Heart failure is an EF below 29%, and usually indicates the heart isn't strong enough to meet the body's demand for oxygenated blood. Dilated left ventricle (thin walls) will reduce the heart's contractility and as consequence a low EF. The problem may not be related to the grafts that appear to remain open....grafts or other therapy is not a cure to CAD only treats the symptoms..
About 6 years ago, I had an EF of 19 to 29% with an enlarged LV and occluded vessels. The RCA 98% blocked was stented, LAD 100% blocked had developed vessels that are a natural bypass, and circumflex 72% occluded was not stented. My heart was dilated due to ischemia (lack of blood flow) and hypokinesis (heart wall movement impaired usually caused by damaged cells from lack of oxygen...mine were revitalized with an increased blood flow to the depleted area).
With medication, etc. may heart has retruned to normal size and my EF is 59%, and I feel OK.