Ketoconazole should not be   given to a patient  being treated with astemizole because:
Ketoconazole, erythromycin and clarithromycin are potent inhibitors of microsomal enzymes. Astemizole, terfenadine and cisapride are the drugs that can block cardiac K +  channels at high concentration. If administered along with microsomal enzyme inhibitors, these drugs can result in torsades de pointes, a serious polymorphic ventricular tachycardia.