If the low cardiac output is due to heart muscle damage from a previous
heart attack, some
medication, drugs, alcohol virus, bacterium, etc.,
heart contractions may not improve very much. High blood pressure can cause the heart to enlarge due to additional workload of pumping against high resistance, and that can reduce contractility. With BP under control, the heart may return to normal.
Five years ago my ventricle pumping capacity was 13-29%.