Experts have set out some key signs that you need to see a doctor as soon as possible as you might have a potentially serious condition . And they say one is a feeling of lightheadedness or fainting as it shows an important part of your body is not working correctly . According to the British Heart Foundation the feeling can be a symptom of heart failure - a condition which up to a million people are currently living with in the UK. A further 200,000 people are diagnosed with it every year, NHS