What Is Coronary Flow Reserve (CFR)?
Also known as myocardial blood flow reserve (MBF), coronary flow reserve (CFR) refers to the ratio between stimulated (maximal) coronary blood flow induced by using a coronary vasodilator and a resting (baseline) flow. In other words, it is a diagnostic that describes the blood flow to the myocardium or heart, specifically the ability of coronaries (blood vessels of the heart) to increase blood supply under stress. As it is a ratio, it is a dimensionless variable.
How Does the Physician Calculate Coronary Flow Reserve (CFR)?
The clinician first measures the coronary flow at rest and the maximum blood flow within the coronary arteries. Once these values are calculated, the coronary artery’s maximum blood flow is divided by the coronary flow at rest.
Why Does a Physician Advise a Coronary Flow Reserve Test?
An individual is advised for this test since it helps the physician assess the presence or severity of any cardiac disease, such as coronary artery disease (CAD). It also helps determine whether an individual is at risk for future cardiac (heart) disease. However, coronary flow reserve (CFR) or myocardial blood flow (MBF) as a stand-alone test cannot distinguish between microvascular disease (disease of the smaller and more delicate blood vessels of the body) and macrovascular disease (disease of the large blood vessels).
How Is a Coronary Flow Reserve Test Done (CFR)?
A physician can use several imaging techniques to assess the coronary flow reserve (CFR). The methods could be either invasive or non-invasive. The invasive techniques include the following procedures -
-
Cardiac Catheterization: Cardiac catheterization, or a traditional angiogram, has been a gold standard test or go-to test for examining the health of a human heart. A traditional coronary angiogram is an invasive procedure that involves passing a catheter into the arteries( blood vessels). At times, doctors may advise a ‘contrast angiography’. It involves the use of a special dye (also known as contrast) injected and helps improve the quality of the images.
The non-invasive methods include
-
Cardiac Nuclear Imaging: Also known as cardiac PET (positron emission tomography), a stress test type that uses Rubidium-82, which acts like a tracer when injected into the blood vessels. This test is also known by other names, such as myocardial perfusion imaging (MPI) and cardiac SPECT (single-photon emission computed tomography).
-
Cardiac Magnetic Resonance Imaging (Cardiac MRI): This is an expensive- time-consuming procedure, and a physician rarely asks for a cardiac MRI (magnetic resonance imaging) to assess the coronary flow reserve.
-
Transthoracic Doppler Echocardiography: It is a reliable way to study coronary flow reserve with the advantage of being non-invasive, readily available, and inexpensive. It can be used in both stenosed and normal epicardial arteries or blood vessels.
What Do the Different Values of Coronary Flow Reserve (CFR) Indicate?
Values greater than 2 or 2.5 fall within the normal range of coronary flow reserve. Values above 2 or 2.5 indicate good heart health and recovery post-surgical interventions or cardiac events. In other words, it shows that when the heart is under stress or exertion of any sort, there is double the amount of blood flow compared to the contrary flow to the heart when it is at rest.
Values less than 2 detect cardiac abnormalities such as epicardial or coronary artery stenosis, which can lead to myocardial ischemia.
What Are the Clinical Applications of the Coronary Flow Reserve (CFR) Test?
The clinical applications of the coronary flow reserve (CFR) test are as follows-
1) Helps detect coronary microvascular disease, such as-
-
Hypertension: It is defined as increased blood pressure, which makes an individual susceptible to heart disease. It is a condition that leads to the wear and tear of blood vessels, consequently causing cardiac problems like myocardial infarctions (MI or heart attacks), aneurysms (weakening or ballooning of the blood vessels), or strokes (brain damage occurring due to disruption in the blood supply of the brain). Hypertension could either be primary or secondary.
-
Diabetes: Diabetes is a condition characterized by increased sugar levels in the blood. This happens because the body has trouble moving glucose (sugar) from the blood into the body’s cells. Over time, high blood sugar or diabetes can damage the blood vessels of the heart and other organs, leading to several health problems.
