Today I made the vilest error of my short-lived medical career. It's every newly qualified doctor's nightmare to hear the crash bleep go off, and to realise that the call is for a patient on the ward next door - that you will almost certainly be the first on the scene, master of ceremonies, however briefly, at the undignified prelude to an inevitable death. This afternoon, a Sunday, as I sat next to a patient (it was ward cover, and I had never seen her before) pondering where best to try and fit a cannula, I heard an almost inaudible sigh. As her posture changed almost imperceptibly, I realised that she was dying. No, she was dead.

This is not supposed to be an earth-shattering moment for a medic. At first, it seems surprising to patients and medical students alike that anyone could die when they are surrounded by so much technology: a machine to make you breathe, a machine to clean your blood, a cupboard of drugs to sweep away the clots. I convinced myself I was managing well on my own. I did my ABC, the mantra of basic life support. The airway was clear, there was no breathing and there was no carotid pulse. She looked pretty dead to me.

I shifted self-consciously into a managerial role, and announced, in what turned out to be rather meek English tones, "Cardiac arrest. Crash team please." Nurses materialised from nowhere. "Crash call?" asked a nursing student by the desk. Well yes, I thought: of course I want a fucking crash call. There is a dead old lady lying here with my tourniquet round her arm. "Yes," I nodded, business-like and calm, and began my chest compressions while someone fetched a bag and mask, and someone else wheeled in the resuscitation trolley.

I heard the call going out on my own bleep. Within 90 seconds, doctors began to trickle in. I had been eyeballing the defibrillator, wondering if I could really trust myself to use it. They were sweaty but experienced, they were the cavalry, and immediately they asked for the notes and the story. I had been with the patient for 10 minutes, long enough to know she needed an IV line, long enough to know from the notes what had happened to her over the past six weeks. I laid out the story and the notes sat on her bedside chair.

Did I mention the relatives? I had forgotten they were there, although it was hard for them not to notice us. This was the moment their loved one passed away, far away from everything that that been her life. I, on the other hand, had a patient, and a list of things to do. The curtains were pulled, and the chest inflations didn't seem to be doing much. The senior house officer hurriedly manoeuvred a plastic tube down her throat.

There was no pulse, and there was "no reversible cause". The defibrillator electrodes were on, and the monitor showed ventricular fibrillation. "VF is the commonest pulseless electrical activity," I thought, inanely, "and the one that is most frequently shockable." A line was placed in both arms, the anaesthetist began struggling to get one into her neck. I allowed myself to be sidelined.

"OK, I'm ready to shock. All clear." It is the responsibility of the person who administers the shock to ensure everyone is clear, the voice of ancient revision recited in my ear. The patient was very clearly very dead ... 200 volts, and she arched into the air. Check: no pulse, no change, VF ... 200 volts, no pulse, no change, except this time a wheeze as the air was forced out of her chest ... 360 volts.

Perhaps I'm not explaining myself properly. What I was watching was horrific: wires in her neck, tubes in her throat, spasms of electrocution. It may even have been painful. It certainly sounded like it from the cracking ribs. It was undignified, and traumatic, and the routine cycled around for what felt like an hour.

Registrars come to arrests to tell you when to stop. Ours came and took one look at the notes, one little look, at the "Not for resuscitation" pink form, ticked, signed, dated, six weeks before, reviewed every week, a form marked: "Not for resuscitation: on grounds of futility, on grounds of quality of life."

Ticked. But not noticed by me. Discussed with nurses, discussed with medical staff, discussed with patient, discussed with relatives, every single one was ticked. So there I stood, thinking: am I guilty of assault, or abuse of a corpse? Was it a crime against this woman, or her family? Was it my fault? Obviously.

I knew nothing of this woman's life. I just marched in at the end with the fireworks, at the most personal moment of her lifetime. "If there is any doubt about resuscitation status, you must always put out a crash call." I don't care. This is the worst thing I have ever done.