University of Southern California Dissertations and Theses

Expansion of emergency medical service regional routing for stroke patient care in the United States from 2000-2010

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Expansion of emergency medical service regional routing for stroke patient care in the United States from 2000-2010

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Content Running head: EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 1
Expansion of Emergency Medical Service Regional Routing for Stroke Patient Care in the
United States from 2000-2010
Natalie Hanks
University of Southern California
Author Note
Natalie Hanks, Department of Preventive Medicine, Keck School of Medicine
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 2
Table of Contents
Abstract 3
Introduction 5
Methods 7
Data Sources 7
Data Analysis 8
Results 10
An increase of states supporting EMS stroke routing protocol 10
Proportion of jurisdictions utilizing EMS routing for stroke care 10
Mortality rate of ischemic stroke before and after protocols for EMS routing of stroke
patients 10
Discussion 12
References 15
Appendix 17
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 3
Abstract
Background: Organized stroke systems of care include preferential emergency medical services
(EMS) routing to deliver suspected stroke patients to designated hospitals. EMS transport of
stroke patients is regulated at the State and County level in the United States. This study aims to
characterize the growth of EMS regional systems of acute stroke care by describing the number
and proportion of nationwide stroke hospitalizations in areas with EMS systems of care over a
10-year period.
Methods: Data on ischemic stroke using ICD-9 coding was collected from the Healthcare Cost
and Utilization Project Nationwide Inpatient Sample (NIS) database from the years 2000-2010.
The NIS contains all discharge data from 1,051 hospitals located in 45 States, approximating a
20-percent stratified sample. Data on EMS systems of care was obtained from a review of
archives, reports, and interviews with state EMS officials.
Results: Nationwide stroke hospitalizations remained constant over the course of the study
period: 583,000 in 2000 and 573,000 in 2010. However from 2000-2003 there were no states or
counties with EMS systems of care. The proportion of US stroke hospitalizations occurring in
jurisdictions with established EMS regional systems of acute stroke care increased steadily from
2004 to 2010 (1%, 13%, 28%, 30%, 30%, 34%, 49%). In 2010, 278,538 stroke hospitalizations,
49% of all US stroke hospitalizations, occurred in areas with established EMS routing, with an
additional 18,979 (3%) patients in regions undergoing a transition to EMS routing.
Conclusions: In 2010, a majority of stroke patients in the United States were hospitalized in an
area with established or transitioning to EMS diversion. This milestone coverage of half the US
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 4
population is a major advance in systematic stroke care, and emphasizes the need for novel
approaches, such as telemedicine, to further extend access to stroke center care to all patients.
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 5
Introduction
As the fourth leading cause of death in the United States, strokes are a primary source of
severe brain damage. During a stroke, two million brain cells die every minute and each minute
that passes without medical attention significantly increases the risk of permanent brain damage,
disability, and death(“Stroke 101 Fact Sheet,” n.d.). Since 1994, the Brain Attack Coalition, a
collaboration of physicians, scientists, and government leaders working to prevent and reduce
strokes and stroke-associated disabilities, has developed several important initiatives to combat
stroke.
Notably in March of 2000, the Brain Attack Coalition published recommendations to
establish comprehensive stroke centers and emphasize the need for rapid and organized stroke
care(Alberts et al., 2000). Due to the time-sensitive administration of the highly potent clot-
dissolving medications, the Brain Attack Coalition emphasized the importance of an extensive
and effective stroke team consisting of emergency medical service (EMS) personnel, nurses, and
a range of physicians, including but not limited to, stroke specialists, radiologists, and
pharmacists(Alberts et al., 2000).
Since the publication of the Brain Attack Coalition's initial recommendations, the Joint
Commission in collaboration with the American Stroke Association (ASA) has certified over 925
primary stroke centers nationally. The Brain Attack Coalition has also released an updated
version of recommendations to reflect improvements in stroke treatment, including the use of
