What to do if patient with dehydration was given5% of dextrose what's the effect on his rbc?

Depends: Some patients with dehydration also have low glucose and might need some IV infusion. In adults we usually infuse D50 or D25, just enough to increase the glucose and usually does not affect the red cells. In Neonates and small infants we can give D5( 5%) that also do not affect the red cells. If you give a large amount of D5 it can decrease the osmolarity of the blood and cause damage to red cell