Mobile stroke units (MSU) are specially designed ambulances with CT imaging capability able to provide faster pre-hospital care to patients in the community. The Melbourne Mobile Stroke Unit is the first MSU service in the Australian region, commencing operations in 2017 in central metropolitan Melbourne. This review describes the challenges of setting up the MSU in a unique Australian setting and initial clinical efficacy of pre-hospital MSU treatment and triage on ischemic and hemorrhagic stroke. We also discuss the current technological and operational limitations of the MSU service and directions for future research.
Mobile stroke units (MSU) are pioneering services incorporating imaging capability and a specialized stroke team in an ambulance platform in order to provide pre-hospital treatment and triage[
The Melbourne MSU was built with a view to local practices and Australian regulations for emergency vehicles
Melbourne mobile stroke unit exterior and interior. mobile stroke unit interior hardware: monitors for viewing and sending imaging in real-time (A); contrast injector (B); storage compartments for medications (C); CereTom scanner (D); contrast warmer (E).
Although previous international MSU services had already been in operation for some years prior to the Melbourne MSU, there were unique challenges to the Australian setting. One example was that the emergency stop button of the CereTom scanner was located on the right side of the machine (if looking from the front of the machine). This usually aids accessibility of the button from the side door in right-hand side driving countries, as the side door would be located on the correct side to avoid opening into traffic. Therefore, operators in other countries can physically stand outside the vehicle during a scan, as the emergency stop button could be easily pressed if necessary. Given the left-hand side driving regulations in Australia, the radiographer instead needed to be inside the vehicle and wear lead gowning for radiation protection.
Further challenges were encountered in the strict no lift policies employed by the local ambulance service (Ambulance Victoria), where all stretchers are hoisted into the ambulance using an electronic floor-mounted loader. This is opposed to the more simplistic manual lifting mechanisms for stretcher hoisting used in some international services. Under normal circumstances the loader mechanism does not allow the stretcher to be lifted or lowered whilst inside the ambulance. However, due to the need for patient positioning to align with the CT scanner, a technical workaround was required to manually disengage the stretcher from the loader whilst inside the MSU. This procedure makes the patient loading workflow more complex and requires two trained paramedics to operate the stretcher loading sequence.
The staffing composition of the Melbourne MSU, with a neurologist/senior stroke fellow, nurse specialist, radiographer, and two paramedics (one with airway and cardiac resuscitation training), was chosen to reflect a standard in-hospital acute stroke team in Australia. The addition of a paramedic with advanced level training was to cater for stroke patients with unstable conscious state or at risk of neurological deterioration. In comparison to international services, this is a large and highly experienced team, especially as some units operate without neurologists or do not have stroke nurse specialists. Many also utilize CT or emergency technicians who generally have less training than their Australian counterparts. This level of experience has undoubtedly enabled optimized workflows on the Melbourne MSU, compared to a typically more junior in-hospital team.
General operational workflows for the Melbourne MSU have been previously described[
The primary dispatch radius of the Melbourne MSU was set at 20-kilometres of the home base of Royal Melbourne Hospital. Within this zone, the MSU transports patients to 9 metropolitan stroke centers, of which 4 have 24 h capability for endovascular thrombectomy and neurosurgery. This radius was based on the estimated average on-scene time of 28 min for Ambulance Victoria attending stroke patients in metropolitan Melbourne (unpublished data, Ambulance Victoria, 2017). As the MSU uses a co-dispatch model, a closer local ambulance crew would arrive and commence extrication with the expectation that the MSU would arrive before they were ready to transport to hospital within this timeframe. Compared to international services, both the Berlin and Houston metropolitan MSUs use a dispatch radius that approximates a 15 min drive[
Following a period of operation, the Melbourne MSU began accepting suspected stroke cases outside the 20-kilometre central Melbourne radius where initial attending paramedics had requested the service. In many of these cases the nearest stroke center was in the direction of the MSU base and there was an opportunity for the service to rendezvous with the initial ambulance prior to arrival at hospital. This approach has been successfully by the Houston MSU to double their standard response radius from the usual 6 miles to 12 miles[
The most prominent feature of MSUs is the ability to provide faster pre-hospital thrombolysis. After the first 365 days of operation, a total of 100 patients had received pre-hospital thrombolysis. Time from dispatch of first ambulance to commencement of reperfusion therapy was compared between MSU and a similar cohort of patients in metropolitan Melbourne presenting via regular ambulance to hospital during MSU operating hours[
