Acute normovolemic hemodilution (ANH) is a widely used blood conservation strategy in cardiac surgery aimed at reducing intraoperative blood loss and the need for allogeneic blood transfusion. However, inadequate or excessive fluid replacement during ANH may lead to hemodynamic instability and other complications. The Pleth Variability Index (PVI) is a noninvasive dynamic parameter that can predict fluid responsiveness and guide goal-directed fluid therapy during surgery. This study aims to evaluate whether performing ANH under intraoperative PVI guidance in cardiac surgery allows more precise fluid management and reduces allogenic blood transfusion and the risk of perioperative complications.

Acute normovolemic hemodilution (ANH) is a blood conservation strategy used to reduce intraoperative blood loss and minimize the need for allogeneic blood transfusion. The technique involves withdrawing a predetermined volume of whole blood from the patient before surgery and replacing it with crystalloid or colloid solutions to maintain intravascular volume. Consequently, blood lost during surgery contains a lower hemoglobin concentration due to hemodilution, and the patient's hemoglobin level is subsequently restored through reinfusion of the collected autologous blood after the surgical procedure [1]. Based on available evidence, the European Society of Anaesthesiology recommends the use of ANH in perioperative bleeding management guidelines to reduce the need for allogeneic blood transfusion [2].

Although ANH is generally considered a safe technique, inadequate or excessive intravenous fluid replacement may lead to several potential adverse effects, including hemodynamic instability, anemia, increased myocardial oxygen consumption due to high cardiac output, dilutional coagulopathy, electrolyte imbalance, and renal dysfunction [3].

The Pleth Variability Index (PVI) is a noninvasive monitoring parameter derived from pulse oximetry that evaluates fluid responsiveness by analyzing respiratory variations in the plethysmographic waveform amplitude [4]. Particularly in patients receiving positive pressure ventilation, PVI can serve as a useful indicator for assessing intravascular volume status. As a dynamic and continuously monitored parameter, PVI has been shown to predict fluid responsiveness and facilitate goal-directed fluid therapy. Studies have demonstrated that PVI-guided fluid management may reduce the total volume of administered fluids compared with conventional fluid administration strategies [5].

Currently, no standardized protocol exists for the implementation of ANH, and its application often depends on institutional or local procedural guidelines [6]. Traditionally, ANH is performed using a 1:3 fluid replacement ratio during acute blood withdrawal, and the amount of blood to be collected is calculated using the formula:

ANH volume = (patient Hb - target Hb) / mean Hb × blood volume,

or is determined according to clinical interpretation of hemodynamic parameters [7].

However, ignoring patient-specific physiological responses may limit the effectiveness of ANH. Therefore, the use of a dynamic parameter such as PVI-whose effectiveness has been demonstrated in intraoperative goal-directed fluid therapy-may improve patient safety and enable a more physiological approach to hemodilution.

The primary aim of this study was to evaluate whether performing acute normovolemic hemodilution under PVI guidance during the intraoperative period in cardiac surgery provides more precise volume management. The secondary aim was to assess its effectiveness in reducing allogenic blood transfusion and the risk of complications.

Inclusion Criteria:

• ASA (American Society of Anesthesiologists) score II-III,
• ejection fraction (LVEF) >45%,
• NYHA (New York Heart Association) class I-III,
• hematocrit value >35%,
• and patients who have not had a myocardial infarction in the last three months.

Exclusion Criteria:

• Emergency or redo surgery requirement,
• LVEF (left ventricular ejection fraction) <35%,
• severe LMCA (left main coronary artery) stenosis (>70%),
• BMI <18.5,
• severe comorbidity (endocarditis, advanced COPD (chronic obstructive pulmonary disease), CKD (chronic kidney disease), anemia, hypoalbuminemia, peripheral artery disease),
• active DAPT /heparin use,
• coagulopathy,
• uncontrolled systemic diseases,
• pregnancy,
• malignancy

Individual participant data will not be shared because of ethical and institutional restrictions related to patient confidentiality.