**2.1 Epidemiology**

Population-based epidemiological studies suggest an incidence of AAS of about thirty cases per one million people per year. Eighty percent is represented by acute aortic dissections, 15% by intramural haematomas and 5% as penetrating ulcers. Seventy percent of the affected population is male with an average of 60 years [3]. A Swedish epidemiological study found the same incidence in an observational period between 1987 and 2002 among 4425 cases. In this study, the incidence of AAS has increased by 50% in men and 30% in women over study period. This may be related to enhanced diagnostics, although a further component could also be the increasing age of the population. Overall, 20% of affected patients died before reaching a medical facility, 30% during the hospital stay and further 20% over the following 10 years [4]. Both circadian and seasonal variations have been observed in the occurrence of AAS, with the peak frequency found between 0800hr and 0900hr, with an increased likelihood during the winter period. The most likely explanation is a link to the circadian variation in blood pressure [5, 6].
