Coronagraphic spectral observations carried out with UVCS on SoHO above polar coronal holes have shown that the velocity distribution is highly anisotropic. Here we examine the influence of the density stratification on the interpretation of such observations. In particular, we investigate the profiles of O VI spectral lines emitted in polar coronal holes. We find that at distances greater than $1~R_{\odot}$ from the solar surface the widths of the emitted lines are significantly affected by the details of the adopted electron density profiles. In particular, the densities deduced by Doyle et al. (1999) from SoHO data result in O VI profiles whose widths and intensity ratio are relatively close to the values observed by UVCS although only isotropic velocity distributions are employed. Hence we expect that the magnitude of the anisotropy of the velocity distribution deduced from UVCS data depends strongly on the adopted density profile.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html