intravenous tissue plasminogen activator (tPA) and advances in brain imaging. Specifically, the
Brain Attack Coalition addressed and reclassified the issue involving the Emergency Medical
Service (EMS) response to patients showing signs of acute stroke. It was recommended that
EMS personnel should transport patients with acute stroke to primary stroke centers, unless there
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 6
is another imminent life-threatening condition, in which case the EMS should transport the
patient to the nearest emergency department. This recommendation was reclassified as Class I,
Level B, meaning the benefits of the treatment clearly outweigh the risks, and so the practice
should be used in light of promising effects (Alberts et al., 2011). This updated classification
highlights the importance of rapid identification and transport of stroke patients who seek
immediate professional treatment.
However, even with improved procedural standards and treatment options for stroke
patients, a significant challenge remains. The EMS system is currently regulated at the state and
county level in the United States, which makes it difficult to establish a uniform protocol for
EMS transportation of stroke patients. Therefore, the aim of this paper is to characterize trends
in EMS regional systems for acute stroke over a 10-year period by describing the number and
proportion of nationwide stroke hospitalizations in areas with EMS systems of care. The
characterization of EMS trends demonstrates the advancement in stroke care through the
growing presence of rapid transportation and treatment of stroke patients through the EMS
system across the nation.
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 7
Methods
Data sources
Data on ischemic stroke using the ninth revision of the International Classification of
Diseases (ICD-9) coding(433.01, 433.11, 433.21, 433.31, 433.91, 434.01, 434.11, and 434.91) were collected using patient discharge information from the Nationwide Inpatient Sample (NIS),
Healthcare Cost and Utilization Project (HCUP), and the Agency for Healthcare Research and
Quality during the years 2000-2010 (“HCUP NIS,” n.d.). The NIS contains data on primary and
secondary patient diagnoses, patient demographics (i.e. age, gender, and race), and hospital
characteristics, such as size, teaching status, and zip code. The NIS patient and hospital data
were collected on 28 states from the year 2000 and on 1,051 hospitals located in 45 states in
2010; this approximates a 20-percent stratified sample of US community hospitals. Given the
increase in the number of states and thus in the number of hospitals covered in the database over
the study period, the NIS team specified a sample and weight strategy required for hospitals and
number of discharges(“Introduction to HCUP NIS,” 2012). Hospital weights were calculated
based on stratification of hospital type; hospital strata were categorized on geographic location
(Northeast, West, Midwest and South), urban or rural region, teaching status, number of beds in
the hospital, and control (public, voluntary, or proprietary hospital). These hospital subtypes
were compared to the national number obtained from the American Hospital Association (AHA) Annual Survey Database in order to extrapolate the NIS sample hospitals to the nationwide
hospitals. The number of community hospitals within each stratum in the nationwide hospitals
was added to the number of community hospitals within each stratum in the NIS sample to
obtain the national hospital weight. Similarly, discharge weights were developed to extrapolate
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 8
the NIS sample discharges to the nationwide discharge. The number of discharges on the
national level were obtained from the AHA(“Introduction to HCUP NIS,” 2012).
Data on EMS systems of care were obtained from a study that reviewed archives,
reports, and interviews with state EMS officials(Saver & Song, 2012).(Note: NIS hospital data
is not reported at the county level in the state of Texas. All Texas data were analyzed using the
state-reported EMS stroke diversion policy in 2005, even though Harris County began EMS
routing in 2000.) Data analysis
Analyses were conducted using Statistical Analysis System(SAS) version 9.2 software.
Cases were patients from the NIS discharge database with diagnosed ischemic stroke, identified
by a review of ICD-9 discharge diagnoses. Based on the NIS hospital identification number,
which denotes the patients' hospital zip code, the hospital was assigned as an area with or
without EMS regional system of acute stroke care by study year (2000-2010). If the protocol for
EMS to transport stroke patients to the nearest primary stroke center was initiated in a given
