The investigators aim to explore obese cardiomyopathy by studying contractile twitch force, sarcomere sensitivity to calcium and mitochondrial function in atrial myocardial samples of patients grouped according to their body mass index, i.e. normal weight, overweight and obese.

Obesity is associated with the development of heart failure, regardless of coronary artery disease or other comorbidities.

Right atrial samples obtained from patients undergoing cardiac surgery and presenting no sign of clinical cardiomyopathy will be grouped according to patient's body mass index. Atrial trabeculae will be explored to dissect out the respective contribution of (i) sarcoplasmic reticulum dysfunction (isometric twitch force contraction analysis), (ii) abnormal sarcomere sensitivity to calcium assessed in permeabilized fibers and (iii) mitochondrial dysfunction to intrinsec contractile dysfunction of myocardium in obese patients.

Inclusion Criteria:

• patient schedulled for coronary artery bypass graft surgery or aortic valve replacement surgery with extracorporeal circulation
• must be over 18 years of age
• must be able to read and understand the consent form in French
• must be in sinus rhythm at inclusion time

Exclusion Criteria:

• medical history of type 1 diabetes
• medical history of sustained atrial arrhythmia
• pregnancy
• class III anti-arhythmic drug

consecutive patients undergoing coronary artery bypass graft surgery or aortic valve replacement surgery with extracorporeal circulation