Currently, NIV(Noninvasive ventilation ) is widely used in patients with hypoxemia and atelectasis after cardiac surgery.About 15% of patients have intolerance during NIV. The main manifestation is that the patient feels uncomfortable, often breaks away from the mask, and even gradually becomes agitated, which eventually leads to the failure of non-invasive ventilation.The patient's intubation rate is as high as 44%, and the mortality rate is 34%.There are many ways to prevent or treat NIV intolerance, of which the most used is to sedate the patient. This study aims to evaluate the effects of remifentanil and dexmedetomidine in the sedation of patients with intolerance to non-invasive mechanical ventilation after cardiac surgery, and to explore the optimal plan for sedation of patients with intolerant non-invasive mechanical ventilation after cardiac surgery.

Inclusion Criteria:

• Sign the informed consent form voluntarily and follow the plan requirements;
• Age from 18 to 75 years old (including 18 and 75 years old), regardless of gender;
• Patients who received non-invasive mechanical ventilation after cardiac surgery and were intolerant of non-invasive ventilation.

Exclusion Criteria:

• A history of allergy to any component of the study drug;
• Visual analogue scale of pain (VAS) score >4；
• Who had taken dexmedetomidine within 8 hours prior to the study;
• Who had used remifentanil within 2 hours before the study began;
• Expectoration difficulties;
• Women who are pregnant or lactating;
• Coma or uncontrollable convulsions;
• Have a history of mental illness or cognitive impairment;
• Delirium before the study began;
• Patients with severe liver dysfunction (CTP grade C);
• Patients with renal insufficiency (patients receiving renal replacement therapy);
• Preoperative left ventricular ejection fraction (LVEF) < 30%;
• History of drug and alcohol abuse;
• Other conditions that the investigator deems inappropriate for participation in this clinical trial.