DLCO 2007 Web version

DLCO 2007 Web version - Diffusion Diffusion...

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Diffusion Diffusion Dr. Gyanendra Agrawal Senior Resident Deptt. of Pulmonary Medicine PGIMER, Chandigarh
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Diffusion Diffusion Primary function of lung–gas exchange Movement of gas across the blood gas interface is by simple passive diffusion “Net transfer of molecules from a zone with high partial pressure to a zone with lower partial pressure” Partial pressure v/s concentration gradient
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Components of diffusion pathway Components of diffusion pathway Gas space within the alveolus Alveolar lining fluid–surfactant rich Tissue barrier – alveolar capillary membrane Plasma layer Diffusion into and within the RBC Uptake of CO by hemoglobin
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Transfer factor Transfer factor CO uptake reflects number of processes–not just diffusion CO uptake is a submaximal value and not truly a ‘capacity’ So, TLco – probably better term
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Indications Indications Specific indications not defined variety of testing procedures in use complexity of physiologic determinants of CO uptake Most commonly used in evaluation of diffuse interstitial lesions suspected emphysema pulmonary vascular obstruction Useful in diagnosis as well as follow up
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Why carbon monoxide ?? Why carbon monoxide ?? Gas that is more soluble in blood than in lung tissues (O 2 and CO) High affinity for hemoglobin (250 times of O 2 ) Transfer is diffusion limited rather than perfusion limited Not present in lung normally CO diffusion is less affected by other factors DL O 2 = DLco x 1.23
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Measuring diffusion capacity Measuring diffusion capacity DLco = CO transferred from alveoli to blood (ml/min) mean alveolar CO pressure–mean capillary CO pressure (mm Hg) Five methods: (1) Single breath method (2) Steady state method (3) Rebreathing method (4) Three Gas Iteration method (5) Intra breath method
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Single breath method Single breath method Most widely used and best standardized of the various methods System design: source of test gas method of measuring inspired and expired vol over time gas analysers System types: with alveolar sample bags with rapid gas analyzer
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Systems with Alveolar Sample Bag Systems with Alveolar Sample Bag Test gas 10% helium (He) 0.3% carbon monoxide (CO) 21% oxygen (O 2 ) Balance nitrogen (N 2 ) Breath hold for approximately 10 sec After breath hold, wash out dead space Collect alveolar sample
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Systems with Continuous Systems with Continuous Rapid Analysis Rapid Analysis Test gas contains methane (CH 4 ) as trace gas Uses rapid gas analyzer to continuously analyze expired gas sample
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Performing the Test Performing the Test Patient connects to instrument Unforced exhalation to RV Valve is opened to test gas Patient rapidly inhales a full breath of test gas Holds breath for about 10 sec Patient exhales without max force After dead space is cleared, sample is collected/analyzed
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DLCO 2007 Web version - Diffusion Diffusion...

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