The aim of this study was to quantify current provision of HEMS in the UK relative to population, geographic area covered, and the degree of social deprivation experienced by populations served. Delivery of HEMS in the UK is compatible with the Inverse Care Law, with provision varying inversely with expected population need just as documented in the original description of the law. Specifically, HEMS resources serving more deprived areas cover larger populations [6]. The total population served by HEMS resources in the UK was not associated with geographic area, an unexpected finding as it would seem logical that helicopter transport would have greater use where populations are more remote. Therefore, in a system delivering equity, more remote services might be expected to serve smaller populations. While these results are statistically and likely operationally significant, the models reflect only a small proportion of the variability observed.

The potential for more deprived areas to have greater need for HEMS or other pre-hospital critical care is consistent with evidence demonstrating the impact of socioeconomic status in pre-hospital care. For example, deprived areas have been shown to experience features associated with poorer outcomes, such reduced likelihood of bystander CPR, and features associated with greater requirement for advanced intervention, with more responses from pre-hospital critical care [17, 18]. The reduced availability of enhanced resource in more deprived communities has also been demonstrated for conventional ambulance transfer to advanced hospital care [19]. Together, these findings demonstrate a mis-match between the expected needs of communities and the resources available for their care.

These findings may be explained by funding structures. The majority of HEMS in the UK are charity funded, and there is evidence across the broader sector that such organisations are more prevalent in more affluent areas [20]. This hypothesis is supported by evidence of a public willingness to pay for helicopter medical services that outstrips their expected benefit in quality adjusted life years when compared to other medical interventions [21]. More affluent communities have greater resources to support such services [22]. These funding structures might also be seen as a continuation of the market forces that were thought to be the basis of Tudor Hart’s Inverse Care Law [6]. Historical factors related to the origins of HEMS in the UK might also have a role, while early services in Cornwall and Scotland served areas of relatively high deprivation, subsequent rapid expansion of services occurred in the relatively affluent South of England [23]. Another explanation may be the concentration of social deprivation in cities, less suited to HEMS operations compared to more rural areas. However, this explanation should be tempered by the presence of longstanding HEMS operations in major UK urban areas, our findings that area of coverage is not associated with populations served, and increasing suburbanisation of social deprivation [24, 25].

There are no other published data assessing the provision of HEMS across the UK by population and sociodemographic characteristics. The study methodology is strengthened by use of national level statistics, with accepted utility in determining allocation of healthcare resource [26], and geospatial modelling, with potential to define and quantify service areas and population characteristics.

However, this study does have limitations. Importantly, we assessed only the availability of helicopter transportation assets, not enhanced care teams, where availability might be by both road and air. No distinction was made between HEMS bases that provide solely pre-hospital critical care at the site of illness or injury, and those where availability for this role might be impacted by an additional remit for inter-hospital transfers. The decile of index of multiple deprivation was determined for each nation, giving a relative deprivation within each nation of the UK, but does not account for differences in the relative deprivation between nations. Assumptions were made that HEMS response to an area would typically be made by the closest resource, and may not reflect local service characteristics; including the occasional use of alternative base locations and standby points. While there is some evidence that areas experiencing higher deprivation have a greater requirement for pre-hospital critical care [18], the interventions that HEMS provides are complex and specialised, and it is not known to what extent requirements match area-level deprivation. We have used geographic and socioeconomic characteristics of the derived service areas as a marker of expected requirements for HEMS, and while these are known to influence the conditions to which HEMS responds [2, 3], other factors may also be important in determining the relative need of each population served.

Further work is required to assess the delivery of enhanced care services by ground assets, and how these services respond to the expected needs of more deprived populations. Policy should consider how to better distribute HEMS resource to better meet expected need, applying these findings and service planning methodology to mitigate health inequalities [27].