ENVIRONMENT
HOST 
AGENT
AGENTIntroduction
to Cardiovascular Epidemiology
The CARDIAC Project
Table
of Content
Cardiovascular Disease
Definition and Goals of Epidemiology
Basic Concepts
Spectrum of Disease
Incidence versus Prevalence
Causation
Prevention
The
CARDIAC Project
Bibliography
Cardiovascular disease comprises
the major disorders of the heart and arterial circulation supplying the heart,
brain, and peripheral tissues (Labarthe 1998). The extent of cardiovascular disease and it’s
impact on the population of the United States is overwhelming. Heart disease has been the leading cause of
death in the United States since 1921 (MMWR 1999). When combined with stroke, these diseases account for approximately
40% of all deaths.
The
impact of cardiovascular disease on the state of West Virginia has been even
more devastating. In 1995, the age-adjusted
mortality rate due to heart disease was 328/100,000 which was 21% higher than the U.S. average. There are many reasons for this discrepancy,
most notably the large number of behavioral risk factors present in West Virginia.
These
figures demonstrate a much better trend in cardiovascular disease than could
be present. Since 1950, the age-adjusted
death rates from cardiovascular disease have declined 60%. Intensive investigation of the cardiovascular
disease epidemic began in the 1940’s following World War II (MMWR 1999).
Many landmark epidemiological studies took place, including the Framingham
Heart Study, which established the major risk factors of high serum cholesterol,
hypertension, smoking, and dietary factors.
Other studies helped to demonstrate a link to socioeconomic status, obesity,
and physical inactivity.
It
wasn’t until the 1960’s that early intervention studies began to ensue. The primary goal was to determine whether lowering
risk factor levels would reduce cardiovascular disease. Two approaches were utilized to satisfy this
goal. One was a “high-risk” approach
which was aimed specifically at those already deemed to be at high risk for
disease. The others were considered
population-wide approaches which were aimed at lowering risk for entire communities.
These prevention efforts and improvements in early detection, treatment,
and care have resulted in a number of beneficial trends (MMWR 1999):
·
A decline
in cigarette smoking among adults aged greater than or equal to 18 years from
42% in 1965 to 25% in 1995.
In spite of the advances being made in the United States and world-wide, declines in West Virginia for heart-rate mortality have been substantially lower. While the rate of heart disease declined 44% nationwide between 1960 and 1990, the decline in West Virginia was only 32% (Thoenen 1993). This has been attributed to the fact that West Virginians consistently report high prevalences of the risk factors associated with cardiovascular disease.
The
West Virginia Behavioral Risk Factor Survey has uncovered a significantly higher
prevalence of hypertension, obesity, and sedentary lifestyle in West Virginia
than in the United States. Furthermore,
West Virginia currently has the highest rate of obesity, the third highest rate
of self-reported hypertension, and the fifth highest rate of cigarette smoking
when compared to the other states. Even though the link between high blood cholesterol
and heart disease has been increasingly recognized among our citizens, more
than one-third of adults in West Virginia reported in 1991 that they had never
had their blood cholesterol level tested (Thoenen 1993).
It
is not difficult to grasp the impact of cardiovascular disease when presented
with statistics such as these. However,
aside from the gross mortality perspective, cardiovascular disease also has
a great effect on the economy. In the
United States, the cost of medical care alone for cardiovascular disease has
been estimated as approximately $100 billion.
This figure corresponds to approximately 3% of the U.S. gross national
product!
It should now be evident just how large a role cardiovascular disease
plays in every facet of society. With
the recent trend toward public health initiatives and preventive medicine, we
have begun to make some progress in reducing the burden of cardiovascular disease.
This fact certainly instills confidence in those who have taken it upon
themselves to attempt to lessen the effects of cardiovascular disease through
screening programs, risk factor modification, and health promotion.
Definition and Goals of Epidemiology
Why does a disease or condition affect some people but not others? This is one of the basic questions that the study of epidemiology attempts to answer. Epidemiology is defined as the study of the distribution of health and disease in groups of people and the study of factors that influence this distribution. What does this definition really mean? How does it pertain to me as a person and as a health sciences student? These are the questions that will attempt to be answered in this essay.
