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.

Background and Rationale Del Nido cardioplegia is widely used in adult valve/aortic surgery to permit prolonged single-dose electrical arrest. The effective protection duration likely varies with patient and procedural factors (e.g., LV function, temperature, hemodilution, surgical complexity, antegrade/retrograde delivery). Many centers empirically re-dose around 60-90 minutes; however, this window may not be optimal for every case. The current study quantifies the protection window by coupling routine care with dense, time-stamped hs-troponin sampling.

Design and Setting Prospective, single-center, observational cohort conducted at Necmettin Erbakan University, Department of Cardiovascular Surgery (Türkiye).

Population Adults 18-80 years scheduled for elective valve and/or thoracic aortic surgery under CPB with ACC in which Del Nido cardioplegia is used per institutional routine. Key exclusions include isolated or CABG-dominant procedures, redo sternotomy, emergency/salvage or preoperative shock, IABP/ECMO, LVEF <35%, eGFR <45 mL/min/1.73 m², significant pulmonary or hepatic disease, major coagulopathy, active infection/sepsis or endocarditis, pregnancy, and preoperative troponin elevation.

Interventions None. Cardioplegia content, dose, route (antegrade/retrograde/combined), temperature, and re-dose decisions are entirely per standard practice at the discretion of the surgical/anesthesia/perfusion team. No experimental therapy is administered.

Sampling Schedule (Biomarkers) Preoperative baseline hs-cTn; intraoperative sampling referenced to ACC at 0, 30, and 60 minutes, then intensified at 75, 90, 105, and 120 minutes. If a re-dose is administered, additional samples at pre-re-dose and +15/+30/+45/+60 minutes. Postoperative hs-cTn at approximately 6, 24, and 48 hours. Intraoperative blood is obtained preferably from the venous reservoir or existing central lines. Each draw ~3-5 mL; total additional volume ~30-40 mL. Exploratory CK-MB may be recorded where available.

Outcomes Primary outcome: intraoperative hs-cTn change-point time (minutes from ACC) identified by segmented (piecewise) trend analysis indicating the earliest sustained acceleration compatible with evolving ischemic injury.

Key secondary outcomes: (i) association between change-point and ACC duration; (ii) presence/timing of Del Nido re-dose and biochemical response to re-dose (slope change pre/post); (iii) early clinical outcomes including 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, re-exploration/bleeding, infection, 30-day MACE, and 30-day all-cause mortality.

Statistical Plan (Summary) Individual time series will be analyzed using segmented (piecewise) regression and/or change-point detection methods. Group-level estimates (mean protection window and variance) will be obtained via mixed-effects change-point models. Associations with durations (ACC, CPB), re-dose timing, and outcomes will use linear/quantile regression and logistic/Poisson/negative binomial models as appropriate. Missing data will be handled per predefined rules and, if needed, multiple imputation. Target enrollment is 80 participants, anticipated to provide ~80% power at α=0.05 to detect a clinically meaningful change-point and slope shift in hs-troponin trajectories.

Safety and Data Handling Risk is minimal and limited to low-volume blood sampling aligned with routine care; no experimental treatment is given. Data are recorded on standardized case report forms and stored in secure, de-identified systems compliant with local regulations. Serious adverse events are reported per institutional/authority requirements.

Significance By quantifying the actual protection window and its variability, the study is designed to inform more rational-potentially individualized-re-dose timing for Del Nido cardioplegia, aiming to reduce both under-protection (late re-dose) and workflow disruption (unnecessary early re-dose).

Inclusion Criteria:

• Age 18-80 years
• Elective valve surgery (aortic, mitral, tricuspid), thoracic aortic surgery, or combined valve + thoracic aortic procedures
• Planned cardiopulmonary bypass (CPB) with aortic cross-clamp (ACC)
• Use of Del Nido cardioplegia per institutional routine
• Ability to provide written informed consent (participant or legally authorized representative)

Exclusion Criteria:

• Isolated CABG or CABG-dominant combined procedures
• Redo sternotomy
• Emergency status (including shock) or preoperative mechanical circulatory support (IABP or ECMO), or anticipated need for such support
• Left ventricular ejection fraction <35%
• Estimated GFR <45 mL/min/1.73 m²
• Moderate-severe chronic lung disease with significant functional limitation, or severe pulmonary hypertension
• Active infection/sepsis or active infective endocarditis
• Severe hepatic dysfunction, major coagulopathy, or bleeding diathesis
• Pregnancy
• Deep hypothermia protocols (<28 °C)
• Procedures without ACC
• Preoperative cardiac troponin above the laboratory upper reference limit
• Any condition that, in the judgment of the treating team, would preclude safe participation or protocol adherence

Single-center, tertiary academic cardiovascular surgery service at Necmettin Erbakan University Meram Medical Faculty Hospital (Konya, Türkiye). Consecutive adults (18-80 years) undergoing elective valve and/or thoracic aortic surgery with cardiopulmonary bypass and aortic cross-clamping, in whom Del Nido cardioplegia is used as part of routine care, are screened and, if eligible, approached for written informed consent. Approximately 80 participants are enrolled and followed through the index hospitalization; 30-day outcomes are abstracted from the medical record/standard follow-up. Observational cohort; no randomization; all treatment decisions per usual care.

De-identified individual participant data underlying the primary and key secondary outcomes (time-stamped hs-troponin values, basic demographics, operative variables, and early postoperative outcomes). Data will be shared in compliance with local regulations (e.g., KVKK) and institutional policies.

Available beginning 6-12 months after the primary publication (or within 12 months after last-patient-last-visit if no publication) and will remain available for 5 years.

Qualified researchers may request access by submitting a brief proposal to the Central Contact. Requests will be reviewed by the study investigators and the institutional Data Access Committee. Upon approval and execution of a Data Use Agreement (non-reidentification, secure storage, academic use only, citation/acknowledgment, and data destruction at project end), de-identified IPD and supporting documents will be transferred via secure link.

Central Contact: Muhammet Talha Ceran, MD - mtceran@gmail.com