Sit down, rest for 15 minutes, add a nose clip with a mouthpiece, collect exhaled breath after breathing smoothly, and measure the partial pressure of exhaled breath CO2. At the end of exhaled breath collection, take arterial blood or arterialized ear blood to measure PaCO2 _ 2. According to the improved Bohr formula: VD/vt = (PaCO _ 2-PECO _ 2)/PaCO _ 2, the ventilation ratio of the ineffective cavity can be calculated, and then VA = ve× (100-VD/vt)%.