Compared to international services, the Melbourne MSU has a somewhat longer time than other busy metropolitan units for some metrics, such as dispatch and scene arrival to thrombolysis[
Another key metric of interest was the ability of the Melbourne MSU to provide very early thrombolysis. Experiences from international MSUs have shown substantial increases in the proportion of patients able to receive thrombolysis within the first hour of known onset, or the “golden hour”, compared to usual hospital pathways[
Finally, the Melbourne MSU is also able to administer supportive medications more efficiently. This includes anti-hypertensive agents pre- and post-thrombolysis, or intravenous idarucizumab to reverse dabigatran prior to thrombolysis[
In contrast to published literature on the benefits of MSUs on intravenous thrombolysis, little has been comparatively reported on endovascular thrombectomy. Although the procedure is performed in hospital, the role of accurate triage and early notification of neuro-interventional services has been a proposed advantage of MSUs[
The Melbourne MSU is therefore one of the very first services to report on the efficacy of MSU-enabled triage in reducing time to EVT commencement[
Further contributions to faster EVT were for time from hospital arrival to EVT commencement, which was 17 min (95%CI: 8-26) faster for MSU patients compared to in-hospital controls. This is likely attributed partly to early pre-hospital notification of neuro-interventional services to prepare the angiography suite for an impending case. Another factor was that one-third of MSU patients received EVT without need for repeat in-hospital imaging. Generally, repeat imaging after arrival in hospital with the addition of CT-perfusion is required if the onset time is > 6 h[
Despite the lack of statistically significant time saving for EVT patients that did not need bypass, the two-thirds that were eligible for thrombolysis still received intravenous therapy substantially faster. Around 10% of all large vessel occlusions managed by the Melbourne MSU did not proceed to the angiography suite due to improving symptoms or recanalisation from thrombolysis. There are almost certainly further cases that proceeded to angiography but did not require intervention due to recanalisation. Analysis of data from two US-based MSUs showed that large vessel occlusion recanalisation solely with thrombolysis was greater than with in-hospital controls (29.4%
In addition to time savings, we also examined the possible disability avoidance of providing earlier thrombolysis and EVT, given improved post-stroke outcomes are highly associated with faster reperfusion therapy[
This is also predicated on a linear assumption of outcome decay with time, which is likely not true as the benefits for reperfusion therapy decrease fastest in the early timepoints[
Whilst the MSU platform was conceived primarily for treatment of ischemic stroke, triage and pre-hospital treatment also benefits patients with intracranial hemorrhage. Of all hemorrhage patients attended to by the MSU in the first 365 operational days, 28% received a CT scan within 60 min of symptom onset and 60% within 90 min. In contrast, 15% and 47% of ischemic stroke patients received thrombolysis within 60 and 90 min respectively on the MSU. This suggests that MSUs can intervene in intracranial hemorrhage at a similarly fast time window as ischemic stroke, which is critical as hematoma growth is highly time-dependent[
Although effective treatments are much more limited for parenchymal intracerebral hemorrhage, a post-hoc analysis of a major trial of aggressive blood pressure management in intracerebral hemorrhage demonstrated improved outcomes if anti-hypertensive therapy is initiated within 2 h of symptom onset[
The other major therapy provided by the MSU is reversal of anti-coagulation associated hemorrhage. The Melbourne MSU provides reversal for patients on therapeutic warfarin using intravenous vitamin K and 3-factor prothrombin concentrate (6% of all intracerebral hemorrhage in the first 365 days,
Similar to large vessel occlusion, pre-hospital diagnosis of intracranial hemorrhage by the MSU allowed improved triage of patients directed to a comprehensive center with neurosurgical units. Our data shows around a quarter of all hemorrhage patients bypassed their local hospital, some of which subsequently underwent emergent neurosurgical intervention. This treatment would have been severely delayed had patients needed a secondary inter-hospital transfer, as transfer processes are even slower than for EVT in Melbourne due to the absence of an established protocol. Such delays are likely to be especially deleterious to younger patients with rapidly progressive hemorrhages, where earlier neurosurgical intervention may delay, reduce, or even prevent disability. Even if patients did not receive surgical intervention, high risk cases require management at a neurosurgical center for neuro-critical care monitoring, especially subarachnoid and subdural hemorrhage. There is also evidence to suggest that hemorrhage cases have better outcomes at neurosurgical centers, regardless of whether surgery was performed[
In addition, almost a third of all patients with parenchymal hemorrhage were recruited into a currently running clinical trial of tranexamic acid administered within 2 h of onset (
Although we have shown that a MSU has been successfully operationalized into the ambulance and stroke services in Melbourne, there are research directions and challenges still to be addressed for future operation.