Epidemiology
as a science has been around for many, many years, even since the time of Hippocrates.
In his work, On Airs, Waters, and Places, Hippocrates
recommended that physicians “attend to the mode in which the inhabitants live
and what are their pursuits, whether they are fond of drinking and eating to
excess, and given to indolence, or are fond of exercise and labor, and not given
to excess in eating and drinking” (Brownson 1998). In so far as is known, this was the first time
that disease was seen as something that could be affected by a person’s lifestyle.
Over
the next 2,000 years diseases were slowly becoming better understood and categorized,
but there was not much interest in their impact on populations.
It was not until 1662 that John Graunt began to analyze weekly births
and deaths in London, quantifying disease in the population.
This was the true beginning of the science of
epidemiology. J. Lind, whose work in 1747 is now regarded as one of the
first experimental trials ever undertaken, led to the discovery of a preventive
measure against scurvy. In 1839, William
Farr became the superintendent of the Statistical Department of the Registrar
General’s Office of England and Wales. He is now considered the “founder of modern disease surveillance”
(Brownson 1998) because of his work in collecting and reporting vital statistics.
One of the greatest early achievements of epidemiology was the work of
John Snow from 1849-1854 which resulted in the halting of the cholera epidemic
in London by discovering the source of the infection.
Many achievements such as these have come and gone since John Snow shut
down the Broad Street water pump, but the true utility of epidemiology has only
recently begun to become realized.
The
overall goals of epidemiology are simple. The
science attempts to understand the causes of disease in a population and what
factors can be manipulated to improve the overall health of the group. There are four major functions of epidemiology
according to Terris (1992):
·
To discover
the agent, host, and environmental factors which affect health, in order to
elucidate the scientific basis for the prevention of disease and injury and
the promotion of health.
As evident by the above four functions, the ultimate
goal of epidemiology is geared toward improving the overall health of the population.
In order to better grasp the impact of this science on our daily personal
and professional lives, we have to be able to clearly understand some of the
basic concepts.
First
we must define disease (and health) so we can see exactly what it is that epidemiologists
strive to achieve when studying populations at risk. The terms disease, illness, and sickness are
sometimes thrown around rather haphazardly, as though they are synonymous. According to Susser in 1973, the true definitions
of these words are:
·
Disease
– a medically definable physiological or psychological dysfunction.
The term health is defined according to mainstream
medicine as the “absence of disease.” The
World Health Organization expanded on this definition in 1948 by defining health
as “a state of complete physical, mental, and social well-being and not merely
the absence of disease or infirmity.” (Gerstman 1998). When studying health and disease, epidemiologists
examine both morbidity and mortality. Morbidity is defined as events related to or caused by disease or
disability. Mortality is simply events
and factors related to death. In attempting
to gain insight into a disease process, factors which increase or decrease morbidity
and mortality must be ascertained.
Another
concept which is important to understand is the spectrum of disease. Any disease
can display a whole range of levels of severity. Some diseases remain silent in some individuals,
yet progressive and fulminating in others. In chronic, noninfectious disease,
this range is known as the spectrum of disease (Gerstman 1998). This range of
manifestation is known as the gradient of infection when dealing with infectious
disease. A good example of this is
coronary artery disease which can exist in one of many different stages:
asymptomatic atherosclerosis, transient ischemia secondary to compromised
coronary circulation, myocardial infarction and death.
It
is this spectrum of disease which has led to one of the most famous metaphors
of epidemiology: the Iceberg Metaphor. Similar
to large icebergs in the ocean, most of the problems due to a certain disease
may be hidden from view (or clinical examination). Again using the example of cardiovascular disease,
many people with this disease never suffer a single symptom until their first
heart attack. It should now be readily
apparent why the study of factors associated with disease is so important.
