This study compares two accepted ways of deciding when adults recovering from open-heart surgery should receive a blood transfusion in the intensive care unit. One approach gives a transfusion when the blood count (hemoglobin) falls below a fixed level that is the same for everyone. The other approach adds each patient's own physiology - such as oxygen levels and lactate - to help decide whether a transfusion is truly needed, within a safe range. The investigators want to learn whether the personalized approach is as safe as the standard approach for major outcomes after heart surgery, while reducing the amount of blood transfused. Participants may also choose to give blood and stool samples to a research biobank for future studies on recovery after cardiac surgery.

Inclusion Criteria:

• Adults 18 years or older scheduled for cardiac surgery using the heart-lung machine (cardiopulmonary bypass), such as bypass, valve, or combined procedures
• Moderate-to-high surgical risk (e.g., EuroSCORE II ≥ 3% or equivalent)
• Allogeneic red blood cell transfusion is considered likely
• Able to provide informed consent and understand randomization to different transfusion thresholds
• Willing to receive blood products and follow study transfusion thresholds from anesthesia induction until hospital discharge or day 28, whichever comes first

Exclusion Criteria:

• Refusal of or contraindication to allogeneic blood (e.g., Jehovah's Witness or formal directive against transfusion), or enrollment in a preoperative autologous donation program
• Emergency or salvage procedures where protocol triggers are impractical (e.g., active arrest, aortic dissection with collapse)
• Heart transplantation, durable ventricular assist device implantation, or surgery solely for VAD insertion
• Off-pump bypass or other procedures not using cardiopulmonary bypass
• Severe preoperative anemia (e.g., hemoglobin < 8 g/dL) or chronic transfusion-dependent anemia
• Conditions preventing protocol adherence (e.g., anticipated massive hemorrhage, known bleeding disorder, very low platelets, uninterruptible dual antiplatelet/anticoagulant therapy)
• Chronic dialysis dependence or established end-stage renal disease (eGFR < 15 mL/min/1.73m² or on maintenance renal replacement therapy) prior to surgery.
• Pregnancy or lactation
• Enrollment in another interventional trial affecting transfusion practice or hemoglobin thresholds
• Unable to provide informed consent (e.g., severe cognitive impairment without a legally authorized representative)