To second order in the heavy quark expansion, these corrections can be expressed in terms of two matrix elements, <>, where b_v is defined as b_v=e&{imbvx}b(x), b(x) being the b quark field in full QCD, v_/mu is the four-velocity of the B meson, m_B its mass and F&{/mu/nu} the gluonic field-strength tensor.