Next,
it is important to understand the concepts of incidence and prevalence. Incidence describes the extent that people,
within a population who do not have the disease, develop the disease during
a specific time period. The key to determining
incidence is to look at the number of new cases of a disease in a specific population over a specific period of time. High levels of incidence would tend to support
the presence of an epidemic (Timmreck 1994). Prevalence is defined as the number of individuals in a population
that have the disease or condition at a certain point in time.
Now
that we have a firm grasp of the basic concepts of health and disease, including
how they are measured within a population, we can discuss the more in-depth
topics of disease causation and risk factors. There are many factors which can influence
the spread, severity, incidence, and prevalence of a disease in a population.
These factors, if when present result in an increase in disease, are
termed causal factors. A more technical definition of a causal factor
would be “any event, condition, or characteristic that increases the likelihood
of disease, caeteris paribus (all
other things being equal)” (Gerstman 1998). Observational study searches for these cause-effect relationships.
There
are three forms of causation: a sufficient
cause, a necessary cause, and a risk factor. Sufficient cause implies that if the cause
is present, the disease will occur. This relationship rarely exists in human populations.
The definition of necessary cause is found to be true more often when
discussing infectious disease. If a
necessary cause is absent, the disease cannot
occur. For example, one cannot contract
measles if the virus is not present in the body. The third form of causation, which is of greatest interest here,
is the risk factor. A risk factor is a characteristic which, if
present and active, clearly increases the probability of a particular disease
in a group of persons who have the factor compared to an otherwise similar group
of persons who do not (Jekel 1996). A
risk factor is not, however, a necessary or sufficient cause. For example, a person that smokes is at greater risk for coronary artery disease,
but smoking in itself is neither a sufficient nor a necessary cause of coronary
artery disease.
This
is a good time in the discussion to clarify a point. When does a factor that is associated
with a disease become a true risk factor? To understand this concept, it is necessary
to discuss the term association. There are three forms of association (Jekel
1996):
·
Direct
causal association – is present when the factor under consideration exerts its
effect without external factors. For
example, a fall down a flight of stairs will cause injury without other causes
being required.
For a factor to be determined to be causal, or to be
defined as a risk factor, it must
be present significantly more often in persons with the disease than in persons
without the disease.
There
is another concept concerning risk factors that is necessary to know: the difference between modifiable and non-modifiable
risk factors. A modifiable risk factor is one that can be
controlled, or changed to a certain degree.
These include things like diet, level of physical activity, tobacco use,
and obesity. These factors can be modified
with education and effort. A non-modifiable
risk factor, on the other hand, cannot be changed. Examples include a person’s age, race, and
genetic make-up. Other examples would
include a person’s family history other co-morbid conditions of a genetic etiology,
such as diabetes. Since no change can
be made to non-modifiable risk factors, most prevention efforts are geared toward
the modifiable ones.
When discussing the causation of disease, it is important to keep in mind all of the factors that can influence the causal relationship. Epidemiologists simplify this interaction by considering the factors of the agent, host and the environment. This comprises the so-called Triangle of Epidemiology:
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Now
we have a handle on some of the more basic terms that we need to be familiar
with in order to jump to the next step in this discussion: Prevention. The ultimate “aim of epidemiology is in support
of major and key public health activities of prevention and control of diseases
in populations.” (Timmreck 1994). With
today’s focus on preventive medicine and public health, it is crucial to understand
exactly what prevention is and how it is implemented in society.
There are three levels of prevention:
primary, secondary, and tertiary, (known as Leavell’s Levels).
Primary prevention focuses on preventing
the disease process from ever occurring by eliminating the causal factor or
increasing resistance in the host. Secondary
prevention interrupts the disease process by detecting and treating it in the
asymptomatic stage. Tertiary prevention limits the physical impairment and social consequences
from symptomatic disease.
Primary prevention aims to promote overall health in the population and to specifically protect the population against some disease processes. Examples of specific active forms of primary prevention include immunization and antimicrobial drug use. More relevant to this discussion are the more subtle forms of primary prevention which include the education of populations at risk on how to prevent the disease process from ever taking place, similar to the goals of the CARDIAC Project. These education efforts can include nutrition counseling, environmental risk modification, and behavioral counseling.
