Overview of Coronary Arterial Pressure Evaluation
When doctors need to find out if a person has epicardial coronary artery disease (CAD), a condition where the arteries that supply the heart muscle with blood get narrow, they run a special test called a left heart catheterization. In this test, they use a procedure known as angiography which is essentially an X-ray of the blood vessels. The doctors then evaluate the images to see how much narrowing of the artery has occurred.
The severely blocked or narrowed areas usually show a decrease in diameter of about 70 percent or more. However, when it comes to areas with intermediate levels of blockages or narrowing (from 40% to 70%), different doctors might make slightly different assessments. These moderately blocked areas may reduce the blood flow to the areas beyond the blockage enough to cause a lack of oxygen in the heart muscle (myocardial ischemia). This could potentially affect how well the left side of the heart pumps or contracts.
That’s why it’s very important to precisely assess how severe these blockages are when patients are in the heart catheterization lab. This will help determine the best possible treatment strategy.
Anatomy and Physiology of Coronary Arterial Pressure Evaluation
Angiography is a medical imaging test that is used to examine the coronary arteries, which are the arteries supplying oxygen and blood to your heart. This test helps to identify any unusual features, major branches, or problems such as build-up of fatty deposits, calcification, clots, or bulging of artery walls. The main arteries that angiography looks at are known as the left anterior descending (LAD), left circumflex (LCx), and the right coronary artery (RCA). The LAD and LCx branch off from the left main coronary artery (LM), which along with the RCA, usually starts from bulges in the main artery (aorta) of your heart.
The LAD and LCx have several offshoots, named diagonals and septals, and obtuse marginals respectively. The RCA often splits into the right posterior descending artery (RPDA) and right posterolateral artery (RPL), and also feeds smaller branches known as acute marginals. All these are investigated during an angiogram, where even pressure measurements can be performed. However, the tinier blood vessels in your heart are not clearly seen during this procedure.
Here’s a bit about how your heart works: the heart’s arteries usually fill with blood when the heart muscles relax between beats. Using the principle of Ohm’s law (which talks about electricity flow), your doctor can see that the pressure and flow of blood in your coronary arteries are directly connected, given there’s no resistance. The difference in pressure between the end of a coronary bed and the start of the aorta is calculated, which can be done by measuring the pressure drop across a partly blocked artery either at rest or during increased blood flow, or by directly measuring blood flow reserve, or by examining how the mean pressure gradient varies with blood flow. The two key tests used to evaluate these physiological factors are the fractional flow reserve (FFR) and the instantaneous wave-free ratio (iFR).
Why do People Need Coronary Arterial Pressure Evaluation
The use of two techniques, called fractional flow reserve (FFR) and instantaneous wave-free ratio (iFR), can help doctors understand how severe a blockage in a coronary artery is (the arteries that supply blood to the heart). These techniques are useful for moderate coronary artery disease, multi-vessel disease, various conditions resulting in sequential blockages in the coronary arteries, non-culprit lesions in acute coronary syndrome (which is a sudden blockage in the blood supply to heart muscle), and in assessing the severity of disease in a vein graft or after an intervention. Major medical societies have provided certain recommendations for these procedures:
1. In 2014, The European Society of Cardiology suggested using FFR to figure out if a coronary lesion (a diseased or damaged area in the coronary artery) is causing significant problems, especially in stable patients, when there’s no other evident sign of reduced blood flow (ischemia). They designated this a Class I recommendation (meaning it’s evidence-based and definitely suggested), with a Level of Evidence A (meaning there’s a lot of good-quality data to support it).
2. In 2012, the American College of Cardiology and the American Heart Association recommended using FFR to guide decisions about revascularization (a procedure that improves blood flow to the heart) when FFR was less than or equal to 0.8. Revascularization typically opens or bypasses blocked arteries to restore blood flow to the heart.
3. In 2017, the Society of Cardiovascular Angiography and Interventions suggested using FFR (with a value less than or equal to 0.8) to guide decisions about revascularization, especially when no form of stress testing is available or if the results of a stress test are unclear.
These guidelines help doctors understand when these techniques should be used, and help guide treatment decisions intended to restore blood flow to the heart.
When a Person Should Avoid Coronary Arterial Pressure Evaluation
There are no particular reasons why a doctor wouldn’t measure your IFR/FFR, which are measurements of blood flow to your heart. However, they should avoid using a drug named Adenosine to help measure FFR if you’re currently having a severe asthma attack.
Equipment used for Coronary Arterial Pressure Evaluation
The small pressure sensor that’s embedded into a wire used in heart procedures can take very accurate pressure measurements. This sensor is made by St. Jude and Philip Volcano and contains something known as piezo-electric sensors, which creates electricity when pressure is applied. This sensor is generally positioned at the beginning of the visible part of the wire tip. The problem with these wires is that their signals may get interrupted at connection points. Normally, these wires have a slight drift in their signal, less than 7 mmHg per hour.
