How to get used to the normal blood pressures. Prevents from pulling fluid.

My mom always feels funny when her BP is close to or below the normal range (120/80). She has always been used to high BP's and 140-160 is usually normal for her. This becomes a problem during dialysis as her BP drops really fast and when it hits the normal range, she starts feeling symptoms of low blood pressure. When that happens, they had to stop pulling fluid off her and wait to start pulling fluid again until her BP goes up again which usually takes about 20 mins or so. An other solution is to lay her flat on her back which increases her BP and they start pulling fluid again. How should she start getting used to handling the normal blood pressures? Any suggestions on this would be helpful. Thanks.