2.8.3 Cardiovascular Drugs

Overview of Cardiovascular Pharmacology

Cardiovascular drugs are a core component of Step 1 pharmacology and are frequently integrated with physiology and pathology of the heart and blood vessels. In this chapter, the focus is on the major classes of drugs that act on the cardiovascular system, their key mechanisms of action, and characteristic adverse effects that are often tested. Detailed disease management and clinical decision making are covered elsewhere, so here you should concentrate on understanding the drug classes and the physiologic targets they modify.

At a high level, cardiovascular drugs can be grouped into agents that affect blood pressure, heart rate and rhythm, myocardial contractility, and blood volume, as well as drugs that modify platelet function and lipids. Many of these agents target autonomic receptors, ion channels, or enzymes involved in vascular tone and cardiac performance, building directly on general pharmacology concepts.

Drugs Affecting the Renin Angiotensin Aldosterone System

ACE Inhibitors

Angiotensin converting enzyme (ACE) inhibitors block the conversion of angiotensin I to angiotensin II. This reduces vasoconstriction and aldosterone secretion, which results in decreased systemic vascular resistance and reduced blood volume. They also inhibit the breakdown of bradykinin, which contributes to vasodilation but also to some characteristic adverse effects.

Typical examples include captopril, enalapril, and lisinopril. They are usually given orally and many are prodrugs that are activated in the liver.

Clinically important adverse effects include chronic dry cough and angioedema due to increased bradykinin, hyperkalemia due to reduced aldosterone, hypotension, and a decline in glomerular filtration in certain renal artery conditions. They are contraindicated in bilateral renal artery stenosis and in pregnancy because of risk to fetal renal development.

Angiotensin Receptor Blockers

Angiotensin II receptor blockers, or ARBs, such as losartan and valsartan, selectively block the angiotensin II type 1 receptor. This prevents the actions of angiotensin II on vascular smooth muscle and the adrenal cortex without directly affecting bradykinin metabolism.

Because they do not increase bradykinin, ARBs have a much lower incidence of cough and angioedema compared with ACE inhibitors, although angioedema can still rarely occur. They share many of the same uses and contraindications, including risk of hyperkalemia and fetal toxicity.

Direct Renin Inhibitors

Aliskiren is the main example of a direct renin inhibitor. It binds to renin and prevents the conversion of angiotensinogen to angiotensin I at the earliest step of the renin angiotensin aldosterone system. This decreases downstream levels of angiotensin I and II and ultimately reduces blood pressure.

Adverse effects overlap with ACE inhibitors and ARBs, including hyperkalemia and renal dysfunction, and it is also contraindicated in pregnancy. It should not be combined with ACE inhibitors or ARBs in patients with diabetes due to increased risk of renal impairment and hyperkalemia.

Diuretics with Cardiovascular Relevance

Thiazide Diuretics

Thiazide diuretics such as hydrochlorothiazide and chlorthalidone are among the most commonly used antihypertensive agents. They act on the distal convoluted tubule in the kidney to inhibit the Na⁺/Cl⁻ cotransporter, which promotes natriuresis and diuresis, reducing plasma volume and cardiac output initially. With chronic use they also reduce peripheral vascular resistance, which sustains blood pressure lowering.

Important adverse effects include hypokalemic metabolic alkalosis, hyponatremia, hyperuricemia that can precipitate gout, hyperglycemia, hyperlipidemia, and hypercalcemia. They may cause allergic reactions in patients with sulfonamide sensitivity.

Loop Diuretics

Loop diuretics such as furosemide, bumetanide, and torsemide act in the thick ascending limb of the loop of Henle by blocking the Na⁺ K⁺ 2Cl⁻ cotransporter. They are potent diuretics and are particularly important in conditions with volume overload, such as acute decompensated heart failure, although heart failure management is treated separately.

Characteristic adverse effects include hypokalemic metabolic alkalosis, ototoxicity, hypomagnesemia, hypocalcemia, dehydration, and gout attacks due to hyperuricemia. They also may lead to sulfonamide induced hypersensitivity.

Potassium Sparing Diuretics

Potassium sparing diuretics such as spironolactone, eplerenone, amiloride, and triamterene have special relevance to cardiovascular disease because they can attenuate remodeling of the heart and blood vessels and help maintain potassium balance when other diuretics are used.

