Out-of-hospital cardiac arrest ( OHCA) is a significant health issue in Scotland. The shape of the problem is that around 3,000 patients each year will have resuscitation attempted after a sudden cardiac arrest in the community, but only around 6% will survive to hospital discharge. In the best performing comparable settings around the world survival is as high as 25%  .
In response to this challenge, in 2015 Scotland launched a National Strategy for out-of-hospital cardiac arrest  . In an unprecedented collaboration, all of Scotland's emergency services, Scottish Government, Third Sector, NHS and Academic partners have come together intent to achieve together what would be too difficult to accomplish separately: to optimise the 'chain of survival' after OHCA thereby saving 1000 additional lives in Scotland by 2020. The aim is that Scotland becomes an international leader in OHCA resuscitation over the next four years.
Solving the problem of OHCA is difficult, but in broad terms it is not complicated. There is a high level of international consensus about the steps required , . The first of these is to create a registry. To build a mechanism to measure current system performance, identify areas for improvement and track progress. This is not a straightforward process, as the system delivering care to victims of OHCA spans multiple agencies and so assembling meaningful data requires the synthesis of information from a range of 'silos'. This is a tractable problem which has been overcome in centres of excellence around the world and marks a foundational step for all those attempting to improve OHCA survival [5-13] . This is a challenge which Scotland's health data architecture should make Scotland well placed to overcome.
Linking the Data
In order to know how many patients fall victim to OHCA each year we begin with details of emergency calls to the Scottish Ambulance Service ( SAS). This gives us the number of incidents where Ambulance Service personnel attempted resuscitation for OHCA. Ambulance Service data collection ceases when the patient is delivered to hospital, and so to calculate the proportion of OHCA survivors we need information about in-hospital care. Information about this part of the patients' journeys are accessible via their Community Health Index ( CHI) number. The CHI number uniquely identifies everyone registered with the healthcare system in Scotland. The backbone of the data included in this report is derived from the linkage of Ambulance Service cases with the patient's CHI number. This transforms 'incidents' into CHI-linked patient records. Once linked, use of the data is restricted to a highly controlled 'safe haven' environment in order to preserve patient confidentiality. We will describe how this linkage process throws up its own challenges, but once complete delivers a rich picture of the way Scotland responds to OHCA (see Figure 14).