Cardiovascular Pharmacology
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Cardiovascular diseases, such as hypertension, angina, heart failure, and arrhythmias, are among the most common chronic conditions treated in healthcare. Pharmacological agents targeting the cardiovascular system aim to manage blood pressure, improve blood flow, and optimize heart function. Understanding these medications is critical for both patient care and success on the INBDE.
ANTIHYPERTENSIVES
Hypertension, or high blood pressure, is a major risk factor for cardiovascular disease. Antihypertensive drugs aim to reduce blood pressure by targeting one or more of the three main factors that regulate blood pressure: cardiac output, vascular resistance, and blood volume.
Diuretics are often the first-line treatment for hypertension. They decrease blood volume by promoting the excretion of sodium and water through the kidneys, which reduces cardiac output and blood pressure.
Diuretics act on specific segments of the nephron, the functional unit of the kidney, to inhibit sodium reabsorption. Examples include:
- Thiazides (hydrochlorothiazide) – act on the distal tubule, reducing sodium reabsorption. They are the most commonly prescribed diuretics for hypertension. Side effects include hypokalemia (low potassium levels).
- Potassium-Sparing Diuretics (spironolactone) – act on the collecting duct, conserving potassium while promoting sodium excretion. These are often used in combination with thiazides to prevent hypokalemia. Side effects may include hyperkalemia (high potassium levels).
- Loop Diuretics (furosemide) – act on the loop of Henle, causing potent diuresis and sodium excretion. They are typically reserved for more severe cases or for heart failure management due to their strong effects.
Calcium channel blockers reduce blood pressure by relaxing vascular smooth muscle, leading to vasodilation, and, in some cases, reducing cardiac contractility. CCBs block the influx of calcium ions into vascular and cardiac muscle cells, which inhibits contraction. Gingival hyperplasia (overgrowth of gum tissue) is a notable side effect of calcium channel blockers and is frequently tested on the INBDE. Examples include:
- Dihydropyridines (amlodipine) – primarily cause vasodilation of peripheral arteries.
- Non-dihydropyridines (verapamil) – act on both the heart and blood vessels, decreasing heart rate and contractility.
Angiotensin Converting Enzyme (ACE) Inhibitors and Angiotensin II Receptor Blockers (ARBs) target the renin-angiotensin-aldosterone system (RAAS), which plays a key role in regulating blood pressure. Both ACE inhibitors and ARBs are first-line treatments for hypertension, especially in patients with diabetes or chronic kidney disease.
- ACE Inhibitors (lisinopril) – block the enzyme angiotensin-converting enzyme (ACE), preventing the formation of angiotensin II, a potent vasoconstrictor. This results in vasodilation and reduced blood pressure. ACE inhibitors also decrease aldosterone levels, which promotes sodium and water excretion. A common side effect is a dry cough, caused by the buildup of bradykinin.
- ARBs (losartan) – block the binding of angiotensin II to its receptor, producing similar effects as ACE inhibitors but without the dry cough.
Beta-blockers (propranolol, metoprolol) reduce blood pressure by decreasing heart rate and cardiac output. They block beta-adrenergic receptors, specifically beta-1 receptors in the heart. Beta-blockers are no longer considered first-line therapy for hypertension but are often used in patients with comorbid conditions such as heart failure or arrhythmias.
ANGINA
Angina occurs when the oxygen supply to the cardiac muscle is insufficient to meet its metabolic demands, often due to coronary artery disease. Anti-anginal drugs aim to improve oxygen delivery or reduce oxygen demand.
Nitroglycerin is a nitrate that provides rapid relief of angina by causing vasodilation and improving blood flow to the heart. Nitroglycerin is converted to nitric oxide (NO) in vascular smooth muscle, which activates a signaling cascade that leads to relaxation and vasodilation. Sublingual nitroglycerin is used for acute anginal attacks due to its rapid onset of action. It is also used prophylactically before activities that may provoke angina. Common side effects include headache, flushing, and hypotension.
Beta-blockers (atenolol, propranolol) are used to reduce the oxygen demand of the heart by decreasing heart rate and contractility. Blockade of beta-1 receptors in the heart reduces myocardial oxygen consumption. Beta-blockers are effective in preventing angina and are often used in combination with other anti-anginal agents. Patients taking beta-blockers may experience heightened sensitivity to epinephrine, leading to increased blood pressure. Avoid excessive use of epinephrine in these patients.
Calcium channel blockers (e.g., amlodipine, verapamil) are effective in both preventing and treating angina by reducing myocardial oxygen demand by decreasing afterload (via vasodilation) and, in some cases, decreasing heart rate and contractility. CCBs are particularly useful in patients who cannot tolerate beta-blockers or nitrates.
ANTI-ARRHYTHMICS
Arrhythmias are disturbances in the heart’s electrical conduction system that lead to irregular heartbeats. Anti-arrhythmic drugs are classified into four main categories based on their mechanisms of action.
- Class I: Sodium Channel Blockers
- These drugs reduce the excitability of cardiac muscle by blocking sodium channels during depolarization.
- Example: Quinidine is used to treat atrial fibrillation and flutter.
- Class II: Beta-Blockers
- Beta-blockers (e.g., propranolol) reduce sympathetic stimulation of the heart, decreasing heart rate and contractility.
- These are commonly used for arrhythmias originating in the atria, such as atrial fibrillation.
- Class III: Potassium Channel Blockers
- These drugs prolong the action potential duration and refractory period by blocking potassium channels.
- Example: Amiodarone, a potent anti-arrhythmic used for both atrial and ventricular arrhythmias.
- Class IV: Calcium Channel Blockers
- These drugs slow conduction through the AV node and reduce heart rate.
- Example: Verapamil is used for atrial fibrillation and supraventricular tachycardias.
Digitalis increases vagal tone, slowing conduction through the AV node. It is particularly useful in patients with both atrial fibrillation and heart failure.