year, then the area was said to be in a transition period for that same year to account for the delay
of several months for actual implementation of stroke routing on the field. The proportion of
stroke patients in jurisdictions with EMS regional system of stroke care was compared to the
proportion of stroke patients in other jurisdictions for each year by comparing the frequency of
patients in each jurisdiction. The frequency of total ischemic patient transportations for that year
is the denominator for comparing the proportions. Then, mortality status, which was obtained
from the NIS database, was compared to the proportion of patients in the various protocols of
EMS regional system. The proportion of deaths from hospitalization of ischemic stroke that
were in areas with EMS stroke routing was compared to the proportion of hospitalized stroke
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 9
deaths in areas without EMS stroke routing. The comparison was made by the frequency of
mortality in each region for each year over the total number of stroke hospitalizations for the
specific region and year.
Mortality analyses were conducted with age-adjustment, and stratification by hospital
type, and specific hospital. Direct age-adjustment was used, standardizing age rates to the 2000
US population according to the US census.
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 10
Results
An increase of states supporting EMS stroke routing protocol.
Although Alabama and Texas were the first states to begin routing acute stroke patients
to stroke-proficient hospitals in 2000, the NIS database documents the first states transitioning to
EMS routing of acute stroke patients beginning in 2004. By 2010, the NIS database included
fourteen states and counties in three separate states that had adopted a protocol for EMS
transportation of acute stroke patients to primary stroke centers (Figure 1).
Proportion of jurisdictions utilizing EMS routing for stroke care.
From 2000 through 2010, 1,009,307 stroke patients were transported in areas that
adhered to EMS regional systems of acute stroke care (Table 1). The proportion of US stroke
hospitalizations in areas with routing protocols for stroke patients compared to areas with EMS
stroke protocols increased from 2004 to 2010 (1%, 13%, 28%, 30%, 30%, 34%, and 49%) (Table
1.).
Mortality rate of ischemic stroke before and after protocols for EMS routing of stroke
patients.
From 2000 to 2010, a total of 458,956 ischemic stroke patients died during
hospitalization in the US. The proportion of patients that died after a stroke in jurisdictions with
no regulations for stroke routing slightly decreased from 2000 to 2010 (9.9%, 9.9%, 9.5%, 9.1%,
8.8%, 8.8%, 8.3%, 8.3%, 7.8%, and 7.4%)(Table 2.). Similarly, there was a slight decrease in
the proportion of deaths after a stroke hospitalization in regions with a set EMS practice for
routing stroke patients from 2004-2010 (11.1%, 8.14%, 8.8%, 8.4%, 8.4%, 7.6%, and 7.9%)(Table 2.). However, the proportion of deaths was consistently higher in the regions with an
established EMS routing stroke procedure. For example, in 2004, 11% of the stroke patients
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 11
died that were in areas that adopted stroke routing versus 9.1% that were in areas without stroke
routing. A similar pattern follows for 2006-2008, and 2010 (percent of mortality in areas with
stroke routing vs. without stroke routing: 8.84% vs. 8.78%, 8.35% vs. 8.25%, 8.38% vs. 8.34%,
and 7.91% vs. 7.40%). However, the total proportion of hospitalized stroke patients dying has
remained relatively constant from 2000 to 2010, differing by 2.2% over the 10-year span. (Table
2). Figure 2 depicts the overall trend in age-adjusted stroke mortality rate in the US population.
There is a decline in mortality rate from 2000 to 2010 with the highest rate of 13.2 deaths per
100 US standard population for year 2000 and the lowest rate in 2010 of 8.85 deaths per 100 US
standard population for year 2000(Table 3).
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 12
Discussion
Adoption of a fixed EMS stroke routing protocol steadily increased over the 2000-2010
study period. In 2010, a majority of stroke patients in the United States were hospitalized in an
area with an established or a transitioning EMS diversion policy for stroke. Half of the US stroke
hospitalizations are in areas that support an extensive practice of EMS regional stroke routing,
which demonstrates the progress towards extended coverage of stroke care. The wide coverage
of stroke routing to primary stroke centers is a major advance for systematic stroke care. Since
the average age of the US population is increasing, the frequency of stroke is predicted to rise as
