Networks With Low-Volume MTCs
If MTCs with lower annual case volumes were accepted, there would be 11 optimized configurations. Two of these could be realized with four helicopters, whereas the remainder would require at least five aircraft. The two configurations requiring only four helicopters are also shown in Table 2 (Configurations J and K). Both had two MTCs, in Glasgow and Edinburgh, and either 15 or 16 TUs. If all 16 hospitals that could become TUs were designated as such (Configuration J in Tables 2 and 3), the Glasgow MTC would be estimated to have moderate volume (494 severely injured patients per year, including secondary transfers), and the Edinburgh MTC would have low volume (381 severely injured patients per year, including secondary transfers). A total of 414 severely injured patients per year would require a secondary transfer. As before, this number includes both undertriaged patients and those who were correctly triaged but could not reach an MTC within 45 minutes. The estimated median access time for patients triaged to MTC care, who would be taken to an MTC, would be 18.2 minutes, which is slightly shorter than for a single-center configuration. The number of exceptions and secondary transfers would also be lower. If the number of helicopters was increased to five, an additional nine configurations would become feasible, all with two MTCs, in Glasgow and Edinburgh, and 10 to 16 TUs.
The implementation of a trauma system with one or two MTCs and with the tasking criteria described is estimated to require an increase in primary helicopter retrievals. The configurations described would necessitate between 2,832 and 4,065 missions to be flown per year, depending on the choice of configuration, equating to approximately nine retrievals per day. However, only 941 to 1,117 of these flights would be for patients triaged to MTC care, the remainder being for patients triaged to TU care, injured in remote locations (Table 3).
Access Time Threshold Sensitivity Analysis
A sensitivity analysis with a 60-minute (as opposed to 45 minutes) access time threshold revealed a small number of additional configurations. Networks with high- or moderate-volume MTCs alone could be optimized with a single MTC in Glasgow, as before, or a single MTC in Edinburgh. However, such a configuration would be associated with markedly longer access times. Networks with low-volume MTCs had two MTCs, in Glasgow and Edinburgh (as before) or in Glasgow and Dundee. However, the latter combination would have been discounted if two fewer patients had been admitted to Dundee. Furthermore, this configuration would also be associated with markedly longer access times. In summary, lengthening the access time threshold to 60 minutes does not alter the conclusions.
This study has shown that a novel notional triage and mathematical optimization methodology can be used to inform the planning of a major national care system. This is the first time such a methodology has been used in this field. That the analysis is based on a complete and large national cohort of prospectively collected data adds to the robustness of the findings.
Our analysis indicates that a trauma system configuration with one MTC, in Glasgow, or two MTCs, in Glasgow and Edinburgh, would be optimal, based on observed data. These findings are at variance with the widely held belief that the geographic distribution and associated long access times would preclude a configuration with a single center.10 While such a configuration would result in a high proportion of patients who could not reach definitive care primarily and a high number of secondary transfers, these results should be viewed in the context of an inclusive trauma system, which would facilitate best possible care, even for those injured in remote areas.
The findings are also at variance with the intuitive assumption that, if two MTCs were required, these would be best placed as far apart as possible, in Glasgow and Aberdeen. These results are explained by the spatial distribution of the incidents and the clustering of case volume in the Glasgow/Edinburgh area in particular, as reported in our previous article,17 which exerts a “gravitational pull.”
Implications for Policymakers
Trauma system design is influenced by a number of forces, which vary with setting. Overdesignating high-level facilities risks diluting experience. In the United States, the principal reasons include the economic benefits and the prestige of trauma center status. In the United Kingdom and in Scotland in particular, there are no economic benefits to a hospital being designated as an MTC, but as the National Health Service is more susceptible to political influences, local opinion can strongly influence decision making. Robust data, as generated by the GEOS study, can help to make the process more objective and transparent.
Following the decision to regionalize trauma care in Scotland, the Scottish Government’s National Planning Forum had originally recommended a trauma system with four MTCs, in Glasgow, Edinburgh, Dundee, and Aberdeen.10 This study shows that such a configuration might not be optimal. For the two solutions identified as optimal in our analysis, for a given minimum MTC case volume, the decision as to which solution is best therefore relates, partly, to what is deemed an adequate center volume. The relationship between case volume and outcome is well recognized, but the improvements in mortality, which are seen with higher case volumes, are probably not the consequence of higher volume per se, but rather the ability to justify a different service delivery framework. A dedicated trauma service to coordinate and deliver care for the severely injured is key to improving outcomes but is only justifiable when there is a sufficient case volume. In addition, mortality is not the only measure of a high-quality service. The precise position of the inflection point on the volume/outcome curve is therefore not known, and it is probable that both configurations described would result in MTCs large enough to sustain a specialist service. Furthermore, choosing between the two optimal configurations identified by the modeling should also consider other factors, such as hospitals’ capacity, and issues that are less quantifiable, such as organizational commitment and resilience. Capacity is difficult to model because it is influenced by the number of admissions and the length of stay. Data on the latter were not available in our prehospital data set. In terms of the number of emergency department attendances alone, however, it seems probable that a configuration with two MTCs, in Glasgow and Edinburgh, would be better able to deal with the predicted increase in volume than a single-MTC configuration.
