This is the most helpless we will ever be. The patients here are mostly unconscious or sedated, lying high on their pillows, faces tilted blindly towards the ceiling while tubes and drips and feeds invade them from every angle. Syringe drivers dose their systems with drugs at timed intervals and the contents of feedbags drip through tubes into their stomachs; monitors trace heart rates, central venus pressure and blood pressure in thin coloured lines that track mountain ranges across a black screen.

On the other side of the beds are breathing machines, flashing up digital breakdowns of breathing patterns, while tubes pass through throats, hands or arms, drugs keep hearts beating, and ventilators gently fill and empty lungs, breath by breath by breath.

'It must look like a torture chamber if you're not used to it,' says Andrea Martin, head nurse of the intensive care unit at London's University College Hospital. 'I suppose I hardly see it any more, but I wonder what relatives think when they first get here.'

In truth, it takes a while to make out the relatives amid the bright lights and fast-moving staff and metres and metres of tubing. Beside a bed near the nurses' station sits an elderly woman, holding tight to her unseeing husband's hand, and staring blankly beyond him. Leaning against a wall nearby, a middle-aged man watches as two nurses move gently round the bed of his relative, suctioning him. He shifts to the other foot after a while. Christmas has come and gone here, but there are no decorations on the ward.

Few of the patients would notice anyway. The intensive care unit (ICU) is reserved for some of the sickest people in a hospital: patients suffering from multiple organ failure. When we are ill, or the body suffers some trauma, normally our own chemicals will begin the battle against the invader while the rest of the system keeps on going. But in cases of truly severe 'insult', as it is called, to the body, such as a car accident, major surgery, or severe infection, the balance between defence and attack is lost, the body starts to do battle with itself and one by one organs are damaged and, eventually, shut down in the crossfire. Lungs, heart, stomach, kidneys, liver: it's hard to believe that things we've never even seen and just assume will always work can let us down so badly.

This is when patients are moved to the ICU: Martin says that part of the reason she came here was because she wondered where the patients who were too sick to be on the ordinary wards went. The ICU is the only place in a hospital that can support those collapsing organs and keep a patient stable enough to administer the other medical treatments they may need urgently.

Fifty or 60 years ago none of this would have been possible: before ventilators existed to keep lungs going when the body had given up on them, a patient would have been left to die. Intensive care has been pioneered over the last half century and turned into a speciality all of its own. The invention of machines and drugs that can keep a restarted heart beating, make failed lungs breathe and force kidneys to function has kept many thousands of patients alive since.

But, as Sara Bunkhall, head of the ICU at UCH points out, the financial pressures are immense. An ICU bed costs about £1,800 a day to keep going. We count up the cost of the machines around just one bed: the ventilator £20,000, each of the four syringe drivers £3,000, the feeding pump £5,000, the beds £3,000 each, the monitoring system for the ward £500,000, a haemo (blood) filter £18,000. What do these new machines do that the old ones didn't? Dr Monty Mythen points to a woman down the corridor. She sits upright in bed, a tube feeding into her throat and multiple lines running through her chest and arms, with a pair of glasses perched on her nose, reading a newspaper.

'Fifty years ago,' says Mythen, 'she would have had to be in an iron lung. Twenty years ago she would have had to be heavily sedated.'

The support machines being used here now are far less intrusive than the old machinery: patients can be conscious and almost comfortable while they are treated. Do the machines save lives then? As you look around at these patients, so paper-white and fragile it seems one moment more of trauma would push them beyond help, the answer must surely be yes. Some doctors say they do, and most intensivists will tell you that they can save many more lives today than they could 20 years ago.

But, says Mythen, slowly: 'There is some evidence to suggest that they may just reduce suffering. That's something, but when it comes to health economics suffering is often acceptable if it's cheap.'

Money, money, money. Anyone in the NHS will tell you that there's not enough of it (in Britain we spend about 6% of our gross domestic product on the health service: most other North European countries spend closer to 9%, while the United States spends more than 10%). The rate of inflation in a hospital is not the same as that of a country: the cost of machinery and drugs multiplies exponentially, not just by a few per cent every year. And while intensive care in this country is under the same financial pressure from which the rest of the NHS suffers, ICUs also put pressure on themselves by insisting on an expensive one-to-one ratio of nurses to patients.

Detractors may suggest that the level of care in Britain is worse than Europe or North America (UCH has the largest ICU in Britain, with 22 beds, while a hospital of similar size in the US will have more than 100), but intensivists point to nurse-patient ratios elsewhere: Mythen has seen patients in the US tied to their beds to stop them dislodging tubes because they are not watched 24 hours a day.

One-to-one care is, in the end, the reason most nurses and doctors come into this field, the reason they take on the higher mortality rates and never-ending pressure (Mythen has had nights when he has been telephoned due to an emergency every 40 minutes). The passage of a patient through this unit is different to any other: relatives need special care and attention as they watch a mother, a brother or child kept alive by pieces of plastic and metal. The daughters of Valerie Cumming, who is being ventilated after breathing difficulties on Christmas Eve, are full of praise for the staff. 'We felt involved from the beginning,' says Jane Fiona. 'They told us, and my mother, even though she was unconscious, exactly what was going on. Everyone has been so supportive, and even though there are such sad things happening here, it's oddly peaceful. We all wish each other well.'

Nurse Martin says: 'Because patients so often come in unconscious, I often think that one of the hardest moments for them must be when they come round and realise where they are.' Behind her, a wall is covered in thank you cards from patients or relatives who have left the ward behind. The nurses stride by, capable, organised, making jokes and being rude about the hospital trust, as NHS staff are. The machines quietly tick over, steadily feeding and breathing and moving the still bodies. New Year's Eve passes, just as Christmas did. But here there are so many clocks ticking to different timescales that hardly anyone notices.