It is difficult to answer your question without more information. Thsi is apparently a congenital anamoly and often the pulmonary vessel and its connection to a small left atrium can be obstructed, etc.
A fast heart rate can be due to the less than normal blood received from the small atrium (reduced capacity for filling) so the heart beats faster to meet the blood/oxygen demand. This could be the underlying cause for fatigue.
The dizziness may be due to a low blood pressure. The salt and water intake will increase the blood pressure. It is difficult to predict the future and the medical significance as there is such a fast advancement in medical technology, etc.
If the condition is serious hasn't there been a discussion of heart transplant or surgery?
If you have more information or any followup questions or comments, you are welcome to respond. Take care, and I wish your daughter well going forward.