Heart disease is scary. In fact, it is the leading cause of death in the United States. Coronary artery disease, the most common form of heart disease, accounted for 382,000 deaths in the United States in 2020 alone.
Even scarier, many people walk around with heart disease without even knowing it!
Fortunately, medicine has evolved to improve our ability to detect and treat heart disease. One such advancement is the nuclear stress test, also known as Myocardial Perfusion Imaging (MPI).
In this article, I’ll cover the basics of a cardiac stress test, what a nuclear medicine stress test is, why it’s performed, how it’s performed, and briefly discuss “negative” and “positive” test results.
Let’s start with a brief overview of coronary artery disease and why it matters.
Heart Basics
The heart is a large muscle in the chest that acts as a pump to move blood throughout the body. Like all muscles, the heart requires oxygen and nutrients to function, with higher levels of activity requiring greater amounts of resources.
The heart consists of four chambers – two atria (top chambers) and two ventricles (bottom chambers). It can be thought of as two separate sides/pumps, right and left. The top chambers (atria) act as small pumps that fill the bottom chambers (ventricles). The ventricles are the larger/stronger pumps that pump blood throughout the body.
The right side of the heart pumps blood through the pulmonary arteries to the lungs, where the blood gets rid of carbon dioxide (CO2) and picks up oxygen (O2).
The blood returns to the left side of the heart, which then pumps the blood to the rest of the body, supplying all of the organs, muscles, etc., with oxygen and nutrients. The left ventricle pumps blood throughout the entire body – to your head, toes, and everything in between – and therefore is a larger/thicker muscle than the right ventricle because it has to pump blood a much longer distance.
What is Coronary Artery Disease?
The coronary arteries are the blood vessels that deliver oxygen and nutrients to the heart. The right coronary artery delivers blood to the right and back sides of the heart and the left coronary artery delivers blood to the left and front sides of the heart.
The left main coronary artery typically splits into a “left anterior descending” coronary artery that supplies the front and left sides of the heart and a “circumflex” coronary artery that wraps around to the back of the heart, supplying the left and back portions of the heart.
Coronary artery disease (CAD) occurs when one or more of the coronary arteries becomes “diseased.”
As we age, “bad” LDL cholesterol is gradually deposited in the wall of the coronary arteries as atherosclerotic plaque. These plaques incite a local inflammatory response, grow over time, can calcify and become rigid, and narrow or even completely block the arteries, like a clogged drain pipe. This narrowing limits the amount of oxygen and nutrients the heart receives. Over time, the vessels may become too narrow to deliver sufficient blood to keep up with the increased demands of the heart when it’s under stress.
Normal Blood Flow
In a heart with normal coronary arteries, the body can regulate the size of the coronary arteries based on the heart’s energy needs. The body can assess how much blood the heart needs and increase or decrease the size of the arteries to deliver more or less blood to match energy requirements.
At rest (low levels of physical activity such as sitting down), the heart doesn’t require much energy. During stress (higher levels of physical activity such as exercising or climbing a flight of stairs), the heart requires more energy, so the body sends a signal to the coronary arteries to expand (dilate) up to ~3-5x larger to allow more blood to be delivered to the heart muscle.
This is a normal physiologic response and is essential to our fight-or-flight response. For example, if you need to run away from a bear, your heart needs to pump faster to deliver more blood to the body (particularly your muscles) so you can run away from the bear. As a muscle itself, the heart also needs more nutrient-enriched blood to sustain more rigorous activity.
The body achieves this by expanding (dilating) the arteries supplying the muscles in use. The heart, therefore, pumps a higher amount (volume) of blood and at a higher rate (your heart beats faster) to significantly increase the amount of blood available to your muscles (whether in your legs or heart) to sustain your run from the bear.
Abnormal Blood Flow
As atherosclerotic plaque builds up, the inside of the blood vessel becomes smaller (more narrow), allowing less blood to pass through and delivering less oxygen and nutrients to the heart.
Diseased blood vessels (generally narrowed by at least 50%) are unable to dilate to accommodate the increased demand and remain narrowed. The heart muscle will still respond to stress by beating faster but will not receive the extra blood needed to sustain the higher activity level.
The portions of the heart supplied by these diseased arteries may experience a state of inadequate blood flow known as ischemia. Prolonged or severely limited blood flow to the heart can progress from ischemia to infarction, where the heart muscle cells begin to die due to a lack of nutrient-enriched blood.
Heart muscle (myocardium) dying (infarction) is where “myocardial infarction,” better known as a “heart attack,” gets its name. Heart function may be affected when portions of the heart muscle die, depending on the location and amount of heart muscle involved.
When Should You Get a Cardiac Stress Test?
Cardiac stress testing is generally appropriate when a person has symptoms of heart disease that typically worsen with physical activity or exertion. This is especially important when the person has risk factors for CAD, such as family history, high cholesterol, previous heart attack, etc.
Common symptoms of CAD include chest pain, chest pressure (known as angina, sometimes described as feeling like an elephant is sitting on your chest), shortness of breath, dizziness or lightheadedness, and arm discomfort.
If you are experiencing any of these symptoms, please inform your doctor or primary ca