Fiddly but instant

The heart problem affecting Tony Blair is called supraventricular tachycardia (SVT). This is an abnormal rhythm of the heart that results in a sudden increase in pulse rate, from the normal 70 to 80 beats per minute at rest, to between 140 and 220 beats per minute.

It is relatively common in otherwise fit and healthy men and women in their thirties and forties, with most cases due to a glitch in the circuitry that carries electric impulses around the muscular heart wall - an abnormality most sufferers were born with, but which only becomes evident later in life, if at all.

SVT attacks come without warning and often only last a few minutes before terminating as abruptly as they started. Symptoms vary, but the most common complaint is an unpleasant feeling of fluttering inside the chest, which can sometimes be accompanied by breathlessness and light-headedness.

Stress, illness, excessive alcohol and too much coffee can all trigger SVT in susceptible individuals, as can physical exertion, but it's the layout of the wiring in your heart, rather than lifestyle, which is most likely to determine whether you develop problems. Intermittent short runs of SVT require no treatment and often disappear as mysteriously as they arrive, but recurrent cases like the Prime Minister's require intervention. Catheter ablation is the best hope of achieving a lifelong drug-free cure. The procedure involves introducing a catheter (a long, thin tube with a metal tip) into the heart via a small incision in the groin. After injecting local anaesthetic, the doctor inserts the catheter into the main vein in the groin and feeds it up through the veins of the abdomen and chest and into the heart.

The progress of the catheter is monitored on a screen, enabling the metal tip to be accurately placed against the heart wall in the area affected by "short-circuiting". This part of the heart wall is then cauterised - either by heating the tip using radio frequency waves, or super-cooling it with liquid nitrousoxide to abolish the abnormal rhythm.

The procedure is fiddly and takes about 90 minutes, but the results are instant and the success rate high. Serious complications are very unusual, but do include the need to insert a pacemaker (because too much tissue is damaged and the heart's normal rhythm is disrupted), something that happens in fewer than one in 100 procedures.

The Prime Minister is likely to be conscious throughout (though probably sedated) and should be back at work by Monday. This should have no lasting effect on his longterm health.