Just like many dogs, when Maddy rests her head on her owner's chest, she listens to his heart and to his breathing.

But Maddy's habit of cuddling up with her adopted "dad" isn't just cute, it could be saving his life.

Maddy the dog began her life being considered nothing more than a "baby-making machine", but now she's a valued member of the family, Invercargill couple Rana and Kevin McFarlane say.

KAVINDA HERATH/FAIRFAX NZ Kevin McFarlane enjoys some puppy love from Maddy who he says has saved his life.

She was found by Invercargill SPCA workers, malnourished, with a litter of puppies under a house.

Two years later, the McFarlane's believe Maddy has saved Kevin's life, and could do so again.

Kevin has a debilitating heart condition which means the bottom left chamber of his heart only works at about 15 per cent of its capacity.

KAVINDA HERATH/FAIRFAX NZ Maddy seems to know Kevin needs help before he realises he's in trouble.

He has a defibrillator implant, which is designed to kick-start his heart if he goes into cardiac arrest.

But he's also been recently diagnosed with chronic obstructive pulmonary disease (COPD), which makes it hard to breathe.

Rana said the disease makes Kevin's lungs "pack up". He has had three episodes during the past year where he has been found unconscious as a result of the condition.

In the last two incidents, the couple have figured out that Maddy has known Kevin needed help before he realised he was in trouble.

She paces around the house, refuses to eat, and tries to get as close to him as possible.

In the most recent episode, Kevin and Rana remembered talking about Maddy's strange behaviour during the hour before Kevin passed out.

After their conversation, Rana went out to the garden, when Maddy began running in and out of the house between Kevin and Rana.

When Rana went inside to make a cup of tea, she discovered Kevin was unconscious on the couch.

They believe Maddy was trying to get Rana's attention, and get help for Kevin.

Maddy stood at the door, waiting, to make sure Kevin was safe with the two ambulances and fire truck which arrived to take him to hospital.

"Once she saw he was getting help, she calmed right down," Rana said.

Because of his illnesses, Kevin spends a lot of time on the couch resting, usually with Maddy lying at his side, her head on his chest or belly.

Kevin's theory is Maddy has become attuned to what his normal breathing is, and when it changes, she notices it, even before he does.

"I have absolutely no doubt at all she will save my life."

Rana said the realisation that Maddy knows when Kevin is about to have an episode means she can relax a bit more.

Kevin, who wears a St John's alarm bracelet, jokes that he may be able to get rid of it because of Maddy.

Maddy isn't just a pet, she is a member of the McFarlane family.

The couple's grandchildren enjoy dressing Maddy up, and even painting her nails.

Maddy, believed to be a staffordshire bull terrier crossed with a pitbull, was evidence that "it's not the breed of the dog, it's the people who've got them," Kevin said.

Despite being neglected for the first two years of her life, the SPCA had worked with her for more than a year before the day the McFarlane's saw her.

The couple went to the Invercargill SPCA to pick up a cat not long before Christmas 2015, but ended up bringing Maddy home instead.

"She was just waiting for us ... there was an instant attraction between the two of them [Maddy and Kevin]," Rana said.

Kevin said all the other dogs in the shelter were jumping up on the bars, begging for attention, while Maddy sat patiently in her kennel.

"She talks with her eyes," he said.

The couple want to encourage everyone to support the SPCA and the work the organisation does, they said.

The SPCA's annual appeal week is on November 11 to 17.