As Lindsay Jones, 22, circled the jogging track for her 10th lap, Amber suddenly perked up.

The 2-year-old, 55-pound yellow Labrador retriever stretched to the end of the leash that was tied up a short distance away. She stared at Jones, who knew that it was her signal to get over to her dog immediately.

Amber nudged Jones, prompting the young woman to lie on the ground. With that, the dog climbed on top of Jones, who knew what to do next.

She checked her heart rate and discovered it was only 40 beats per minute, not the 150 beats she expected after her run.

"I would have definitely passed out if she hadn't alerted me," said Jones said.

Another emergency averted by her trained cardiac alert dog.

Jones suffers from two heart conditions: neurocardiogenic syncope, in which the nerves that control her blood pressure stop sending impulses and force her blood pressure and heart rate to drop; and postural orthostatic tachycardia syndrome (POTS), which causes her heart rate to spike as her blood pressure drops.

"I have one condition that causes my blood pressure to spike and I pass out, and one that causes it to drop and I pass out," said Jones, who lives in the University City section of Philadelphia.

Before Jones was partnered with Amber in June, her frequent passing out, more than once a week, caused 12 concussions. "I couldn't tell it was coming," she said. "I'd be in the kitchen cooking and hit my head on the floor. Since I've had Amber, I've not passed out yet."

Amber has the ability to detect Jones' changing heart rate and warns her with an extended stare, or by walking until her leash it fully outstretched, getting her owner to lie down until her blood pressure and heart rate stabilize.

"If it's high, she lays between my legs and rests her head on my thigh and if it's low, she lies all the way on top of me with her chest on my chest," Jones said. "Until my heart rate stabilizes, she won't stand up."

Amber has both improved and changed life for Jones, a graduate student at the University of Pennsylvania studying environmental biology.

"I didn't think going to Penn would be a likely option for me because my family wouldn't be comfortable with me moving across the country with my condition," said Jones, who is now able to live on her own, away from her family in Colorado.

How Amber knows that her owner is in trouble is a bit of a mystery.

"While science has not exactly determined how the dogs do this, the tight bond formed by the dog and human is one where each is very sensitive to changes in the other," said Rob Danoff, physician and program director for the Aria Jefferson family medicine residency.

"The dog's sense of smell can detect changes in heart rate, blood pressure, and blood sugar, even though the chemical changes in the body are minute. Also, since dogs' sense of hearing is so amazing and sensitive, some can actually hear the sound of the heartbeats and alert the human if the heart rate is going too fast or too slow."

Not every dog is up for the task. Cardiac alert dogs must have an innate ability for sensing blood-pressure and heart-rate changes, said Tonya Guy, associate director at Canine Partners for Life, which matched Amber and Jones.

It can take up to two years to figure out whether the dog has the ability. For the first year, puppies are raised by volunteers who teach them obedience, house manners, and life skills.

At 14 months, the dogs move to the training facility to learn the specific skills they may need to assist a person with a disability, including retrieving items, tugging off clothing, turning lights on and off, opening doors, taking steps one at a time, and support work to help someone replace a cane or a crutch or use mobility aids.

"They are in the kennel for nine months to a year, working on the foundation of all those skills to find out what the dog is most efficient at doing so they can do the job they love, they are good at, and the person needs," said Anne Savo, a trainer at Canine Partners for Life.

To discover which dogs will be successful at cardiac alerting, they are sent home with a man who has multiple seizures a day. "He watches how they will react," said Guy. "Some dogs will sleep through and not even know he's having a seizure while others will alert to it right away. If the dogs are able to alert to seizures, they are able to alert to a cardiac condition."

Dogs without that ability are still able to provide service for people with other types of needs.

Canine Partners for Life, based in Cochranville, is one of the few groups in the country training and placing cardiac alert dogs. It trains mostly Labrador retrievers, a breed that has proven especially adept for service. It has so far matched 18 cardiac service dogs with recipients, with 10 applicants on the waiting list.

It costs $30,000 to train, place, and offer follow-up support for each dog, but recipients are asked to pay only what they can afford, typically $1,000 to $3,000. The rest is covered by private donations. Jones raised $2,500 from friends and family for Amber.

The group began training dogs for this purpose in 2006 when Marty Harris, who suffers from a severe form of neurocardiogenic syncope, approached them.

"I'm one of those cases where science hasn't caught up with the condition," said Harris, who lives in Boston. In the 10 years that Adele, Harris' service dog, has lived with her, "she has prevented fainting, concussions, falling down stairs, and all the things that go with fainting," Harris said.

Adele has recently retired to house-pet status, and keeps Harris' new service dog, Hector, in line.

"If he isn't doing something to her satisfaction, she'll push him out of the way and do it with him watching her," Harris said. "She's still the boss."

Adele and Harris are the subjects of a new documentary, Letting Go of Adele, currently being screened in film festivals with the goal of being picked up for broader release. "The documentary is a good opportunity to help people around the world see the way service dogs change lives for the better," she said.