Heterogeneity index (β) = D2/Lm in the following groups: mechanical ventilation with a protective strategy (V_T = 6 ml/kg) for 2 h (control); V_T = 6 ml/kg during hour 1 followed by an abrupt increase to V_T = 22 ml/kg throughout hour 2 (no adaptation time); V_T = 6 ml/kg during the first 30 min followed by a gradual V_T increase up to 22 ml/kg for 30 min, then constant V_T = 22 ml/kg during hour 2 (shorter adaptation time); and a more gradual V_T increase, from 6 to 22 ml/kg during hour 1, followed by V_T = 22 ml/kg during hour 2 (longer adaptation time). Boxes show the interquartile (25 to 75%) range, whiskers encompass the range (minimum to maximum), and horizontal lines represent median values of eight animals/group. *versus control (P < 0.05); #versus shorter adaptation time group (P < 0.05). D2, central moments of mean linear intercept; Lm, mean linear intercept.