Please explain why is it that the rbc, hemoglobin and hematocrit level of a patient with congenital heart disease is decreased?

Nutrition. The most common reason that hemoglobin and hematocrit are decreased is iron deficiency. In most cases, this is due to either inadequate nutrition (low iron intake) or blood loss (including frequent blood sampling.) neither of these reasons is truly caused by the heart defect, but children with congenital heart disease are at increased risk for either problem.