The way we now think about changes in aging is the rule of fourths (Figure 1-1). This rule states that, of changes often attributed to normal aging by the general public (and in past decades by the medical profession), about one fourth is due to disease, one fourth to disuse, one fourth to misuse, and only about one fourth to physiologic aging.

Figure 1-1 The rule of fourths. In the past, all of the decline in function that occurs between young adulthood and old age was called normal aging. We now know that approximately one fourth can be attributed to disease, one fourth to disuse (e.g., sedentary lifestyle, lack of mental stimulation), and one fourth to misuse (e.g., smoking, injuries from contact sports, and adverse effects of prescription and/or recreational drugs). Only about one fourth can be attributed to physiologic aging.

The job of health care providers is to determine whether and to what extent a new symptom is caused by each of these etiologic categories and then develop a treatment plan.

• If the problem is disease, then medical treatment is indicated.

• If the problem is disuse, it can often be cured with an activity regimen.

• If the problem is misuse, prior damage cannot be reversed but steps can be taken to prevent deterioration and to preserve function.

• If the problem is physiologic aging, then steps should be taken to adapt and compensate for the disability.

Normal physiologic changes

Whereas much of the change seen with aging results from causes other than physiologic aging, some changes are inevitable. Table 1-1 lists and describes many common physiologic changes noted with aging. Among the many notable changes are the following:

TABLE 1-1

Some Common Anatomic and Physiologic Changes with Aging (in the Absence of Identified Disease)

System or Function Affected	Change Noted
Body composition: Percent body water Percent body fat	Decreased Increased
Brain: Weight Anatomy	Decreases by 7% “Atrophy” commonly noted
Sleep patterns	Markedly reduced stage 3 and 4 sleep More frequent awakenings; reduced sleep efficiency
Vision: Lens accommodation Amount of light reaching the retina Color perception	Markedly reduced after age 40-50 Diminished by up to 70% Reduced intensity (especially of blues and greens)
Hearing	Acuity declines beginning about age 12, with decline steepest in high pitches (>5000 Hertz [cycles/second])
Taste: Number of taste buds Changes in preferences	Reduced by 70% Increased tolerance for very sweet and very salty foods (as a result of reduced perception)
Cardiac function: Maximum heart rate	Reduced from about 195 to about 155 beats/min
Renal perfusion	Decreased by 50%
Bone mineral content	Diminished by 10%-30%
Prostate gland anatomy	Size increases by 100%
Sexual function: Men Women	Reduced intensity and persistence of erections; decreased ejaculate and ejaculatory flow Menopause; reduced lubrication; vaginal atrophy

Adapted from Sloane PD. Normal aging. In: Ham RJ, Sloane PD, Warshaw GA, editors. Primary care geriatrics: A case-based approach. 4th ed. St. Louis: Mosby-Yearbook; 2002, p. 15-28.

• The age at which reading glasses are needed because of reduced lens elasticity is between 42 and 50.

• Vestibular sensitivity gradually increases until about age 60, which is one of the reasons why adults have increasing trouble on amusement park rides as they age.

• Fertility in women peaks between 15 and 25 and declines thereafter, with menopause typically occurring about age 50.

• Reaction time tends to increase with age (which explains why teenagers are usually far better at games of speed—including many video games—than older persons).

• The amount of sway a person will experience if asked to stand still with his or her eyes closed is high in early childhood, is minimized between about ages 15 and 16, and then gradually increases beyond age 60.

• Ankle jerk reflexes are increasingly diminished or absent with older age, in the absence of detectable musculoskeletal pathology.

• Bone density plateaus between ages 20 and 50, then gradually declines, with the slope of decline being more rapid in women than in men.

The list of physiologic changes with age is long, and the clinical implications vary from merely interesting to very important. Also, the line between “normal physiology” and changes caused by other factors is frequently blurry.

What is important for the clinician to recognize is that aging does result in real, profound changes. Many of these changes cannot be reversed, and the older person will need to make adjustments. An important role of primary care clinicians is therefore to help provide access to the variety of mechanisms that can help compensate for bodily change and preserve function. In addition, the clinician may need to help the patient successfully make changes in goals and lifestyle that will help him or her successfully adjust to aging.

Functional reserve

All body systems tend to have functional ability over and above what is used during everyday activities; this is called functional reserve. For example, the average adult’s cardiac output is around 5 L/min when sedentary, whereas the heart of a trained athlete is capable of generating 40 to 50 L/min.² All other key body systems, such as the kidney, the lungs, the liver, and the brain, have reserve capabilities as well, so significant impairment from disease, disuse, misuse, or physiologic aging is needed to result in impaired function during normal activity.

