The goal of this observational study is to understand how oxygen consumption after heart surgery relates to blood flow problems and organ injury. The study focuses on patients over 18 years old who are having planned heart surgery with a heart-lung machine (cardiopulmonary bypass). The main questions the study aims to answer are: 1. How does oxygen consumption in the early hours after surgery relate to lactate levels (a sign of low oxygen supply to the tissues)? 2. How is oxygen consumption linked to signs of poor blood flow and organ injury (such as heart, kidney, liver, brain, and gut damage)? Researchers will measure oxygen consumption after surgery using a technique called indirect calorimetry. They will also track blood flow and oxygen use during surgery and check for signs of organ injury the day after the procedure. The study will include 65 participants. People with certain health conditions, like severe anemia, high lactate levels before surgery, or needing intensive care or extra oxygen supply before surgery, will not be included. By understanding how oxygen consumption relates to blood flow and organ injury, this research may help to better manage patients after heart surgery and reduce complications.