Lung Stress and Strain During Mechanical Ventilation

Objective: Tidal volume (VT) and volume of gas caused by positive end-expiratory pressure (VPEEP) generate dynamic and static lung strains, respectively. Our aim was to clarify whether different combinations of dynamic and static strains, resulting in the same large global strain, constantly produce lung edema. Design: Laboratory investigation. Setting: Animal unit. Subjects: Twenty-eight healthy pigs. Interventions: After lung computed tomography, 20 animals were ventilated for 54 hours at a global strain of 2.5, either entirely dynamic (VT 100% and VPEEP 0%), partly dynamic and partly static (VT 75–50% and VPEEP 25–50%), or mainly static (VT 25% and VPEEP 75%) and then killed. In eight other pigs (VT 25% and VPEEP 75%), VPEEP was abruptly zeroed after 36–54 hours and ventilation continued for 3 hours. Measurements and Main Results: Edema was diagnosed when final lung weight (balance) exceeded the initial weight (computed tomography). Mortality, lung mechanics, gas exchange, pulmonary histology, and inflammation were evaluated. All animals ventilated with entirely dynamic strain (VT 825 ± 424 mL) developed pulmonary edema (lung weight from 334 ± 38 to 658 ± 99 g, p < 0.01), whereas none of those ventilated with mainly static strain (VT 237 ± 21 mL and VPEEP 906 ± 114 mL, corresponding to 19 ± 1 cm H2O of positive end-expiratory pressure) did (from 314 ± 55 to 277 ± 46 g, p = 0.65). Animals ventilated with intermediate combinations finally had normal or largely increased lung weight. Smaller dynamic and larger static strains lowered mortality (p < 0.01), derangement of lung mechanics (p < 0.01), and arterial oxygenation (p < 0.01), histological injury score (p = 0.03), and bronchoalveolar interleukin-6 concentration (p < 0.01). Removal of positive end-expiratory pressure did not result in abrupt increase in lung weight (from 336 ± 36 to 351 ± 77 g, p = 0.51). Conclusions: Lung edema forms (possibly as an all-or-none response) depending not only on global strain but also on its components. Large static are less harmful than large dynamic strains, but not because the former merely counteracts fluid extravasation.