Spironolactone and eplerenone are aldosterone receptor antagonists in the collecting duct. Amiloride and triamterene block epithelial sodium channels in the same nephron segment. The major adverse effect of all potassium sparing diuretics is hyperkalemia, which becomes especially dangerous when combined with other agents that increase serum potassium.

Spironolactone also has antiandrogen effects that can cause gynecomastia, impotence, and menstrual irregularities. Eplerenone is more selective for mineralocorticoid receptors and has fewer endocrine adverse effects.

Calcium Channel Blockers

Dihydropyridines

Dihydropyridine calcium channel blockers, such as amlodipine, nifedipine, and nicardipine, primarily target L type calcium channels in vascular smooth muscle. This leads to vasodilation, especially of arterioles, and a decrease in peripheral vascular resistance. The cardiac conduction system is relatively spared at therapeutic doses compared with the non dihydropyridines.

Their vasodilatory effect can produce reflex tachycardia, flushing, headache, and peripheral edema, particularly in the lower extremities. Gingival hyperplasia is a classic adverse effect that is often tested. Short acting dihydropyridines, like immediate release nifedipine, can cause marked hypotension and reflex sympathetic activation and are generally avoided in chronic hypertension management.

Non Dihydropyridines

Verapamil and diltiazem are non dihydropyridine calcium channel blockers that have prominent effects on the heart. They reduce conduction through the atrioventricular node, slow heart rate, and decrease myocardial contractility by blocking L type calcium channels in cardiac tissue.

These drugs are particularly important in arrhythmia management where control of AV nodal conduction is desired. Because they can depress cardiac function, they are contraindicated in severe left ventricular dysfunction, certain conduction blocks, and with beta blockers in patients with preexisting conduction disease.

Adverse effects include bradycardia, atrioventricular block, constipation especially with verapamil, hypotension, and negative inotropic effects that may worsen heart failure symptoms.

Beta Blockers in Cardiovascular Therapy

Cardioselective and Nonselective Agents

Beta blockers are a heterogeneous group, but from a cardiovascular perspective, they reduce heart rate, decrease myocardial contractility, and lower blood pressure through decreased cardiac output and inhibition of renin release from the kidney. Cardioselective beta blockers, such as metoprolol, atenolol, and bisoprolol, preferentially block $\beta_{1}$ receptors at usual doses. Nonselective agents, such as propranolol, block both $\beta_{1}$ and $\beta_{2}$ receptors.

Some beta blockers, like carvedilol and labetalol, also block $\alpha_{1}$ receptors and provide additional vasodilation. Others, such as pindolol and acebutolol, possess partial agonist activity at beta receptors, which can lead to less resting bradycardia.

Adverse effects include bradycardia, atrioventricular block, fatigue, sexual dysfunction, and bronchospasm, particularly with nonselective agents in patients with asthma or chronic obstructive pulmonary disease. Beta blockers can mask symptoms of hypoglycemia in diabetic patients on insulin.

Abrupt Withdrawal and Clinical Cautions

Chronic beta blockade upregulates beta receptors and sensitizes tissues to catecholamines. Abrupt discontinuation can precipitate rebound tachycardia, hypertension, or angina and can worsen ischemic events. Tapering is recommended when possible. Beta blockers are relatively contraindicated in acute decompensated heart failure with fluid overload and in severe bradycardia or high grade atrioventricular block.

Vasodilators and Drugs that Modify Vascular Tone

Direct Arterial Vasodilators

Hydralazine is a direct arterial vasodilator that relaxes arteriolar smooth muscle. It significantly reduces afterload. A common compensatory response is reflex tachycardia, which can provoke angina in susceptible patients. Hydralazine can cause drug induced lupus like syndrome, particularly in slow acetylators, and may produce headache, flushing, and fluid retention.

Minoxidil is another potent arterial vasodilator. It acts by opening potassium channels in vascular smooth muscle. Because it is very potent, it is usually combined with a diuretic and a beta blocker to counteract fluid retention and reflex tachycardia. Hypertrichosis, or excessive hair growth, is a notable adverse effect and is the reason topical formulations are used for hair loss.

