The process of aging leads to a host of physiological changes in the respiratory system, impacting the results of pulmonary function tests (PFTs). These changes are part of a normal, healthy aging process and should not always be confused with lung disease. Understanding the distinction is crucial for accurate diagnosis and management. The lung reaches its peak maturity around the age of 20 to 25, after which a slow but steady decline begins. This article details the specific PFT values that decrease with age and the underlying anatomical and physiological reasons.
The Physiological Basis of Age-Related Lung Changes
Several factors contribute to the predictable decline in pulmonary function seen with increasing age. These changes affect the mechanics of breathing and the efficiency of gas exchange.
Changes in Respiratory Mechanics
- Decreased Lung Elastic Recoil: The lungs lose their natural elasticity over time, primarily due to the degradation of elastin fibers in the lung parenchyma. This causes the lungs to become more compliant (less stiff) and reduces the driving pressure available for forceful expiration, which is similar to the changes seen in emphysema.
- Stiffening of the Chest Wall: As the lungs lose recoil, the chest wall becomes more rigid, with calcification of the rib cage and changes in the thoracic vertebrae. This stiffening reduces overall respiratory system compliance and increases the work required to breathe.
- Weakened Respiratory Muscles: The diaphragm and other respiratory muscles can lose mass and strength with age. This weakening impacts the ability to inhale and exhale fully and forcefully, affecting overall lung volumes and flows.
Changes in Gas Exchange
- Enlargement of Alveolar Spaces: Aging leads to the dilation of alveoli and enlargement of the airspaces, sometimes referred to as “senile emphysema”. This reduces the surface area available for gas exchange.
- Ventilation-Perfusion Mismatch: The loss of lung elasticity causes small airways to close prematurely, especially during exhalation. This creates an imbalance between ventilation (airflow) and perfusion (blood flow), increasing the physiological dead space and decreasing oxygenation.
PFT Values That Decline with Age
Several key PFT measurements predictably decrease with advancing age due to the physiological changes described above. The rate of decline can vary based on individual factors such as sex, height, and smoking history.
Forced Expiratory Volume in 1 Second ($FEV_1$)
This measures the amount of air a person can exhale during the first second of a forceful breath. $FEV_1$ is one of the most consistently measured PFT values that declines with age. After peaking in the mid-20s, the decline is roughly 30 mL/year in healthy non-smokers and can be significantly faster in smokers. The rate of decline also tends to accelerate after age 70.
Forced Vital Capacity ($FVC$)
$FVC$ measures the total amount of air exhaled during a maximal forceful breath. It also declines with age due to decreased lung elasticity and muscle strength, reducing the total volume of air that can be moved. The rate of decline in $FVC$ is often slightly slower than that of $FEV_1$, contributing to the change in the $FEV_1/FVC$ ratio.
$FEV_1/FVC$ Ratio
The ratio of $FEV_1$ to $FVC$ naturally decreases with age because the decline in $FEV_1$ is typically more rapid than the decline in $FVC$. This is a crucial consideration when diagnosing obstructive lung diseases like COPD. Using a fixed ratio (e.g., $FEV_1/FVC < 0.70$) for older adults can lead to an overdiagnosis of airway obstruction, as a low ratio may simply reflect the normal aging process.
Diffusing Capacity of the Lung for Carbon Monoxide ($DL_{CO}$)
$DL{CO}$ measures the ability of the lungs to transfer gas from inhaled air to the bloodstream. As people age, the alveolar surface area decreases and capillary density can be reduced, causing a decline in $DL{CO}$. This indicates a less efficient gas exchange, which can contribute to lower arterial oxygen levels in older individuals.
Maximal Inspiratory and Expiratory Pressures ($MIP/MEP$)
These measurements reflect the strength of the respiratory muscles. Both maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) decrease with age as respiratory muscle strength diminishes. This reduction is more pronounced in men than in women.
Comparison of Key PFT Changes with Age
| PFT Value | Direction of Change | Underlying Cause(s) |
|---|---|---|
| Forced Expiratory Volume in 1s ($FEV_1$) | ↓ Decrease | Loss of lung elastic recoil, weakened respiratory muscles |
| Forced Vital Capacity ($FVC$) | ↓ Decrease | Reduced lung elasticity, increased chest wall stiffness, weakened muscles |
| $FEV_1/FVC$ Ratio | ↓ Decrease | $FEV_1$ declines faster than $FVC$ |
| Diffusing Capacity of the Lung ($DL_{CO}$) | ↓ Decrease | Reduced alveolar surface area, decreased capillary blood volume |
| Residual Volume ($RV$) | ↑ Increase | Premature airway closure due to lost elastic recoil traps air |
| Functional Residual Capacity ($FRC$) | ↑ Increase | Shift in the balance between lung and chest wall recoil |
| Total Lung Capacity ($TLC$) | → Unchanged | The increase in RV is offset by the decrease in FVC |
| Maximal Inspiratory Pressure ($MIP$) | ↓ Decrease | Weakening of inspiratory muscles like the diaphragm |
| Maximal Expiratory Pressure ($MEP$) | ↓ Decrease | Weakening of expiratory and abdominal muscles |
Interpreting PFTs in Older Adults
Because age-related changes can mimic early-stage lung disease, clinicians must use age-specific reference equations when interpreting PFTs in older patients. The interpretation requires considering individual variables such as age, sex, height, and ethnicity, as well as the patient's specific symptoms and clinical history. For example, the American Thoracic Society recommends defining obstruction with an $FEV_1/FVC$ below the 5th percentile for the adult population, a more conservative definition than the fixed ratio often used. Additionally, older adults may limit their physical activity to avoid symptoms like dyspnea, making it easy to overlook a respiratory problem. A low threshold for PFT testing is often recommended.
Conclusion
While a decline in PFT values like FEV1, FVC, and DLCO is an expected part of normal aging, it is important to accurately interpret these changes in the context of a patient's overall health. This decline is primarily driven by the loss of lung elastic recoil, stiffening of the chest wall, and weakening respiratory muscles. Measures of residual volume and functional residual capacity, in contrast, tend to increase. For clinicians, using age-adjusted reference values is crucial to avoid misdiagnosis, especially for conditions like COPD. By considering the full clinical picture, healthcare providers can differentiate the effects of aging from underlying pathology and provide appropriate care for their older patients.
Disclaimer: This article provides general information and is not a substitute for professional medical advice. Always consult a healthcare professional for diagnosis and treatment.
