Considering that the variation of the critical temperature can be explained if the impurity potential acts only by the non-spin-flip part, in the presence of either a pure d-wave or anisotropic s-wave gap symmetry, and inserting the pair-breaking time dependence on temperature, we obtain a quantitative expression for the isotope effect coefficient a as a function of measured critical temperature T c maximal critical temperature T and degree of anisotropy of the energy gap. Our result predicts the achievement of a large range of values in a for different ratios T/T c . Introducing the dependence of the critical temperature and pair-breaking time on dopant content for a single Cu-O layer superconductors, we can describe the variation of the isotope effect coefficient with nonstoichiometry for these materials. The result for the case of anisotropic s-wave gap symmetry is in good agreement with the experimental data.