Nitroprusside and Other Parenteral Vasodilators

Sodium nitroprusside is an intravenous vasodilator used in situations that require rapid control of blood pressure. It releases nitric oxide, which activates guanylate cyclase in smooth muscle, increasing cyclic GMP and causing both arterial and venous dilation. Because it affects both sides of the circulation, it reduces preload and afterload and can rapidly lower blood pressure.

Nitroprusside can generate cyanide and thiocyanate as metabolites. With prolonged infusion or in renal impairment, cyanide toxicity can develop and presents with lactic acidosis, altered mental status, and cardiovascular collapse. Close monitoring is required.

Other intravenous vasodilators, such as nicardipine and clevidipine, are dihydropyridine calcium channel blockers specifically formulated for rapid blood pressure reduction. Fenoldopam is a dopamine D1 receptor agonist that causes vasodilation of peripheral arteries and promotes natriuresis.

Nitrates and Nitrite Therapy

Mechanism of Organic Nitrates

Organic nitrates such as nitroglycerin and isosorbide dinitrate are metabolized to nitric oxide in vascular smooth muscle. Nitric oxide activates guanylate cyclase and raises intracellular cyclic GMP, which leads to dephosphorylation of myosin light chains and smooth muscle relaxation.

At therapeutic doses, nitrates mainly dilate veins, which decreases venous return and reduces preload. This reduces myocardial oxygen demand and is a key concept. They also modestly dilate coronary arteries and can improve blood flow to ischemic regions, although that is not their primary benefit.

Administration, Tolerance, and Adverse Effects

Nitroglycerin can be given sublingually, orally, transdermally, or intravenously. Sublingual forms act quickly because they bypass first pass metabolism. Long acting preparations are used for chronic prophylaxis but are associated with tolerance. To reduce tolerance, a daily nitrate free interval is often used so that enzyme systems can recover.

Common adverse effects include headache, flushing, hypotension, and reflex tachycardia. When combined with phosphodiesterase 5 inhibitors like sildenafil, nitrates can cause profound hypotension due to excessive cyclic GMP accumulation.

Antiarrhythmic Drugs

Antiarrhythmic drugs are categorized based on their primary electrophysiologic action on cardiac myocytes or conduction tissue. The Vaughan Williams classification divides them into class I through class IV, along with miscellaneous agents. Only the core features relevant to cardiovascular pharmacology are outlined here, while detailed electrophysiology is treated in general pharmacology and physiology.

Class I: Sodium Channel Blockers

Class I antiarrhythmics block fast sodium channels and affect phase 0 depolarization in non nodal cardiac tissue. They are subdivided into IA, IB, and IC based on their effect on the action potential duration.

Class IA drugs, such as quinidine, procainamide, and disopyramide, moderately block sodium channels and also inhibit some potassium channels. They prolong the action potential duration and the QT interval. Quinidine can cause cinchonism, which includes tinnitus and visual disturbances. Procainamide is associated with drug induced lupus like syndrome. All IA drugs carry a risk of torsades de pointes due to QT prolongation.

Class IB drugs, including lidocaine and mexiletine, weakly block sodium channels and shorten the action potential duration. They preferentially affect ischemic or depolarized tissue and are used especially in ventricular arrhythmias associated with ischemia.

Class IC drugs, such as flecainide and propafenone, strongly block sodium channels with minimal effect on action potential duration. They markedly slow conduction but have little effect on repolarization. These agents can be proarrhythmic, especially in structural heart disease.

Class II: Beta Blockers

Class II antiarrhythmics are beta blockers that decrease sympathetic activity on the heart. They slow conduction through the sinoatrial and atrioventricular nodes by reducing cyclic AMP and calcium currents. This is particularly useful for rate control in supraventricular tachyarrhythmias. Adverse effects and cautions follow the general beta blocker profile described earlier.

Class III: Potassium Channel Blockers

Class III agents primarily block potassium channels and prolong repolarization, which increases the action potential duration and the effective refractory period. Amiodarone, sotalol, dofetilide, and ibutilide are key examples.

Sotalol has both beta blocking and class III effects and can prolong the QT interval, with risk of torsades de pointes. Dofetilide and ibutilide also prolong the QT interval and require monitoring.

Amiodarone is very important for many arrhythmias and has a broad spectrum of actions that include properties of all four major classes. It prolongs the action potential duration but is less likely to cause torsades than other class III agents. However, it has numerous noncardiac toxicities involving the lungs, thyroid, liver, and skin, which are key exam topics.

