Myocardial Perfusion SPECT Medication

Updated: Feb 24, 2016
  • Author: Ricardo Cardona, MD; Chief Editor: Gowthaman Gunabushanam, MD, FRCR  more...
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Medication

Medication Summary

A radioisotope is used for this type of diagnostic test used in nuclear medicine. The most commonly used tracers are thallium-201 thallous chloride (thallium), techntium-99m 2-methoxy-isobutyl-isonitrile (MIBI) and technetium-99m 1,2-bis [bis(2-ethoxyethyl)phosphino] ethane (tetrofosmin). Thallium-201 and the 2 technetium-labeled radiopharmaceuticals (MIBI and tetrofosmin) are commercially available.

As also mentioned above, a pharmacological stress with vasodilators is the procedure of choice for patients unable to exercise adequately or for those with LBBB or paced rhythm. The 2 more commonly used vasodilators are adenosine and dipyridamole.

Dobutamine infusion is commonly used when dynamic exercise is not feasible and contraindications to vasodilator stress are noted.

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Radiopharmaceuticals Medication Class

Thallium-201

Thallium-201 was introduced as a perfusion tracer because of its high (about 80%) first pass myocardial extraction fraction and provides a good estimate of myocardial blood flow. [35] The myocardial uptake of thallium-201 is a sarcolemmal membrane Na/K ATPase-dependent active process requiring cell membrane integrity. It is therefore indicative of myocardial viability. Thallium-201 uptake by myocytes is a reflection both of regional perfusion and viability. Its physical half-life is prolonged (73 h), limiting the overall amount that can be administered.

Technetium-99 Labeled Radiotracers

Technetium-99m sestamibi, a lipophilic cationic compound, and Tc-99m tetrofosmin, a diphosphine agent, are the Tc-99m labeled radiotracers. The uptake of these agents across myocytes cell membranes is by passive diffusion and generally parallels myocardial perfusion, in contrast to the active uptake thallium-201. [36] The uptake of these compounds depend on the presence of intact electrochemical gradients across sarcolemmal and mitochondrial membranes. A significant difference from thalium-201 is that these agents show minimal distribution within the myocardium. They also have a shorter half-life of 6 hours.

Another less commonly used agent is teboroxime. It differs from thallium and sestamibi in that its uptake appears to be relatively insensitive to myocardial injury. [37] Therefore, teboroxime may serve as a pure blood flow agent that does not depend on cellular viability. In contrast to sestamibi, teboroxime is characterized by rapid uptake and washout, with a clearance half-time in the myocardium of only a few minutes.

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Vasodilators

Class Summary

The two more commonly used vasodilators for pharmacologic myocardial perfusion imaging is adenosine and dipyridamole. These drugs produce primary coronary vasodilation that is mediated by the A2A adenosine receptors. [38] Adenosine and dipyridamole are equally effective diagnostically, but adenosine has the advantages of a short half-life, rapid reversal of side effects after the test is completed, and possibly more predictable vasodilation. On the other hand, because of its longer half-life, patients can receive a dipyridamole infusion and then undergo any level of supplemental exercise.

Adenosine (Adenoscan)

Adenosine is a potent vasodilator in most vascular beds, except in renal afferent arterioles and hepatic veins where it produces vasoconstriction. It is indicated as an adjunct to thallium-201 myocardial perfusion scintigraphy in patients unable to exercise adequately.

Dipyridamole

Dipyridamole injection is a coronary vasodilator. The mechanism of vasodilation has not been fully elucidated, but may result from inhibition of uptake of adenosine, an important mediator of coronary vasodilation. It is indicated as an alternative to exercise in thallium myocardial perfusion imaging for the evaluation of coronary artery disease in patients who cannot exercise adequately.

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Inotropic Agents

Class Summary

Dobutamine is indicated as a stress agent in patients who are not candidates for vasodilator stress testing including those with reactive airways disease or those who have recently taken theophylline or have recent significant caffeine intake.

Dobutamine

Dobutamine is a potent beta1-receptor agonist and a week beta2-receptor agonist. The beta1 stimulation produces positive inotropic and chronotropic effects, and the beta2 stimulation produces peripheral vasodilation.

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