Effects of Aging on the Respiratory System

Introduction

1. Structural / Anatomical Changes

A. Lung Parenchyma ("Senile Emphysema")

• Loss of elastin fibers occurs with age. Unlike elsewhere in the body, this elastin is NOT replaced by collagen, making the lung more compliant (less stiff).
• Alveolar walls thin and alveolar ducts enlarge - a process resembling emphysema but without inflammatory destruction (hence "senile emphysema").
• Alveolar surface area decreases, reducing available area for gas exchange.
• The lung's reduced elastic recoil is the single most consequential structural change, with downstream effects on airways, volumes, flow rates, and gas exchange.

B. Chest Wall

• Calcification of costal cartilages and intervertebral discs → increased chest wall stiffness (reduced compliance).
• Kyphosis develops from vertebral changes and loss of disk height.
• The diaphragm flattens and its mechanical advantage is reduced.
• These changes directly oppose the increased lung compliance, with net effect of reducing efficiency of the breathing pump.

C. Airways

• Conducting airways: Central airway diameter increases slightly with age, but small peripheral airways lose structural support as surrounding lung elastic tissue weakens. Small airways depend on elastic tethering by surrounding parenchyma to remain patent - as this support diminishes, they collapse at higher lung volumes.
• Ciliary function: Ciliary motility in both upper and lower airways decreases significantly with aging, impairing mucociliary clearance and increasing susceptibility to infection.
• Upper airway: Soft tissue laxity of the pharynx, weakened genioglossal and hypopharyngeal muscles predispose to upper airway obstruction during sleep.

2. Lung Volumes and Capacities

Clinical Significance of Closing Capacity

• In the supine position, CC exceeds FRC by approximately age 44
• In the upright position, CC exceeds FRC by approximately age 65

3. Flow Rates and Spirometry

• FEV1: Declines approximately 25-30 mL/year after age 25-30 in men, slightly less in women.
• FVC: Also declines, but less steeply than FEV1.
• FEV1/FVC ratio: Decreases slightly, reflecting primarily small airway collapse limiting expiratory flow.
• Peak Expiratory Flow Rate (PEFR): Declines with age.
• Mid-expiratory flow rates (FEF 25-75%): Reduced, reflecting small airway disease.
• Flow-volume loop: Shows progressive flow limitation during forced exhalation - in a 70-year-old, the last ~45% of exhalation is flow-limited vs. only ~20% in a 30-year-old.

4. Gas Exchange

A. PaO2 Decline

• The alveolar-arterial oxygen gradient (A-a gradient) increases with age due to V/Q mismatch.
• Estimated normal PaO2 = 100 - (age/3) mmHg (a commonly used approximation).
• In an 80-year-old, a PaO2 of 70 mmHg may be "normal," whereas in a young adult it would indicate pathology.

B. PaCO2

• PaCO2 is maintained throughout life in healthy elderly, because increased dead space ventilation is compensated by a slight increase in total minute ventilation. Any rise in PaCO2 with aging suggests disease.

C. Mechanisms of V/Q Mismatch

• Loss of alveolar surface area
• Reduced elastic tethering of small airways → uneven ventilation distribution
• CC exceeding FRC → dependent zone airway closure during tidal breathing
• These changes are modest at rest but become clinically significant during exercise or illness.

5. Respiratory Muscle Function

• Diaphragm and accessory muscles undergo age-related sarcopenia (muscle mass loss).
• Maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) decline by ~25% between ages 20 and 70.
• Increased chest wall stiffness and flattened diaphragm further reduce mechanical efficiency.
• Net result: increased work of breathing at rest and greatly increased work during any stress.
• Elderly patients fatigue more quickly when minute ventilation must increase (e.g., fever, pain, pneumonia), predisposing to respiratory failure.

6. Ventilatory Control / Chemosensitivity

• Hypoxic ventilatory response (HVR) decreases by approximately 50% with aging (some studies suggest even more at night).
• Hypercapnic ventilatory response (HCVR) decreases by approximately 40-50% with aging.
• The reduced response is not explained by reduced muscle strength or mechanics alone - airway occlusion pressure (P0.1) measurements confirm a reduced central ventilatory drive.
• The mechanism is debated: may be peripheral chemoreceptor dysfunction (carotid body) or impaired central respiratory center processing.
• Clinical consequence: Elderly patients may NOT increase minute ventilation appropriately in the face of hypoxemia or hypercapnia - resulting in blunted recognition of respiratory failure, delayed presentation, and increased perioperative risk.

7. Upper Airway and Protective Reflexes

• Cough reflex: Threshold elevated and peak cough flow reduced. Reduced ability to clear secretions.
• Swallowing reflex: Delayed and less coordinated. The sensorimotor cortex and brainstem circuits governing swallowing both show age-related changes.
• Result: Greatly increased risk of aspiration, a significant cause of community-acquired pneumonia in the elderly and a major contributor to postoperative pulmonary complications.
• Sleep-disordered breathing: Up to 75% of people over age 65 have some degree of sleep-disordered breathing, related to pharyngeal muscle laxity. This places them at heightened postoperative hypoxia risk.

8. Pulmonary Circulation

• Pulmonary vascular resistance increases slightly with age due to vessel wall changes (intimal fibrosis, media thickening).
• Pulmonary arterial pressure rises modestly at rest, more markedly with exercise.
• Pulmonary capillary bed diminishes with alveolar surface area reduction.
• These changes may limit maximal cardiac output and contribute to exercise intolerance.

9. Defense Mechanisms and Immunity

• Mucociliary clearance is impaired (reduced ciliary beat frequency and mucociliary transport rate).
• Alveolar macrophage function declines, reducing phagocytic capacity.
• Immunosenescence (reduced T-cell function, impaired antibody responses) increases susceptibility to respiratory infections (influenza, pneumococcal pneumonia, tuberculosis reactivation).
• Overall respiratory defense is compromised at multiple levels.

10. Summary Table of Key Changes

11. Clinical Implications

1. Perioperative risk: Elderly patients require longer pre-oxygenation, higher FiO2 intraoperatively, PEEP, and aggressive pulmonary toilet.
2. Drug effects: Sensitivity to respiratory depressants (opioids, benzodiazepines) is exaggerated because baseline ventilatory reserve is low.
3. Disease threshold: Age-related functional loss can be unmasked by even mild disease (pneumonia, COPD exacerbation) triggering frank respiratory failure.
4. Spirometric interpretation: Reference values must be age-adjusted. A "normal" FEV1 in a young adult may represent significant disease in an elderly patient.
5. Oxygen therapy: Standard pulse oximetry targets (SpO2 > 94%) must be maintained in elderly patients because their reduced HVR means they cannot compensate autonomously for desaturation.

Conclusion

• Fishman's Pulmonary Diseases and Disorders, 2-Volume Set - Chapter 18: Aging of the Respiratory System
• Barash's Clinical Anesthesia, 9e - Chapter 34: Pulmonary Aging
• Morgan & Mikhail's Clinical Anesthesiology, 7e - Chapter 43: Respiratory System