Watson, a 15-month-old cocker spaniel sponsored by Ipswich-based insurance company WTW, is being trained by Hearing Dogs for Deaf People and was staying with volunteer dog trainer Jeannette Godsell, 58, in Marchwood, Southampton, when the incident happened.
Ms Godsell began to feel unwell after returning home from a training session at a railway station and sat down on the sofa.
She said: “Watson is usually a really chilled dog, so I couldn’t understand why he wouldn’t stop nudging me.
“I tried to go upstairs and lie down, but he blocked the stairs by standing in front of them.
“In the end, I gave up and sat back down on the sofa.”
Watson then went into the garden and began barking, alerting Ms Godsell’s neighbour, Sue, who came to check on her.
Despite Ms Godsell’s insistence that she was just experiencing indigestion, Sue called an ambulance.
Paramedics quickly arrived and performed an ECG, revealing that she was having a heart attack.
Ms Godsell was taken to the hospital, where she underwent emergency surgery to have a stent fitted.
She said: “It’s terrifying to think what would’ve happened if Watson hadn’t stopped me from going upstairs, where I would’ve been alone.”
The doctors told Ms Godsell she was within the ‘golden hour’ for treatment and said her chances of survival would have been much lower if she had arrived at the hospital just 30 minutes later.
Watson is now preparing to move on to the next stage of his training, where he will be matched with a deaf partner.
Ms Godsell said: “He’ll always have a piece of me – the heart he helped protect.
“It might sound dramatic, but he literally saved my life.
“I want the world to know how amazing he is.”