The Aging Process

Aging is a natural consequence of life. It leads to anatomic, biochemical, and physiologic alterations in every organ system (Table 101-1). Common degenerative disorders and diseases include atherosclerotic and cerebrovascular disease; chronic obstructive pulmonary disease; emphysema; diabetes mellitus; arthritis; emotional, mood, and memory disorders (e.g., depression, Alzheimer’s disease); and impaired thermoregulation. Polypharmacy (including excesses in medication intake) is a problem. Accidents due to unsteadiness, falls, forgetfulness, and operating machinery and vehicles take their toll. The cumulative effect of these changes on elders subjected to stressful environments produces a major increase in health risk and health care demand.^4,15

Anatomic changes are organ specific, because organs appear to age independently and not necessarily in parallel fashion. Furthermore, laboratory tests must be carefully interpreted. For example, glomerular filtration rate (GFR) and renal blood flow (RBF) decrease with age, but many elders have normal serum creatinine levels because there is a concomitant loss of muscle mass due to aging and, as a result, lower creatinine production. Physical and physiologic alterations may occur slowly. They may not be apparent for many years, yet result in “silent” functional and anatomic changes.

The degree of loss of function in various physiologic systems can be approximated by using the “1% rule,” which states that “most organ systems lose function at roughly 1% per year after the age of 30 years.”³⁰ Some age-related biologic changes and their resulting functional changes are listed in Table 101-1.

Inasmuch as aging reflects the effects of the passage of time, injuries and illnesses that occur along the path of life may produce cumulative anatomic scars, which, when combined with the degenerative changes of aging, may result in a functionally impaired older adult. The risk may be greatly exaggerated by wilderness ventures. The challenge to health professionals is to determine the extent of that risk and take appropriate action.

Etiology of the Aging Process

Some individuals age faster than others. Lifestyle and genetic predisposition are the most commonly incriminated “causes,” but this does not explain the biologic basis for aging.²⁵ Genetic events may somehow determine longevity. For example, there is an increased risk for development of Alzheimer’s disease in the presence of an allele of apolipoprotein E (ApoE) gene, which encodes a carrier of cholesterol.² Selman and colleagues have proposed that deletion of ribosomal S6 protein kinase 1 (S6K1) leads to an extended life span and resistance to age-related processes in mice.⁴⁴ Nevertheless, support for genetic determination of tissue longevity is scant. Most researchers believe that the processes of aging are multifactorial. Speculative theories suggest that damage to deoxyribonucleic acid (DNA) and proteins occurs from a variety of sources, such as reactive oxygen species, including superoxide, the hydroxyl radical, and hydrogen peroxide. According to Hoeijmakers, damage to DNA has implications both for cancer and acceleration of the aging process.²⁶

Hayflick observed that each body cell undergoes a finite number of divisions, roughly 50, after which the cell dies.²⁵ This number of divisions is known as the “Hayflick limit.” Each cell replication has a span of about 18 months and may be a predictor of individual human longevity. For example, with a cellular life span of 18 months, 50 doublings would result in a human life span of 75 years. Variation in the duration of each doubling has been speculated to be a predictor of length of life.

The Hayflick limit is a function of caps, called telomeres, at each end of the chromosomes. Each cap is a DNA-protein complex, which shortens after each cell division, protected by a cellular enzyme, telomerase. Eventually, after about 50 divisions, the telomeres become too short for further division and the cell dies.

Molecular concomitants of aging include alteration in chromosomal structure, mitochondrial deterioration, DNA cross-linking and single strand breaks, decline in DNA methylation, and loss of telomeric sequencing.

Other theories on the aging process include cellular changes, autoimmune mechanisms, neuroendocrine factors, and even a “biological clock,” but Strehler⁴⁴ feels that the cause should explain the progressive deleterious and intrinsic changes universal within the species. For example, some animals, usually cold-blooded fish or amphibians, which may grow to an indeterminate size, may also have an indeterminate life span, whereas warm-blooded animals with a limited or fixed size after maturation may die at a more predictable time and at an actuarially determined rate.⁴⁵

Questions concerning the nature and cause of aging include the following:

In perhaps the clearest summation of these theories, Hayflick suggests that the ultimate effect from the many factors influencing and affecting human life is that we simply exceed our reserve capacity. This lends support to the mountaineering dictum, “Always keep your reserve,” which is particularly appropriate for elders.

Definition of Elders

Why Some Elders Venture Into the Wilderness

Hard work, family and fiscal responsibilities, and delayed gratification are characteristics of Western industrialized cultures. As retirement years approach and leisure time increases, seniors begin to increase their participation in recreational and outdoor activities.

