After a cardiac operation, there is reversible myocardial dysfunction
that also involves a metabolic disorder. In patients with cardiac fail
ure, care must be taken to reduce the strain on the heart by minimizin
g systemic oxygen uptake, which is, in fact, the main determinant of c
ardiac output. Inotropic support may improve cardiac output and tissue
oxygenation in cardiac failure, but it also increases myocardial stre
ss directly by increasing myocardial demands and indirectly by increas
ing systemic energy demands. Mixed venous oxygen saturation reflects t
he balance between cardiac output and systemic oxygen consumption and
indicates whether cardiac output can adequately provide the peripheral
tissues with oxygen. This physiologic view toward the treatment of po
stoperative cardiac failure helps us avoid overtreatment, that is, sti
mulating cardiac output more than necessary for adequate tissue oxygen
ation. In this way, the strain on the heart can be reduced and myocard
ial recovery, enhanced.