What is diffusing capacity and why is it important?

The diffusing capacity of the lung (as a whole) is often used as an indicator of clinical conditions, and, in a clinical or laboratory environment, carbon monoxide is used to determine this measure. In this video from our Blood Gas Analysis Essentials course, we'll take a look at the importance of diffusing capacity as a measure of gas exchange and consider the clinical factors that affect it.

Master arterial blood gas analysis based on an understanding of relevant physiological principles. You’ll cover the crucial factors that determine the oxygenation of blood in the lungs, as well as oxygen transport and delivery to peripheral tissues. Learn about the interplay between blood gas and acid-base analysis, and how carbon dioxide affects arterial pH. This course complements our Acid-Base Essentials course.

So far, we've only spoken about the diffusing capacity at the alveolar capillary membrane. But often in the clinic, we use the diffusing capacity of the lung as a whole as an indicator of clinical conditions. To measure diffusing capacity of the lung in the lab or clinic, we use carbon monoxide and measure the ability of the body to absorb carbon monoxide from a single breath.

This is called the lung carbon monoxide diffusing capacity or DLCO. And as we have seen, the flux of a gas or diffusion of the gas across a membrane is dependent on factors that affect the rate of diffusion in the surface area where diffusion is taking place. When DLCO is below the predicted reference range, it becomes a clue to the presence of a physiologic problem.

But a variety of physiological factors can affect the value normally measured for DLCO. Increases in DLCO occur with increases in body size, such as in patients with increased height, increased body weight, and increased body surface area. DLCO increases with increasing lung volume, likely due to increased blood volume or increased alveolar capillary membrane surface area.

The ratio of DLCO to alveolar volume, known as crows constant, may be used as an index in patients with emphysema-related lung hyperinflation to indicate the primary effect of the disease on gas exchange. As a person ages, DLCO increases to a maximum at about 20 years and then declines about 2% per year thereafter.

For comparable age and body size, women have a lower DLCO than men by about 10%. DLCO increases during exercise. In a healthy individual, pulmonary vessels dilate during exercise, a process known as recruitment, increasing the area for diffusion. DLCO is higher in the supine position than in the sitting position. DLCO is also higher in the sitting position compared with standing.

The likely explanation is increases in capillary blood volume with sitting versus standing, and with lying versus sitting. But now that you know what to watch out for, you can use DLCO as a sign of something wrong. Let's take a look at some of the most important examples. In patients with chronic obstructive pulmonary disease or COPD, DLCO is reduced.

The changes are more profound in patients with emphysema than in those with chronic bronchitis, likely related to greater loss of alveolar surface area in emphysema and, therefore, less area for gas diffusion. Restrictive pulmonary diseases are a broad category that includes not only diseases influencing the chest wall, pleural space, and respiratory muscles, but also pulmonary parenchymal disorders.

However, diffusing capacity is generally preserved in restrictive diseases, unless the pulmonary parenchyma is also affected. Widening of the pulmonary interstitium or flooding of the alveoli with fluid impairs gas movement across the alveolar capillary membrane and is reflected in a decreased DLCO. Thrombotic or embolic occlusion of branches of the pulmonary artery reduces DLCO, as do disorders of the pulmonary capillaries, for example, vasculitis.

Severe pulmonary hypertension may be associated with obliteration of the pulmonary capillaries, and a reduction in DLCO. Acute alveolar hemorrhage is associated with an increase in DLCO. This is secondary to hemoglobin in the alveoli, serving as a sink for carbon monoxide used during measurement.

When alveolar hemorrhage is chronic or recurrent, resulting lung fibrosis reduces DLCO. Through an increase in capillary red blood cells, polycythaemia results in an increase in DLCO. Anemia with reduced red cell mass reduces DLCO. After all this attention on O2 What about CO2 exchange across the lung in health and disease. The solubility of CO2 in tissues is about 20 times that of O2, and the rate of diffusion across the alveolar capillary membrane is about 20 times that of O2.

Therefore, mild abnormalities of the pulmonary parenchyma are usually not associated with an increased arterial alveolar gradient for CO2. With more advanced thickening of the alveolar capillary membrane, and a reduction in DLCO to about 25% or less of normal, an arterial alveolar gradient for CO2 may arise.

So measurement of DLCO provides a sensitive, albeit nonspecific measure of the overall gas exchange function of the lung, and serves as a useful clinical measure in the search for underlying lung disease. Overlooking a reduced DLCO can delay early diagnosis and treatment of a disease.

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