Class IV: Calcium Channel Blockers

Verapamil and diltiazem represent class IV antiarrhythmics. They act on L type calcium channels in nodal tissue and slow conduction through the atrioventricular node. They are used to control ventricular rate in atrial fibrillation and other supraventricular tachycardias. Their adverse effects reflect those already discussed for non dihydropyridine calcium channel blockers.

Miscellaneous Antiarrhythmic Agents

Adenosine is a very short acting nucleoside used intravenously for acute termination of certain supraventricular tachycardias involving the atrioventricular node. It activates adenosine receptors, increases potassium efflux, hyperpolarizes nodal tissue, and transiently blocks conduction through the atrioventricular node. Patients often experience a brief sense of chest discomfort, flushing, or dyspnea.

Digoxin, a cardiac glycoside, has both inotropic effects and actions on the atrioventricular node, and magnesium is sometimes used in specific arrhythmias such as torsades de pointes. Their detailed pharmacology, including toxicity, is discussed in other sections.

Antiplatelet and Anticoagulant Drugs with Cardiac Relevance

Antiplatelet Agents

Antiplatelet therapy is central in the management of coronary artery disease and prevention of thrombotic events.

Aspirin irreversibly inhibits cyclooxygenase 1 in platelets, reducing thromboxane A₂ production and impairing platelet aggregation for the life of the platelet. Gastrointestinal irritation, bleeding, and hypersensitivity reactions are important adverse effects.

P2Y₁₂ receptor inhibitors, such as clopidogrel, prasugrel, and ticagrelor, block ADP mediated activation of the glycoprotein IIb/IIIa complex. This decreases platelet aggregation. Clopidogrel is a prodrug activated by hepatic cytochrome P450 enzymes and shows variable efficacy in some patients. Adverse effects include bleeding and, rarely, thrombotic thrombocytopenic purpura.

Glycoprotein IIb/IIIa inhibitors, including abciximab, eptifibatide, and tirofiban, are potent intravenous agents that block the final common pathway of platelet aggregation. They are used in interventional settings and carry a significant bleeding risk.

Anticoagulants

Systemic anticoagulants reduce thrombin generation and fibrin formation, indirectly protecting the heart from thromboembolic events associated with atrial fibrillation and mechanical heart valves.

Heparins, including unfractionated heparin and low molecular weight heparins, enhance antithrombin activity, which inactivates thrombin and factor Xa. Bleeding and heparin induced thrombocytopenia are key adverse effects.

Direct oral anticoagulants target either factor Xa or thrombin directly. Rivaroxaban and apixaban inhibit factor Xa, while dabigatran inhibits thrombin. These agents are widely used in nonvalvular atrial fibrillation. Bleeding is the major adverse effect, and specific reversal agents exist for some.

Warfarin inhibits vitamin K epoxide reductase, reducing synthesis of vitamin K dependent clotting factors II, VII, IX, and X, as well as proteins C and S. Its narrow therapeutic index, numerous drug and dietary interactions, and need for monitoring of the international normalized ratio are classic Step 1 concepts. Teratogenicity and skin necrosis are important adverse effects.

Lipid Lowering Drugs

Statins

Statins, such as atorvastatin and simvastatin, inhibit HMG CoA reductase, the rate limiting enzyme of cholesterol synthesis in the liver. This decreases intracellular cholesterol and upregulates LDL receptors, leading to increased clearance of LDL from the blood. Statins are the most effective agents for lowering LDL cholesterol and are foundational in cardiovascular risk reduction.

Adverse effects include hepatotoxicity with elevated liver enzymes and myopathy, which in rare cases can progress to rhabdomyolysis. The risk of muscle toxicity increases when statins are combined with drugs that inhibit their metabolism.

Other Lipid Modifying Agents

Ezetimibe inhibits intestinal absorption of cholesterol, reducing delivery of cholesterol to the liver and leading to upregulation of LDL receptors.

Bile acid sequestrants, such as cholestyramine, bind bile acids in the intestine and prevent their reabsorption. This also promotes increased LDL receptor expression but can cause gastrointestinal discomfort and may interfere with absorption of other drugs and fat soluble vitamins.

Fibrates, such as gemfibrozil and fenofibrate, activate PPAR alpha and increase lipoprotein lipase activity, which decreases triglycerides. They can cause gallstones and myopathy, particularly when combined with statins.