well (Heidenreich et al., 2011). The projected growth in stroke-prone individuals emphasizes the
need for novel approaches, such as telemedicine, support of the EMS transportation of acute
stroke patients to primary stroke centers by the national EMS leadership, and increased
recognition and certification of stroke centers across the US. These advancements focus
on further extending access to stroke care to all patients.
The mortality rate among stroke hospitalization shows a continual decrease over the ten
year span. There has been great success in the implementation of policies supporting EMS
stroke routing, the continued certification of primary stroke centers(Diamond, Wilson-Stronks,
& Jacobs, 2010; “Facts about Primary Stroke Center Certification,” 2011), and the practice and
advancement of stroke therapies(Wojner, Persse, Alexandrov, & Grotta, 2003; Albers, Clark,
Madden, & Hamilton, 2002), such as intravenous recombinant tissue plasminogen activator
(Hacke et al., 2004). Expanding coverage of and patient access to more comprehensive stroke
care teams has resulted in better outcomes for stroke victims(Rymer, Thurtchley, & Summers,
2003).
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 13
However, the proportion of fatalities after stroke hospitalization is higher in EMS routing
areas, which does not reflect the success of the EMS protocol. These data illustrate a few
limitations in the study. The proportion of mortality by region and year was not adjusted for age
or tested for statistically significant differences. The slight difference in the percentage of stroke
routing and non-routing mortality might be from random error. Also, the legislation adopting
EMS regional systems of stroke care was regulated on the state or county level. However, the
actual implementation of this practice at the local EMS level was not assessed and investigated
in this study; geographic areas implementing EMS includes areas that may not have primary
stroke centers available. Another limitation is the potential variation in patients’ stroke
conditions that were transported in areas with routing compared to areas without routing to
primary stroke centers. The disparity might be caused from the early adoption of EMS stoke
routing protocols by densely-populated areas. If the EMS in certain regions mainly transported
patients with severe stroke to primary stroke centers, then comparing the proportion of mortality
in this jurisdiction to another jurisdiction, that mostly transported less severe stroke patients,
would not be appropriate. Another contributing factor is that mortality includes patients that
were transported from one hospital to another who may have died shortly after, even if stroke
was not the main complication. These limitations suggest that more valid comparisons may be
achieved using different stroke outcomes to better compare the effect of the EMS stroke routing
policies. For example, examining the proportion of tPA administration in areas with and without
a fixed EMS regional stroke care team might be a better outcome to compare the different
regions. Investigating administration and timing of administration of tPA would reflect the
effectiveness of EMS stroke care because tPA should be administered ideally within 3 hours of
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 14
stroke onset. Therefore, the variation in stroke conditions between the areas, including
transportation of patients between hospitals, would be eliminated.
The progress towards extended coverage of stroke care is demonstrated by the fact that
half of the US stroke hospitalizations are in areas that support an extensive practice of EMS
regional stroke routing. Extended access to comprehensive stroke care for all stroke patients in
the US is an achievable goal, if stroke organization leaders, national EMS and medical directors,
and legislators continue to work towards a proficient and an effective stroke care system.
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 15
References
Albers GW, Clark WM, Madden KP, Hamilton SA.(2002). ATLANTIS trial: results for patients
treated within 3 hours of stroke onset. Alteplase Thrombolysis for Acute
Noninterventional Therapy in Ischemic Stroke. Stroke. 33, 493–495.
Alberts M. J., Hademenos G., Latchaw R.E., Jagoda A., Marler J. R., Mayberg
M. R.…Walker, M. D. (2000). Recommendations for the establishment of primary stroke
centers. Journal of American Medical Association. 283, 3102–3109.
Alberts, M. J., Latchaw, R. E., Jagoda, A., Lawrence, R., Wechsler, T. C., George, M. C.
…Walker, M. D. (2011). Revised and Updated Recommendations for the Establishment
of Primary Stroke Centers: A Summary Statement From the Brain Attack Coalition.
Stroke. 42, 2651-2665.
