  Emission lines from highly-excited states (n >= 5) of H- and He-like ions
have been detected in astrophysical sources and fusion plasmas. For such
excited states, R-matrix or distorted wave calculations for electron-impact
excitation are very limited, due to the large size of the atomic basis set
needed to describe them. Calculations for n >= 6 are also not generally
available. We study the behaviour of the electron-impact excitation collision
strengths and effective collision strengths for the most important transitions
used to model electron collision dominated astrophysical plasmas, solar, for
example. We investigate the dependence on the relevant parameters: the
principal quantum number n or the nuclear charge Z. We also estimate the
importance of coupling to highly-excited states and the continuum by comparing
the results of different sized calculations. We provide analytic formulae to
calculate the electron-impact excitation collision strengths and effective
collision strengths to highly-excited states (n >= 8) of H- and He-like ions.
These extrapolated effective collision strengths can be used to interpret
astrophysical and fusion plasma via collisional-radiative modelling.
