Ejection fraction is calculated by dividing the amount of blood your heart pumps out per beat (stroke volume) by the total amount of blood in the chamber just before it contracts (end-diastolic volume), then multiplying by 100 to get a percentage. The formula is straightforward: EF = (SV ÷ EDV) × 100. A normal left ventricular ejection fraction is 50% or higher, meaning your heart pushes out at least half the blood it holds with each beat.
What makes things more complex is how doctors actually measure those two volumes inside a beating heart. Several imaging techniques exist, each with different strengths and trade-offs in accuracy.
The Core Formula
The two numbers you need are the end-diastolic volume (how much blood fills the ventricle when it’s fully relaxed) and the end-systolic volume (how much blood remains after the ventricle contracts). Subtract the second from the first and you get the stroke volume, which is the amount of blood ejected in one heartbeat. Divide that stroke volume by the end-diastolic volume, multiply by 100, and you have the ejection fraction as a percentage.
For example, if your left ventricle holds 120 mL of blood when full and 50 mL remains after it squeezes, the stroke volume is 70 mL. Dividing 70 by 120 gives 0.58, or an ejection fraction of 58%. That number tells your doctor how efficiently your heart is pumping.
How Echocardiography Measures It
An echocardiogram (heart ultrasound) is the most common way ejection fraction gets measured. The standard approach is called the biplane Simpson method, sometimes referred to as the “method of disks.” The technician captures two-dimensional images of your left ventricle from two different angles, traces the inner border of the heart muscle in each view, and the software divides those outlines into a stack of thin slices. Each slice is treated as a small elliptical disk, and the volumes of all the disks are added together to estimate the total volume of the chamber.
This process happens twice: once when the ventricle is fully expanded (end-diastole) and once when it’s fully contracted (end-systole). The software then plugs those two volumes into the formula to produce your ejection fraction. Getting accurate results requires clear images of the heart’s inner wall in both its most relaxed and most contracted states, which is why the technician spends time finding the best viewing angles during your exam.
Cardiac MRI: The Gold Standard
Cardiac MRI is considered the most accurate way to measure ejection fraction. The scanner takes a series of cine images (essentially short video clips) in multiple cross-sectional slices through the heart, covering the entire left ventricle from top to bottom. Each slice is about 6 mm thick, spaced every 10 mm. The result is a detailed three-dimensional map of the ventricle at every phase of its contraction cycle, allowing precise measurement of both the filled and emptied volumes.
Because MRI doesn’t depend on finding a clear ultrasound window through your ribs, it works reliably regardless of body type. It’s typically reserved for cases where the echocardiogram images are poor quality or when precise measurement really matters, such as before deciding on a device implant.
MUGA Scans Use Radioactive Tracers
A MUGA scan (also called radionuclide angiography) takes a fundamentally different approach. Instead of imaging the heart muscle directly, a small amount of radioactive tracer is attached to your red blood cells. A camera then counts how much radioactivity is present in the ventricle when it’s full versus when it’s contracted. The ejection fraction is calculated by comparing end-diastolic counts to end-systolic counts, after subtracting background radiation.
MUGA scans are commonly used to monitor patients receiving chemotherapy drugs that can damage the heart, because the test is highly reproducible from one scan to the next. However, when compared against cardiac MRI, MUGA results can vary substantially for individual patients. One study found that while the average difference between the two methods was only about 1.5 percentage points, the range of disagreement for any given patient was wide enough (roughly 16 to 19 points in either direction) that 35% of cancer patients were misclassified when using a 50% threshold.
Why Results Can Vary Between Tests
Ejection fraction isn’t as precise a number as it might seem. Several factors introduce variability, and it’s common for repeat measurements to differ by a few percentage points even when nothing has changed in your heart.
- Body type and lung disease: Obesity, chronic lung conditions, and narrow rib spacing can block the ultrasound beam, making it harder to see the heart’s inner wall clearly. Poor image quality leads directly to less accurate volume estimates.
- Irregular heart rhythms: Atrial fibrillation and frequent extra beats cause the heart to fill differently from one beat to the next. Since the measurement captures only a few beats, the result may not represent your heart’s typical performance. Techniques that rely on syncing images to your heart’s electrical signal are especially affected.
- Operator differences: Tracing the inner border of the heart muscle involves judgment calls. Different technicians may draw slightly different outlines, particularly around structures like the small muscles inside the ventricle. This human variability is one of the biggest sources of measurement error in echocardiography.
- Breathing and movement: Patient movement during image acquisition can distort the heart’s outline in ultrasound, MRI, and CT alike, reducing the accuracy of the calculated volumes.
These sources of error matter clinically because treatment decisions often hinge on specific percentage cutoffs. A measured EF of 38% versus 42% could determine whether you qualify for certain therapies.
What the Numbers Mean Clinically
The 2022 guidelines from the American Heart Association and American College of Cardiology classify heart failure into categories based on ejection fraction:
- 50% or higher: Preserved ejection fraction. The heart squeezes normally, though heart failure symptoms can still be present if the ventricle is too stiff to fill properly.
- 41% to 49%: Mildly reduced ejection fraction. A gray zone where the heart’s pumping ability is below normal but not severely impaired.
- 40% or below: Reduced ejection fraction. This is the threshold where the diagnosis of classic systolic heart failure applies and where the strongest evidence exists for specific medications.
There’s also a category called “improved ejection fraction” for people whose EF was previously 40% or below and has since risen above that level with treatment. This distinction matters because stopping heart failure medications after improvement can cause the EF to drop again.
Treatment Thresholds Tied to EF
Certain devices and therapies are approved only when the ejection fraction falls below specific numbers. Cardiac resynchronization therapy, a specialized pacemaker that coordinates the heart’s contractions, generally requires an EF below 35% along with evidence of electrical delay in the heartbeat and persistent symptoms despite medication. Implantable defibrillators follow similar thresholds.
For patients who already have a standard pacemaker and develop worsening heart failure with an EF dropping below 40%, an upgrade to a resynchronization device may be considered. On the other end, patients with an EF above 50% are generally not candidates for these devices. These hard cutoffs are why accurate, reproducible measurement of ejection fraction carries real consequences for the care you receive.
Right Ventricle Ejection Fraction
Most discussions of ejection fraction focus on the left ventricle, which pumps blood to the entire body. But the right ventricle, which sends blood to the lungs, has its own ejection fraction. Measuring it is trickier because the right ventricle has an irregular, crescent-shaped geometry that doesn’t lend itself to the same geometric assumptions used for the left side.
Nuclear imaging techniques have an advantage here because they don’t depend on the chamber’s shape. They simply count radioactivity during filling and emptying. The main challenge is that the right atrium (the chamber just above) overlaps with the right ventricle in the images, contaminating the count data. Techniques using separate tracing regions for the filled and emptied states, or methods that subtract the atrial signal from the data, help improve accuracy. Cardiac MRI has increasingly become the preferred method for right ventricular assessment because it can capture the chamber’s complex shape without geometric assumptions.