PCSK9 inhibitors, like evolocumab and alirocumab, are monoclonal antibodies that increase LDL receptor recycling and markedly lower LDL cholesterol. Injection site reactions and cost are notable considerations.

Drugs Used in Heart Failure

Positive Inotropic Agents

Digoxin is a classic positive inotropic agent that inhibits the Na⁺/K⁺ ATPase on cardiac myocytes. This increases intracellular sodium, which reduces the activity of the Na⁺/Ca²⁺ exchanger, resulting in higher intracellular calcium and increased contractility. Digoxin also enhances vagal tone to the atrioventricular node and can slow conduction.

The risk of digoxin toxicity is a frequent exam focus. Toxicity can present with gastrointestinal symptoms, visual disturbances, and various arrhythmias. Hypokalemia, renal impairment, and interactions with certain drugs increase the risk.

Catecholamines such as dobutamine and phosphodiesterase 3 inhibitors like milrinone provide short term increases in contractility and are used intravenously, typically in acute or advanced heart failure settings. They increase cyclic AMP in cardiac myocytes but carry a risk of arrhythmias and increased mortality with chronic use.

Neurohormonal Modulators

In addition to ACE inhibitors, ARBs, and aldosterone antagonists, newer classes modify neurohormonal pathways in heart failure.

Neprilysin inhibitors, combined with an angiotensin receptor blocker in sacubitril valsartan, prevent breakdown of natriuretic peptides and bradykinin. This promotes vasodilation and natriuresis. Adverse effects include hypotension, hyperkalemia, and risk of angioedema. They should not be combined with ACE inhibitors because of the increased risk of bradykinin mediated effects.

Ivabradine inhibits the funny current, $I_{f}$, in the sinoatrial node, slowing heart rate without negative inotropic effects. It can cause visual phenomena such as luminous phenomena or phosphenes.

Pulmonary Hypertension and Special Vascular Agents

Pulmonary Vasodilators

Drugs used in pulmonary arterial hypertension target pathways that regulate vascular tone in the pulmonary circulation.

Endothelin receptor antagonists, such as bosentan, block endothelin 1, a potent vasoconstrictor and mitogen. This leads to pulmonary vasodilation and reduced vascular remodeling. Hepatotoxicity and teratogenicity are important adverse effects.

Phosphodiesterase 5 inhibitors, including sildenafil and tadalafil, inhibit breakdown of cyclic GMP in pulmonary vascular smooth muscle, thereby enhancing nitric oxide mediated vasodilation. They are also used for erectile dysfunction.

Prostacyclin analogs, such as epoprostenol and iloprost, mimic prostacyclin, a vasodilator and inhibitor of platelet aggregation. They improve symptoms and hemodynamics in pulmonary arterial hypertension but require complex administration and can cause flushing, jaw pain, and hypotension.

Other Vasoactive Agents

Alpha adrenergic agonists and antagonists, though primarily covered in autonomic pharmacology, have cardiovascular significance. Alpha blockers such as prazosin lower peripheral vascular resistance through vasodilation and can cause orthostatic hypotension, while alpha agonists like phenylephrine cause vasoconstriction and are sometimes used to manage hypotension.

Integration and Adverse Effect Patterns

In cardiovascular pharmacology, understanding patterns of adverse effects and interactions is as important as memorizing mechanisms. Several recurring themes are highly testable.

First, drugs that raise serum potassium, such as ACE inhibitors, ARBs, direct renin inhibitors, and potassium sparing diuretics, can together produce dangerous hyperkalemia. Second, drugs that prolong the QT interval, including class IA and class III antiarrhythmics and some other agents, share a risk of torsades de pointes. Third, many potent vasodilators produce reflex tachycardia, which can worsen ischemic heart disease if not accompanied by beta blockade.

Exam questions often present cardiovascular drugs in combination, designed to probe your knowledge of these interactions. For example, a patient given both a nitrate and a phosphodiesterase 5 inhibitor, or someone on warfarin starting a new medication that interferes with its metabolism, are classic scenarios.

By focusing on mechanisms that connect directly to cardiovascular physiology, and by recognizing adverse effect clusters and contraindications, you can approach cardiovascular pharmacology questions systematically, relating each drug back to its main target in the heart or vessels.