In General anesthesia, two main ways of pain control are used intra-operatively, one is opioids, the other is epidural. Many colleges prefer using epidural analgesia because it is potent and more effective , and also the benefits of stress block, less depression of breath. however, the epidural technique often causes more drop of blood pressure, especially in dehydrated patients. The investigators designed the trial to see if the benefit weigh out the potential risk of epidural in isolated hypertensive patients.