Cardiovascular Pharmacology (Antihypertensives & Antiarrhythmics)

Angiotensin-converting enzyme inhibitors (lisinopril, enalapril, ramipril, captopril) block endothelial peptidyl dipeptidase, preventing the conversion of angiotensin I to angiotensin II while simultaneously inhibiting the degradation of bradykinin and substance P. Accumulation of bradykinin potentiates nitric oxide and prostacyclin vasodilation but causes a dry, non-productive cough in 5–20% of patients and potentially life-threatening C1-esterase-independent angioedema of the lips, tongue, and glottis (requiring immediate discontinuation and transition to an angiotensin II type 1 [AT₁] receptor blocker such as losartan, valsartan, or candesartan after a washout period). In the renal microcirculation, angiotensin II preferentially constricts the efferent glomerular arteriole; blocking angiotensin II dilates the efferent arteriole, lowering intraglomerular filtration pressure and attenuating hyperfiltration-induced proteinuria in diabetic nephropathy. However, a transient 15–25% rise in serum creatinine is expected upon initiation, and ACEi/ARBs are strictly contraindicated in bilateral renal artery stenosis (where GFR depends entirely on efferent constriction) and across all trimesters of pregnancy (causing fetal renal dysgenesis, oligohydramnios, and Potter sequence).

Voltage-gated L-type calcium channel blockers are divided into vascular-selective dihydropyridines (DHPs: amlodipine, nifedipine, nicardipine, clevidipine, nimodipine) and myocardium/node-selective non-dihydropyridines (verapamil > diltiazem). Dihydropyridines preferentially dilate peripheral and coronary arteriolar smooth muscle without depressing SA/AV nodal conduction; their primary dose-limiting adverse effect is pre-capillary arteriolar dilation without venodilation, which elevates capillary hydrostatic pressure and causes peripheral ankle edema (alleviated by adding an ACEi/ARB to dilate post-capillary venules, not loop diuretics), along with reflex tachycardia and gingival hyperplasia. Non-dihydropyridines block L-type Ca²⁺ channels in the SA and AV nodes and ventricular myocardium, slowing AV conduction in atrial fibrillation and terminating AV nodal reentrant tachycardia, but are contraindicated in HFrEF (negative inotropy) and causes constipation (verapamil). Direct arteriolar vasodilators—hydralazine (cGMP-mediated arteriolar relaxation, associated with slow-acetylator drug-induced lupus with anti-histone antibodies) and minoxidil (opens ATP-sensitive K⁺ channels causing hyperpolarization, hypertrichosis, and pericardial effusion)—trigger intense baroreceptor reflex tachycardia and RAAS fluid retention, mandating co-administration of a β-blocker and a loop diuretic.

Class I antiarrhythmics block fast inward voltage-gated Na⁺ channels (I_Na) during phase 0 depolarization in atrial, His-Purkinje, and ventricular myocytes, exhibiting use-dependence (greater blockade at faster heart rates because channels spend more time in open and inactivated conformations). Class IC agents (flecainide, propafenone) have the slowest dissociation kinetics (τ >10 s), producing marked phase 0 slope depression and QRS widening without altering action potential duration (APD); they are used as "pill-in-the-pocket" rhythm control for paroxysmal atrial fibrillation in structurally normal hearts, but the landmark CAST trial demonstrated that Class IC drugs are strictly contraindicated in post-MI or ischemic cardiomyopathy patients due to fatal ventricular proarrhythmia. Class IA agents (quinidine, procainamide, disopyramide) have intermediate dissociation kinetics and concurrently block repolarizing K⁺ channels, prolonging both the QRS duration and the QT interval (predisposing to torsades de pointes); procainamide induces anti-histone drug-induced lupus, and quinidine causes cinchonism (tinnitus, headache, thrombocytopenia). Class IB agents (lidocaine, mexiletine) have ultra-fast dissociation kinetics (τ <1 s) and selectively bind inactivated Na⁺ channels in depolarized ischemic ventricular tissue, shortening phase 3 repolarization (APD) and treating acute post-MI or digoxin-induced ventricular arrhythmias.

Class III antiarrhythmics block rapid delayed rectifier K⁺ channels (I_Kr) during phase 3 repolarization, prolonging the effective refractory period (ERP) and QT interval. While pure Class III agents (dofetilide, ibutilide) and mixed Class II/III sotalol carry high risks of dose-dependent torsades de pointes (especially during bradycardia due to reverse use-dependence), amiodarone combines Class I, II, III, and IV properties with a remarkably low incidence of torsades despite substantial QT prolongation because it homogenizes transmural dispersion of repolarization across ventricular layers. However, amiodarone is an amphiphilic, heavily iodinated molecule (~37% iodine by weight) with an elimination half-life of 25 to 100 days that accumulates in adipose, pulmonary, hepatic, corneal, and dermal tissues. Chronic amiodarone therapy mandates baseline and serial 6-month monitoring of pulmonary function tests (DLCO) and chest radiographs (for fatal interstitial pneumonitis and pulmonary fibrosis), TSH and free T₄ (for Jod-Basedow hyperthyroidism or Wolff-Chaikoff hypothyroidism), and hepatic transaminases, while also recognizing corneal verticillata microdeposits, blue-gray photodermatitis, and potent CYP2C9/P-gp inhibition (requiring immediate 30–50% dose reduction of warfarin and digoxin).

