14 Introduction to Preventive Medicine
Sections 1 and 2 of this text focus on epidemiology and biostatistics, two basic sciences for preventive medicine and public health. This section (3) focuses on the theory and practice of preventive medicine. Preventive medicine and public health share common goals, such as promoting general health, preventing specific diseases, and applying epidemiologic concepts and biostatistical techniques toward these goals. However, preventive medicine seeks to enhance the lives of individuals by helping them improve their own health, whereas public health attempts to promote health in populations through the application of organized community efforts. Although this section (Chapters 14-23) emphasizes preventive medicine and Section 4 (Chapters 24–Chapter 30) focuses on public health issues, a seamless continuum binds the practice of preventive medicine by clinicians, the attempts of individuals and families to promote their own and their neighbors’ health, and the efforts of governments and voluntary agencies to achieve analogous health goals for populations.
Western medical education and practice have traditionally focused on the diagnosis and treatment of disease. Diagnosing and treating disease will always be important, but equal importance should be placed on the preservation and enhancement of health. Although specialists undertake research, teaching, and clinical practice in the field of preventive medicine, prevention is no longer the exclusive province of preventive medicine specialists, just as the care of elderly persons is not limited to geriatricians. All clinicians should incorporate prevention into their practice.
Health is more difficult to define than disease. Perhaps the best known definition of health comes from the preamble to the constitution of the World Health Organization: “Health is a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.” This definition is strengthened by recognizing that any meaningful concept of health must include all dimensions of human life, and that a definition must be positive, not only the absence of disease. Nevertheless, the definition has been criticized for two weaknesses: (1) its overly idealistic expectation of complete well-being and (2) its view of health as static, rather than as a dynamic process that requires constant effort to maintain.
In the 1960s, Dubos1 noted that “the states of health or disease are the expressions of the success or failure experienced by the organism in its efforts to respond adaptively to environmental challenges.” Environmental challenges have also been called “stress.” Stress denotes any response of an organism to demands, whether biologic, psychological, or mental.2 Researchers who developed the concept of stress correctly understood that different stressors could induce stress that is either helpful (eustress) or harmful (distress). Good health requires the presence of eustress in such forms as exercise (for the heart, muscles, and bones) or infant stimulation. An individual in good health also may experience some distress, but in the interest of maintaining good health, this must be limited to a level to which the organism can adapt.3 An individual may adapt successfully to environmental stressors in the short term, but a requirement for constant, major adaptation may exact a serious toll on the body, particularly on the lungs and the neural, neuroendocrine, and immune systems. The ongoing level of demand for adaptation to stressors in an individual is called the allostatic load on an individual, and it may be an important contributor to many chronic diseases.4
Often what matters most to people about their health is how they function in their own environment. The inability to function at a satisfactory level brings many people to a physician more quickly than does the presence of discomfort. Functional problems might impinge on a person’s ability to see, to hear, or to be mobile. As Dubos5 states, “Clearly, health and disease cannot be defined merely in terms of anatomical, physiological, or mental attributes. Their real measure is the ability of the individual to function in a manner acceptable to himself and to the group of which he is a part.” Breslow6 describes health as “both (1) the current state of a human organism’s equilibrium with the environment, often called health status, and (2) the potential to maintain that balance.”
However health is defined, it derives principally from forces other than medical care. Appropriate nutrition, adequate shelter, a nonthreatening environment, supportive relationships, and a prudent lifestyle contribute far more to health and well-being than does the medical care system. Nevertheless, medicine contributes to health not only through patient care, but also indirectly by developing and disseminating knowledge about health promotion, disease prevention, and treatment.
Measures of health status can be based on mortality, on the impact of a particular disease on quality of life, and on the ability to function. Historically, measures of health status have been based primarily on mortality data (see Chapter 2). Researchers assumed that a low age-adjusted death rate and a high life expectancy reflected good health in a population. Another way to account for premature mortality in different age groups is the measure of years of potential life lost (YPLL). This measure is used mainly in the field of injury prevention. In YPLL, deaths will be weighted depending on how many years a person might have lived if he or she had not died prematurely. This measure gives more weight to deaths occurring in young people.
Using measures of mortality alone has seemed inadequate as an increasing proportion of the population in developed countries lives to old age and accumulates various chronic and disabling illnesses. An appropriate societal goal is for people to age in a healthy manner, with minimal disability until shortly before death.7 Therefore, health care investigators and practitioners now show increased emphasis on improving and measuring the health-related quality of life. Measures of the quality of life are subjective and thus more challenging to develop than measures of mortality. However, efforts to improve the methods for measuring quality of life are ongoing.8
An example of such a measure is a health status index. A health index summarizes a person’s health as a single score, whereas a health profile seeks to rate a person’s health on several separate dimensions.9 Most health indices and profiles require that each subject complete some form of questionnaire. Many health status indices seek to adjust life expectancy on the basis of morbidity, the perceived quality of life, or both. Such indices also can be used to help guide clinical practice and research. For example, they might show that a country’s emphasis on reducing mortality may not be producing equal results in improving the function or self-perceived health of the country’s population. When clinicians consider which treatments to recommend to patients with a chronic disease, such as prostate cancer, this approach allows them to consider not only the treatment’s impact on mortality but also its side effects, such as incontinence and impotence. Describing survival estimates in terms of the quality of life communicates a fuller picture than survival rates alone.
