The goal of this clinical trial is to learn about in postoperative acute kidney injury in cardiac surgery. The main questions it aims to answer are:

• Intraoperative Transesophageal Ultrasound Doppler venous imaging can be used to predict the occurrence of cardiac surgery associated acute kidney injury (CSA-AKI ) in the early stage by observing the changes of renal vein, portal vein and hepatic vein blood flow patterns, calculating the renal vein stasis index and portal vein pulsatility index, and evaluating the degree of venous congestion.
• Controlled low central venous pressure (CLCVP) technology can reduce the occurrence of CSA-AKI by reducing venous congestion and increasing renal perfusion pressure.

Participants will receive Transesophageal Ultrasound Doppler during cardiac surgery. If the participant is assigned to the intervention group, if the central venous pressure of the participant is ≥ 10mmHg 30 minutes after the end of cardiopulmonary bypass, he will receive nitroglycerin pumped to apply the controlled low central venous pressure (CLCVP) technology; If participants are assigned to the control group, no intervention measures will be taken.

The researchers will compare the intervention group with the control group to see the occurrence of AKI and AKD after cardiac surgery.

Inclusion Criteria:

Patients aged above 18 years undergoing coronary artery bypass graft or heart valve surgery under cardiopulmonary bypass.

Exclusion Criteria:

1. Patients with CKD history, end-stage renal disease and renal transplantation before operation
2. Used nephrotoxic drugs within two weeks before operation
3. Known Renal Artery Stenosis and Renal Malformations
4. There are contraindications for transesophageal ultrasound Doppler evaluation of esophageal space occupying lesions