What is a nuclear stress test? Cardiology Basics

What is a nuclear stress test? Cardiology Basics

Usual stress test for the evaluation of coronary artery disease is a treadmill exercise ECG. It is commonly part of general health check-up and is widely available. But there are certain situations when the exercise ECG is not interpretable as in a person with left bundle branch block. Then a nuclear stress test will be useful.

While exercise ECG tells you that there is a probability of obstructions in coronary arteries, it most often does not identify the region of myocardium supplied by the obstructed vessel.

Nuclear stress test will tell you which is the region of myocardium that is affected. If there is already significant damage to the myocardium, nuclear stress test will also give an indication on whether the damage is reversible if the blood vessels are opened up with a procedure. The procedure could be either a coronary angioplasty or coronary artery bypass graft surgery. This is known as myocardial viability assessment.

A radioactive tracer like technetium (99mTc) sestamibi is injected into a vein and a nuclear scan using a gamma camera is obtained after about 20 minutes. This will show the amount of radioactive tracer in each part of the myocardium at rest.

After that those who can walk on a treadmill will be connected with electrodes for monitoring ECG during the test. Treadmill slope and speed are typically increased every three minutes and maximal exercise without chest pain or ECG changes is achieved. This protocol for treadmill test is known as Bruce Protocol. The test is terminated earlier in case of significant chest pain, other major symptoms, or ECG changes. The ECG printouts are analysed by the supervising physician.

At maximal exercise, one more injection of radioactive tracer is given. One more nuclear scanning is done after waiting about 20 minutes with the gamma camera. The rest images are compared with the exercise images by the nuclear medicine specialist.

Those who are unable to exercise on the treadmill may be given medications to stress the heart as injection or infusion instead and nuclear stress images of the heart obtained as described above. Dobutamine and adenosine are two commonly used medications for this purpose. Dobutamine increases the heart rate and myocardial contractility thereby increasing the workload of the heart. Adenosine dilates the non-diseased coronary vessels and produces coronary steal, diverting blood flow from the diseased regions. Regadenoson is a newer vasodilator agent which has selective action on A2A receptors and lesser side-effects [1].

Caffeinated drinks are avoided for 24 hours prior to the procedure. Any medications containing caffeine has also to be avoided. Asthmatics on theophylline preparation will have to hold it for 48 hours. Methylxanthines theophylline and caffeine block adenosine binding due to antagonistic action at A2A receptors and can reduce the coronary vasodilation effects of adenosine. Of course, adenosine cannot be given in those with reactive airways disease with significant wheezing. Activation of the A2B and A3 receptors in bronchial tree can lead to bronchospasm. Activation of the A1 receptor results in decreased atrioventricular conduction and adenosine cannot be given in those with atrioventricular conduction disturbances in the absence of a pacemaker [1]. Such patients can be given dobutamine infusion. Cardiac patients will be asked not to take nitrates on the day of the test. All these are followed strictly as per the advice of the individual’s treating physician as the exact medical condition may mandate some modifications.

Reference

1. Hassan Mehmood Lak; Sagar Ranka; Amandeep Goyal. Pharmacologic Stress Testing. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-.