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How a Mobile Stroke Unit Performs in the Densest of Cities

Article In Brief

Patients treated with a mobile stroke unit in New York City had significantly shorter times than patients treated with conventional care on three measures: mean dispatch-to-thrombolysis, onset-to-thrombolysis, and ambulance arrival-to-thrombolysis.


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The use of a mobile stroke unit (MSU) to transport stroke patients cut time-to-thrombolysis even in the most densely populated borough of the most densely populated city in the US, New York City, a December 17, 2019 study in the Journal of the American Heart Association found.

The researchers—who compared an MSU from New York's Weill Cornell Medical Center and Columbia University Irving Medical Center to a conventional ambulance transport operating within two catchment areas in Manhattan during alternating two-week periods from October 2016 to September 2017—found that the dispatch-to-thrombolysis times were significantly reduced in patients transported by the MSU.

The patients were part of the METRONOME (Metropolitan New York Mobile Stroke) registry with suspected acute ischemic stroke whose primary outcome was dispatch-to-thrombolysis time.

MSUs have gained traction in several US cities since their introduction in Houston in 2014. Yet some stroke experts have expressed skepticism about their efficacy in densely populated urban settings with large numbers of proximate dedicated stroke centers and complex traffic patterns.

In these areas, the study authors wrote, “a given emergency scene location may be close to a high number of stroke-capable hospitals, resulting in shorter travel times for emergency transport vehicles. This could potentially reduce or nullify the beneficial effect of MSU care on thrombolysis time that has been demonstrated in less densely populated regions.”

Lead study author Matthew E. Fink, MD, FAAN, chair of neurology at Weill Cornell Medical Center, said that of the many geographic, demographic, and logistical challenges to emergency stroke response in a location like Manhattan, the most daunting is traffic.

“Conventional ambulances have to make a round trip from the hospital to the patient, then back to the hospital. That's twice as long as the one-way trip the MSU has to make, because the MSU is a primary stroke center where patients are treated at the scene,” he said.

The study found longer ambulance arrival-to-hospital arrival times for the MSU because of the interval during which the vehicle remained stationary at the scene to administer CT scans and tissue plasminogen activator (tPA) thrombolytic therapy.

At the time of the study, New York City's MSU was in its first year and was the first MSU program on the east coast.

“I thought that Manhattan, with its high population and large share of elderly residents, was actually a perfect location for an MSU,” Dr. Fink said.

New York City's MSU program is now participating in the BEST-MSU multi-city trial of MSU operations, which runs through 2021.

Study Design, Findings

Dr. Fink and his co-authors conducted a prospective cohort study of a New York City–based MSU program that was launched in October 2016 by New York Presbyterian (NYP) Hospital and the Fire Department of New York (FDNY). A single MSU operated only in Manhattan from two separate, non-overlapping medical center catchment areas—Weill Cornell Medical Center and Columbia University Irving Medical Center. The MSU was on-service within the Weill Cornell catchment area for two “on-weeks”; then, during the following two weeks, the on- and off-service centers switched, resulting in an alternating biweekly schedule.

All conventional ambulances in the study, including those owned by individual hospitals, operated under the city's 911 emergency medical services (EMS) system and were registered with the FDNY. The MSU, while not an official municipal ambulance unit, functioned as an institutional unit authorized by the FDNY to operate on the EMS network.

To compile the comparison group, a vascular neurologist at each of the two medical centers prospectively reviewed an FDNY database of all EMS ambulance call reports of ambulance transports to each medical center's emergency department. This review flagged all reports of transports that occurred during each campus' two off-weeks, and that were coded as an FDNY EMS call type of “CVA” (cerebrovascular accident, with stroke symptoms that are evaluated >5 hours from symptom onset) or “CVA-C” (cerebrovascular accident–critical, with stroke symptoms that are evaluated within 5 hours of symptom onset or in whom the last known “well time” is unknown).

For each flagged patient, three vascular neurologists then reviewed all available history and physical examination findings to determine whether the patient would have been eligible for MSU transport had the MSU been available at the time when they were evaluated. Patients who would have been eligible for MSU care were added to the METRONOME registry as part of the conventional care group. The review team was blinded to prehospital care-related time metrics, including dispatch-to-thrombolysis time.

In total, 85 patients met the study criteria, including 66 patients transported via MSU and an additional 19 patients via conventional ambulance in the comparison group. Because seasonal weather in New York City can affect traffic and ambulance travel times, investigators conducted a post-hoc sensitivity analysis adjusting the primary analysis for the season of each patient's ambulance transport date as an additional variable.

