Diffusing capacity

Diffusing capacity of the lung (DL) (also known as Transfer factor is another expression for the formerly used diffusing capacity.) measures the transfer of gas from air in the lung, to the red blood cells in lung blood vessels. It is part of a comprehensive series of pulmonary function tests to determine the overall ability of the lung to transport gas into and out of the blood. DL, especially DLCO, is reduced in certain diseases of the lung and heart. DLCO measurement has been standardized according to a position paper by a task force of the European Respiratory and American Thoracic Societies. V ˙ C O = Δ [ C O ] ∗ V A Δ t {displaystyle {dot {V}}_{CO}={frac {Delta {}*V_{A}}{Delta {t}}}} .     ( 4 ) P A C O = V B ∗ F A C O O {displaystyle P_{A_{CO}}=V_{B}*F_{A_{CO_{O}}}} .     ( 5 ) D L O 2 = V ˙ O 2 P A O 2 − P a O 2 ≃ V ˙ O 2 P A O 2 − P v O 2 {displaystyle D_{L_{O_{2}}}={frac {{dot {V}}_{O_{2}}}{P_{A_{O_{2}}}-P_{a_{O_{2}}}}}simeq {frac {{dot {V}}_{O_{2}}}{P_{A_{O_{2}}}-P_{v_{O_{2}}}}}}     ( 1 ) D L C O = V ˙ C O P A C O {displaystyle D_{L_{CO}}={frac {{dot {V}}_{CO}}{P_{A_{CO}}}}} .     ( 2 ) 1 D L C O = 1 D M + 1 θ ∗ V c {displaystyle {frac {1}{D_{L_{CO}}}}={frac {1}{D_{M}}}+{frac {1}{ heta *V_{c}}}} .     ( 3 ) Diffusing capacity of the lung (DL) (also known as Transfer factor is another expression for the formerly used diffusing capacity.) measures the transfer of gas from air in the lung, to the red blood cells in lung blood vessels. It is part of a comprehensive series of pulmonary function tests to determine the overall ability of the lung to transport gas into and out of the blood. DL, especially DLCO, is reduced in certain diseases of the lung and heart. DLCO measurement has been standardized according to a position paper by a task force of the European Respiratory and American Thoracic Societies. In respiratory physiology, the diffusing capacity has a long history of great utility, representing conductance of gas across the alveolar-capillary membrane and also takes into account factors affecting the behaviour of a given gas with hemoglobin. The term may be considered a misnomer as it represents neither diffusion nor a capacity (as it is typically measured under submaximal conditions) nor capacitance. In addition, gas transport is only diffusion limited in extreme cases, such as for oxygen uptake at very low ambient oxygen or very high pulmonary blood flow. The diffusing capacity does not directly measure the primary cause of hypoxemia, or low blood oxygen, namely mismatch of ventilation to perfusion: The single-breath diffusing capacity test is the most common way to determine D L {displaystyle D_{L}} . The test is performed by having the subject blow out all of the air that they can, leaving only the residual lung volume of gas. The person then inhales a test gas mixture rapidly and completely, reaching the total lung capacity as nearly as possible. This test gas mixture contains a small amount of carbon monoxide (usually 0.3%) and a tracer gas that is freely distributed throughout the alveolar space but which doesn't cross the alveolar-capillary membrane. Helium and methane are two such gasses. The test gas is held in the lung for about 10 seconds during which time the CO (but not the tracer gas) continuously moves from the alveoli into the blood. Then the subject exhales. The anatomy of the airways means inspired air must pass through the mouth, trachea, bronchi and bronchioles (anatomical dead space) before it gets to the alveoli where gas exchange will occur; on exhalation, alveolar gas must return along the same path, and so the exhaled sample will be purely alveolar only after a 500 to 1,000 ml of gas has been breathed out. While it is algebraically possible to approximate the effects of anatomy (the three-equation method), disease states introduce considerable uncertainty to this approach. Instead, the first 500 to 1,000 ml of the expired gas is disregarded and the next portion which contain gas that has been in the alveoli is analyzed. By analyzing the concentrations of carbon monoxide and inert gas in the inspired gas and in the exhaled gas, it is possible to calculate ( D L C O ) {displaystyle (D_{L_{CO}})} according to Equation 2. First, the rate at which CO is taken up by the lung is calculated according to: