What are the primary causes of hyperinflated lungs?

Primary Causes of Hyperinflated Lungs

The primary causes of hyperinflated lungs include chronic obstructive pulmonary disease (COPD), asthma, airflow obstruction, reduced lung elastic recoil, and body habitus factors, with COPD being the most common pathological cause. 1

Pathophysiological Mechanisms of Lung Hyperinflation

Obstructive Lung Diseases

• COPD is the most common cause of pathological lung hyperinflation, characterized by airflow limitation, reduced elastic recoil, and destruction of lung parenchyma 1
• Emphysema leads to destruction of alveolar walls, resulting in loss of lung elastic recoil, decreased carbon monoxide transfer capacity, increased static lung compliance, and increased total lung capacity 1
• Asthma causes hyperinflation through bronchoconstriction, airway inflammation, and mucus plugging that limit expiratory airflow 2

Dynamic Hyperinflation

• Expiratory flow limitation during tidal breathing is the primary trigger for functional residual capacity (FRC) to increase 2
• When airways narrow, expiration becomes incomplete before the next inspiration begins, leading to air trapping and dynamic hyperinflation 3
• This results in intrinsic positive end-expiratory pressure (PEEPi), which acts as an inspiratory threshold load that must be overcome by respiratory muscles 1

Static Hyperinflation

• Reduction in elastic recoil of the lung due to parenchymal destruction (emphysema) leads to static hyperinflation 4
• This causes permanent enlargement of airspaces distal to terminal bronchioles without obvious fibrosis 1
• Two major patterns of emphysema contribute to hyperinflation: panacinar (entire acinus destruction) and centriacinar (respiratory bronchioles, alveolar ducts, and central alveoli destruction) 1

Clinical Manifestations and Diagnosis

Physical Examination Findings

• Signs of chronic overinflation include loss of cardiac dullness, decreased cricosternal distance, and increased anterior-posterior diameter of the chest 1
• Rhonchi (wheezes), especially on forced expiration 1
• Hyperinflated chest configuration with horizontal ribs and increased intercostal spaces 1

Pulmonary Function Testing

• Abnormal FEV1 (<80% of predicted) with an FEV1/VC ratio of <70% strongly suggests COPD with hyperinflation 1
• Increased residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC) are hallmarks of hyperinflation 5
• Even in mild airway obstruction (GOLD stage I), RV and FRC can be increased to 135% and 119% of predicted values, respectively 5

Imaging

• Chest radiography may show flattened diaphragms, increased retrosternal airspace, and hyperlucent lung fields 1
• However, it's important to note that mild emphysema cannot be reliably diagnosed radiographically 1

Non-Pathological Causes of Apparent Hyperinflation

Body Habitus

• Tall and thin individuals naturally have different chest wall configurations that can mimic hyperinflation on imaging 6
• In these patients, the lungs appear more elongated and the diaphragm sits lower in the chest, creating a radiographic appearance similar to pathological hyperinflation 6
• This should be distinguished from true hyperinflation by correlating with clinical symptoms and pulmonary function tests 6

Physiological Consequences of Hyperinflation

Respiratory Mechanics

• Increased work of breathing due to decreased lung compliance at higher lung volumes 4
• Diaphragmatic flattening and mechanical disadvantage 3
• Increased energy consumption of inspiratory muscles at any given level of minute ventilation 1

Gas Exchange

• Ventilation/perfusion (V'/Q') inequality is the major mechanism impairing gas exchange and leading to arterial hypoxemia 1
• In severe COPD, various abnormal V'/Q' distributions may be found, with some lung units having very high V'/Q' (emphysematous regions) and others having very low V'/Q' (partially blocked airways) 1

Cardiovascular Effects

• Pulmonary vascular resistance increases due to compression of alveolar vessels by hyperinflated lungs 4
• Right ventricular afterload increases, potentially leading to right heart dysfunction 1
• Left ventricular function may be compromised due to mediastinal shift and increased negative intrapleural pressure 4

Clinical Significance

• Hyperinflation is a key determinant of morbidity and mortality in COPD, partially independent of the degree of airflow limitation 7
• It correlates better with symptoms and limited exercise capabilities than airflow measurements in patients with COPD 3
• Severe hyperinflation is the major physiologic cause of hypercapnic respiratory failure and difficulty weaning from mechanical ventilation 3