The U.S. National Park System studied the frequency of recreational activities in the National Parks from 1982 to 1983 and found, in order of frequency, the following activities: driving for pleasure, sightseeing, walking for pleasure, picnicking, stream/lake/pool swimming, and motor boating.⁹

A repeat survey, completed by the National Park Service in 1994, found that walking has become the most popular single outdoor activity and now involves over 134 million participants. Walking is especially attractive for an older person because it is healthful, self-paced, and socially rewarding when enjoyed with others. With population increases of persons age 70 and older, predictions indicate that many new trails and walking venues will be needed. The most striking change in activities is the dramatic increase in bird watching, having risen from 21 million in 1982 to 54 million in 1994, an increase of 155%. Growth in participation within other activities includes hiking (94%), backpacking (73%), downhill skiing (59%), and primitive area camping (58%).¹⁰

Nash concluded that the personal reasons for elders to venture into the wilderness are for enjoyment of nature, physical fitness, tension reduction, tranquility and solitude away from noise and crowds, experiences with friends, enhancement of skill and competency, and excitement or even the thrill of risk-taking³⁶ (Figure 101-2, online). Some ventures, however, such as off-road motorcycling, kayaking, and cross-country skiing, take place in difficult and inaccessible areas under extraordinary environmental circumstances (cold, altitude, rough terrain). If an emergency occurs in such locations and circumstances, search and rescue services or medical intervention may be required.

FIGURE 101-2 President George H. W. Bush lives up to his remark, “Even old guys can still do stuff!,” on his 75th and 80th birthday parachute jumps.

(From Perspectives, Newsweek, June 21, 1999. Courtesy George Bush Presidential Library.)

Physical Activities of Elders in the Wilderness

For elders, the physical and environmental demands of certain activities may be excessive. The physical workload of a wilderness venture depends on its nature and the characteristics of its component parts. For example, is the venture a walk or a climb? What is the nature of the terrain? What is the altitude? What is the ambient temperature? It is prudent for elders to examine plans for prospective wilderness ventures and to select activities consistent with their personal capacity, skill, and tolerance.

Wilderness activities can be classified according to their physical, technical, and environmental characteristics. The skill, judgment, and capacity of all participants, elders included, can be matched with the characteristics of the venture to determine whether individual capacity is adequate for the demands.

Classifying Wilderness Ventures

One classification scheme for wilderness ventures ranges from those that are extremely demanding and risky undertakings to those with minimal physical and environmental requirements (see Table 101-2). Although this classification is very general, it can be helpful in examining prospective participants and matching demands of the wilderness task or activity facing that individual.^12,14

Physiologic Workload of Wilderness Activities

Workload of wilderness ventures and functional capacity of individuals can be defined with a degree of precision by using measurements of energy demands from indirect calorimetry derived from oxygen consumption techniques. Oxygen consumption may be expressed as , calories (1000 cal = 1 Kcal or 1 C [Cal]) or as metabolic equivalents (METs).

The term MET, considered to be the energy cost of sitting, is defined by convention as 3.5 mL oxygen per kilogram of body weight per minute. Multiples of the MET are used to define the energy cost of various activities. For example, walking 2 miles an hour on a level, smooth surface requires 7 to 9 mL O₂/kg/min or approximately 2 METs; 3 miles an hour requires 10.5 mL O₂/kg/min or 3 METs, and so on. Lists of the energy cost of various activities given in calories or METs are available. Most activities of daily living (ADLs) require an energy expenditure of less than 3 METs. When a task is complex, there may be several levels of energy cost of various components.

Armed with the energy cost of a planned activity, a venture planner can proceed with a comparison of the task requirements and the functional capacity of the participant. The average 40-year-old man in this country can reach a 10-MET activity at the point of exhaustion, the PWCmax. Derived from this is a useful scale of 1 to 10; that is, 1 MET is the energy cost of sitting, increasing to 10 METs as the PWCmax of the average 40-year-old man. Tolerance to physical activity among women, increasing for several decades, is now within 1 to 2 METs of that of men. Well-conditioned athletes can reach a PWCmax of around 13 to 23 METs.

A natural decline in PWCmax occurs with age and results in a loss of roughly 1 MET of PWCmax per decade after 40 years of age. The average maximal functional capacity of a healthy 70-year-old man is approximately 7 METs.

It has been empirically observed that a healthy individual can function at an energy expenditure of about 25% to 40% of personal PWCmax over an 8-hour period of time without being too exhausted to get up the next morning and repeat that level of energy expenditure. A healthy person with a 10-MET PWCmax should be able to handle roughly a 3-MET activity for 8 hours.

These figures are very general approximations only, and the circumstances of each prospective venturer need to be considered individually. Of course, skill and experience with any activity are the best predictors of a safe and successful venture.

