Terrier-cross Molly took months to recover from a skin condition after being neglected by her previous owner.
Vets then discovered she had a third degree heart block which could potentially shorten her life.
Big-hearted members of the public rushed to donate to the appeal, launched by the RSPCA's Coventry branch.
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Since Molly, who has featured on Channel 5's The Dog Rescuers, was signed over to the RSPCA, areas of hair loss she suffered from her skin condition have regrown.
However, she suffered her latest setback when her "heart rate plummeted dangerously slow" on the day she was being neutered.
Danni Holder, kennel supervisor at the Coundon Wedge Drive centre, where Molly has been since May 2016, said: "Molly's heart is beating too slow and if she doesn't have the pacemaker fitted her heart will become worse and will lessen her lifespan.
"She has been on such a journey and we would love her to have the happy ending to her story that she deserves."
When the charity launched its appeal on Thursday morning, Ms Holder said: "We are appealing for people to help us in our appeal to fundraise £2,500 for this so we can mend Molly's broken heart."
Donations flooded in and by 13:30 GMT on Thursday the charity had raised more than its original fundraising target.
A £2,500 charity appeal to fit a rescue dog with a pacemaker has smashed its target.