Mechanical ventilation (MV) is a clinical therapy that becomes necessary when the ventilatory capacity of patients is depressed by pathophysiological reasons or post-surgery conditions like cardiac surgery. 22.7% of patients undergoing cardiac surgery require mechanical ventilation over 24 to 48 hours. [1]. In these patients, the mortality rate may exceed 40%. It is known that the main cause of failure in weaning is an imbalance between the ability of respiratory muscles, mechanical ventilation and respiratory controller actions. This paper presents the assessment of respiratory muscles coupling as an indicator of work of breathing in mechanically ventilated cardiovascular postoperative patients. The approach includes correlograms and multichannel singular spectrum analysis of diaphragm, intercostal and sternocleidomastoid, showing that over 70% of cases had linear and nonlinear coupling between muscles. Patients that had a failed weaning also presented high muscle coupling probably due to the ventilator settings.