Hi vooda,
Cardiac output (EF) will change depending on the resistance the heart is pumping against (afterload). The heart pressures, heart dimensions, heart wall thickness will influence preload (filling) and if abnormal will compromise CO.
An echo will provide heart dimensions and a good source for an EF estimate. An angiogram is good source for determining pressures. I had an EF (according to written report) of 29% when entering ER. The next day with an angiogram the report states an EF of 13%.
If you had a heart attack (MI), there probably was heart muscle damage. It would impair pumping contractions and a lower CO. This would cause your system to compensate by enlarging the heart and wall thickness would increase, etc. If your medication is appropriate, it should reduce preload and afterload resulting in a more efficient CO, and there should be stability to your condition.
If the echo is less accurate it is because the outer and inner margins are somewhat "fuzzy" and the dimensions may be slightly inaccurate (resulting in smaller or larger borders outlined manually with a transducer for equipment software to calculate). But at the very most only an error of 10%.
One can feel absolutely well with an EF below 29% and with medication to increase the EF feel less well. Not unusual at all. The medication improves longevity but for some people there is an intolerance.