Both the single- and two-MTC system configurations (but, in fact, also the four-MTC configuration proposed by the Scottish Government) would require an increase in aeromedical retrievals, although a proportion of the primary helicopter retrievals predicted by this study were for patients triaged to TU care, with a low probability of major trauma who were injured in remote locations. Some of these patients might not always require helicopter transport or could be taken to a local emergency hospital, if a degree of provider judgment was applied. The anticipated need for increased aeromedical retrieval reflects Scotland’s geography and a probable underprovision of lift capacity, given the population characteristics. The cost of operating additional helicopters may seem substantial but should be viewed in the context of setting up and running additional MTCs, which is also considerable. The combined set-up cost for four MTCs is estimated to be in the region of £12 to £17 million ($19–$27 million). A detailed health economic analysis is in planning.
Strengths and Weaknesses
The strengths of the study lie in its prospective, systemwide, population-based design; its use of actual incident location data; and its application of multiobjective optimization to network analysis, which enables multiple, conflicting objectives to be considered. The use of prehospital triage decisions implicitly considers both undertriage and overtriage and thus provides a realistic model of patient flow.
Several previous studies18–20 have attempted to quantify access to trauma center care using “isochrone analysis” of census data, relying on the assumption that the distribution of the injured population mirrors that of the population in general, which does not always hold true.21 These issues are overcome by network analysis, a well-established technique for solving siting problems in operations research, which was used by Branas et al.22,23 in their seminal Trauma Resource Allocation Model for Ambulances and Hospitals (TRAMAH) study. However, the TRAMAH study was limited by the use of retrospectively calculated severity scores, obtained from trauma registries, to stratify injury severity. In reality, patient flow is determined by prehospital triage decisions, as used by the GEOS modeling. The positive predictive value of triage for determining severe injury is limited, and the resulting overtriage has implications for transport services and hospitals’ capacity.
Nevertheless, mathematical modeling also has limitations. No provision was made for provider judgment in triage, and the model is dependent on the sensitivity of Steps 1 and 2 of the Field Triage Decision Scheme for detecting severe injury. We used a conservative estimate of 45.5%, derived from a large multicenter study from the United States.16 It is possible that the performance of the Field Triage Decision Scheme in Scotland differs because of variations in case mix and application. Calculated drive times are estimates and did not consider the effects of weather, both of which may impact on the accuracy of the data. We considered making allowances for no-fly weather conditions, but the available data on which to base such modeling are limited, particularly as the entire flight path—rather than just the incident location—would have to be considered. Similarly, we did not make allowances for weather-related decreases in driving speeds. However, the model was built on a large data set, collected over a full year, to account for any seasonal variation in the geographic distribution of the incidents. It is possible that the injuries observed over this time may not be representative of what would happen every year, but the profile of the data is similar to that seen in previous years,24–26 which provides reassurance that the results are generalizable.
Future Research and Other Applications
This study adds to the literature on trauma system design, and while the data in this study pertains to the configuration of a trauma system for Scotland, the methodology could easily be adopted and adapted to other settings. In particular, the technique could also be used to compare mathematically optimized configurations with existing ones, to provide a form of quality assurance of the configuration of existing trauma systems. This latter application could help to address the issue of trauma center proliferation, which is an increasingly recognized problem in North America. Furthermore, trauma is not the only time-critical condition that requires complex care delivered by a hierarchical clinical network. The effect of hospital volume on outcome following percutaneous coronary intervention for myocardial ischemia, thrombolytic therapy for stroke, and the repair of ruptured abdominal aortic aneurysms is well recognized,27–29 and these treatments might also benefit from geospatially optimized systems of care.
This study has shown that a novel combination of notional triage, network analysis, and mathematical optimization methodology can be used to inform the planning of a major national care system. Scotland’s nascent trauma network would be optimally configured with one or two MTCs, and the latter configuration, in particular, seems feasible with regard to the capacity of the proposed centers and the additional need for aeromedical retrieval resources. Whatever configuration is eventually implemented, it should be carefully and continuously evaluated.
The methodology described here is not only applicable to trauma care or to Scotland. Whether explicitly considered or not, there is a geographic dimension to the design of any clinical network. The need to balance conflicting objectives—such as accessibility, center case volumes, and need for aeromedical transport—is a particular feature of networks caring for patients with highly acute conditions.
J.O.J. conceived, designed, and managed the study; assembled, analyzed, and interpreted the data; and wrote the draft and final versions of the manuscript. J.J.M. contributed to the design of the study, the interpretation of the data, and the writing of the draft and final versions of the manuscript. H.W. contributed to the design of the study, wrote the software, analyzed the data, and contributed to its interpretation and the writing of the draft and final versions of the manuscript. S.H. contributed to the design of the study and the writing of the draft and final version of the manuscript. R.L. contributed to the design of the study, the implementation of the notional triage, the extraction and assembly of the data, the interpretation of the results, and the writing of the draft and final versions of the manuscript. J.D.H. contributed to the writing of the draft and final version of the manuscript. M.K.C. contributed to the design of the study, the analysis and interpretation of the data, and the writing of the draft and final versions of the manuscript. J.O.J. was the chief investigator. All authors reviewed and approved the final manuscript.
J.O.J. is a member of the Major Trauma Oversight Group (MTOG), the Scottish national trauma system implementation group. J.O.J. does not receive financial support from MTOG.
The Health Services Research Unit receives funding from the Chief Scientist Office of the Scottish Government Health and Social Care Directorates. J.O.J. receives academic salary support from NHS Research Scotland.
The GEOS study was funded by the North of Scotland Planning Group. The opinions expressed in this article are those of the authors alone.
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Keywords:© 2015 Lippincott Williams & Wilkins, Inc.
Trauma systems; geographic information systems; multiobjective optimization