BMC Public Health 2007, 7:348 http://www.biomedcentral.com/1471-2458/7/348 of any of the events will be registered in the data collection protocol: transitory ischaemic accident, ictus, angina, myocardial infarction, symptomatic abdominal aneurysm and mortality attributable to vascular causes. The main variable of response will be the appearance of cardiovascular events as a combined compound variable [27] made up of the following diseases: Morbidity -Coronary disease: 1) angina, 2) acute myocardial infarc- tion -Cerebrovascular disease: 1) transient ischaemic attack, 2) ictus -Symptomatic aneurysm of the abdominal aorta These events will be confirmed by a cardiologist, neurolo- gist and vascular surgeon, respectively. Mortality -Ictus -Myocardial infarction -Sudden death attributable to cardiovascular events -Aneurysm of the abdominal artery -Complications of PAD. The mortality will be studied by: control of the computer- ised clinical history, telephone interview with a relative and personal or telephone interview with general practi- tioner responsible for the patient. If adequate information is not obtained by the above sources, the emergency departments (SEM) and emergency paramedical services (061) and, if necessary, the statistical registries of deaths (Department of Health) will be consulted. Statistical analysis Sample size The sample size for the cohort study has been calculated with an alpha risk of 0.05, a beta risk of 0.20 in a bilateral contrast, taking a relationship of exposure (altered AAI) compared to no exposure (normal AAI) of 1:9 into account. To do this 301 subjects in the group of patients diagnosed with peripheral arterial disease (pathologic AAI) and 2709 patients in the group without peripheral arterial disease (normal AAI) are required to detect a min- imum relative risk of the main variable of response of 2, accepting an incidence of the cardiovascular disease stud- ied in the general population of 5.2% [24]. The rate of loss to follow up has been calculated to be of 10%. This sample size also ensures correct estimation of the populational prevalence of asymptomatic AAI in our medium with a precision of 2%, accepting an alpha risk of 0.05 in the most unfavourable case of having to detect a prevalence of 50%. Data analysis This will be performed with the Stata 9 statistical package after the introduction of the data in a database (Acces). Quantitative variables will be compared with the Stu- dent's t test and analysis of variance (ANOVA) will be per- formed, using the corresponding non parametric tests when necessary. Qualitative variables comparisons will be determined using the Chi squared test. An analysis of survival of each vascular event will be per- formed according to the presence/absence of PAD with the Kaplan-Meier curves. Cox multivariate regression models will be used to compare the probability of having a cardiovascular event in the follow up cohorts, adjusting for the necessary covariables. The relative risk (hazards ratio) will be analysed with a confidence interval of 95%. Discussion The aim of the PERART study is to evaluate the real preva- lence of peripherial arterial disease in the general popula- tion which remains to be determined in Spain and is poorly known in countries with a low cardiovascular risk. These data will be the main contribution of this study since, to date only data of series of patients and very selected populations have been established. With the present study we hope to demonstrate that the silent forms of peripheral arterial disease may be a marker of cardiovascular risk with a better adjustment than the methods based on risk tables since these are indicative of real damage in the cardiovascular system still in presymp- tomatic stages. It is therefore important to keep in mind that the AAI may have a predictive value which is higher than the tradi- tional methods of calculating cardiovascular risk since these methods are based on probabilistic mathematical equations while the AAI detects early stages of atheroscle- rotic disease. On being a systemic disease, the detection of peripheral involvement is probably indicative of asymp- tomatic involvement of other vascular territories, and thus, the determination of AAI may have diagnostic and prognostic significance in the detection of asymptomatic cerebrovascular disease. Another interesting aspect is the follow up of the patients with a high AAI, which may also indicate a certain risk of having cardiovascular events if associated with arterial calcification [32]. Page 5 of 7 (page number not for citation purposes)