A new look at the stress index for lung injury

Ranieri and colleagues ^1,2 have proposed a simple mathematical equation (P_aw(t) ≈ at^b + c) where a, b, and c are constants that relates airway pressure, P_aw(t), over inspiration (0 ≤ t ≤ t_i) in constant flow mechanical ventilation to ventilator-induced lung injury. They have correlated b with low-volume stress (b ≤ 0.9), high-volume stress (b ≥ 1.1), and minimal stress (0.9 ≤ b ≤ 1.1) in the lungs. In this paper we examine the theoretical underpinnings of this empirical equation and show that it has a relationship to the upper and lower inflection points of the P_elastic - V curve during inspiration. An alternative equation (P_aw(t) ≈ αt/γ+βt + Λ) that comes from the theoretical analysis of this type of mechanical ventilation is presented. Both equations are compared using experimental data.