Obstructive lung diseases
Obstructive lung diseases are usually caused by increased airflow resistance, usually due to an obstruction at some point. Four types are important:
- Chronic bronchitis
Emphysema and chronic bronchitis usually occur together, so they’re together called chronic obstructive pulmonary diseases, or COPD. They occur together because the etiology for both is the same: smoking.
The airway obstruction means that these patients struggle to breathe air out. Their FEV1 / FVC ratio (the Tiffeneau index), which measures how much of their total lung volume they can breathe out in one second, is significantly decreased.
Emphysema is characterised by abnormal permanent enlargement of alveolar spaces because alveolar septa are destroyed. This causes the lung to become abnormally inflated.
We must recall the structure of acini in the lung. These structures are comprised of a respiratory bronchiole, alveolar duct and the alveoli themselves. You can see the structure on the figure below.
Now, we can talk about morphology. Four types exist, of which two are important:
- Centriacinar emphysema affects the respiratory bronchioles but not the alveoli themselves. This type is associated with smoking. It’s more common in the upper lobes of the lung. Smoking causes neutrophils to produce enzymes that degrade the alveolar wall, like elastase.
- Panacinar emphysema affects the alveolar duct and alveoli. This type is associated with a genetic α1-antitrypsin deficiency, and not smoking. α1-antitrypsin is a protein that usually inhibits the effects of elastase. When this inhibitor is deficient will elastase produced by neutrophils act unopposed to destroy the connective tissue of the lung. It’s more common in the lower lobes of the lung.
Structure of acini and the two most important types of emphysema. Remember that the centriacinar type is associated with cigarettes.
The obstruction in emphysema occurs when the walls of terminal bronchioles also lose elastic tissue which causes them to collapse during expiration. This “traps” air in the alveoli, which causes the lungs to become hyperinflated.
The first symptom is dyspnoea. The hyperinflation causes the lung to expand, and the chest with it. The antero-posterior diameter of the chest increases, which gives the morphology called barrel chest. The destruction of the alveolar wall also decreases the total alveolar surface available for gas exchange, which decreases the diffusion capacity. Patients with emphysema are often hunched over, as they try to use accessory breathing muscles to help force the air out. Skinny, barrel-chested, hunched-over, dyspnoeic people are classic emphysema patients, often called pink puffers. Coughing is not a characteristic symptom.
Hypoxia-induced pulmonary vasoconstriction causes pulmonary hypertension with eventual cor pulmonale.
While the diagnosis of emphysema is based on morphology is the diagnosis of chronic bronchitis based on clinical features. Chronic bronchitis is defined as the presence of a persistent productive (“wet”) cough for at least 3 consecutive months in at least 2 consecutive years.
Chronic bronchitis is most commonly caused by smoking but can also be caused by air pollution and dust exposure. The major pathological changes are mucous gland hypertrophy and hyperplasia and hypersecretion of mucus, which obstructs the airways. The mucosa is often oedemic and covered by mucinous and possibly purulent secretion.
The ratio of mucous glands to the whole thickness of the wall in the bronchi is called the Reid index. It’s usually 0.4 (meaning that 40% of the whole thickness of the bronchial wall is made up of mucous glands), but in chronic bronchitis the glands are hyperplastic and hypertrophic, so this number is increased.
People suffering from chronic bronchitis can have episodes of acute worsenings of the symptoms, called acute exacerbations. These episodes can be caused by bacterial infection, like by haemophilus, or by viruses, allergens or smoke.
The most important symptom of chronic bronchitis is productive cough, initially without outflow obstruction. Only some patients develop severe enough obstruction that it starts to produce symptoms like respiratory failure. These patients are often obese and cyanotic, which has given them the nickname blue bloaters.
Bronchial asthma, or simply just asthma, is a chronic inflammatory disorder of the airways that causes recurrent episodes of wheezing, breathlessness and cough. It’s characterised by intermittent and reversible airway obstruction, eosinophilic inflammation of the bronchi, bronchial smooth muscle hypertrophy and hyperreactivity, and increased mucus secretion.
We distinguish two types: atopic and nonatopic asthma.
Atopic (extrinsic) asthma is associated with allergy and eczema and is often hereditary. It involves a hypersensitivity type I reaction, which has two phases: the early phase and the late phase. In the early phase will there be bronchoconstriction and increased mucus production, while in the late phase is characterised by inflammation. When these reactions occur repeatedly will this lead to structural changes in the bronchial wall, called airway remodelling, which is when smooth muscle and mucus gland hypertrophy occurs.
Nonatopic (intrinsic) asthma has no association with allergy, and the pathomechanism is unknown. Viral infection may trigger it. It usually begins in adulthood.
An asthmatic attack involves severe dyspnoea with wheezing; the main difficulty lies in expiration because the bronchial smooth muscle is spasming. This spasm can be relieved by bronchodilators. Between these episodes are patients usually free from significant obstruction.
Bronchiectasis means permanent dilation of the bronchi that’s caused by destruction of the smooth muscle and supporting elastic tissue. It’s a secondary condition that usually occurs due to chronic necrotizing infections, often due to:
- Bronchial obstruction
- Foreign bodies
- Thick mucus (cystic fibrosis)
- Necrotizing bacterial infections
- Immunodeficient people
- Staphylococcus aureus pneumonia
- Klebsiella pneumonia
Obstruction interferes with the airways’ clearing mechanisms. This makes the airways a much better place for infections, causing chronic necrotizing infections. Proteolytic enzymes and reactive oxygen species produced by leukocytes then damage the walls.
The most distal airways are most severely dilated, up to 4x their normal diameter. A common finding during autopsy is that the airways can be opened all the way to the periphery of the lung, because the distal airways are so dilated. In healthy lungs are the distal airways so small that they can’t be opened during autopsy.
The most common clinical symptom is severe, persistent productive cough with purulent sputum, possibly haemoptysis. Abscesses and empyema of the airways can occur.
78. Infectious disorders of the lower airways
80. Chronic restrictive lung diseases