy dissipated in the respiratory system did not lead to appreciable differences in VILI.