Doctors use a combination of non-invasive tests and, when needed, an invasive procedure called cardiac catheterization to determine whether you need a stent. The process typically moves in stages: initial screening narrows down the likelihood of a significant blockage, and if suspicion is high enough, a catheter-based angiogram provides the definitive answer. A blockage of 70% or more in a coronary artery is the general threshold where stenting becomes likely.
It Usually Starts With a Stress Test
Before anyone threads a catheter into your heart, your doctor will almost certainly order a less invasive test first. The most common starting point is a stress test, which checks how your heart performs under exertion. You’ll walk on a treadmill or ride a stationary bike while your heart rate, blood pressure, and electrical activity are monitored. In some versions, a radioactive tracer or ultrasound imaging is added to create pictures of blood flow through your heart muscle.
The purpose is straightforward: if blood flow looks normal during exercise, a significant blockage is unlikely, and you probably don’t need further testing. A negative exercise test in low-risk patients is associated with excellent long-term outcomes. But if the test reveals areas of your heart that aren’t getting enough blood during exertion, that’s a strong signal to move to the next step. For people with a high probability of heart disease based on symptoms and risk factors, doctors may skip directly to imaging or catheterization.
CT Angiography: A Non-Invasive Look Inside
Coronary CT angiography (CCTA) uses a specialized CT scanner to create detailed 3D images of your coronary arteries without inserting a catheter. You lie in the scanner, receive an injection of contrast dye through an IV in your arm, and hold still for a few seconds while the machine captures images. The whole scan takes minutes.
CCTA has become an increasingly important “gatekeeper” that helps doctors decide whether an invasive angiogram is actually necessary. Studies estimate that 20 to 70% of patients being evaluated can safely avoid an invasive catheterization based on CT results alone. If the CT scan shows clean or minimally narrowed arteries, you’re unlikely to need a stent, and the workup can stop there. If it reveals concerning narrowing, the next step is usually a catheter-based angiogram to get a closer look and potentially treat the blockage in the same session.
Cardiac Catheterization: The Definitive Test
This is the gold standard for diagnosing coronary artery blockages, and it’s the test that can lead directly to stent placement. During a cardiac catheterization (also called a coronary angiogram), a cardiologist inserts a thin, flexible tube called a catheter into a blood vessel, typically through your wrist or groin, and guides it up to your heart.
Once the catheter is in position, contrast dye is injected through it into your coronary arteries. An X-ray camera records video as the dye flows through, creating a real-time map of your arteries that reveals exactly where narrowing or blockages exist, how severe they are, and which vessels are affected. You’ll be awake but sedated during the procedure. When the dye is injected, you may feel a brief flush of warmth through your chest, which is normal and passes in seconds. The cardiologist may ask you to hold your breath or turn your head at certain points to help position the catheter.
The procedure typically takes 30 to 60 minutes. If the images show a blockage severe enough to warrant treatment, the cardiologist can often place a stent right then, during the same session, rather than scheduling a second procedure.
How Doctors Decide the Blockage Is Severe Enough
Finding a blockage isn’t the same as needing a stent. Doctors weigh several factors before making that call.
The most straightforward measure is the percentage of the artery that’s blocked. A blockage of 70% or more generally triggers stent placement. Below that threshold, many people don’t even experience symptoms, and medication alone often manages the condition effectively. Some doctors may suggest stenting for blockages under 70%, but clinical guidelines emphasize that for stable patients (those not having a heart attack), stenting has not been shown to improve survival compared to medication. What it does reliably improve is symptoms, particularly chest pain that limits daily activity.
Pressure Testing Inside the Artery
When the angiogram shows a blockage that’s borderline, somewhere in the gray zone where images alone don’t make the decision obvious, the cardiologist can measure how much the blockage actually restricts blood flow. This is done by passing a tiny pressure-sensing wire through the catheter and across the narrowed area. The wire compares the blood pressure on either side of the blockage to produce a ratio.
A ratio below 0.80 indicates the blockage is significant enough to reduce blood flow in a meaningful way, which supports the case for stenting. Values above 0.80 suggest the blockage isn’t causing enough flow restriction to justify a stent, even if it looks concerning on the angiogram. This pressure-based approach prevents unnecessary stenting by distinguishing blockages that look bad from blockages that actually are bad in terms of limiting blood supply to heart muscle.
Imaging From Inside the Artery
In some cases, the cardiologist uses a miniature ultrasound probe or a light-based imaging device threaded through the catheter to see the artery wall in cross-section. These tools measure the exact size of the remaining opening inside the artery. An opening smaller than about 1.6 to 1.9 square millimeters is predictive of a flow-limiting blockage. This internal imaging also helps the cardiologist choose the right stent size and plan how to position it.
Emergency vs. Planned Evaluation
The path to a stent looks very different depending on whether you’re having a heart attack or being evaluated for stable symptoms like occasional chest pain with exertion.
If you arrive at the emergency room with signs of a heart attack, particularly the type caused by a completely blocked artery (called a STEMI), the goal is to open that artery as fast as possible. You’ll be taken to the catheterization lab for an emergency angiogram, and if the blockage is confirmed, a stent goes in immediately. Rapid treatment in this scenario directly improves survival. Guidelines recommend that this happen within 120 minutes of arriving at the hospital. If a catheterization lab isn’t available quickly enough, clot-dissolving medication is used as a bridge.
For less severe heart attacks where the artery isn’t completely blocked, timing depends on your risk level. High-risk patients typically undergo catheterization within 24 hours. Intermediate or low-risk patients may be evaluated within 48 to 72 hours.
For stable symptoms, the process is more deliberate. You’ll go through the screening steps described above: stress testing, possibly a CT scan, and then catheterization only if those results warrant it. If a stent is placed for stable chest pain, the primary benefit is symptom relief rather than preventing a future heart attack.
Blood Work and Preparation Before the Procedure
Before a catheterization, your doctor will order blood tests to check your kidney function, since the contrast dye used during the procedure is filtered by your kidneys and can cause damage in people with reduced kidney function. The key number is your estimated glomerular filtration rate (eGFR), a measure of how well your kidneys are filtering. If your eGFR is low, the medical team may take extra precautions like using less contrast dye or hydrating you with IV fluids before and after the procedure. People with diabetes, heart failure, or anemia face higher risk of kidney-related complications from contrast dye.
You’ll also have your blood clotting checked, since the procedure involves accessing a blood vessel. Expect to fast for several hours beforehand, and your doctor will review your medications, particularly blood thinners, to decide what to adjust.