To Your Good Health: Understanding the mechanisms behind a nuclear stress test

I had a comment about your recent column on nuclear stress tests. As an 87-year-old person who has experienced atrial fibrillation and a mild stroke, I have had two nuclear stress tests done. In both cases, I told the supervising nurse I wanted to go to the max limits that they would allow. Even though I attempted to prepare by briskly walking up the hill close to my home and thought I was doing well on the treadmill, I failed to reach the final parameters that they were seeking. To get me there, I was injected with something — the name escapes me — that is supposed to achieve the same result as the final stage of a nuclear stress test. As I recall the discussion, the nurse stated that many couldn’t get as far as I did. Medications were a way for them to obtain the needed results.

Stress testing may be done for several reasons, but the most common is to determine whether there are blockages in the arteries in the heart that are severe enough to limit blood flow.

When ordering a stress test, the clinician has to decide how to stress the heart and get information from it.

In people who can exercise — meaning they don’t have an orthopedic or neurological condition that keeps them from being able to move — then using exercise is the best approach in almost all cases.

Exercise is safer since you can stop doing it at any time, and it also provides useful information. People who can exercise for a longer time — 9 minutes on a standard protocol called a Bruce — have a very good prognosis; whereas if you can’t make it to 3 minutes, this is a poor prognosis.

The treadmill gets steeper and faster every 3 minutes on a Bruce protocol so that eventually even an Olympic athlete has to give up. The goal is to get to maximum exercise where a person simply can’t go any further.

The person who runs the stress test looks for any sign of incipient heart damage through an electrocardiogram, a blood pressure monitor, or symptoms so that the test can be stopped if it seems dangerous.

Despite trying really hard, some people cannot get their heart rate and blood pressure high enough where the stress test can diagnose heart disease. In this case, the exercise stress test is converted to a pharmacologic stress test.

A medication, such as dobutamine, is given to make the heart beat faster and stronger until the adequate workload is achieved.

Many of my patients tell me that this is uncomfortable, and more than once, I have heard someone say, “My heart was going to jump out of my chest.” If there are signs of danger, the medication can be stopped, and in some cases, a reversal agent is given.

Once the heart is at an adequate level of exertion, abnormalities in the blood flow can be detected by the EKG.

However, the test’s sensitivity can be improved by looking at blood flow through a radioactive chemical injected in the blood, which can measure blood flow to various regions of the heart more precisely.

The sensitivity can also be improved by examining the heart’s wall movement through echocardiography.

In other cases, a more limited stress test is done to determine the safety of exercise. This is often done after a heart attack to determine which exercises a person can do as part of their cardiac rehab.