`I felt so faint I thought I must be dying, but I was just hyperventila ting.'

A couple of years ago, I had to call my GP in the middle of the night. I felt so weak and faint that I thought I might be dying. He took my pulse and blood pressure, listened to my heart and then told me that he could find nothing amiss.

On that occasion I was reassured, but I still thought there must be something wrong. Over the next few months, staff at the surgery became used to me in the waiting room, often with my head on my knees. Apart from the faintness, it was the mental symptoms that really frightened me: confusion and feelings of unreality - as if I were watching myself from a distance. I thought I must be going mad. The more anxious I became, the worse I felt.

I was referred to a hospital physician who carried out blood tests and X-rays, but who could find nothing wrong. He did, however, notice that the top part of my chest moved very rapidly when I breathed and he referred me, privately, to Dr Claude Lum, an authority on respiratory disorders.

Dr Lum told me my problem was caused by a common condition that doctors had been treating for over 25 years. I was suffering from hyperventilation, or over-breathing.

It is estimated that a tenth of the population suffers from hyperventilation. Specialists believe the incidence of this debilitating and frightening condition is on the increase, because of the more stressful lifestyles of the Nineties. Yet I learnt that sufferers can be completely cured by correcting faulty breathing patterns

Hyperventilation is essentially the habit of breathing in, and out, too much. As everyone knows, we breathe in to obtain oxygen and out to expel the waste gas, carbon dioxide. When we run for a bus or climb mountains, the body responds by taking in more oxygen and expelling more carbon dioxide.

People who hyperventilate take in excess air. They do not use the extra oxygen, but they do lose an excessive amount of carbon dioxide when they breathe out, and blood levels of the gas fall. The imbalance of oxygen and carbon dioxide upsets the delicate acid/alkaline balance in the blood, which in turn affects all body tissues, particularly the brain and nervous system - hence the wide variety of physical and mental symptoms that may occur.

Apart from those I experienced, hyperventilation can cause headaches, poor memory, ringing sounds in the ears, dizziness, panic attacks, chest pains, yawning, tingling and numbness. A heightened sensitivity to loud noises and bright lights is also common.

Stress and excitement can bring it on. One of my daughter's friends, in her early twenties, often hyperventilates when she is overexcited and laughs too much. She becomes so badly affected that she cannot speak or move. Fortunately, her friends now recognise the situation and put a paper bag over her nose and mouth until she begins to breathe normally again. The bag catches the exhaled air, high in carbon dioxide, and this is inhaled back into the lungs on the next breath. But this treatment is only useful in acute attacks.

The consultant sent me to a physiotherapist to learn how to breathe correctly - not easy for someone who had been over-breathing for many years. I was taught to breath from my diaphragm. Keeping my chest as still as possible, I learned to breathe in, pushing my stomach muscles out, and then exhale slowly, allowing the stomach muscles to relax.

She explained that I should aim for eight to 10 breaths a minute, and she timed me with her stopwatch. I practised this breathing at home, for 10 minutes about four times a day. Although I had been a little sceptical, I had to admit that after a few weeks I was definitely feeling better.

Diet, surprisingly, is also important. Symptoms are much worse if the blood sugar is low. A stable blood sugar level requires a regular supply of protein (60g to 70g a day), which should be spread over the three main meals with mid-morning and mid-afternoon snacks, such as cheese or egg. Coffee is definitely out, since it is a strong stimulant to breathing, although moderate amounts of weak tea are permitted.

Doctors at Papworth Hospital in Cambridge have found that breathing retraining cures 75 per cent of patients completely within 12 months. Of the rest, 20 per cent are left with occasional mild symptoms. Only one in 20 cases proves intractable.

I feel fine now - although I still practise the breathing exercises about once a day, just to be on the safe side.

Dr Lum is a former consultant in chest medicine and respiratory physiology at Papworth and Addenbrooke's Hospitals. He first recognised the illness in 1965, in suspected heart attack patients, after tests had proved their hearts and lungs were normal. Although retired, he is regarded as a leading authority on the condition.