Diamond LC, Wilson-Stronks A, Jacobs EA.(2010). Do Hospitals Measure Up to the National
Culturally and Linguistically Appropriate Services Standards? Medical Care. In Press.
Retrieved March 25, 2013 from:
http://dx.confex.com/dx/10/webprogram/Paper2474.html
Facts about Primary Stroke Center Certification. (2011). Retrieved March 25, 2013 from:
http://www.jointcommission.org/assets/1/18/Facts_about_Primary_Stroke_Center_Certifi
cation.pdf.
Hacke W, Donnan G, Fieschi C, Kaste M, von Kummer R, Broderick JP,…Hamilton, S. (2004).
Association of outcome with early stroke treatment: pooled analysis of ATLANTIS,
ECASS, and NINDS rt-PA stroke trials. Lancet. 363, 768 –774.
HCUP Nationwide Inpatient Sample (NIS). Healthcare Cost and Utilization Project (HCUP).
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 16
2000-2010. Agency for Healthcare Research and Quality, Rockville, MD.www.hcup-
us.ahrq.gov/nisoverview.jsp
Heidenreich P. A., Trogdon J. G., Khavjou O. A., Butler J., Dracup K., Ezekowitz E., …Woo, J.
(2011) Forecasting the future of cardiovascular disease in the United States: a policy
statement from the American Heart Association. Circulation. 123, 933-944.
Introduction to the HCUP Nationwide Inpatient Sample (NIS). (2012). Retrieved March 25,
2013 from http://www.hcup-us.ahrq.gov/db/nation/nis/NISIntroduction2010.pdf
Rymer MM, Thurtchley D, Summers D.(2003). Expanded modes of tissue plasminogen
activator delivery in a comprehensive stroke center increases regional acute stroke
interventions. Stroke. 34, e58–e60.
Saver, J., and Song, S. (2012). Growth of Regional Acute Stroke Systems of Care in the United
States in the First Decade of the 21st Century. Stroke. 43, 1975-1978.
Stroke 101 Fact Sheet. Retrieved March 25, 2013, from
http://www.stroke.org/site/DocServer/STROKE101_2009.pdf?docID=4541
Wojner AW, Persse D, Alexandrov AV, Grotta J. (2003). Weaving a web for stroke treatment:
the Houston Paramedic/Emergency Stroke Treatment Outcomes Study (HoPSTO).
Stroke. 34,248A.
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 17
Appendix
Figure 1
Figure 1. Depicts the distribution of EMS regional systems with acute stroke routing to primary
stroke centers for states in the NIS database from 2000 to 2010 in the United States. The figure
is adapted from Saver and Song’s article (Saver & Song, 2012).
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 18
Table 1
Table 1. The frequency and proportion of stroke patients in regions without (No EMS), with
(EMS), or in transition of having(In-Transit EMS) EMS stroke routing protocol and total
number of hospitalized stroke patients in the US by year.
Frequency of Stroke Patients
Percent of Stroke Patients by
Year
No EMS EMS
EMS In-
transit
Total No EMS EMS
EMS In-
transit
2000 582954 0 0 582954 100 0 0
2001 580078 0 0 580078 100 0 0
2002 569887 0 0 569887 100 0 0
2004 459644 7372.02 65215.5 532232 86.36 1.39 12.25
2005 367911 69681.9 88083.3 525676 69.99 13.26 16.76
2006 382256 146074 1241.1 529571 72.18 27.58 0.23
2007 351403 156695 12227.7 520326 67.54 30.11 2.35
2008 368810 163267 32087.6 564165 65.37 28.94 5.69
2009 264436 187681 95525.8 547643 48.29 34.27 17.44
2010 275043 278538 18979.3 572560 48.04 48.65 3.31
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 19
Table 2
Percent of Survival by Year Percent of Mortality by Year
No
EMS EMS
EMS In-
transit
Total
No EMS EMS
EMS In-
transit
Total
2000 90.14 0 0 90.14 9.86 0 0 9.86
2001 90.11 0 0 90.11 9.89 0 0 9.89
2002 90.49 0 0 90.49 9.51 0 0 9.51
2004 90.92 88.93 91.55 90.97 9.08 11.07 8.45 9.03
2005 91.24 91.86 90.85 91.26 8.76 8.14 9.15 8.74
2006 91.22 91.16 94.23 91.21 8.78 8.84 5.77 8.79
2007 91.75 91.65 92.35 91.73 8.25 8.35 7.65 8.27
2008 91.66 91.62 90.54 91.58 8.34 8.38 9.46 8.42
2009 92.2 92.42 92.25 92.28 7.8 7.58 7.75 7.72
2010 92.6 92.09 92.31
92.34
7.4 7.91 7.69
7.66
Table 2. Data on the total proportion of survival and mortality of hospitalized stroke patients in
the US and the proportion by regions without (No EMS), with (EMS), or in transition of having
(In-Transit EMS) EMS stroke routing protocol for each year.
Table 3
Year Age-adjusted
mortality rate
(per 100) 95% Confidence
Interval
2000 12.24 8.65, 15.82
2001 13.2 9.71, 16.69
2002 12.21 9.08, 15.34
2003 12.8 9.30, 16.30
2004 12.31 9.13, 15.49
2005 10.58 7.78, 13.37
2006 9.84 7.18, 12.51
2007 10.29 7.42, 13.16
2008 10.25 7.66, 12.84
2009 10.38 7.45, 13.30
2010 8.85 6.60, 11.09
Table 3. Age-adjusted mortality rate per 100 for ischemic stroke in US by year.
EXPANSION OF EMS REGIONAL ROUTING FOR STROKE CARE 20
Figure 2
Figure 2. Graphs the mortality rate of ischemic stroke by year in the US.
0
2
4
6
8
10
12
14
1998 2000 2002 2004 2006 2008 2010 2012
Age-Adjusted Mortality rate (per 100) Year
Mortality Rate of Ischemic Stroke

