Heart failure is classified into three groups based on the measurement of left ventricular ejection fraction (LVEF). This is the percentage of left ventricular (LV) contents pumped out with each contraction of the LV. This classification determines how each patient should be treated. 

Based on evolving understanding, the following classification is used internationally today: 

• LVEF ≤ 40%: HFrEF = heart failure with reduced LVEF or systolic heart failure.
• LVEF 41-49%: HFmrEF = heart failure with mildly reduced LVEF.
• LVEF ≥ 50% + left ventricular hypertrophy / dilatation of the left atrium / diastolic dysfunction: HFpEF, heart failure with preserved LVEF or diastolic heart failure.

In recent years, an additional group was defined: HFimpEF, heart failure with improved ejection fraction. 

Depending on the underlying cause, heart failure can be reversible provided correct diagnosis and treatment, with significant improvement in heart function and disappearance of heart failure symptoms. (Reversible versus non-reversible causes of heart failure)

Patients with HFimpEF are those with symptomatic heart failure with a baseline LVEF ≤ 40%, a ≥10 % increase from baseline LVEF, and a second measurement of LVEF > 40%.

Nowadays, we also recognise that patients with a baseline LVEF of 41-49% who have an improved LVEF to ≥50% can be categorised as HFimpEF. 

Further research on this subject is in progress and has already shown that when LVEF improves, the prescribed heart failure therapy should not always be interrupted just like that, as deterioration of heart function and progression to symptomatic heart failure may occur afterwards.