Secondary
prevention refers to the act of diagnosing a disease process while in the asymptomatic
stage and treating it at that point. If you remember the Iceberg Metaphor from above, you can appreciate
just how important this level of prevention can be. Imagine all of the people with atherosclerotic heart disease which
could be helped if their disease process was discovered early enough. It is this level of prevention in which screening
programs play a large role. Screening
is the process of identifying a group of people who are at high risk for having
asymptomatic disease or who have a risk factor that puts them at high risk for
developing a disease or becoming injured (Jekel 1996).
Screening usually takes place in a community setting.
A screening test is employed which helps to uncover those in the population
which are at high risk of developing a disease.
It is important to note at this point that a screening test is not diagnostic of the disease, but only identifies a person at high
risk. When a positive screening test
is found, a diagnostic test must follow it.
For example, a fingerstick cholesterol measurement is a screening test.
If the total cholesterol is found to be elevated, a more in-depth diagnostic
test should be undertaken, specifically a fasting lipid profile.
If the diagnostic test is also found to be abnormal, a specific treatment
protocol can be initiated to eliminate or reduce the probability of the disease
process progressing.
There
are several requirements which must be met before a screening program can be
considered appropriate (Jekel 1996).
·
The disease
must be serious enough to warrant a screening program.
·
The disease
must not be too rare or too common.
The epidemiology and prevention
of cardiovascular disease involves the understanding of the causes, identification
of means of prevention, and monitoring of populations to assess the changing
burden of the disease and the measurable impact of interventions to control
them (Labarthe 1998). It has been shown
and is now widely accepted that the atherosclerotic process begins in childhood.
It has been shown through various studies that approximately 30% of the
population has atherosclerotic changes present in their vessels by the age of
20 (Labarthe 1998).
These findings have led to
further investigation of the onset and progression of atherosclerosis from childhood.
The most famous of the resulting studies was the Bogalusa, Louisiana
study by Berenson in the 1970’s. This
study set out to uncover the presence of factors related to atherosclerotic
disease present in school age children. The major accomplishments of the Bogalusa
Heart Study can be summarized as follows (Berenson 1991):
·
Discovery
that the major etiologies of adult heart disease, atherosclerosis, coronary
heart disease, and essential hypertension begin in childhood. Documented changes occur by 5 to 8 years of
age.
These
discoveries led to the now widely held belief that intervention should begin
in childhood if there is to be a real impact on the incidence of cardiovascular
disease in adults (Berenson 1991).
The Coronary Artery Risk Detection
in Appalachian Communities (CARDIAC) Project is based upon this premise.
It is designed to examine the effectiveness of universal blood cholesterol
screening in rural Appalachia to identify children and their parents at risk
of developing premature coronary heart disease (CHD).
The study consists of obtaining a family history, a physical activity,
dietary and anxiety profile, non-fasting finger stick blood cholesterol, blood
pressure, and anthropometric measurements from pre-pubertal children across
West Virginia.
If the blood cholesterol test is abnormal (>200mg/dL), a fasting lipid profile is drawn from the subject (and the subject’s parents) to assay for dyslipidemia. If this is found to be present, a referral is made to the family’s primary care physician or the patient can visit one of several consultative preventive cardiology clinics around the state for follow-up care and the development of a treatment plan. Subjects are informed of a proper nutritional plan for CHD prevention as well as risk factor education concerning tobacco avoidance and the importance of exercise, etc.
There
are several overall aims of the CARDIAC Project.
One is to establish the first statewide cardiovascular disease community
intervention project in the nation by the year 2003.
The most ambitious goal is to decrease the age-adjusted rate of death
due to heart disease in West Virginia to the national average by the year 2010.
This is based upon the premise that identification of dyslipidemic children
will provide the opportunity to identify parents at risk of developing premature
CHD, and to interrupt the inevitable sequelae of unhealthy lifestyle factors
by health promotion and the introduction of concepts of wellness.
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