Fractional Area Change Calculator: LV Function

The fractional area change (FAC) calculator works out the percentage change in the cross-sectional area of the left ventricle between diastole and systole, using two area measurements you enter in cm². You get the FAC percentage, which is a standard echocardiography index of left ventricular systolic function. This page explains the formula, a worked example, and how to interpret the result in clinical context.

Formula

The fractional area change is calculated with the following formula:

FAC (%) = [(LVEDA – LVESA) / LVEDA] × 100

Where:

• LVEDA = left ventricular end-diastolic area (cm²), measured at the frame just before mitral valve closure.
• LVESA = left ventricular end-systolic area (cm²), measured at the frame just before mitral valve opening.

The result is a percentage. A higher percentage means the ventricle shrinks more during contraction, indicating better systolic function. The same formula is used regardless of the imaging view (e.g., apical four-chamber or two-chamber), as long as both areas are measured in the same view and at the correct phases of the cardiac cycle.

Worked Example

Using the default values from the calculator above (LVEDA = 25 cm², LVESA = 15 cm²):

1. Subtract the systolic area from the diastolic area: 25 – 15 = 10 cm².
2. Divide by the diastolic area: 10 ÷ 25 = 0.4.
3. Multiply by 100 to get a percentage: 0.4 × 100 = 40%.

The calculator returns 40.0% as the fractional area change, and 10.0 cm² as the absolute area change. This matches the formula output exactly.

Why Fractional Area Change Matters

FAC is one of several echocardiographic measures of left ventricular systolic function. It is particularly useful when the endocardial border is well visualized but the ventricle has an abnormal shape, such as after a myocardial infarction, because it does not assume a geometric model like the Teichholz or Simpson methods do. FAC is also used in the assessment of right ventricular function, where it is calculated from the right ventricular area in the apical four-chamber view.

The American Society of Echocardiography (ASE) guidelines consider a normal left ventricular FAC to be greater than 50%. Values below this threshold suggest reduced systolic function. For the right ventricle, a FAC below 35% indicates systolic dysfunction. These cutoffs are based on population studies and are widely accepted in clinical practice.

How to Measure the Areas Correctly

Accurate FAC depends entirely on how the areas are traced. The following steps are standard for left ventricular FAC measurement:

• Use the apical four-chamber view, with the left ventricle centered and the apex visible.
• Trace the endocardial border in the frame with the largest cavity (end-diastole), typically just before mitral valve closure.
• Trace the endocardial border in the frame with the smallest cavity (end-systole), typically just before mitral valve opening.
• Include the papillary muscles and trabeculae in the cavity area (i.e., trace along the compacted myocardium, not the trabeculated edge).
• Do not include the left atrium; the trace should stop at the mitral valve annulus.

Most ultrasound machines have a built-in tracing tool that calculates the area automatically once you trace the border. The same view and the same tracing convention must be used for both diastole and systole.

Common Mistakes and Pitfalls

Several errors can lead to inaccurate FAC values:

• Using different views – measuring diastole in the four-chamber and systole in the two-chamber view gives meaningless results.
• Incorrect frame selection – choosing a frame slightly before or after true end-diastole/end-systole will under- or overestimate the areas.
• Including the left atrium – the atrial area is not part of the ventricular cavity; including it inflates the diastolic area and lowers FAC.
• Excluding papillary muscles – this changes the traced area by a few cm² and can shift the FAC by several percentage points.
• Unit errors – the formula requires cm². If your machine reports areas in mm² (common in some settings), divide by 100 before entering them.

Also note that FAC is load-dependent: it can be reduced in hypovolemia or increased in hypervolemia, even with normal contractility. It is not a pure measure of contractility, but rather of global systolic performance.

Limitations of Fractional Area Change

FAC has important limitations. It assumes the traced area is representative of the whole ventricle, which may not hold in regional wall motion abnormalities. It also requires good endocardial definition; in patients with poor acoustic windows, the tracing may be unreliable. Unlike ejection fraction (EF), which uses volumes, FAC is a 2D measurement and does not account for longitudinal shortening. However, studies have shown that FAC correlates reasonably well with EF, and it is faster to obtain because it avoids geometric assumptions.

FAC is also used in the assessment of right ventricular function, where it is calculated from the right ventricular area in the apical four-chamber view. The same formula applies, but the normal cutoff is lower (≥35%).

Frequently Asked Questions

What is a normal fractional area change?

For the left ventricle, a normal FAC is greater than 50%. For the right ventricle, a normal FAC is greater than 35%. These values are based on ASE guidelines and are used to classify systolic function as normal or reduced.

How is fractional area change different from ejection fraction?

Ejection fraction uses ventricular volumes (end-diastolic volume and end-systolic volume) and is expressed as a percentage. FAC uses cross-sectional areas instead of volumes. EF is more comprehensive because it accounts for the entire 3D chamber, but it requires geometric modeling or 3D imaging. FAC is simpler and does not assume a geometric shape, making it useful in abnormally shaped ventricles.

Can I calculate FAC from a single image?

No, you need two images: one at end-diastole and one at end-systole. Both must be from the same view (e.g., apical four-chamber). The difference in area between these two frames is the basis of the calculation.

What does a low FAC mean?

A low FAC indicates reduced systolic function of the ventricle. For the left ventricle, a FAC below 50% suggests systolic dysfunction. This can be due to conditions like myocardial infarction, cardiomyopathy, or valvular disease. It is not a diagnosis itself but a marker that warrants further investigation.

Does FAC change with age?

Yes, like most cardiac function parameters, FAC can decline slightly with age. However, the normal cutoff of 50% for the left ventricle is generally applied across adult age groups. In elderly patients, a borderline value should be interpreted in the context of other findings.

Why is my FAC different from the machine’s calculation?

Most ultrasound machines calculate FAC automatically after you trace the areas. If your manual calculation differs, check the units (cm² vs mm²), the tracing inclusion (papillary muscles), and the frame selection. Small differences in tracing can cause a few percentage points of variation.

Related

• Dry Ounces to Cups Calculator: Free & Easy
• Dry Ounces to Cups Calculator: Quick Conversion
• Dirt Yard Calculator: How Many Yards Do You Need?

in a_d | Diastolic area (LVEDA) | cm² | 25 | 0
in a_s | Systolic area (LVESA) | cm² | 15 | 0
out fac | Fractional area change | % | ((a_d - a_s) / a_d) * 100 | 1
out area_change | Area change | cm² | a_d - a_s | 1