Incidence of OHCA
Number of patients with OHCA in Scotland
On average 3,000 people per year suffer from an OHCA where resuscitation is attempted by the responding ambulance crews. This translates to 58 per 100,000 resident population. This is a slightly higher figure than both the 53 per 100,000 resident population published by the English OHCA registry based on data from 2014  and the 49 patients per 100,000 resident population reported by the European Registry of Cardiac Arrest ( EuReCa ONE)  .
Worldwide there is variation in the reported incidence of OHCA with some other countries reporting similar numbers to Scotland, for example 58 per 100,000 of resident population in Finland  and 56 per 100,000 per resident population in North America.
It is important to note that although the incidence of OHCA for Scotland quoted above is derived from all OHCA where resuscitation was attempted, all other analysis is based on a slightly smaller dataset. Throughout the remainder of this report, the 'denominator' number used to calculate proportions - for example, the proportion of survivors after OHCA - refers to OHCA where resuscitation was attempted by SAS, and which we were able to link via CHI to ascertain survival outcome ( see 'Linking the Data').
Sex and age distribution of OHCA patients
Of all patients who had an OHCA, 62% were males and 38% were females. These results are in line with published data from other countries across the world describing higher proportions of OHCA among males [17-20] .
Figure 1 shows the age distribution (each age group is expressed as a percentage of all OHCA cases in our dataset) of OHCA patients across five-year age groups for males and females. Due to the very small number of OHCA cases in the lowest and highest age bands, some of these groups have been combined further to protect patient confidentiality. This figure also illustrates that OHCA is more common among males compared with females. Of note, the proportion of OHCA cases is lowest in children between 5 and 10 years old, however, all age groups - including very young children - are at some risk of OHCA. To illustrate this further, 10% of all OHCA patients are less than 45 years old.
Figure 1: Distribution of OHCA cases by age and sex expressed as a percentage of the total number of OHCA between January 2011 and March 2015
Figure 2 shows the incidence of OHCA - number of cases per 100,000 resident population - for the different age groups and for males and females separately. Mid-year population estimates of the Scottish population published by National Records Scotland are used to estimate incidence  .
Figure 2: Incidence of OHCA by age and gender, expressed per 100,000 resident population between January 2011 and March 2015
A comparison of the distributions for males and females indicates that male OHCA patients are on average younger. This is also reflected in an average (mean) age of 66.1 years for males and 69.8 years for females. The results shown in Figures 1 and 2 are consistent with reports from other countries about the age and sex distribution of OHCA , , [22-24].
SIMD status of OHCA patients
The Scottish Index of Multiple Deprivation ( SIMD) is a measure designed to identify area concentrations of multiple markers of deprivation. For the purpose of disseminating data, Scotland is divided into areas called 'datazones', each with a population of around 500 to 1,000 residents. The SIMD ranks the 6,505 datazones that cover Scotland from most deprived (ranked 1) to least deprived (ranked 6,505). Ranking is based upon markers including current income, employment, health, education, skills and training, housing, geographic access and crime. These SIMD-ranked datazones can then be split into quintiles reflecting the most deprived 20% of the population ( SIMD1) up to the least deprived 20% ( SIMD5)  .
In our linked dataset, the SIMD quintile was available for over 97% of OHCA cases included in the analysis (no information was available for 207 patients). Figure 3 shows the proportion of OHCA patients in each SIMD quintile. The proportion of OHCA involving people living in the most deprived SIMD quintile was twice as high as the proportion of people in the least deprived SIMD quintile (28% versus 14%). The majority of relevant academic literature has shown that a low socio-economic status is associated with a higher incidence of OHCA , .
Figure 3: Percentage of OHCA cases across the SIMD quintiles. SIMD1: most deprived, SIMD5: least deprived
Figure 4 shows the (mean) average age at which patients suffered an OHCA in each SIMD quintile. For both males and females the mean age increases across the SIMD quintiles. This indicates that people who live in a more deprived area who have an OHCA are younger than people who live in less deprived areas who have an OHCA (average age for males in SIMD1 and SIMD5 is 62.5 and 70.3 respectively, and for females 66.6 and 72.6). The age difference across the SIMD quintiles is most pronounced for males and the mean age at which males suffer OHCA is consistently younger than for females.
Figure 4: (Mean) Average age of OHCA cases across the SIMD quintiles by sex
Number of patients with OHCA in urban and rural areas
The Scottish Government Urban Rural Classification  is based on two key criteria - settlement size and drive time to major settlements. The six categories distinguish between urban, rural and remote areas (see Table 1). Table 1 shows the proportions of all people who live in Scotland across the categories and the proportion of OHCA patients who lived in the different urban rural categories. Over 73% of all OHCA patients live in an urban area. The last column of the table shows the incidence per 100,000 population by the six categories. This shows that the incidence is highest in the large urban areas and lowest in remote rural areas. Note that these are crude incidences not adjusted for age, sex or SIMD. Furthermore, the urban rural classification is based on patients' home addresses, not necessarily the locations where the OHCA happened. In contrast, data from the Irish OHCA registry showed no difference in incidence between urban and rural areas  . For practical reasons in some analysis a simpler urban (combining categories 1 and 2) or rural (combining categories 3-6) classification is used instead of the six categories. More detailed analysis is planned to investigate regional and urban/rural differences in incidence and outcome of OHCA in Scotland.
Table 1: Description of urban and rural classification and proportions and incidence of OHCA cases
|Category||Description||Proportion of total population of Scotland||Percentage of OHCA's||Incidence per 100,000 population|
|1. Large urban areas||Settlements of over 125,000 people||34.7||34.7||41.0|
|2. Other urban areas||Settlements of 10,000-125,000 people||35.2||38.4||44.6|
|3. Accessible small towns||Settlements of between 3,000 and 10,000 people and <30 min drive of a settlement of >10,000 people||9.4||8.9||38.7|
|4. Remote small towns||Settlements of between 3,000 and 10,000 people and with a drive time of >30 min to a settlement of >10,000 people||3.5||3.1||36.2|
|5. Accessible rural||Settlements of <3,000 people and within 30 min drive of a settlement of >10,000 people||11.3||10.3||37.2|
|6. Remote rural||Settlements of <3,000 people and with a drive time of >30 min to a settlement of >10,000 people||5.9||4.6||31.9|