  We consider in this paper some class of perturbation for the semilinear wave
equation with critical (in the conformal transform sense) power nonlinearity.
Working in the framework of similarity variables, we find a Lyapunov functional
for the problem. Using a two-step argument based on interpolation and a
critical Gagliardo-Nirenberg inequality, we show that the blow-up rate of any
sigular solution is given by the solution of the non perturbed associated ODE,
namely $u" = u^p$.
