In this paper, we complete the stability analysis of various human respiratory nonlinear time delay models introduced in (Vielle & Chavet 1998), (Kollár & Turi 2004), (Batzel & Tran 2000a), and (Batzel & Tran 2000b). More precisely, we present a detailed mathematical analysis of the stability of the nonlinear model trivial equilibrium, an estimate of its region of attraction and exponential estimates of the solutions starting in this region. The proposed approach is constructive and it is based on the use of Lyapunov-Krasovskii functionals of complete type for time-delay systems with a cross term in the time derivative.