Patients treated with MSU care had significantly shorter times than patients treated with conventional care on three measures: mean dispatch-to-thrombolysis, onset-to-thrombolysis, and ambulance arrival-to-thrombolysis times. Compared with patients in the conventional care group, patients in the MSU group had a mean decrease in dispatch-to-thrombolysis time of approximately 29.7 minutes (95% CI, 6.9-52.5).

Within the estimated difference in dispatch-to-thrombolysis time, patients in the MSU group had a mean increase in dispatch-to-ambulance arrival time of 6.5 minutes (95% CI, 2.4-10.6; p=0.002), offset by a mean decrease in ambulance arrival-to-thrombolysis time of 36.2 minutes (95% CI, 58.5 to 13.9; p=0.001). Differences in dispatch-to-thrombolysis time for patients in the MSU group also occurred despite significantly longer ambulance arrival-to-hospital arrival times (due to stationary vehicle status during CT scan and treatment) and longer distances traveled from pick-up to the accepting hospital.

Within the thrombolysis-treated population, the study encountered a higher rate of final diagnosis of stroke mimic among patients who received thrombolysis on the MSU. This was despite the fact that demographic and clinical characteristics, including rates of symptomatic intracranial hemorrhage, were not significantly different between the two groups.


“Conventional ambulances have to make a round trip from the hospital to the patient, then back to the hospital. Thats twice as long as the one-way trip the MSU has to make, because the MSU is a primary stroke center where patients are treated at the scene.”—DR. MATTHEW E. FINK

Overall, the study found that fewer patients were transported by the MSU over a 12-month period than in the published accounts of other MSU programs operating in less densely populated areas. Investigators believe that the novelty of New York City's MSU, and the possibility that recognition of acute stroke by emergency dispatchers may be insufficient, could both have contributed to that discrepancy.

Expert Commentary

James C. Grotta, MD, FAAN, director of Stroke Research at the Clinical Institute for Research and Innovation at Memorial Hermann-Texas Medical Center, director of the Mobile Stroke Unit Consortium, and founder of the country's first MSU program in Houston, said the study's results were consistent with those of other MSU data, which has found that the use of mobile stroke units decreases time to thrombolysis.

“This study proved that that's true even in a city like New York, that has a stroke center around the corner from almost any location,” said Dr. Grotta, who is the lead investigator on the BEST-MSU trial.

Dr. Grotta noted that the structure of city EMS systems is one factor that may set New York City apart from other urban areas. “Houston is more like most American cities than New York is,” he said. “Our MSUs can treat 10 times the number of patients that New York City can, for two reasons: We have the cooperation of all of the city's stroke centers, so we deliver to all hospital systems. And we have one citywide EMS system that we can coordinate with. Coordination between the EMS systems in New York is more complicated, since many of the hospitals have their own ambulances.”

The biggest challenge to cities establishing MSU programs, both Drs. Grotta and Fink acknowledged, is money—because the programs save money in the long term but lose it in the near term, and, as Dr. Grotta emphasized, reimbursement is still inadequate. “You need outcomes data to convince payers to pay more for an MSU transport,” he said.

Peter A. Rasmussen, MD, medical director of the Distance Health Cleveland Clinic, who directed the Cleveland Clinic's mobile stroke unit program when it began operations a few weeks after Houston's opening in 2014, said that while the study highlighted only one aspect of the benefits of the mobile stroke program—time-to-thrombolysis—its success points to three other benefits as well.

“You're getting the benefit of a stroke expert as soon as possible; I don't know how you gauge the value of an expert eye so quickly,” Dr. Rasmussen said. “You get near-perfect triage, so in a city like New York, you can get patients to centers of excellence for different types of stroke—whether it's hemorrhagic or ischemic stroke, whether it's a primary stroke center or a comprehensive stroke center the first time, and that reduces the subsequent cost of transport. And there's the recognition of a patient that's an interventional mechanical thrombectomy candidate and making sure that patient gets to a thrombectomy-capable center.”

For Drs. Grotta and Rasmussen, though, the primary message of the New York City study was its success. “Many people think that a mobile stroke unit is impractical, doesn't save time, and is hard to operate,” Dr. Grotta said. “Now this study has shown that you can make it work in the toughest kind of place. Even in New York City, in fact, it's extremely practical, and it does save time.”


Dr Fink serves as an editor for Relias Learning. Dr. Grotta receives consulting fees from Frazer, Ltd., a manufacturer of mobile stroke units. Dr. Rasmussen reports stock holdings or proceeds from Mehana Medical, Perflow Medical, and Blockade Medical.

Link Up for More Information

• Kummer BR, Lerario MP, Hunter MD, et al. Geographic analysis of mobile stroke unit treatment in a dense urban area: The New York City METRONOME registry J Am Heart Assoc 2019;8(24):e013529.