The management of small abdominal aortic aneurysms: a computer simulation using Monte Carlo methods.

Many small abdominal aortic aneurysms can now be identified by ultrasound screening and it is necessary to decide whether the risks of enlargement and rupture justify elective surgery. A computer simulation of the behaviour of small aneurysms has been constructed using Monte Carlo methods to model patterns of enlargement and rupture. The effect of policy with regard to the observation and timing of intervention has been evaluated for different patient groups. The results demonstrate the value of early intervention in otherwise fit patients with expected operative mortality rates of 5% or below, whereas, for older patients (over 70 years old) 6-monthly screening and operation if the aneurysm exceeds 5 cm is suggested. Higher risk patients with expected operative mortality of over 10% may be better treated conservatively up to an aneurysm diameter of 7 or 8 cm. The method used for simulation is flexible, being easily adjusted to take account of new information, and may have applications in other areas of clinical decision making.