Have you ever wondered what happens to your heart when you exercise, run, or climb stairs? Your heart needs more oxygen than it normally would. To meet this increased need for oxygen, your coronary arteries, which supply blood to your heart, need to pump more blood. So, your heart's ability to pump more blood when the need increases, is simply called Coronary Flow Reserve (CFR). This is like the 'reserve power' of your heart.
What is Coronary Flow Reserve (CFR)?
Simply put, Coronary Flow Reserve (CFR) is a measure of how much blood flow through your heart's coronary arteries can be increased during peak exertion. This can give your doctor valuable information about the health of your heart, especially the condition of the arteries that supply blood to your heart (coronary arteries).
Think about it this way. When you're sitting in a chair, your heart needs a certain amount of blood. But when you suddenly run after a bus, your heart's work increases dramatically. Your heart then needs three to four times more oxygen than normal. A healthy heart's blood vessels can meet this increased demand, increasing blood flow instantly. The CFR test measures your heart's ability to do this.
This test helps to identify early whether you have coronary artery disease or are at risk of developing a condition such as a heart attack in the future.
How is the CFR value calculated?
The way this is calculated is very simple. Your doctor measures the blood flow to your heart in two ways.
1. At-rest blood flow: This measures the amount of blood flowing to the heart when you are at rest, without any exertion.
2. Maximum blood flow: The heart is forced to pump blood at maximum effort (a drug is given for this), and the maximum amount of blood that flows at that time is measured.
Then, this maximum blood flow is divided by the resting blood flow.
CFR = (Maximum Blood Flow) / (Resting Blood Flow)
For example, if your maximum blood flow is three times your resting flow, your CFR value will be 3.
What is an average CFR value?
A healthy person's CFR should be between 2.0 and 2.5 . This means that the heart should be able to pump at least twice as much blood as it can when it is at rest. In the healthiest person, this value can be 3, 4, or even higher.
- If the CFR value is less than 2.0:This means that there may be some problem with the blood vessels in the heart. Perhaps the blood vessels are narrowed (blocked). People like this are at higher risk of developing heart disease in the future.
- If the CFR value is greater than 2.0: It indicates that the functioning of the blood vessels in the heart is at a good level.
But one thing to remember is that this CFR value naturally decreases slightly with age. Therefore, the normal value for a young person may be different from the normal value for an older person. Only your doctor can accurately interpret your results.
How to do the CFR test?
There are two main methods for performing this test: one is an invasive method, which involves inserting a device into the body, and the other is an external method, which involves performing it non-invasively.
| Test method | Simple explanation |
|---|---|
| Invasive - Cardiac Catheterization | In this, a very thin tube (catheter) is passed through a vein in your arm, leg, or neck to the coronary arteries of your heart. A special wire is then passed through to measure the speed of blood flow. This can be done at the same time as an angiogram. |
| Non-invasive method - PET Scan | This is a special scan. In this test, you are given a very small amount of a radioactive substance (radiotracer) through an IV. Then, a PET scanner measures blood flow by observing how the substance moves through the heart. |
| Other methods - Cardiac MRI | This is also an external scan, but this method is rarely used to measure CFR. |
How should you prepare before the test?
Your doctor will explain this to you. Generally, you should be careful about these things.
- Medication Information: Tell your doctor about all the medications you are taking (both prescription and over-the-counter). Some medications can affect blood clotting and you may be asked to stop taking them the day before the test.
- Allergies: Be sure to tell us if you have any allergies to any medications or contrast dye.
- Fasting: If you are having a cardiac catheterization or PET scan, you will be advised to refrain from eating or drinking for 6-8 hours before the test.
- For an MRI: You cannot wear any metal objects (jewelry, watches) on your body. If you have a device like a pacemaker inside your body, ask your doctor in advance if it is safe for an MRI.
What to expect during the test?
Regardless of the test method you have, an IV tube (cannula) will be placed in your arm. The steps of the test will look like this.
1. Resting state measurement: First, the blood flow in your heart is measured while you are calm and at rest.
2. Giving a drug: Next, a drug (vasodilator) that dilates the blood vessels in your heart and maximizes blood flow is given through an IV. The most commonly used drug is Adenosine . When this is given, blood flow to the heart increases by 4-5 times within 2-3 minutes.
3. Peak measurement: When the blood flow reaches its peak after the drug is given, the blood flow is measured again.
4. Comparison: Finally, these two metrics are compared to calculate your CFR value.
If you are having a cardiac catheterization, you may be given a sedative to help you sleep lightly. This will help you feel less uncomfortable. When having a scan, it is very important to remain still to get clear images.
What happens after the test?
After the test, you can live normally, but you need to be careful about some things.
- If you have had a cardiac catheterization, you will need to keep the arm or leg where the catheter was inserted upright for about an hour to prevent bleeding.
- The doctor will advise you to drink more water to help the contrast dye leave your body more quickly.
- If you have been given a sedative, you cannot drive that day. It is essential that someone accompany you home.
Are there any risks or side effects to this test?
A CFR test is generally a very safe test, but as with any medical test, there are some very small risks.
- Catheter-related risks:During cardiac catheterization, blood clots, damage to blood vessels, or bleeding can very rarely occur.
- Side effects of adenosine: Some people may experience the following for a few seconds after being given this medicine:
- Abnormal heart rhythm
- Hypotension
- Difficulty breathing
- Chest discomfort
These symptoms disappear completely as soon as the medicine wears off (within a minute) . There is nothing to worry about.
Important: Adenosine is not suitable for people with asthma because it can cause bronchospasm . If you have asthma, your doctor will use another medicine instead.
What do the results mean and when will we know?
Your CFR is a number that tells you how much blood your heart can pump when it is stressed.
It can take a few days for the doctor to analyze the test results and provide a report. Sometimes, if the results are borderline, other specialists may be consulted.
If your results indicate a risk of heart disease, your doctor will provide you with a treatment plan to reduce that risk.
Take-Home Message
- Coronary Flow Reserve (CFR) is a measure of the 'reserve power' your heart has to increase blood flow during times of stress.
- This test can provide important information about the health of the heart's blood vessels. It can also help identify future heart disease risks early.
- Generally, a CFR value greater than 2.0 is a healthy characteristic.
- Be sure to tell your doctor about any medications you are taking and any allergies you may have before the test.
- This is a very safe test. If you have any questions about the test, don't hesitate to ask your doctor .
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