AGING LUNG- Review Article
Introduction
Case Scenario-1:
I recollect the story of an 80 year old retired nurse referred as a case of interstitial lung disease. She started experiencing shortness of breath on walking or climbing stairs for the last 3 years which is gradually progressing, affecting her house hold work. As she is living alone she has to attend to all her personal needs including cooking food. The referring physician detected fine crackles in the left lung base and advised X Ray chest which was reported as normal. Her cardiac work up was also normal. A spirometry attempted recorded poor values due to improper effort. On evaluation her respiratory system appeared normal for her age and the basal crackles detected in the left lung base is part of aging process and not due to any lung disease. This is important as the closing volume increases as age advances and left lower lobe being compressed by the left ventricle has more impact than right lower lobe. But it is difficult to convince a person who is self dependant for years and developed symptoms only recently. An HRCT thorax was advised which also reported normal with age related changes. It is important to have knowledge on lung function decline as age advances and all activities should be modified to ones comfort level. Trying to stretch it beyond a certain level to satisfy our needs make one symptomatic. Another thing which can be tried here is to build exercise tolerance. This has to be started early in life so that the impact of aging process on respiratory system will be slower.
Structural and functional changes with aging
Chest wall is vital for optimal lung function. Changes to the spine, muscles, and ribs with aging impact normal lung function. As people normally age, narrowing of the intervertebral disc spaces causes increased kyphosis of the spine. This curvature decreases the space between the ribs and creates a smaller chest cavity4. After age 40, the kyphosis angle begins to increase more rapidly in women than men, from a mean of 43° in women aged 55–60 years, to a mean of 52° in women 76–80 years of age5. It is reported that with increasing vertebral angle there is a significant decline in the forced expiratory volume in 1 second (FEV1) and vital capacity (VC). The angle of the muscle fibers in relation to the ribs may also affect the efficiency and decrease the movement of the lower ribs during inspiration6 . These changes are structural and based on the origin and insertion of the muscles.
Aging is associated with reduced inspiratory and expiratory muscle strength. Overall muscle function in the body decreases by 2% annually as we age7. The most important respiratory muscle, the diaphragm, undergoes structural changes such as flattening, fibrosis and regional weakness leading to functional decline. Respiratory muscle decline can lead to an inability to ventilate in the face of increasing demands, such as that seen in respiratory disease. There is also evidence that at the cellular level, the muscles of elderly individuals have less mitochondrial adenosine triphosphate reserves to sustain a sudden increase in metabolic demand8. If an elderly person becomes ill with pneumonia, and therefore has increased metabolic demands for oxygen in the setting of decreased respiratory muscle strength and decreased cellular energy reserve, he or she may not be able to meet those demands. This leads to an increased risk of respiratory failure in older individuals9.
With aging there is a decreased ability to clear mucus from the lungs. Two mechanisms primarily contribute to this decline: 1) reduced cough strength and 2) alterations in the body’s ability to clear particles in the airways. Any decrease in the strength of the respiratory muscles will greatly impact an individual’s ability to generate the force required for an effective cough10. Aging is associated with reduction of both inspiratory and expiratory muscle strength. There is age-related atrophy of muscle fibers, termed sarcopenia, which may also explain the reduced respiratory strength in the elderly. The decrease in muscle fiber strength can be as high as 20% by age 7011.
The elasticity of the lung is reduced. Elasticity is the important factor for inflation and deflation of lungs. Once it is reduced the natural capacity to inhale and exhale effectively is affected. Along with that the alveoli are enlarged. This leads to retention of more air in the lungs. This is called senile emphysema. These changes are much more in smokers when compared to non smokers.
Physiological changes with aging:
Number of alveoli, alveolar ducts, and capillary segments are stable once adulthood is reached and total lung volume remains the same12. There will be reduction in functional capacity of the lung. The alveolar-capillary surface area increases13 and elasticity decreases, resulting in an increase in resting functional residual capacity and an increase in end-expiratory lung volume. In healthy older adults, these functional changes may only be felt during extreme exertion14, but age-related changes in capacity must be accounted for when diagnosing lung disease.
Case Scenario-2:
A 75 year old nonsmoker used to complain of exertional breathlessness. He had multiple consultations for this and is using inhaled bronchodilator on a presumptive diagnosis of COPD. Otherwise healthy person, he has no identifiable cardiovascular morbidity. Looking at his occupation which he is doing for the last 50 years, it is evident that he is overstretching his respiratory reserve at this age. He is working as electric welder and has to manage metal rods and sheets which precipitate his breathlessness. This scenario indicates that although there may be global reduction in lung function, it is slow in development and the individual can cope up with these changes by life style modification, so that there is less work load on respiratory system.
FVC and FEV1, or the ratio of FEV1/FVC, change in a linear manner with age. The reference values inferred from younger populations are inaccurate when applied to older adults and can lead to a misdiagnosis of COPD15. These physiologic changes to the lungs contribute to changes in lung function and susceptibility to disease (Table 1) 12, 13. Lung diseases such as COPD and pulmonary fibrosis increase with age, as does the incidence of pneumonia16. In some cases, such as pulmonary fibrosis, the development of disease is clearly linked to cellular senescence such as mitochondrial dysfunction17.
