We analyse the internal dynamics of star-forming H ii regions and their efficiency in interacting with the ISM. We have selected the nearby (4.5 Mpc) spiral galaxy M 83, and from GHαFaS Fabry-Perot observations of the central 3.4' × 3.4' region, extracted the integrated Hα emission-line profiles of a sample of 136 isolated H ii regions. We investigate the effect of the instrumental response and find that, although commonly this effect is subtracted quadratically, this correction is not adequate for deriving reliable emission line profiles. Instead, we convolve a Gaussian with the observed instrumental response before fitting to the line profile. Quadratic subtraction overestimates the velocity dispersion, σ, by ≍ 6 km-1 (left panel). When performing the same study fitting two Gaussians the secondary component tends (>80% of the cases) to be smaller and narrower, and uncertainties to increase by a factor of three.