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Chapter 3 – The pulmonary function laboratory
Figure 3.4 Typical system configuration for the measurement of rebreathing pulmonary diffusing capacity.
As the patient rebreathes the gas mixture in the bag, a modified mass spectrometer
continuously analyzes it during both inspiration and expiration. During this rebreathing
procedure, the carbon monoxide disappears from the patient-bag system; the rate at which this
occurs is a function of the lung diffusing capacity. The helium is inert and insoluble in lung
tissue and blood and equilibrates quickly in unobstructed patients, indicating the dilution level
of the test gas. Acetylene, on the other hand, is soluble in blood and is used to determine the
blood flow through the pulmonary capillaries. Carbon monoxide is bound very tightly to
hemoglobin and is used to obtain diffusing capacity at a constant pressure gradient across the
alveolar-capillary membrane. Decreased lung diffusing capacity can occur from the
thickening of the alveolar membrane or the capillary membrane as well as the presence of
interstitial fluid from edema. All these abnormalities increase the barrier thickness and cause a
decrease in diffusing capacity. In addition, a characteristic of specific lung diseases is
impaired lung diffusing capacity. For example, fibrotic lung tissue exhibits a decreased
permeability to gas transfer, whereas pulmonary emphysema results in the loss of diffusion
surface area.
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