Jane,
Any decrease in Qd will result in a decrease in clearance. This decrease will be more pronounced with a lower blood flow than with a higher Qb because it will have a greater reduction in clearances.
The affect on the patient would be the same as any symptoms of underdialysis: lower energy levels, lower appetite, restless legs, etc. It is hard to say if these symptoms would increase just because the Qd was 600 instead of 800.
I beleive all patients who dialyzer 3 times a week are underdialyzed to some extent. Even with high clearances, this treatment causes such dramatic swings in chemistries and fluid shifts that they still have many of the same symptoms.
Time is more important than high per-minute clearances!
Curt