AbleCommerce Shopping Cart Software

I have a stored proc where I have a output parameter as a return value.

I use
ISQLQuery query = NHibernateHelper.CreateSQLQuery("exec StoredProcName");
query.SetParameter("BasketId", _Basket.Id);
query.SetParameter("ProductName", AddProductName.Text.Trim().ToString());
query.SetParameter("Price", Convert.ToDecimal(AddProductVariablePrice.Text.Trim()));
query.SetParameter("Quantity", Convert.ToInt16(AddProductQuantity.Text.Trim()));
query.SetParameter("rtn", 32); /*where rtn is a output parameter*/

query.ExecuteUpdate();



Then how can I get the return value of @rtn from it.