Rationale:Near-infrared spectroscopy (NIRS) is a noninvasive bedside tool for monitoring regional cerebral oxygen saturation (rSO2). The sinus conversion of atrial fibrillation (AF) was shown to be responsible for increasing rSO2. However, the reason for this improvement has not yet been clearly explained.Patient concerns:We report the case of a 73-year-old woman who underwent cardioversion during an off-pump coronary artery bypass under NIRS and live hemodynamic monitoring.Interventions:Unlike previous studies that failed to control and compare all conditions during procedures, this case showed real-time fluctuating hemodynamic and hematological values, such as hemoglobin (Hgb), central venous pressure (CVP), mean arterial pressure (MAP), cardiac index (CI), left ventricular end-diastolic pressure (LVEDP), and SVO2.Outcomes:The rSO2increased immediately after cardioversion and decreased during the obtuse marginal (OM) graft and after AF was obtained. However, no other hemodynamic data showed the same or opposite directional changes in the rSO2.Lessons:Significant instantaneous changes were observed in rSO2using NIRS after sinus conversion, without obvious hemodynamic alterations in the systemic circulation or other monitoring values.