FFR, or Fractional Flow Reserve, is the ratio of pressure on the far side of a blockage (Pd) to the pressure on the side closer to the heart (Pa) when the arteries are fully relaxed or dilated.
On the other hand, iFR equipment, which stands for Instantaneous Wave-Free Ratio, works based on how the heart generates waves. During a certain part of the heart’s rest period or diastole, called a wave-free period (WFP), the pressure within the coronary artery and the flow of blood decreases uniformly and the resistance in the smaller blood vessels, or microvascular resistance, is at its lowest. Therefore, iFR uses this WFP to measure the pressure within the coronary artery without having to stimulate the artery to dilate.
Who is needed to perform Coronary Arterial Pressure Evaluation?
When the pressure of your heart’s arteries is measured, it’s usually performed by a specialized heart doctor, known as an interventional cardiologist. During this procedure, they’ll have other medical professionals helping them, such as a trainee cardiologist (also known as a cardiology fellow) or a technician, and a nurse. Their role is to make sure that everything goes smoothly and safely during the procedure, while also looking after your general health.
Preparing for Coronary Arterial Pressure Evaluation
Before starting the process, it’s important to make sure all the needed tools and computer programs are ready and set up.
How is Coronary Arterial Pressure Evaluation performed
Firstly, the doctor will obtain access to your arteries either through the radial artery (wrist) or the common femoral artery (in the groin). This is done to introduce a special guiding catheter into the needed blood vessel.
A pressure wire, used to measure blood flow, is unpacked under clean conditions, prepared with saline (a clean solution similar to our body fluids), and then set to zero to ensure accurate measurements. Depending on the twists and turns of your blood vessels, the tip of the wire might be shaped accordingly. The wire is then advanced (moved forward) through an entry point and the guiding catheter into your heart artery. The doctor will make sure that the pressure recording taken by the wire at the opening of the guiding catheter looks normal.
The pressure wire is then parked, right after the narrowed part of your vessel. The doctor records a measurement called the iFR. An iFR less than or equal to 0.89 might make your doctor consider to perform a procedure to clear the narrowed part of the vessel.
In a similar way, for another measurement known as FFR, the wire is placed after the narrowed segment of your vessel. A drug called adenosine is given to you. This drug is used to increase the blood flow to your heart for a short while. The doses may vary based on the blood vessel being examined or if it is given through an infusion.
The FFR calculation is then obtained. An FFR less than 0.80 usually means that there is significant narrowing in your blood vessel. Based on these measurements, your doctor will decide the best treatment plan for you. These procedures are carried out while you are under local anesthesia so that you do not feel any discomfort.
Possible Complications of Coronary Arterial Pressure Evaluation
Undergoing PCI, or Percutaneous Coronary Intervention – a procedure commonly used to treat blocked heart arteries, can sometimes lead to complications. These complications are similar to those that can occur during any kind of heart procedure and may include vessel dissection (tear in a blood vessel), perforation (hole in a blood vessel), embolism (blockage caused by a blood clot or another substance), and coronary spasm (sudden tightening of a heart artery).
Other complications that might occur during a diagnostic test called a catheterization, where a tube-like device called a catheter is threaded through your blood vessels to your heart, can include vascular complications (problems with blood vessels), contrast induced nephropathy (a type of kidney damage caused by the dye used in the test), stroke (when blood flow to a part of the brain is cut off), myocardial infarction (another name for a heart attack), arrhythmias (irregular heartbeats), and in rare cases, death.
What Else Should I Know About Coronary Arterial Pressure Evaluation?
Large medical studies have been conducted to understand the best treatment for patients with heart disease. These studies looked at total death rate, heart attack occurrences, or the need for further heart surgery within one to two years in patients treated with a specific type of heart disease treatment known as iFR/FFR guided PCI.
The DEFER trial included 325 patients and found that those with an FFR value greater than 0.75 did not seem to benefit from percutaneous coronary intervention (PCI – a non-surgical procedure used to unblock narrowed blood vessels of the heart). On the other hand, patients with FFR less than 0.75 seemed to feel better.
The FAME trial included 1005 patients. Patients were either treated with FFR-guided PCI or just PCI based on angiography (a type of X-ray used to check blood vessels). The study found that those in the FFR-guided group had a lower rate of death, heart attacks, or the need for further heart surgery after 12 months, and this advantage was still present at 24 months.
The FAME-2 trial included 1220 patients with an FFR value of less or equal to 0.80. Patients were either given PCI and optimal medical therapy (OMT) or just OMT. The study was stopped early because those on OMT alone had a higher need for urgent heart surgery.
Two other trials, DEFINE-FLAIR and IFR SWEDEHEART, showed that iFR is just as effective as FFR in preventing major negative heart events after one year. A further detailed analysis of these two trials found that more people were able to avoid further heart surgery with iFR than with FFR, but the overall outcomes were similar in both groups.
