According to the Textbook of Critical Care Nursing (Thelan, L, and others: Chapter 11, Cardiovascular Anatomy & Physiology, p. 150; 1990, Mosby) ejection fraction is "the ratio of the stroke volume ejected from the left ventricle per beat to the volume of blood in the left ventricle at the end of diastole (left ventricular end-diastolic volume). It is expressed as a percent, normal being at least greater than 50%. Both ejection fraction and LVEDV are widely used clinically as indices of contractility and cardiac function."
Ejection fraction can be estimated through echocardiography and more precisely measured during cardiac catheterization. When you review a chart look for the cardiologist's progress note on the date the heart cath was done and you'll see the EF recorded, as well as the extent of coronary artery occlusion and valve problems. Also, look for the echo report. The EF will be recorded there. Essentially, the lower the EF, the sicker the patient. I've seen some EF's as low as 10-11% in folks with really poopy pumps.
After cardiac surgery drugs like dobutamine and epi can be used to increase heart contractility and blood flow. If the patient becomes bradycardic or junctional the epicardial pacer can be used to increase heart rate and perfusion. I assume most or all of your post-op hearts are monitored hemodynamically with a Swan-Ganz catheter. In which case all of your efforts to maximize EF would be reflected in cardiac output and cardiac index measurements.
It's getting late, so I'll leave the QTc explanation to another colleague, or if no one posts, I'll review and post in a few days.
Good luck, study hard, keep reading, don't get discouraged if you don't understand something the first time or two. Over time you will understand if you persist. I hope this helps.