CABGx3, essentially, is triple bypass done to reperfuse parts of the heart that are not receiving enough blood flow. Usually veins are harvested from a patients inner thighs, AKA saphenous vein grafting, and used as the bypass. Your care plan should focus on things such as: impaired circulation, fluid overload, and infection, respectfully.
You did everything possible. You called the doc when you had your gut feeling, and your charge nurse called RRT when she noticed a change in your pts condition. Sometimes its just a patients time to go. Do not beat yourself up over your patients passing. It is normal for you to have feelings of sorrow; we are nurses and we're compassionate about our pts well-being, therefore we feel this way after something like this occurs. The feeling of guilt will soon dissipate and you'll notice you'll will be giving the same advice to another nurse with the same issue some day. I'm with the comments above.