A young puppy in Clarksville is fighting for her life as she battles a rare heart condition.Isabella Marie is a 10-week-old Orkney-Chihuahua mix that needs surgery that will repair a hole in her heart.The puppy's owners passed the sick dog on to Second Chance and Happy Tales after they learned of her condition."When you listen to the heart it sounds like a train," Inge Irby explained.According to Irby, who works for the rescue organization, Isabella has a heart valve that doesn't close all the way."The blood flow is not going all the way in or out like it's supposed to," she continued. Irby told Nashville's News 2 the puppy needs open heart surgery which costs more than $5,000 to repair her heart or she could die. "It's not fair for her to die, it's not her fault, she is born with a birth defect," Irby said. While Isabella looks healthy and enjoys playing with her best friend Sophia, both of their futures remain uncertain. Just one day after receiving Isabella, someone found Sophia, a pure bred Chihuahua, living by a nearby river.The 15-week-old Chihuahua is also in need of a family to give her a home forever.Irby said she considers both of the animals like her children. "For us, it's a child, it's a creature with the right to live," she said.Sophia will be placed up for adoption in a few weeks, while Isabella will be available for adoption after she undergoes the operation.Donations to help with the expense of Isabella's surgery can be made at any U.S. Bank branch or by visiting Second Chance and Happy Tail's Web site.