The 52-year-old, from Stonehaven in Aberdeenshire, seemed to be fit and healthy all her life and took part in numerous sports, including squash and running.
Ms Macfarlane was diagnosed last year with hypertrophic cardiomyopathy, (HCM), which is an inherited condition.
Her diagnosis came almost 20 years after a brief episode of irregular heart rhythm in her 30s, the cause of which was not found.
"In hindsight it absolutely frightened me that I had run and played squash and I had this time-bomb inside me," she told BBC Scotland.
According to new research from the British Heart Foundation, more than 50,000 Scots are thought to be carrying a faulty gene that puts them at high risk of heart disease.
The majority of those affected are undiagnosed and unaware that they may be at risk of a sudden heart attack.
Each week in the UK about 12 seemingly healthy people aged 35 or under are victims of sudden cardiac death with no explanation.
Ms Macfarlane says she had first noticed irregular heart beats and palpitations when she was in her 30s but nothing was found.
"Then in my mid-40s I started getting more severe palpitations, light-headedness and I passed out a couple of times," she says.
"I was referred back to the cardiology department at Aberdeen Royal Infirmary, who did various tests and echo scans, ultrasounds, a treadmill test and they could not find anything at all.
"There was certainly nothing lifestyle related like blocked arteries or anything so it was a bit of a mystery."
Ms Macfarlane says she was "starting to feel like I was some kind of hypochondriac" but doctors persevered with their tests.
Eventually she was fitted with a reveal monitor, which was implanted into her body for two years.
"For 18 months it didn't pick up anything," she says.
"Then in the last six months it picked up what's called tachycardia, which is a very fast heart beat.
"They were able to analyse this and see that it was something a bit sinister."
Another episode of fainting last June led to an MRI scan which showed that the wall of her left ventricle, which is the lower chamber of the heart, was thickened.
That suggested the possibility of an inherited heart condition.
She was sent for a gene test which gave the positive result.
Ms Macfarlane says that her father had died of a heart condition but doctors could not tell from his notes whether he had carried the faulty gene.
They also tested Ms MacFarlane's son and her brother to see if they had the gene. Both tested negative.
As a result of her diagnosis, Ms Macfarlane was fitted with a cardiac defibrillator.
She says: "That gave me a complete new lease of life because I had lost all my confidence, knowing that I had the condition that could possibly lead to sudden cardiac death.
"The cardiac defibrillator constantly monitors my heart and if the rhythm goes off or my heart stops for any reason it will kick in and I am extremely lucky to have that."
Helga Macfarlane says she is "horrified" to think that she was unknowingly living with a faulty gene that put her at high risk of coronary heart disease or sudden death.