As a CT PACU nurse there are many reasons that there is always an increased risk for a decreased C.O.
Remember the heart is stopped (usually unless they are an off pump case) during the CABG and the temperature is dropped to decrease the oxygen demands of the body while the patient is on Pump (bypass)....which can be a couple of hours. They can suffer an intra-op MI, the pericardial cavity can fill up with blood if there is any bleeding leading to cardiac tamponade. There can be so much bleeding that the patient becomes hypovolemic and goes onto hemorrhagic shock. They are hypothermic.
Arrhythmias are another reason causing a decrease in C.O and depending on how the Right atrium was cannulated A Fib is VERY common. Ventricular arrythmias are also "common" for the heart is NOT accustomed to being removed from the chest and being man handled....so they myocardium is annoyed....and that shows up in arrythmias.
The pleural effusion is also common post op from the trauma to the lungs especially when the internal mammary artery is used. There is a naturally occurring artery in the left chest called the Left internal mammary is a common cause of pleural effusions. If you ever saw how far the chest is open during bypass....you would understand.
Hopefully the cardiac function is improved post cabbage by restoring/improving circulation to the myocardium once normothermia is restored......but it can be changed again with intra-op, post op MI's.
Why is the SVR changed post op?? Well what happen to the blood vessels when the patient is hypothermic....they constrict therefore causing an increase of the SVR. The entire blood supply is leaving the body (bypass) and being returned causing other stress related responses. The patient is given meds Peri-operatively that can cause anaphylaxis...specifically protamine to reverse the heparin given Peri-operatively.
It can change if the patients suffers a MI or is cardiogenic shock.....or hemorrhagic shock. The bodies own flight/flight response to stress will also affect the SVR.
These are a few.....I hope it helps. check out this site it might help. http://www.ccmtutorials.com/cvs/Shock/index.htm