Abstract (if available)

Abstract Background: Organized stroke systems of care include preferential emergency medical services (EMS) routing to deliver suspected stroke patients to designated hospitals. EMS transport of stroke patients is regulated at the State and County level in the United States. This study aims to characterize the growth of EMS regional systems of acute stroke care by describing the number and proportion of nationwide stroke hospitalizations in areas with EMS systems of care over a 10-year period. ❧ Methods: Data on ischemic stroke using ICD-9 coding was collected from the Healthcare Cost and Utilization Project Nationwide Inpatient Sample (NIS) database from the years 2000-2010. The NIS contains all discharge data from 1,051 hospitals located in 45 States, approximating a 20-percent stratified sample. Data on EMS systems of care was obtained from a review of archives, reports, and interviews with state EMS officials. ❧ Results: Nationwide stroke hospitalizations remained constant over the course of the study period: 583,000 in 2000 and 573,000 in 2010. However from 2000-2003 there were no states or counties with EMS systems of care. The proportion of US stroke hospitalizations occurring in jurisdictions with established EMS regional systems of acute stroke care increased steadily from 2004 to 2010 (1%, 13%, 28%, 30%, 30%, 34%, 49%). In 2010, 278,538 stroke hospitalizations, 49% of all US stroke hospitalizations, occurred in areas with established EMS routing, with an additional 18,979 (3%) patients in regions undergoing a transition to EMS routing. ❧ Conclusions: In 2010, a majority of stroke patients in the United States were hospitalized in an area with established or transitioning to EMS diversion. This milestone coverage of half the US population is a major advance in systematic stroke care, and emphasizes the need for novel approaches, such as telemedicine, to further extend access to stroke center care to all patients.

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Asset Metadata

Creator Hanks, Natalie E. (author)

Core Title Expansion of emergency medical service regional routing for stroke patient care in the United States from 2000-2010

School Keck School of Medicine

Degree Master of Science

Degree Program Applied Biostatistics and Epidemiology

Publication Date 05/09/2013

Defense Date 05/09/2013

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag EMS,OAI-PMH Harvest,stroke

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Mack, Wendy Jean (committee chair), Cen, Yong (committee member), Mack, William (committee member)

Creator Email nhanks@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-253444

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