Mariateresa Guanziroli

Mechanical Power and Development of Ventilator-induced Lung Injury.

Background:The ventilator works mechanically on the lung parenchyma. The authors set out to obtain the proof of concept that ventilator-induced lung injury (VILI) depends on the mechanical power applied to the lung. Methods:Mechanical power was defined as the function of transpulmonary pressure, tidal volume (TV), and respiratory rate. Three piglets were ventilated with a mechanical power known to be lethal (TV, 38 ml/kg; plateau pressure, 27 cm H2O; and respiratory rate, 15 breaths/min). Other groups (three piglets each) were ventilated with the same TV per kilogram and transpulmonary pressure but at the respiratory rates of 12, 9, 6, and 3 breaths/min. The authors identified a mechanical power threshold for VILI and did nine additional experiments at the respiratory rate of 35 breaths/min and mechanical power below (TV 11 ml/kg) and above (TV 22 ml/kg) the threshold. Results:In the 15 experiments to detect the threshold for VILI, up to a mechanical power of approximately 12 J/min (respiratory rate, 9 breaths/min), the computed tomography scans showed mostly isolated densities, whereas at the mechanical power above approximately 12 J/min, all piglets developed whole-lung edema. In the nine confirmatory experiments, the five piglets ventilated above the power threshold developed VILI, but the four piglets ventilated below did not. By grouping all 24 piglets, the authors found a significant relationship between the mechanical power applied to the lung and the increase in lung weight (r2 = 0.41, P = 0.001) and lung elastance (r2 = 0.33, P < 0.01) and decrease in PaO2/FIO2 (r2 = 0.40, P < 0.001) at the end of the study. Conclusion:In piglets, VILI develops if a mechanical power threshold is exceeded.

Dissipated energy during protective mechanical ventilation

From literature we know that a cornerstone of the protective lung ventilation in Acute Respiratory Distress Syndrome (ARDS) patients[1] and during general anesthesia[2] is a low tidal volume. On the other hand, driving pressure seems to be the variable that best stratifies mortality risk[3]. Our hypothesis is that the combination of volume and pressure, that is the energy dissipated into respiratory system, is the main determinant of a ventilator-induced lung injury (VILI).