Clinically significant impairment in function occurs when demands exceed functional reserve. As people age, patterns of disease, disuse, misuse, and physiologic aging combine to decrease functional reserve. Among the losses in functional reserve that have particularly common implications in geriatric care are the following clinical situations:

• Delirium is common in postoperative older persons, because brain functional reserve capacity is overwhelmed by the stress of the surgery and the persistence of anesthetic agents in the central nervous system and the bloodstream.

• Nocturia is almost ubiquitous in older persons, largely because of changes in bladder physiology (decreased capacity and increased residual volume) combined with altered control of fluid excretion (related to low nighttime antidiuretic hormone levels and increased nighttime natriuretic polypeptide levels).

• An older person will often fall when a younger person would not, because neuromuscular mechanisms to reestablish equilibrium from a minor perturbation (e.g., tripping on the edge of a rug) are impaired, often by a combination of disuse and normal aging changes in nerve conduction and vibratory sensation in the feet.

When functional reserve is impaired, the clinician should work with the patient to explore ways to improve this capacity and thereby to improve function. For a patient with chronic obstructive pulmonary disease, for example, solutions include continuous low-flow oxygen and pulmonary rehabilitation exercises. For a patient with impaired brain reserve, minimizing sedating and anticholinergic medications is often the best approach.

Reduced stamina and fatigue

One of the inexorable physiologic declines with aging is reduced stamina. An insidious reduction in stamina occurs, beginning in one’s 20s and terminating in advanced age (Figure 1-2). Of course, this gradual decline in stamina and fatigue can be accelerated by disease, disuse, and misuse; however, gradual decrease in stamina and need for more frequent rest is a universal phenomenon as one ages. This is why medical interns in their 20s can pull an all-night shift much more easily than they would be able to if asked to follow the same schedule when in their 50s.

Figure 1-2 Inability to party all night is one of the earliest signs of aging. With increasing age and disability, diminished stamina can progress to the point where the older person no longer has enough energy to complete the necessary activities of daily living. This severe fatigue is the cardinal symptom of the frailty syndrome.

When reduced stamina and fatigue are so great that they become the defining feature of one’s physiologic status, we refer to this as frailty. A commonly accepted, evidence-based definition of frailty is that of Fried et al.; it defines frailty as the occurrence of three or more of the following: unintentional weight loss (10 lbs in past year), self-reported exhaustion, weakness (reduced grip strength), slow walking speed, and low physical activity.³ One can see that all but the first of these criteria are manifestations of reduced stamina and fatigue. Frailty is a common and important geriatric syndrome (see Chapter 29).

Increased physiologic diversity

Another characteristic of aging is that, with advancing age, physiologic diversity increases. Indeed, the range of “normal” (i.e., the range that encompasses the performance of 95% of people) becomes increasingly wide as populations age. When we say, for example, “Jason is a normal 5-year-old,” we have a pretty good idea what Jason can and cannot do. The same is true at age 20. However, with each advancing decade the range of normality becomes wider (Figure 1-3), to the point that saying, “George is a normal 75-year-old,” tells you practically nothing about George other than how long ago he was born.

Figure 1-3 As people age they become more diverse. This explains why individualized care rather than protocol-based care is especially important in the geriatric population.

This increased diversity with age has many clinical implications. For example, it is easy to develop age-related guidelines for children, because, except in rare cases of chronic or developmental illness, age predicts performance in children. In older adults, however, age is not very helpful in determining health care norms or needs. In geriatrics, however, age-related protocols and guidelines are virtually nonexistent, and the clinician must individualize most aspects of assessment, goal setting, and care planning.

Because of increasing diversity with age, protocols and guidelines are less useful in geriatric care than for younger ages, and instead care must be individualized.

Environment and function

An often unappreciated aspect of aging is the importance of the environment in which an older person is asked to live and function. Indeed, the environment within which one lives and functions can make the difference between being independent and being unable to carry out basic everyday activities.

In health care it is useful to think about three distinctive types of environments: the physical environment, the social environment, and the organizational environment.⁴ The physical environment refers to the physical setting in which a patient lives; it includes such things as size and decor of spaces, lighting, temperature, acoustic properties, and access to outdoors. The social (caregiving) environment refers to the people who interact directly with the patient—how they approach the person and what they do. The organizational environment refers to rules and regulations that affect a patient’s life, such as when they can eat, whether and how they can go outdoors, and what type and amount of services they receive. Needless to say, these environmental characteristics can have a huge impact not only on function but on quality of life for a dependent older person.

How living units are designed can greatly influence an older adult’s independence. Fortunately, with the aging of the population has come increased interest in housing design features that make it easier for persons who have a variety of disabilities to function. For example: