CKM matrix elements are free parameters of the Standard Model which are determined by combining experimental and theoretical input. On the theory side, this requires the computation of hadronic matrix elements (ME) in lattice simulations. The lattice correlators can suffer from excited states contamination, i.e. additional contributions from multi-particle states with the same quantum numbers as the initial and/or final state of the matrix element. This can lead to an over- or underestimation of the ME and thus to a systematic error in the results of CKM matrix elements. In this talk, I present how Heavy Meson Chiral Perturbation Theory can be used to estimate the excited states contamination of B meson correlation functions relevant for flavour physics.