Indications for pharmacologic agents
Pharmacologic stress tests are indicated in the following scenarios:
- Inability to exercise due to physical limitations.
- Contraindications to exercise, such as unstable angina, severe aortic stenosis, decompensated heart failure, or aortic aneurysm.
- Baseline ECG abnormalities rendering ST-T interpretation difficult, including left bundle branch block (LBBB) or pacemaker rhythms.
- Atrial fibrillation or atrial flutter
- Patients on medications such as nitrates, beta-blockers, or calcium channel blockers
- Recent acute myocardial infarction (AMI)
Pharmacologic agents in perfusion imaging
Vasodilators (adenosine, dipyridamole, regadenoson)
These agents exert vasodilatory effects, increasing coronary blood flow by relaxing vascular smooth muscle. They are contraindicated in patients with bronchospastic lung disease, or a history of significant reactive airway disease. Contraindications also include second-degree or third-degree AV block, sinus node dysfunction without a pacemaker, systolic blood pressure below 90 mmHg, and known hypersensitivity to the agent. Aminophylline is the antidote for regadenoson toxicity.
Adenosine and regadenoson work primarily by activating A2A adenosine receptors, which leads to coronary vasodilation. This effect is more pronounced in healthy arteries, which can dilate more than stenotic arteries, which are already dilated. This results in flow heterogeneity and perfusion defects in areas supplied by the disease artery. Thus, vasodilators replicate the effects of exercise on coronary arteries, without the need for physical activity.
Advantages of regadenoson over adenosine
Regadenoson has advantages over adenosine:
- Regadenoson is a selective A2A receptor agonist: A2A receptors are responsible for coronary vasodilation. This selectivity minimizes activation of A1, A2B, and A3 receptors, which are associated with adverse effects such as bronchoconstriction and atrioventricular block. Adenosine is an agonist on A1, A2A, A2B and A3 receptors.
- Fixed-dose administration: Regadenoson dosing is not weight-based, simplifying administration.
Dipyridamole
Dipyridamole has multiple targets that contribute to its vasodilatory effects. It inhibits phosphodiesterases, particularly PDE3 and PDE5, which break down cAMP and cGMP. It also inhibits the cellular reuptake of adenosine into endothelial cells. Further, it inhibits adenosine deaminase, which is responsible for breaking down adenosine. These actions result in increased extracellular adenosine concentrations, causing vasodilation.
Dobutamine
Dobutamine is a chronotropic and inotropic agent. It increases heart rate, myocardial contractility, and blood pressure, thereby simulating the effects of exercise. However, dobutamine is rarely used in current practice due to its less favorable safety profile and the preference for vasodilators like adenosine, dipyridamole, and regadenoson, which are generally better tolerated and easier to administer.
