The goal of this clinical trial is to learn if giving inhaled nitric oxide during heart surgery can prevent acute kidney injury (AKI) in patients who are at high risk of developing this complication. AKI is a serious problem after heart surgery and can lead to longer hospital stays, higher costs, and increased risk of death or long-term kidney disease. Currently, there is no effective medicine to prevent AKI after heart surgery.

The main questions it aims to answer are:

• Does inhaled nitric oxide reduce the chance of developing AKI within 7 days after heart surgery compared to placebo?
• Does inhaled nitric oxide help preserve kidney function up to 90 days after surgery?

Researchers will compare inhaled nitric oxide (the device is turned on) to a placebo (the same device is connected to the breathing circuit but not turned on, so no nitric oxide is given). The device panel is covered so that both participants and the research team do not know who receives active treatment or placebo (double-blind).

Participants in this study will:

• Have the nitric oxide device connected to their breathing circuit during heart surgery; they will receive either active nitric oxide or no nitric oxide (device off)
• Provide blood and urine samples to test kidney function during and after surgery
• Be followed for 90 days after surgery to monitor kidney health and any side effects

This is a single-center, prospective, randomized, double-blind, placebo-controlled superiority trial. It evaluates whether intraoperative inhaled nitric oxide (iNO) at 80 ppm reduces the incidence of acute kidney injury (AKI) within 7 days after cardiac surgery with cardiopulmonary bypass (CPB) in high-risk patients (Mayo Clinic renal risk score ≥2).

Intervention: Patients receive inhaled nitric oxide at 80 ppm via the ventilator circuit during CPB and until the end of surgery.

Control: The same device is connected but remains turned off (no iNO delivery). The device screen is covered in both groups to maintain blinding.

Randomization and blinding: Randomization (1:1) uses a blocked scheme with allocation concealment. Patients, surgeons, anesthesiologists, ICU staff, and outcome assessors are blinded. A dedicated gas therapist (not otherwise involved) manages the device.

Standardized management: All patients receive standardized anesthesia, CPB, and KDIGO-recommended AKI prevention bundle (MAP>55 mmHg, goal-directed perfusion).

Safety monitoring: Intraoperative methemoglobin (blood gas) and nitrogen dioxide (device readout) are monitored with predefined safety thresholds for dose adjustment.

Exploratory biomarker substudy (optional): Blood and urine samples are collected at perioperative time points to measure biomarkers of kidney injury and inflammation.

Follow-up: In-hospital outcomes (complications, ventilation duration, length of stay) and telephone follow-up at 30 and 90 days for major adverse kidney and cardiovascular events.

Sample size: Approximately 330 participants (1:1) are planned.

Statistical analysis: The primary analysis compares 7-day AKI incidence between groups with a two-sided α=0.05. Secondary outcomes will be analyzed using appropriate parametric or non-parametric methods.

Inclusion Criteria:

• Age ≥18 years
• Scheduled for elective cardiac surgery with cardiopulmonary bypass, including valve surgery and/or coronary artery bypass grafting (CABG)
• Mayo Clinic renal risk score ≥2

Exclusion Criteria:

• Emergency surgery
• Cardiac or major vascular surgery requiring deep hypothermic circulatory arrest
• Use of potentially nephrotoxic drugs (e.g., radiocontrast agents, aminoglycosides, amphotericin) within 24 hours before surgery
• Preoperative heart failure or low cardiac output syndrome requiring life support devices (ECMO, Impella or other left ventricular assist devices, mechanical ventilation), or left ventricular ejection fraction <30%, or other equivalent severe cardiac dysfunction
• Receipt of inhaled nitric oxide or inhaled prostacyclin within 7 days before randomization
• End-stage renal disease with eGFR <15 mL/min, or currently receiving renal replacement therapy, or prior kidney transplantation
• Hemophilia A or B
• Other end-stage chronic disease with estimated life expectancy <1 year (as assessed by the attending physician)
• Pregnancy
• Active endocarditis or other active infection

The investigators do not plan to share individual participant data (IPD). The informed consent form signed by participants did not include provisions for sharing de-identified data with external researchers beyond the primary study team.

The gas therapist who operates and adjusts the nitric oxide(NO) delivery device is unblinded. All other parties including patients, surgeons, anesthesiologists, ICU staff, outcome assessors, and data analysts are blinded. The device screen is covered with an opaque material in both the active and placebo groups to maintain blinding. The unblinded gas therapist does not participate in any other study procedures (e.g., patient management, data collection, or outcome assessment).