Hydration Guided by Remote Dielectric Sensing (ReDS) System for Preventing Acute Kidney Injury in Elderly Patients With Renal Insufficiency for Coronary Angiography and Intervention

Inclusion Criteria: * Age ≥ 60 years; * Patients with unstable angina pectoris are scheduled for percutaneous coronary intervention； * Patients with chronic renal disease (estimated glomerular filtration rate \< 90 ml/min); * Sign the informed consent form. Exclusion Criteria: * 1\. Acute decompensated severe heart failure or cardiogenic shock; * 2\. Malignant tumors, severe renal failure (estimated glomerular filtration rate \< 30 ml/min); * 3\. Respiratory failure; * 4\. Used contrast media within one week; * 5\. Have allergic to contrast medium