Respiratory Diseases

First post for PHYL3002 after the study break! This post will touch on a lot of topics that I've spoken about before.

Types of Lung Disease

Lung diseases can be classified into several different categories: infections, tumours, chronic and pulmonary vascular diseases. For this post, we will be mainly looking at chronic lung diseases, like asthma and COPD.

There are also several other types of respiratory disease that don't necessarily involve the lung. For example, haematological disorders such as anaemia can reduce the oxygen-carrying capacity of the blood. Neuromuscular diseases can also impact the diaphragm and other muscles involved in breathing.

Back to chronic lung diseases! Chronic lung diseases can be broken down into two main categories: restrictive disease (in which compliance is reduced and changes in lung volume are reduced) and obstructive disease (in which resistance is increased and flow rate is reduced). Obstructive disease can then be broken down into reversible (i.e. responds well to bronchodilators) and non-reversible. An example of a restrictive disease is pulmonary fibrosis, an example of a reversible obstructive disease is asthma and an example of non-reversible obstructive disease is COPD. (I'll go into all of these in more detail).

Lung Function Tests

As mentioned above, restrictive diseases are marked by reduced compliance and obstructive diseases are marked by increased resistance. To directly measure compliance, you need to know intrapleural pressure, whereas to directly measure resistance, you need to know alveolar pressure. Unfortunately, these parameters are difficult to measure. Plethysmography, which uses a "body box," can measure these, but plethysmographs are expensive and not commonly found. Hence, we have to use other measures to help us monitor these diseases.

Firstly, we can use spirometry to measure different volumes. I've explained spirometry and the values that it can measure here. I've also described the pattern of lung volume changes in obstructive vs. restrictive diseases here.

Spirometry can't measure all lung volumes- we need to use the helium dilution technique to measure functional residual capacity (and by extension residual volume and total lung capacity), as explained here. Sometimes a little bit of CO is used at the same time in order to determine the diffusion capacity of the lung. This is because CO has similar diffusion properties to O₂. (The amount of CO used is obviously low so we don't kill the patient while we're at it.) This gives us a value called D_LCO, or the CO diffusion capacity of the lung.

Commonly used indices for resistance include PEF (peak expiratory flow), FEV₁, FEV₁/FVC ratio and flow-volume loops. I've discussed PEF and flow-volume loops, as well as how they change in disease states, here. FEV₁ was discussed here.

Blood gases can also be tested in order to test lung function. pH, arteriolar O₂ and CO₂ can all be measured. These measurements can also be used to calculate AaDO₂, or the alveolar-arterial oxygen difference.

Yet another test that can be done is the nitrogen washout test. This tests whether or not the alveoli have filled up relatively evenly. In the nitrogen washout test, the patient inspires 100% oxygen and then exhales. Normally, the graph of expired N₂ will start pretty flat, as the alveolar dead space will still be filled with the 100% oxygen that was just inhaled. As exhalation continues, though, the expired N₂ will increase. Eventually, when the alveoli are reached, a plateau phase will be reached in which the exhaled N₂ remains constant for a bit. If there is non-uniform ventilation, however (i.e. the alveoli do not fill up evenly), there will be no plateau phase: instead the expired N₂ will increase steadily.

Examples of Lung Diseases

Asthma

See previous post: The Respiratory System

Asthma is a reversible obstructive disease marked by airway wall thickening, smooth muscle thickening, mucus production and infiltration by inflammatory cells, such as eosinophils. As mentioned above, it is considered to be reversible because it can be helped by bronchodilators.

Bronchitis

See previous posts: COPD and Respiratory Pathophysiology 1

Another thing to be aware of is that broncholitic asthma is a type of chronic bronchitis that can see some improvements via a bronchodilator. It is possible, however, that broncholitic asthma is simply chronic bronchitis and asthma together in the same person.

Emphysema

See previous posts: COPD and Respiratory Pathophysiology 1

COPD

COPD is basically an umbrella term that covers bronchitis and emphysema. Both can cause fun stuff like obstruction, V'/Q' mismatch, pulmonary hypertension and right heart failure.

Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis, in contrast to the diseases covered above, is a restrictive disease. As the name suggests, it is the formation of scar tissue (fibrosis) in the lung for no apparent reason ("idiopathic" is a fancy term for saying "we don't know"). While many lung volumes, like TLC, FRC, RV and FVC can fall, FEV₁/FVC ratio is often increased in this group (due to a normal FEV₁ and reduced FVC).