The investigators aim to evaluate the correlation between PetCO2, PaCO2, and PtcCO2 in patients with respiratory diseases. The investigators also evaluate the correlation between PetCO2, PaCO2, and PtcCO2 changes over time.

Partial pressure of carbon dioxide(PCO2) is mainly evaluated by partial pressure of arterial carbon dioxide (PaCO2), and is used as an index for prognosis and NPPV introduction in chronic respiratory failure. Arterial blood gas analysis for PaCO2 measurement is the most accurate test value, but it is invasive and frequent tests are difficult and cannot be performed at home. The transcutaneous PCO2 (PtcCO2) is a non-invasive measurement and its measured value is close to PaCO2, and it can be measured continuously, which is useful for monitoring patients with chronic respiratory failure. However, PtcCo2 has not been applied at home due to technical problems (necessity of calibration, periodic drift, etc.). The end-tidal pressure of expired CO2 (PetCO2) can be measured non-invasively, easily and continuously. PetCO2 is mainly used for patients undergoing mechanical ventilation, and its accuracy and stability have been evaluated. The investigators decided to investigate whether PetCO2 is useful for monitoring respiratory status in patients with respiratory disorder.

Inclusion Criteria:

• Subjects with respiratory disorder.
• Subjects who are clinically stable for the last 2 weeks

Exclusion Criteria:

• Subjects who required treatment with antibacterial agents or oral/intravenous steroids for pneumonia or exacerbation of respiratory disease in the last 2 weeks.
• Subjects with severe complications such as cardiovascular disease, liver disease, renal disease, and neurological disease.

Subjects with respiratory disease such as COPD, IP