The availability of cerebral imaging is the unique feature of MSUs that enables pre-hospital stroke treatment. The current CereTom CT scanner has been the standard hardware in almost all first-generation MSUs due to its portability, significantly lower weight, self-mobility, and ability to run from battery power. The trade-off for these features is that there are limitations in image quality, scanner capabilities, and adequate coverage of the head and neck[
The overall experience in operating the CereTom scanner in the Melbourne MSU has been that whilst standard non-contrast CT brain sequences are adequate to exclude contraindications to thrombolysis, the substantially slower acquisition time compared to in-hospital scanners allow greater patient movement and resultant artefact. This makes interpretation difficult and may necessitate repeat imaging in some cases, although this rarely affected the ability to administer thrombolysis on the MSU.
The smaller bore hole where the head of the patient rests inside the scanner also presents unique challenges. First, this does not permit imaging of most of the neck and thorax as they cannot fit in the scanner, hence CT-angiogram covers only the intracranial vessels and not usually the common carotid bifurcation or thoracic vessels. This missing information is useful to diagnose underlying stenosis or occlusion of extracranial vessels and, in cases of large vessel occlusion, may help neuro-interventional services plan the procedural approach for EVT. Second, the required head positioning for the small bore hole excludes patients with significant upper spinal kyphosis from being scanned as they are unable to maneuvered into the scanning position. In a similar fashion, the scanner is unable to fully scan the lower posterior fossa in patients placed in a cervical collar or with short necks, as these require the shoulders to enter the bore hole to fully image the brain. Thrombolysis cannot be given with incomplete images; therefore such patients may be unable to benefit from MSU management.
Another limitation is the quality of some CT-angiogram images. The fast acquisition time of in-hospital scanners allow tracking of a test contrast bolus to determine optimal contrast delivery individualized for the patient. Whilst bolus tracking is possible on the CereTom scanner, the slow acquisition time makes this impractical in routine MSU service. As a result, the Melbourne MSU uses set contrast delay timings that are likely to be appropriate for the majority of patients but results in suboptimal CT-angiogram images in some cases. This may lead to difficulty in diagnosing large vessel occlusion and need for repeat in-hospital imaging, which will delay commencement of EVT.
The final limitation is the lack of whole-brain CT-perfusion capability. Due to the low number of slices that the CereTom scanner can perform on a 1 cm thick CT-perfusion slab, the end imaging result is adjudged not sufficient for routine clinical use. The consequences are that firstly, the MSU is unable to provide thrombolysis for suitable patients > 4.5 h from onset[
Therefore, there is a need for next generation CT scanning devices that not only deliver better imaging quality and capabilities to better approximate in-hospital scanners, but also maintain the portability that allows fixing into a MSU without needing a chassis that would make operation in crowded cities impractical.
Many of the US-based MSUs utilize telemedicine-based services where the medical practitioner can view an audio-visual feed of the patient and receive transmitted CT images remotely. This has practical advantages in allowing the medical practitioner to conduct other work in between telemedicine consultations and provides potential for one stroke doctor to manage multiple MSUs. However, potential downsides include slower management decisions, as the medical practitioner must rely on other personnel to gather relevant information, as well as potential delays in transmission of imaging. In the worst-case scenario of technical failure, the medical practitioner may be entirely unable to view the patient or not be able to receive imaging. With no alternatives on-board the vehicle, this would force the patient to go to hospital for definitive management, which would be delayed by the aborted MSU review.
The largest telemedicine validation study published by the Houston MSU group found a statistically significant difference in time to thrombolysis decision making between the onboard and telemedicine neurologist, although the absolute median time difference was 3 min (18 min
Further considerations for telemedicine are that much of the initial assessment and decision making for scanning occurs outside the vehicle, whereas the Melbourne MSU audio-visual equipment are fixed inside the back cabin. Waiting for extrication and loading into the vehicle to allow telemedicine connectivity may unduly lengthen the time that the MSU spends at each patient, making the workflow inefficient for cases that are unlikely to be treatment eligible. Portable telemedicine equipment (such as electronic glasses or body cameras) that can be brought away from the vehicle are preferred in this setting but maintaining mobile signal connectivity for a good quality video signal may be challenging. Future implementation of a telemedicine system therefore relies greatly upon technological solutions that offer very high reliability, rapid imaging transmission, and immediate troubleshooting.
