There is a variety of medical conditions which can cause a person to collapse; in Mr Haughey's case, they are added to by the fact he has terminal prostate cancer.
Any process which interferes with the oxygen or blood supply or the delivery of glucose to the brain can cause a patient to lose consciousness.
Permanent brain damage can result if resuscitation is not started within 4-5 minutes.
The medical bulletin from Beaumont Hospital yesterday morning referred to a "life-threatening cardiac condition".
We also know Mr Haughey was unconscious when he arrived at the accident and emergency department and had to be resuscitated. There are a number of fast, abnormal heart rhythms which could cause collapse.
The most dangerous is ventricular fibrillation which cau sed the two large chambers of Mr Haughey's heart to contract in a chaotic fashion, with little flow of blood from the heart to the brain and other vital organs.
The immediate treatment for this condition is to give an external electric shock using a defibrillator.
While almost certainly administered in the accident and emergency department, defibrillation may also have been used by the ambulance crew shortly after their arrival in Kinsealy.
Treatment with intravenous anti-arrhythmic drugs and a correction of any biochemical disturbance will follow in the intensive care unit.
How might Mr Haughey's cancer affect his medical condition?
He is weakened by it and so his reserves will be depleted in terms of recovery.
Because he is terminally ill, he is likely to be taking strong painkillers; these can cause depression of the respiratory or breathing centre in the brain and may have been an indirect reason why he was ventilated after admission.
As Mr Haughey's cancer progresses, it may have led to uraemia, a condition which reflects the declining ability of the kidneys to clear a substance called urea from the body.
This in turn can cause elevated potassium levels in the blood which are a major cause of the heart rhythm disturbance, ventricular fibrillation, the reason for his sudden collapse.
A key factor in Mr Haughey's immediate prognosis was how quickly he could be taken off the ventilator.
Once this was done and the heart rhythm regulated, there is no reason why he cannot make a full recovery.
There is a possibility of a recurrence, although he will be given medication to guard against this. However, as a 75-year-old with terminal prostate cancer, he remains a very sick man who will require ongoing medical treatment for his condition.