Tachycardia due to nitrates in a patient with angina pectoris is blocked by?
**Core Concept**
Tachycardia due to nitrates in a patient with angina pectoris is a clinical scenario where nitrates, commonly used for angina management, paradoxically increase heart rate instead of reducing it. This phenomenon is attributed to the complex hemodynamic effects of nitrates, which involve the release of nitric oxide (NO) leading to vasodilation and subsequent reflex tachycardia.
**Why the Correct Answer is Right**
The reflex tachycardia induced by nitrates is primarily mediated through the baroreceptor reflex. When nitrates cause vasodilation, particularly in the venous system, it leads to a decrease in venous return to the heart. This reduction in ventricular preload triggers the baroreceptors in the carotid sinus and aortic arch to detect the decrease in blood pressure. As a result, the baroreceptors send signals to the brain, which responds by increasing sympathetic tone and heart rate to compensate for the perceived decrease in blood pressure. The correct answer is the drug that blocks this baroreceptor reflex-mediated tachycardia.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because beta-blockers would actually exacerbate the reflex tachycardia by further reducing heart rate, which would activate the baroreceptors and worsen the tachycardic response.
**Option B:** This option is incorrect because calcium channel blockers would also not be effective in blocking the baroreceptor reflex-mediated tachycardia. They would instead cause vasodilation and further reduce blood pressure, potentially worsening the reflex tachycardia.
**Option C:** This option is incorrect because ACE inhibitors would not directly block the baroreceptor reflex-mediated tachycardia. They would instead reduce blood pressure and increase cardiac output, potentially worsening the reflex tachycardia.
**Clinical Pearl / High-Yield Fact**
In clinical practice, it's essential to recognize the potential for nitrates to induce tachycardia in patients with angina pectoris. To mitigate this effect, concomitant administration of beta-blockers or other agents that can block the baroreceptor reflex, such as clonidine or doxazosin, may be considered.
**Correct Answer:** C. Clonidine.