This single-center, prospective, observational cohort study quantifies the effective myocardial protection window of Del Nido cardioplegia during adult open-heart surgery performed under cardiopulmonary bypass (CPB) and aortic cross-clamp (ACC). Without altering routine care, time-stamped high-sensitivity cardiac troponin (hs-cTn) measurements will be obtained at predefined intraoperative and early postoperative intervals to identify the inflection (change-point) at which biochemical evidence of ischemic injury begins to rise. Eighty adults undergoing elective valve and/or thoracic aortic procedures with Del Nido cardioplegia will be enrolled. The primary endpoint is the intraoperative hs-cTn change-point time referenced to ACC. Secondary endpoints include associations between change-point and ACC duration, the presence/timing of any re-dose, and early clinical outcomes (e.g., low cardiac output syndrome, maximum VIS in the first 24 h, new arrhythmia or pacemaker need, acute kidney injury by KDIGO, ventilation hours, ICU/hospital length of stay, 30-day MACE and mortality). All cardioplegia choices (dose, route, temperature, re-dose decisions) remain per standard practice; no experimental therapy is administered. Risks are minimal and limited to small-volume blood sampling coordinated with routine draws.