Dispatch accuracy is a current issue for Melbourne MSU operation. In our region, the currently used emergency dispatch algorithm only has a 32% concordance with a paramedic diagnosis of stroke or TIA. Not surprisingly, the Melbourne MSU is cancelled by either the first responding paramedic crew or by the MSU personnel in 60% of cases, mainly due to low likelihood of stroke. This is normally while the vehicle is en route to a case or, in some instances, after the MSU has physically arrived at the address.
This low specificity for stroke is particularly problematic for a single resource like the MSU, as the service cannot attend concurrent dispatches or may be much further away from subsequent dispatched cases. The Melbourne MSU is only attached to 65% of all potential dispatches in the 20-kilometre dispatch zone and therefore cannot attend all stroke cases within the central operating area. Poor dispatch specificity worsens this situation further if the vehicle is continually dispatched to low likelihood cases, further limiting the availability of the MSU to attend patients who are eligible to receive pre-hospital stroke treatments.
The Berlin MSU group developed a new local dispatch algorithm derived from the most specific stroke symptoms after retrospective analysis of emergency calls[
Future research in this area must address stroke dispatch in a whole systems approach, both for MSU and regular ambulance dispatch. A multi-tiered approach is required, including consensus for changes to internationally used emergency algorithms, review of local dispatch practices, training of emergency call takers and development of dispatch screening procedures used by each MSU service. This optimization is critical to allow best utilization of the limited number of MSUs worldwide and ensure these high-resource services are cost-effective.
MSUs around the world have consistently demonstrated substantial time savings in commencement of thrombolysis, with evidence for EVT time saving also shown by the Melbourne service. In almost all instances this has led to clear benefits in functional outcomes in published literature[
Outside of clinical trials, the Berlin MSU group has conducted an interim observational analysis of the effect of MSU operation on functional outcomes compared to in-hospital controls[
Preliminary health economic analyses of the Melbourne MSU have been carried out for the first full calendar year of operation[
Similar analyses of potential cost-effectiveness have been conducted by the Berlin MSU service, which showed an incremental cost-effectiveness ratio of €32,456 per quality-adjusted life year[
Overall, initial estimates of the cost-effectiveness of MSUs appear to be comparable with many interventions outside of stroke. Of note, these analyses largely comprise the effect of reperfusion therapy for ischemic stroke without consideration of benefits in other areas, such as intracranial hemorrhage. No doubt the upcoming randomized control trials will provide confirmatory evidence of clinical benefits and allow a wider analysis of cost-effectiveness.
The Melbourne MSU service provides an exciting frontier of pre-hospital stroke care management. The service has been successfully integrated into local Ambulance Victoria operations and has delivered substantial benefits for reperfusion therapy in ischemic stroke and management of intracranial hemorrhage. Future research directions involve evolution of imaging and technical capabilities of MSU services in addition to long term health and cost benefits.
Substantial contribution to conception, design of the project, and performed data analysis and interpretation: Zhao H, Campbell BCV, Foster S, Stephenson M, Coote S, Langenberg F, Easton D, Donnan GA, Davis SM
Data that supports the findings of this research are available upon reasonable request by a suitably qualified investigator.
The Melbourne Mobile Stroke Unit and associated projects received funding from the Australian Commonwealth Government, Victorian State Government, Royal Melbourne Hospital Neurosciences Foundation, Stroke Foundation, The Florey Institute of Neurosciences and Mental Health, University of Melbourne, Boehringer Ingelheim, and private donation.
Dr. Zhao discloses grants from the Australian Commonwealth Government (MRFF1194787) and the University of Melbourne and personal fees from Boehringer Ingelheim. Prof. Davis discloses grants from the National Health and Medical Research Council and personal fees from Abbott, Boehringer Ingelheim, and Medtronic.
Data collection for the Melbourne Mobile Stroke Unit is approved by the Melbourne Health Human Research Ethics Committee with waiver of patient consent (QA2017169).
Not applicable.
© The Author(s) 2021.