Classification of Individuals Considering Wilderness Ventures

Two basic physiologic factors affect the physical relationship between the venture and the participant: (1) the physical workload of the activity and (2) the capacity of the individual to tolerate the physical workload as influenced by the environmental stressors. Physical demands of the venture were classified previously. A similar classification assists in matching the participant with the venture (Table 101-3). The populations presenting to the physician may fit the distribution curve shown in Figure 101-3, where the groups defined in Table 101-3 are graphed. These classifications serve as starting places for more precise recommendations for the individual presenting to the physician for advice concerning physiologic capacity to tolerate the physical demands of a specific adventure.^12,14,15

TABLE 101-3 Classification of Participants in Wilderness Ventures

FIGURE 101-3 Classifying prospective participants according to physical characteristics, functional capacity, and experience helps match the candidate with demands of the venture. Here, prospective participants are arranged with an idealized representation of participants divided by standard deviations according to the classifications in Table 101-3.

Matching Individuals With Wilderness Physical Activities

Individuals who plan to participate in wilderness ventures probably have already been through a form of natural selection. For example, a person who aspires to participate in an expedition to Mt Everest will in all likelihood have already participated in a similar activity and will have proved his or her capacity to function at an extreme level of performance. This person would most likely be in participant group A or B. An individual who is healthy but has not recently been involved in vigorous activities and has become “deconditioned” may be in group C. The motivation may be a desire to reaffirm youth or vigor in some form of exciting or hazardous activity. These individuals deserve the scrutiny of alert organizers with, perhaps, an assessment before the venture in order to consider risk factors or occult health problems.

Particular attention should be directed toward any individual at risk of illness or injury (group D), even though manifesting evidence of disease may not be superficially apparent. For example, cardiovascular risk factors, such as smoking, a fat-laden diet, and high blood pressure, may warrant detailed medical examination to determine level of functional capacity considered safe for that individual. An examination may even disclose the presence of diseases, asymptomatic or symptomatic.

Group E includes persons with definite manifestations of illness. Supervised outdoor activities can still be of value in such cases and have even been used as a form of physical therapy and rehabilitation for persons with various illnesses, including cardiovascular disease. Persons in this category should be individualized in their assessment and require a high degree of medical evaluation and supervision.

Physical Conditioning to Prepare Elders for Wilderness Ventures

Elders considering wilderness activities should consider the appropriateness of their participation in a contemplated venture. After a medical and functional examination, the classification may suggest the need for a physical conditioning program to increase individual reserve capacity. An individual can tolerate an energy cost of approximately one-fourth to one-third of his or her maximal physical work capacity over a period of 8 hours. If the demands of a venture exceed predicted tolerance, PWCmax can be enhanced by 15% or more by a 3- to 6-month graduated conditioning program. A conditioning program should begin with a warm-up session that includes flexion and extension exercises, followed by a cardiovascular conditioning component and a cool-down component. The aerobic component should last at least 20 minutes at a level of about 50% of individual PWCmax. Some persons can learn to monitor the pulse rate corresponding to this level of activity as a monitoring technique. Mouth breathing, which usually occurs at about 60% of maximal work capacity, is also a useful body signal for marking the intensity of the activity. As intensity increases, catecholamines increase. Above 70% of maximal work capacity, the catecholamine level may be hazardous for an individual with subtle cardiovascular disease. Consequently, if the subject keeps the intensity of the activity below that requiring mouth breathing, the aerobic activity should be in the range below 60% of the individual maximal capacity, a relatively safe range in healthy individuals. This level of conditioning is helpful and may be reasonably safe for seniors, although not at the level necessary for conditioning competitive athletes. If cardiovascular disease is present, a conditioning program should be individualized and supervised. A structured program is an ideal time for teaching an individual to read his or her personal body signals, a collection of markers for physiologic responses to activity (Box 101-1).

The American College of Sports Medicine has developed a position paper on exercise and physical activity for older adults. This should be reviewed by any serious student of the subject.¹ Recommendations include endurance training to help maintain and improve cardiovascular function and strength training to help offset loss of muscle mass. Standards for exercise programs for subjects with cardiovascular disease have been available from the American Heart Association since 1979.¹⁷ Additional benefits from regular exercise include improved bone health, reduction of risk from osteoporosis, improved postural stability, and increased flexibility and range of motion. Psychological benefits include a feeling of well-being, relief of symptoms of depression often associated with older adults, and a general joie de vivre.

Environmental Stresses and Elders

Environmental variables encountered in the great diversity of outdoor wilderness activities may produce significant physiologic stresses. These variables include extremes of heat and cold, high altitude, water immersion, tropical humidity, desert aridity, and ultraviolet exposure. The common denominator in nearly all wilderness ventures is physical activity, often at extreme levels. To compound the complexity of physical activity influenced by environmental stress, the physician may have to care for a senior afflicted with subclinical or manifesting disease. When the physiologic demands from environmental stresses are added to the increased and prevalent degenerative conditions and diseases associated with aging, risk for illness and injury is multiplied. The complete package of age, conditioning, environment, nature of the activity, and experience must be considered when an elder is advised or treated in the wilderness.