Measures of Lung Function Changes With Age
Total lung volume No change
Alveoli (number) No change
Alveoli (size) Increases
Alveoli (elasticity) Decreases
Functional residual capacity Increases due to changes in elasticity and enlargement of airways
End-expiratory lung volume Increases due to changes in elasticity and enlargement of airways
Apparent diffusion coefficient Increases due to airspace enlargement
FEV1/FVC Decreases due to changes in elasticity, airspace enlargement, and other physiologic changes
Airspace wall surface area per unit volume of lung tissue Decreases due to airspace enlargement
Table-1: Showing physiological changes in lung function due to aging
Immunological changes with aging
Lung, over the course of an average life is exposed to particulates, ozone, aerosols, infections, allergens, and pollutants. In addition to this some people are exposed to cigarette smoke, radiation, drugs and medications, mechanical injury, and exposure to industrial pollutants. Consequently, tissue repair is of paramount importance in the lungs as we age. Inappropriate or ineffective repair can lead to fibrosis, dysplasia or remodeling. Tissue repair and remodeling are orchestrated by epithelial cells, immune cells, fibroblasts, and progenitor cells, all of which undergo age-related changes that affect function. It is found out that there is basal activation of the innate immune system in aged individuals in the absence of an immunologic threat18. This phenomenon, referred to as “inflammatory aging”, is marked by elevated levels of tissue and circulating pro-inflammatory cytokines in aged subjects. Related to inflammatory aging is the blunted immune response, known as “immune-senescence”, following a pathogenic threat or tissue injury18. Multiple studies have established reduced levels of mediators such as TNF-α, IL-6, interferon-γ, nitric oxide, monocyte chemoattractant protein-1, and macrophage inflammatory protein-1α after different types of antigenic stimulation in aged animals19. The interplay between inflammatory aging and immune-senescence, leads to disruption in the balance of pro- and anti-inflammatory mediators in favour of a pro-inflammatory environment with advanced age, which subsequently retards an appropriate adaptive immune response. This imbalance of immune mediators, delayed immune activation, and protracted course of inflammation may result in increased morbidity and mortality in aging individuals following infection, environmental exposures, or systemic injury.
Pathological changes with aging
Chronic respiratory diseases, such as asthma, emphysema, chronic bronchitis, bronchiectasis, and interstitial lung diseases have higher mortality among people aged 65 years and older. Many respiratory diseases develop exclusively in the elderly. Similarly many diseases have poor out come in the elderly when compared to their younger counterparts. Pneumonia severity is assessed by CURB65 score. According to this score individual aged 65 or more are at risk of developing severe pneumonia. Resolution of pneumonia is delayed and mortality is very high in elderly individuals. COPD usually starts after the age of 50 years. The prevalence of COPD is two to three times higher in people over age 60. The increased burden of COPD seen in the elderly population may be due to age-associated changes in the structure and function of the lung, increasing the susceptibility to COPD. It is a progressive disease and cause significant morbidity after 65 years of age. Acute infective exacerbations, hospitalizations and ICU admissions are more in the elderly. It is reported that infective exacerbations due to fungal infections is more common in elderly COPD patients20. Idiopathic pulmonary fibrosis is another disease of the elderly. Due to immunological injury and repair, progressive fibrosis develops. This causes irreversible damage to the lungs leading to compromised respiratory function. Lung cancer also is frequently manifested in the elderly individuals. Due to exposure to smoke and irritants there will be dysplastic changes. Due to poor defense in the elderly, irritants are not effectively removed from the respiratory tract. This can lead to metaplastic and neoplastic changes as age advances.
Many systemic diseases of the elderly are also linked with poor respiratory function. It is well proved that development of Type-2 diabetes, a disease of the elderly, is linked to functional decline of the lungs. Individuals whose FEV1 and FVC are at the lower end of the normal range develop many diseases of elderly such as cardiovascular disease, type 2 diabetes and cognitive decline, earlier than those with normal lung function. A chest wall change such as increased kyphosis due to spondylosis reduces respiratory compliance. Dementia and Parkinsonism are frequent causes for aspiration related lung diseases in the elderly. It is reported that low lung function is an accelerant to the aging process; and in turn can be considered as a predictor for development of any of the age-related conditions. Consistent with this statement, a study of middle-aged adults found that lower FEV1 and FVC at baseline correlated with lower cognitive function and an increased risk of being hospitalized for dementia.
Interventions to reduce respiratory morbidity in the elderly
• Avoid smoking: Cigarette smoking is found to be the most important risk factor for COPD, Idiopathic pulmonary fibrosis and lung cancer. Smoking also increases the chance of development of respiratory infections such as pneumonia and tuberculosis.
• Avoid air pollution: Indoor and outdoor air pollutants can damage the lungs. Secondhand smoke, outdoor air pollution, chemicals in the home and workplace can cause or worsen lung diseases such as asthma and COPD.
• Exercise: Regular exercise can improve the exercise tolerance and patients feel better and comfortable at that functional level.
• Rehabilitation: Especially vocational rehabilitation is essential as age advances. It is important to understand that the decline in pulmonary function occur as age advances and the same level of work load will not be tolerated at an advanced age..
• Weight reduction: Abdominal fat can impede the diaphragm's ability to fully expand the lungs. A combination of both healthy eating and exercise will help in maintaining the optimal weight.
• Immunization: It is better to have prophylactic influenza and pneumococcal vaccine after the age of 65 years. This is especially important in patients suffering from COPD and other chronic lung diseases.
Conclusion
Lung health is intimately associated with good health in older adults. There are many age-associated changes in the respiratory system. An efficient respiratory system is essential for health and longevity, but age brings about changes that reduce its efficiency. As the body ages, respiratory muscles lose strength, lung tissues lose elasticity, the alveolar surface area diminishes and lung capacity is reduced. As age increases, the respiratory system is less able to expel inhaled irritants and pathogens due to a reduced cough reflex and a decline in muco-ciliary clearance. There are many complex changes in immunity with aging that increase susceptibility to infections. It is pertinent to know the changes associated with age in order to make a proper clinical diagnosis and offer treatment. Unnecessary investigation and medication may further worsen the respiratory function. Proper counseling and precautionary measures may help the patient to lead a comfortable life with an aged lung.
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