-
Cardiomyopathies: They are diseases of the heart muscle tissue that most often lead to progressive heart failure with significant morbidity and mortality. They could be either primary (defect is primarily within the heart muscle) or secondary (heart muscle dysfunctions due to a cause secondary to other problems). Typically, there are three patterns of cardiomyopathies- dilated cardiomyopathy, restrictive cardiomyopathy, and hypertrophic cardiomyopathy.
-
Aortic Stenosis: It is a valvular heart disease that occurs when the aortic valve (the primary outlet for pumping from the heart to the body) becomes abnormally narrowed. It can occur because of the wear and tear of the blood vessels and the gradual build-up of calcium deposits which stiffen and restrict the valve, preventing it from opening correctly. As the valve narrows, it exerts more pressure on the heart as it needs to pump harder to push blood throughout the body. When left untreated, aortic stenosis can lead to a severe decrease in the quality of life, heart failure, and even death.
2) Helps detect macrovascular diseases, such as -
-
Epicardial Coronary Stenosis: It is a medical condition in which the epicardial coronary arteries are stenosed or have narrowed. Coronary arteries are also known as epicardial coronary arteries since they run along the heart’s outer surface, the epicardium.
-
Re-Stenosis of the Coronary Blood Vessels (Arteries): Refers to a condition wherein the part of the coronary artery which was previously treated with a stent experiences an obstruction or narrowing again.
3) It helps assess prognosis in the following conditions-
-
Coronary Artery Disease: Also referred to as CAD, is a condition that affects the heart and is one of the most prevalent heart diseases. The coronary arteries struggle to supply the heart with adequate blood, oxygen, and nutrients. Cholesterol deposits build up within the walls of the blood vessels and make them narrow, decreasing blood flow to the heart. This eventually can lead to chest pain, dyspnea, or heart attack.
-
Cardiac Transplantation: The heart has minimal capacity to repair itself, and when it is severely damaged, it is replaced with another heart. It is a complex and intricate procedure but can help improve critical patients’ quality of life.
-
Cardiomyopathies: They are diseases of the heart muscle tissue that most often lead to progressive heart failure with significant morbidity and mortality.
4) Helps evaluate the effect of pharmacological interventions done while treating cardiac or heart conditions.
What Are the Prerequisites for an Individual Undergoing the Coronary Flow Reserve Test?
The physician takes a complete medical and drug history before performing this test. They would assess any possible drug interactions or allergies to any drug or the contrast material (in case of cardiac catheterization). If the coronary flow reserve is being measured via a PET (positron emission tomography) scan or cardiac catheterization, the individual is instructed to refrain from eating for six to eight hours before the procedure.
What Are the Risks and Side Effects Associated With the Coronary Flow Reserve Test (CFR)?
The coronary flow reserve (CFR) test or the myocardial blood flow (MBF) reserve test is a safe procedure with minimal side effects. On rare occasions, individuals undergoing this test can risk injuring their coronary arteries if a catheter is used to assess the coronary blood flow. The risks include the formation of a blood clot within the coronary blood vessels, a tear in the blood vessels, spasm to the heart tissue, or bleeding to the heart and its associated structures.
Who Should Avoid Getting a Coronary Flow Reserve (CFR) Test?
Asthmatic individuals should avoid getting this test since the drug Adenosine (a vasodilator that increases the coronary blood flow by four to five times) can lead to spasms of the airway (bronchospasms), making it difficult for the asthmatic individual to breathe.
The drug adenosine, a vasodilator, is also associated with other side effects which last for a concise duration. They include-
-
Arrhythmias (abnormal or irregular heartbeats.).
-
Hypotension (decreased blood pressure).
-
Dyspnea (shortness of breath).
-
Pain and discomfort in the chest.
Conclusion
A coronary flow reserve (CFR) or myocardial blood flow (MBF) is a diagnostic test used to assess the blood flow to the coronary arteries under stress. This helps a clinician understand whether the heart and its associated structures can get adequate blood, oxygen, and nutrients under any stress due to an underlying condition, thereby helping identify individuals at risk of cardiac disease. It also helps assess the prognosis of the treatment plan in individuals with coronary artery disease.
