The primary goal of the imaging and outcomes component of the Todd and Karen Wanek Family Program for HLHS is to develop better methods of evaluating heart function in HLHS. Echocardiography (cardiac ultrasound) is the most common clinical test ordered and ejection fraction is the “number” most often used to evaluate the performance of heart muscle. However, the best echocardiographic methods for assessing heart function were designed for the left ventricle in individuals with two ventricles (or at least a single left ventricle).
In HLHS, the right ventricle (RV) is the only cardiac pump and the geometric shape of the right ventricle is vastly different from that of the left. This difference makes consistent and accurate functional assessment of the RV and its ejection fraction in HLHS patients is a challenge. Our objective is to identify either a new way to calculate ejection fraction in HLHS or to define an alternative measurement that doctors can substitute for an RV ejection fraction.
People affected with HLHS, who participate in our research programs, are helping Dr. Patrick O’Leary, a pediatric cardiologist and principle investigator of the clinical and imaging component of the Todd and Karen Wanek Family Program for Hypoplastic Left Heart Syndrome, and his colleagues to tackle this challenge.
“Currently, most doctors still use a subjective assessment of RV wall motion to estimate the ejection fraction in HLHS. As a result, there can be a wide variation in the value assigned to the right ventricle by different doctors," says Dr. O'Leary. "We are investigating several secondary measures to assess right ventricular function as substitutes for ejection fraction. These include not only equations describing the change in RV dimension during contraction, but also the motion of the tricuspid valve during the heartbeat, and even microscopic changes in the deformation of a single heart muscle segment. We expect that at least one of these new methods or possibly new 3-dimensional echo techniques will eliminate the variability in echocardiographic evaluation of RV function and improve our ability to predict the long term health of the RV in HLHS."
How is ejection fraction calculated during an echocardiogram?
Assessment of ventricular function and calculation of ejection fraction by echocardiography begins with a qualitative review of the patient’s ultrasound images. The reviewing physician evaluates how much the blood-filled cavity of the ventricle decreases in size during each heartbeat. Next, the physician compares this to the standard of what should be normal (based on past experience) and estimates the ejection fraction. When examining a left ventricle, several objective measurements and formulas can then be used to confirm the initial estimate. Unfortunately, there is no formula for objectively measuring right ventricular volume or ejection fraction using 2-dimensional echocardiography.
“Ejection Fraction is a simple concept,” says Dr. O'Leary. “You take the volume within a ventricle at the beginning of a heartbeat (when the ventricle is full) and subtract the volume that is left over when the heart has finished its contraction. The difference is called the stroke volume. Stroke volume is the amount of blood that is ejected from the heart as flow into the body during each heartbeat. The stroke volume is then divided by the beginning volume of the ventricle, and that result is multiplied by 100 to give the percentage. That percentage represents the fraction of blood the ventricle ejected into the artery during that heart beat.”
The problem with assigning a specific value to the ejection fraction of a right ventricle by echocardiography is that there is no simple way to measure the volume of the right ventricle. Therefore, doctors will often stop with the initial “estimate” ejection fraction. This subjective approach produces a wide range of values, even among experienced physicians.
Cardiac magnetic resonance imaging (cMR) can acquire an image of the entire ventricle, allowing a 3-dimensional reconstruction to be made. Right ventricular volume (both before and after contraction) can actually be measured using these reconstructions and then an ejection fraction is calculated. There is still some variation between examiners, but it is less than with an echocardiographic “estimate." This is why we are using cMR ejection fraction to validate the new echocardiographic measurements that we are evaluating in our research program.
What should my ejection fraction be?
A good ejection fraction for someone with Hypoplastic Left Heart Syndrome would be between 50 and 60 percent. Many individuals with HLHS have right ventricular ejection fractions that are somewhat less than this range but do not have any significant limitations. One of our goals is to better understand what is “good function” for the person with HLHS and how to best clinically describe it so that we can recognize worsening function, and treat it, hopefully at an earlier point than we do today.
The Todd and Karen Wanek Family Program for Hypoplastic Left Heart Syndrome (HLHS) is a collaborative network of specialists bonded by the vision of delaying or preventing heart failure for individuals affected by congenital heart defects including HLHS. The specialized team is addressing the various aspects of these defects by using research and clinical strategies ranging from basic science to diagnostic imaging to regenerative therapies.