Beat-to-beat Evaluation of Atrial Pacing to Test Fluid Responsiveness

The goal of this observational study is to learn whether atrial pacing-induced preload variation can identify fluid responsiveness in adult patients admitted to the intensive care unit after cardiac surgery. The main questions it aims to answer are: Can beat-to-beat atrial pacing induce measurable changes in stroke volume that predict fluid responsiveness? Do stroke volume changes measured by transthoracic echocardiography (LVOT VTI) and arterial waveform analysis identify patients who respond to a 500 mL fluid bolus? Researchers will compare fluid responders and non-responders, defined by a ≥10% increase in stroke volume after a 500 mL Ringer's lactate fluid bolus, to determine whether pacing-induced hemodynamic changes predict fluid responsiveness. Participants will: Undergo a standardized atrial pacing protocol (80-120 bpm in 10 bpm increments) before and after a routine 500 mL Ringer's lactate fluid bolus. Undergo transthoracic echocardiography and continuous arterial pressure monitoring during the pacing protocol. Have routine ICU hemodynamic data collected during their admission.

Inclusion Criteria: * Aged 18 years or older. * Underwent CABG surgery, without concomitant valve surgery. * Admitted to the ICU during the first postoperative day. * Equipped with surgically placed temporary atrial pacemaker wire. * Scheduled for a 500 mL bolus of balanced solution (Ringer's lactate) within the first two postoperative hours for clinical reasons. * Able to provide preoperative consent at the outpatient clinic before the surgery takes place. * Assessed by the attending intensivist as hemodynamically stable and (thereby) eligible for the study. Exclusion Criteria: * Presence of atrial fibrillation or other arrhythmias precluding atrial pacing. * Absence of atrial pacemaker wire. * Suspected cardiac tamponade or significant postoperative bleeding (e.g., \>400 mL blood loss in the first hour postoperatively). * Use of extracorporeal life support (ECLS) during the observation period. * Acute changes in vasopressor or inotrope dosage within the observation period. * Significant changes in body position, ventilator settings, or pacing mode during the study protocol. * Known contraindications to echocardiographic monitoring or arterial wave form analysis.