The Coronation Street star told the Sunday Mirror the 19-year-old 'was crying in agony' and that he told her: "I'm so scared mum."
Speaking to promote a new British Heat Foundation campaign, Kym, 38, said: "It was heart-breaking. I haven't seen him cry since he was little. He was so frightened and so was I, but I tried to be strong for him."
The terrifying incident happened in June last year shortly after David complained to his mum about a headache and feeling exhausted.
At first, Kym thought her son had a virus - but when he then told her his neck was hurting too, she took him straight to hospital near their home in Greater Manchester.
However, David's condition grew increasingly worse as he struggled to breathe.
And when ECG results and blood tests came through, they appeared to confirm the doctor's suspicion of a heart attack, leaving Kym fearing her son was about to die.
A few days later, Kym was told that David hadn't had a heart attack but was suffering from a serious condition called myopericarditis - an inflammation of the heart and surrounding sac.
Kym said: "When he was diagnosed, I asked the nurse how serious it was.
"She said it could prove fatal. I was a wreck and couldn't believe what was happening."
Fortunately, after lots of rest and medication, David has now made a full recovery.
Kym painfully lost her youngest son Archie just minutes after he was born back in February 2010.
The soap star and her then husband Jamie Lomas were both left devastated.
At the time, they released a joint statement reading: "Archie is our beautiful angel and we will miss him so much. Thank you all for your support."
Kym is certainly no stranger to heartache - the soap star has also struggled with her father David's battle with heart problems.
During her interview with the Sunday Mirror, she admitted that her son's illness brought back painful memories.
When she was just 18-years-old, the actress helplessly watched her father suffer from a heart attack.
When he fell ill, David Senior died for three minutes before doctors were able to resuscitate him.
Afterwards, he had quadruple bypass surgery and was then forced to quit his career as a joiner.
Kym's dad had another mild heart attack two years ago before stress issues triggered an irregular heartbeat. However, his health is now stable.
As a thank you to the British Heart Foundation for all its support to people who suffer with coronary problems, Kym is fronting their new Dechox campaign, urging people to quit chocolate in March.
Kym said: "Dad still can't pick up [my youngest daughter] because she can't put any stress on his heart - but he's still here.
"The British Heart Foundation is very important to my dad and it means a lot to him that I'm supporting them."
Kym wasn't paid for her interview with The Sunday Mirror. Instead, the paper made a donation to the British Heart Foundation on her behalf.
Sign up for the Dechox campaign at bhf.org.uk/dechox.