In this study, the investigators propose to investigate a novel technique for fluid responsiveness prediction. It is based on the occurrence of an extra systole, which induces a preload variation: Extra systoles are comprised by, first, the premature/ectopic beat with decreased cardiac preload, then, the post-ectopic beat with moderately increased preload.

Consequently, the post ectopic beat is associated with a Frank-Starling curve right shift but is otherwise a normal sinus beat. As such, the post-ectopic beat elucidates and predicts the hemodynamic effect of increasing preload, i.e. giving fluids

Inclusion Criteria:

• Patients requiring elective CABG or OPCAB surgery
• Patient's age ≥ 18 years

Exclusion Criteria:

• Patient refusal
• Pregnancy
• EF < 35%
• End stage kidney failure (defined by the need for haemodialysis)
• Patients with atrial fibrillation or frequent and coupled extra systoles (e.g. trigemini)