The subject should breathe calmly for 4-5 cycles after clamping on the nasal clip and holding the mouthpiece, and after the end-tidal baseline is stable, instruct the subject to exhale completely to the residual air volume level, and then make the subject inhale quickly and evenly completely to the total lung volume level, and it is recommended to finish inspiration within 2s. It is recommended that expiration be completed within 2-4s. Pulmonary diffusion index 1, pulmonary carbon monoxide diffusion (DLCO) and pulmonary carbon monoxide diffusion factor (TLCO) pulmonary carbon monoxide diffusion (DLCO) is the amount of CO transferred from the alveoli to the alveolar capillaries and bound to Hb (mL or mmol) per unit time (1min) and per unit pressure difference (1mmHg or 0.133kPa), and its unit is mL/(min. mmHg) or mmol/(min. kPa), is the primary indicator of pulmonary diffusion. Because pulmonary diffusion capacity is not only affected by capillary membrane, but also by capillary blood flow, therefore, some scholars propose to use transfer factor (TL) instead of diffusion volume (DL), and the detection method, unit and meaning are the same as DLCO. 2, alveolar ventilation volume (VA) refers to the effective ventilation volume per minute of inhaled air that can reach the alveoli and gas exchange, used to estimate the lung volume of carbon monoxide in the lung can diffuse and pass through the alveolar capillary membrane, the unit is mL, VA in normal subjects is roughly equal to TLC. 3, lung carbon monoxide diffusion to alveolar ventilation ratio (DLCO/VA), also known as diffusion volume per alveolar volume or specific diffusion volume, because the diffusion volume is affected by alveolar ventilation, a decrease in alveolar ventilation can lead to a decrease in DLCO, so the subject’s alveolar ventilation (VA) should be considered when evaluating diffusion function to exclude the effect of lung volume on diffusion volume, the unit is L/(min. mmHg) or mL/(min. kPa). DLCO/VA is commonly used clinically as a correction, and DLCO/VA makes it easier to distinguish pathophysiological changes in the lungs. However, because the relationship between DLCO and VA is not linear and significantly less than 1:1, it does not accurately correct for volume effects. 4. Carbon monoxide diffusion per liter of alveolar volume (KCO or Krogh factor) The unit is mL/(min. kPa) and has the same meaning as DLCO/VA. 5. Corrected DLCO values (DLCOc) are commonly corrected for hemoglobin (Hb), partial pressure of oxygen of inhaled gas (PIO2) and carboxyhemoglobin (COHb).