Some myocardial infarctions (MI) occur as the first manifestation of coronary artery disease. These are termed 'unheralded' events as they have not been preceded by other forms of coronary artery disease. Unheralded MIs are important because of the high likelihood of missed opportunities for prevention. The proportion of MIs that are 'unheralded' is unknown.

This study aims to quantify the proportion of MIs that occur 'unheralded' and also give an estimate of the incidence of 'unheralded' MI in the UK, compared to 'heralded' MI and those with angina of recent onset (MIs with premonitory symptoms).

This study is part of the CALIBER (Cardiovascular disease research using linked bespoke studies and electronic records) programme funded over 5 years from the NIHR and Wellcome Trust. The central theme of the CALIBER research is linkage of the Myocardial Ischaemia National Audit Project (MINAP) with primary care (GPRD) and other resources. The overarching aim of CALIBER is to better understand the aetiology and prognosis of specific coronary phenotypes across a range of causal domains, particularly where electronic records provide a contribution beyond traditional studies. CALIBER has received both Ethics approval (ref 09/H0810/16) and ECC approval (ref ECC 2-06(b)/2009 CALIBER dataset).

Using data from MINAP and GPRD, patients experiencing their first myocardial infarction (MI) will be separated into three categories:

• 'Heralded MI': patients who have been diagnosed or treated for cardiovascular disease for more than 90 days before MI.
• 'MI with premonitory symptoms': patients who have been diagnosed or treated for cardiovascular disease within the 90 days prior to MI.
• 'Unheralded MI': patients without a cardiovascular disease diagnosis or treatment at any time before MI.

Patients with MI will be identified in the linked GPRD-MINAP record. Using GPRD population denominators, this study will establish the absolute incidence rates of first 'unheralded' MI by calendar year, age, sex, season, MI subtype (ST-elevation MI and non ST-elevation MI) and region and compare these to the rates of first 'heralded' MIs and first 'MI with premonitory symptoms'.

Inclusion Criteria:

• Patients in GPRD practices which are deemed "up to standard" by GPRD criteria will be included if their practice agreed to be linked to the MINAP dataset.
• Age over 18.

Exclusion Criteria:

• Patients will be excluded after experiencing their first MI.
• Patients will be excluded if they do not fulfil one of the inclusion criteria.

The study population is comprised of patients registered at those GPRD practices that agreed to the linkage with the MINAP database, and whose practices are "up to standard" according to GPRD criteria. Practices taking part in the GPRD are chosen to be representative of all UK practices, and 98% of people in the UK are registered with a GP. Therefore the GPRD should be a representative sample of the UK population.