Objective: Follow-up of CO2 level during open heart surgery is crucial in terms of monitoring and management of the metabolic status. In this prospective observational study,measurement of end tidal CO2 and arterial CO2 levels in adult open heart surgery was compared with transcutaneous CO2 monitoring which is a non-invasive method. Material and Method: The study included 22 ASA II-III patients with an age range of 30-80 years, who will undergo elective open heart surgery. Demographic data, ASA scores, and comorbidities were recorded. Along with routine monitoring of heart surgery, TcCO2 was placed on deltoid muscle and measurements were obtained pre- and post-induction, and during and following cardiopulmonary bypass (CPB). Results: No difference was observed between pre- and post-induction terms in all three CO2 values, however a significant decrease was observed during and after cardiopulmonary bypass period in transcutaneous CO2, arterial CO2 and end tidal CO 2 values. Conclusion: During CPB period of heart surgery, conditions, such as hypothermia, hemodilution, and lower arterial blood pressure, may negatively affect the optimal measurement conditions, such as optimal temperature related issues, high capillary perfusions, and calibration issues. The lower TcCO2 values obtained especially during and following KPB period were thought to be caused by issues related with the device. Because of such challenging issues,, instead of non-invasive CO2 measurement method, CO2 monitoring by arterial blood gas samples, which are already obtained during heart surgery anesthesia with invasive artery catheterization, is thought to be a better option.