A shear-flow-ballooning transformation is used to demonstrate that the presence of a poloidal shear Bow can suppress the ballooning interchange instabilities in a high-beta straight tokamak. It is shown that, with the toroidicity ignored, complete suppression of these instabilities requires the coexistence of both finite poloidal Alfven velocity shear and flow shear, in such a way that the latter is greater than the former. The DIII-D parameters [Gohil et al., Phys. Rev. Lett. 61, 1603 (1988)] are used to compare the experimental results and the above prediction, and an agreement is found.