inclusion criteria: * the age ranges from 18 to85 (inclusive), regardless of gender. * patients who still have refractory hypoxia after sufficient standard treatment for severe and critical covid-19 pneumonia following the suggestion of the chinese diagnostic criteria for covid-19 pneumonia in the diagnosis and treatment plan for covid-19 pneumonia (tentative tenth edition). * 20kg/m2≤body mass index(bmi)≤30 kg/m2. * volunteer to participate in this clinical study and sign the written informed consent

inclusion criteria: * the age ranges from 18 to85 (inclusive), regardless of gender. * patients who still have refractory hypoxia after sufficient standard treatment for severe and critical covid-19 pneumonia following the suggestion of the chinese diagnostic criteria for covid-19 pneumonia in the diagnosis and treatment plan for covid-19 pneumonia (tentative tenth edition). * 20kg/m2≤body mass index(bmi)≤30 kg/m2. * volunteer to participate in this clinical study and sign the written informed consent

inclusion criteria: the age ranges from 18 to85 (inclusive), regardless of gender. patients who still have refractory hypoxia after sufficient standard treatment for severe and critical covid-19 pneumonia following the suggestion of the chinese diagnostic criteria for covid-19 pneumonia in the diagnosis and treatment plan for covid-19 pneumonia (tentative tenth edition). 20kg/m2≤body mass index(bmi)≤30 kg/m2. volunteer to participate in this clinical study and sign the written informed consent

inclusion criteria: the age ranges from 18 to85 (inclusive), regardless of gender. patients who still have refractory hypoxia after sufficient standard treatment for severe and critical covid-19 pneumonia following the suggestion of the chinese diagnostic criteria for covid-19 pneumonia in the diagnosis and treatment plan for covid-19 pneumonia (tentative tenth edition). 20kg/m2≤body mass index(bmi)≤30 kg/m2. volunteer to participate in this clinical study and sign the written informed consent