Pharmacology
Summary
This lesson discusses the medical applications, mechanism of action, and side effects of nitrates. Nitrates, such as nitroglycerin, are often used in the management of stable angina due to their ability to relax vascular smooth muscle, causing marked venodilation. This venodilation reduces the heart's preload, left ventricular end-diastolic volume, and consequently, the myocardial oxygen demand, thus easing angina symptoms. Nitrates also have a smaller impact on arterial vasodilation, which lowers systemic vascular resistance. While sublingual nitroglycerin is effective for immediate symptom relief, oral nitrates serve a prophylactic purpose, preventing anticipated angina episodes.
Nitrates are used in treating additional medical conditions such as vasospastic angina, acute coronary syndrome and hypertensive emergencies. The side effects of nitrates are mainly related to venous pooling of blood due to increased venous capacitance, which can cause symptoms like hypotension, syncope, reflex tachycardia, headaches, and flushing. These side effects may be managed with beta blockers or calcium channel blockers. Furthermore, patients can develop a tolerance towards nitrate therapy necessitating a daily nitrate-free interval. Nitrates are contraindicated in patients with right-sided myocardial infarctions, hypertrophic cardiomyopathy, and in those taking phosphodiesterase inhibitors.
Lesson Outline
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FAQs
Nitrates, such as nitroglycerin, serve as cornerstone treatments for angina due to their vasodilatory properties. Nitrates primarily decrease myocardial oxygen consumption through marked venous dilation, which subsequently reduces cardiac preload. Nitrates exert minimal effects on arterioles, minimizing a rise in cardiac afterload. Collectively, these actions lessen the heart's workload and provide relief from anginal discomfort.
Sublingual nitroglycerin is often administered on the onset of an angina attack because it is rapidly absorbed through the oral mucosa, leading to almost immediate relief. Oral nitroglycerin has a low bioavailability due to extensive first pass metabolism, making sublingual nitroglycerin inidicated for acute symptoms relief.
Nitrate-induced vasodilation can lead to a sudden drop in blood pressure. The body responds to this drop through reflex sympathetic activation, a compensatory mechanism that aims to raise BP by increasing heart rate and vascular tone. This reflex can lead to undesirable symptoms like tachycardia. Calcium channel blockers and beta blockers are commonly utilized to prevent reflex sympathetic activation.
Nitrates, though beneficial in many cardiac conditions, is associated with several adverse effects due to their vasodilatory properties. These include venous pooling of blood, which can manifest as orthostatic hypotension and syncope. This pronounced venous dilation may also lead to reflex tachycardia. Additionally, patients might experience throbbing headaches and facial flushing. In more severe scenarios, overexposure to nitrates can induce methemoglobinemia, a condition where hemoglobin is oxidized, rendering it incapable of effectively carrying oxygen to body tissues.
