The goal of this single-center, prospective, double-blind randomized controlled clinical trial is to explore preventive interventions for chronic postoperative pain (CPSP) in patients undergoing minimally invasive cardiac surgery. The main questions it aims to answer is: the incidence of CPSP is high and severe, and there is no safe and effective method for prevention and treatment. The participants in the dexamethasone palmitate (DXP) group will receive a single paravertebral space infusion of 21 ml of the first dose of analgesia, consisting of 0.5 percentage ropivacaine 20 ml and dexamethasone palmitate 1ml. The participants in dexamethasone sodium phosphate (DXM) group will receive a single infusion of a paravertebral space first dose of 21 ml of analgesic containing 0.5 percentage ropivacaine 20 ml and dexamethasone sodium phosphate 1ml. Then both groups use a continuous paravertebral nerve block pump for postoperative analgesia. The liquid of the pump is 250 ml 0.2 percentage ropivacaine. The background of the pump is set to 5 ml/h and PCA is set to 5 ml at an interval of 30 min.

Minimally invasive cardiac surgery has the advantages of small injury, rapid recovery and international leadership. But the incidence of chronic pain after operation is high, which is reported to be as high as 70 percentage in thoracic intercostal surgery, seriously affecting the quality of life of patients after surgery. Uncontrolled acute pain after surgery poses the greatest risk factor for CPSP. Procedure-specific postoperative pain management (PROSPECT) guidelines recommend that the use of paravertebral nerve block as the first choice for multimodal analgesia in intercostal surgery can significantly reduce the incidence of postoperative acute pain. But there is not enough evidence to prove that it can reduce the incidence of CPSP. Choosing a reasonable combination of analgesics in nerve block to inhibit the production and transmission of multi-source nociceptive signals caused by intercostal nerve injury and inflammatory stimulation is one of the research hotspots to prevent the transformation from acute pain to chronic pain.

Dexamethasone can play an anti-inflammatory effect by reducing the sensitivity caused by neuroinflammation, which has also been confirmed by small sample studies. But a large number of literatures point out that a single use of dexamethasone cannot significantly reduce the incidence of CPSP. This may be due to the fact that the half-life of dexamethasone is only 48 hours, and single use is not enough to exert anti-inflammatory effects; long-term large-scale use can increase blood sugar concentration, leading to complications such as poor wound healing. From this, the investigators infer that finding a means or preparation to prolong the action time and efficacy of dexamethasone while reducing side effects may become an important means of preventing CPSP.

Dexamethasone palmitate is a kind of dexamethasone nanoparticles with long curative effect, good effect and few side effects. It plays a very significant therapeutic role in the late stage of inflammation and has been proved to prevent the progression of inflammation. And there have been studies on the injection of dexamethasone palmitate in epidural or perineural space, but the safety and efficacy of dexamethasone palmitate in paravertebral space need to be further studied.

The primary aim of this study is to compare whether single injection of dexamethasone palmitate and dexamethasone sodium phosphate around the paravertebral nerve combined with continuous paravertebral nerve block for postoperative analgesia can reduce the incidence of chronic pain 3 months after operation.

Inclusion Criteria:

1. NYHA class I-III
2. 18 years to 65 years
3. Undergoing intercostal incision cardiac surgery

Exclusion Criteria:

1. Urgent surgery
2. BMI≧35kg/m^2
3. Non-first cardiac surgery
4. Local anesthetic allergy
5. Skin damage or infection at the puncture site
6. Dysfunction of liver, kidney and blood coagulation
7. Previously had chronic pains