Life expectancy traditionally is defined as the average number of years of life remaining at a given age. The metric of quality-adjusted life years (QALY) incorporates both life expectancy and “quality of life,” the perceived impact of illness, pain, and disability on the patient’s quality of life.10 For example, a patient with hemiparesis from a stroke might be asked to estimate how many years of life with this disability would have a value that equals to 1 year of life with good health (healthy years). If the answer were that 2 limited years is equivalent to 1 healthy year, 1 year of life after a stroke might be given a quality weight of 0.5. If 3 limited years were equivalent to 1 healthy year, each limited year would contribute 0.33 year to the QALY. Someone who must live in a nursing home and is unable to speak might consider life under those conditions to be as bad as, or worse than, no life at all. In this case the weighting factor would be 0.0 for such years.
Healthy life expectancy is a less subjective measure that attempts to combine mortality and morbidity into one index.11 The index reflects the number of years of life remaining that are expected to be free of serious disease. The onset of a serious disease with permanent sequelae (e.g., peripheral vascular disease leading to amputation of a leg) reduces the healthy life expectancy index as much as if the person who has the sequela had died from the disease.
Other indices combine several measures of health status. The general well-being adjustment scale is an index that measures “anxiety, depression, general health, positive well-being, self-control, and vitality.”12 Another index is called the life expectancy free of disability, which defines itself. The U.S. Centers for Disease Control and Prevention (CDC) developed an index called the health-related quality of life based on data from the Behavioral Risk Factor Surveillance System (BRFSS).13 Using the BRFSS data, CDC investigators found that 87% of U.S. adults considered their health to be “good to excellent.” Also, the average number of good health days (the number of days free of physical and mental health problems during the 30-day period preceding the interview) was 25 days in the adults surveyed.14
Several scales measure the ability of patients to perform their daily activities. These functional indices measure activities that directly contribute to most people’s quality of life, without asking patients to estimate the quality of life compared to how they would feel if they were in perfect health. Such functional indices include Katz’s activity of daily living (ADL) index and Lawton-Brody’s instrumental activities of daily living (IADL) scale. These scales have been used extensively in the geriatric population and for developmentally challenged adults. The ADL index measures a person’s ability independently to bathe, dress, toilet, transfer, feed, and control their bladder and bowels. Items in the IADL scale include shopping, housekeeping, handling finances, and taking responsibility in administering medications. Other scales are used for particular diseases, such as the Karnofsky index for cancer patients, and the Barthel index for stroke patients.
The natural history of disease can be seen as having three stages: the predisease stage, the latent (asymptomatic) disease stage, and the symptomatic disease stage. Before a disease process begins in an individual—that is, during the predisease stage—the individual can be seen as possessing various factors that promote or resist disease. These factors include genetic makeup, demographic characteristics (especially age), environmental exposures, nutritional history, social environment, immunologic capability, and behavioral patterns.
Over time, these and other factors may cause a disease process to begin, either slowly (as with most noninfectious diseases) or quickly (as with most infectious diseases). If the disease-producing process is underway, but no symptoms of disease have become apparent, the disease is said to be in the latent (hidden) stage. If the underlying disease is detectable by a reasonably safe and cost-effective means during this stage, screening may be feasible. In this sense, the latent stage may represent a window of opportunity during which detection followed by treatment provides a better chance of cure or at least effective treatment, to prevent or forestall symptomatic disease. For some diseases, such as pancreatic cancer, there is no window of opportunity because safe and effective screening methods are unavailable. For other diseases, such as rapidly progressive conditions, the window of opportunity may be too short to be useful for screening programs. Screening programs are detailed in Chapter 16 (see Table 16-2 for screening program criteria).
When the disease is advanced enough to produce clinical manifestations, it is in the symptomatic stage. Even in this stage, the earlier the condition is diagnosed and treated, the more likely the treatment will delay death or serious complications, or at least provide the opportunity for effective rehabilitation.
The natural history of a disease is its normal course in the absence of intervention. The central question for studies of prevention (field trials) and studies of treatment (clinical trials) is whether the use of a particular preventive or treatment measure would change the natural history of disease in a favorable direction, by delaying or preventing clinical manifestations, complications, or deaths. Many interventions do not prevent the progression of disease, but instead slow the progression so that the disease occurs later in life than it would have occurred if there had been no intervention.
In the case of myocardial infarction, risk factors include male gender, a family history of myocardial infarction, elevated serum lipid levels, a high-fat diet, cigarette smoking, sedentary lifestyle, other illnesses (e.g., diabetes mellitus, hypertension), and advancing age. The speed with which coronary atherosclerosis develops in an individual would be modified not only by the diet, but also by the pattern of physical activity over the course of a lifetime. Hypertension may accelerate the development of atherosclerosis, and it may lead to increased myocardial oxygen demand, precipitating infarction earlier than it otherwise might have occurred and making recovery more difficult. In some cultures, coronary artery disease is all but unknown, despite considerable genetic overlap with cultures in which it is hyperendemic, showing that genotype is only one of many factors influencing the development of atherosclerosis.
After a myocardial infarction occurs, some patients die, some recover completely, and others recover but have serious sequelae that limit their function. Treatment may improve the outcome so that death or serious sequelae are avoided. Intensive changes in diet, exercise, and behavior (e.g., cessation of smoking) may stop the progression of atheromas or even partially reverse them.