Energy dissipation during expiration and ventilator-induced lung injury: an experimental animal study

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 parenchyma 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, positive end-expiratory pressure (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 versus 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.NEW & NOTEWORTHY Energy dissipation within the respiratory system is a factor promoting ventilator-induced lung injury (VILI). In this animal study, we modulated the expiratory flow, reducing the energy dissipated in the system. However, this reduction happened mostly across the endotracheal tube, and only partly in the respiratory system. Therefore, in healthy lungs, the advantage in energy dissipation does not reduce VILI, but the advantages might be more relevant in diseased lungs under injurious ventilation.