Mechanical power thresholds during mechanical ventilation: An experimental study

The extent of ventilator-induced lung injury may be related to the intensity of mechanical ventilation-expressed as mechanical power. In the present study, we investigated whether there is a safe threshold, below which lung damage is absent. Three groups of six healthy pigs (29.5 +/- 2.5 kg) were ventilated prone for 48 h at mechanical power of 3, 7, or 12 J/min. Strain never exceeded 1.0. PEEP was set at 4 cmH(2)O. Lung volumes were measured every 12 h; respiratory, hemodynamics, and gas exchange variables every 6. End-experiment histological findings were compared with a control group of eight pigs which did not undergo mechanical ventilation. Functional residual capacity decreased by 10.4% +/- 10.6% and 8.1% + 12.1% in the 7 J and 12 .1 groups (p = 0.017, p < 0.001) but not in the 3J group (+1.7% +/- 17.7%, p = 0.941). In 3 J group, lung elastance, PaO2 and PaCO2 were worse compared to 7 J and 12.1 groups (all p < 0.001), due to lower ventilation-perfusion ratio (0.54 +/- 0.13,1.00 +/- 0.25, 1.78 +/- 0.36 respectively, p < 0.001). The lung weight was lower (p < 0.001) in the controls (6.56 +/- 0.90 g/kg) compared to 3. 7, and 12 J groups (12.9 +/- 3.0, 16.5 +/- 2.9, and 15.0 +/- 4.1 g/kg, respectively). The wet-to-dry ratio was 5.38 +/- 0.26 in controls, 5.73 +/- 0.52 in 3 J, 5.99 +/- 0.38 in 7 J, and 6.13 +/- 0.59 in 12 J group (p = 0.03). Vascular congestion was more extensive in the 7 J and 12 J compared to 3 J and control groups. Mechanical ventilation (with anesthesia/paralysis) increase lung weight, and worsen lung histology, regardless of the mechanical power. Ventilating at 3 J/min led to better anatomical variables than at 7 and 12 J/min but worsened the physiological values.