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.