**Question:** Tachycardia due to nitrates in a patient with angina pectoris is blocked by-

A. Beta-blockers
B. Calcium channel blockers
C. Alpha-blockers
D. Digoxin

**Core Concept:**

Tachycardia is a rapid heart rate that can be caused by various factors, including nitrates, which are commonly used in the management of angina pectoris (chest pain related to reduced blood flow to the heart muscle due to coronary artery disease). In the context of nitrate-induced tachycardia, we are discussing the mechanism of action of nitrates and their effects on cardiac autonomic control.

**Why the Correct Answer is Right:**

Nitrates are vasodilators that work by increasing intracellular levels of cyclic guanosine monophosphate (cGMP) through the activation of soluble guanylate cyclase. This leads to relaxation of smooth muscle in the coronary arteries, improving blood flow to the heart and thus alleviating chest pain in patients with angina pectoris.

However, nitrates also have a direct cardiac effect, stimulating the release of catecholamines (such as adrenaline) from the adrenal medulla, leading to increased cardiac contractility and heart rate (tachycardia). This is because cGMP increases the sensitivity of cardiac myocytes to catecholamines.

**Why Each Wrong Option is Incorrect:**

A. Beta-blockers (e.g., propranolol) are used to block the effects of catecholamines on the heart. However, they would not address the direct cardiac effects of nitrates.

B. Calcium channel blockers (e.g., nifedipine) are used to block the depolarization of cardiac myocytes and reduce contractility. They do not address the increased sensitivity of cardiac myocytes to catecholamines due to nitrates.

C. Alpha-blockers (e.g., doxazosin) are used to block the effects of alpha-adrenergic agonists on the heart. They would not address the direct cardiac effects of nitrates.

D. Digoxin is a cardiac glycoside that increases cardiac contractility by enhancing the action of ATPase and Na+/K+-ATPase pump in cardiac myocytes. It does not address the direct cardiac effects of nitrates.

**Clinical Pearl:**

In patients with angina pectoris, it is crucial to differentiate between the direct cardiac effects of nitrates (stimulating catecholamine release and increasing cardiac contractility) and the indirect effects through vasodilation (improving coronary blood flow). Treatment options should focus on addressing the direct cardiac effects of nitrates, such as beta-blockers, and not calcium channel blockers or alpha-blockers, which would not address the increased sensitivity of cardiac myocytes to catecholamines due to nitrates.

Digoxin treatment is also not appropriate, as it increases cardiac contractility without addressing the direct cardiac effects of nitrates.

**Why Beta-Blockers are Right:**

Beta-blockers, such as propranolol (