Each nephron segment employs a specific apical solute cotransporter that dictates both diuretic potency and divalent cation handling. In the proximal convoluted tubule, acetazolamide inhibits luminal and intracellular carbonic anhydrase (types IV and II), blocking NaHCO₃ reabsorption and producing a self-limited alkaline diuresis with hyperchloremic non-anion-gap metabolic acidosis (Type 2 RTA physiology); it treats idiopathic intracranial hypertension (pseudotumor cerebri), acute mountain sickness, and acute angle-closure glaucoma, but alkaline urine precipitates calcium phosphate nephrolithiasis. In the thick ascending limb of the loop of Henle, loop diuretics (furosemide, bumetanide, torsemide, and non-sulfonamide ethacrynic acid) inhibit the apical Na⁺-K⁺-2Cl⁻ (NKCC2) cotransporter, abolishing the lumen-positive transepithelial electrical potential (+10 mV) that normally drives paracellular Ca²⁺ and Mg²⁺ reabsorption ("Loops Lose Calcium"). Conversely, in the early distal convoluted tubule, thiazide diuretics (hydrochlorothiazide, chlorthalidone, indapamide, metolazone) inhibit the apical Na⁺-Cl⁻ (NCC) cotransporter, lowering intracellular Na⁺ and enhancing basolateral Na⁺/Ca²⁺ exchange to increase proximal and distal Ca²⁺ reabsorption (treating idiopathic hypercalciuria, nephrolithiasis, and osteoporosis).

All diuretics acting upstream of the cortical collecting tubule (acetazolamide, loop diuretics, and thiazides) increase distal delivery of Na⁺ and tubular flow to aldosterone-sensitive principal cells. Enhanced apical Na⁺ entry via epithelial sodium channels (ENaC) creates a lumen-negative electrical potential that drives K⁺ secretion through ROMK channels and H⁺ secretion by α-intercalated cells—causing hypokalemic metabolic alkalosis with loops and thiazides, alongside hyperuricemia (competitive organic anion transporter OAT secretion block precipitating gout), hyperglycemia, and dyslipidemia (thiazides). In contrast, potassium-sparing diuretics act directly on the cortical collecting tubule to prevent K⁺ and H⁺ wasting (causing hyperkalemic normal-anion-gap metabolic acidosis [Type 4 RTA physiology]). Spironolactone and eplerenone are competitive cytosolic mineralocorticoid receptor antagonists (with eplerenone avoiding spironolactone's anti-androgenic gynecomastia and mastodynia), whereas amiloride and triamterene directly plug apical ENaC channels (making amiloride the drug of choice for Liddle syndrome and lithium-induced nephrogenic diabetes insipidus).

Neurohormonal blockade is the cornerstone of survival in heart failure with reduced ejection fraction (HFrEF). Sacubitril/valsartan (ARNI) combines AT₁ receptor blockade with neprilysin inhibition, preventing the degradation of endogenous atrial and brain natriuretic peptides (ANP/BNP), bradykinin, and adrenomedullin to augment cGMP-mediated natriuresis and reverse myocardial remodeling (requiring a strict 36-hour washout from any ACE inhibitor to prevent severe angioedema). Only three specific beta-blockers—carvedilol, sustained-release metoprolol succinate, and bisoprolol—plus mineralocorticoid receptor antagonists (spironolactone/eplerenone), SGLT2 inhibitors (dapagliflozin/empagliflozin), and the combination of hydralazine plus isosorbide dinitrate (in self-identified Black patients with NYHA class III–IV HFrEF) reduce mortality. By contrast, digoxin inhibits sarcolemmal Na⁺/K⁺-ATPase to raise intracellular Na⁺, slowing NCX1 Ca²⁺ extrusion and increasing SR Ca²⁺ stores (positive inotropy) while augmenting vagal tone at the AV node; it reduces HF hospitalizations without improving survival and exhibits narrow therapeutic toxicity (nausea, yellow-green xanthopsia halos, hyperkalemia, and bidirectional ventricular tachycardia, exacerbated by hypokalemia and reversed by digoxin immune Fab).

Antianginal pharmacotherapy restores myocardial oxygen balance by decreasing myocardial oxygen demand (determined by heart rate, ventricular contractility, and left ventricular wall stress T = (P × r) / 2h) or reversing coronary vasospasm. Organic nitrates (sublingual nitroglycerin, isosorbide mononitrate/dinitrate) are converted by mitochondrial aldehyde dehydrogenase-2 into nitric oxide (NO), which activates guanylyl cyclase (↑ cGMP → myosin light-chain dephosphorylation) predominantly in capacitance veins. Venous pooling decreases right and left ventricular preload (end-diastolic volume and radius r), reducing wall stress and myocardial O₂ consumption. However, nitrates alone trigger baroreceptor reflex tachycardia and reflex positive inotropy; pairing a nitrate with a β-blocker or non-DHP CCB (verapamil/diltiazem) blunts reflex tachycardia while nitrates prevent beta-blocker-induced end-diastolic volume expansion. Chronic continuous nitrate exposure causes sulfhydryl depletion tachyphylaxis (requiring a daily 10–12 hour nitrate-free interval) and Monday disease in industrial munitions workers. Nitrates and riociguat are absolutely contraindicated within 24 hours of sildenafil/vardenafil or 48 hours of tadalafil (PDE-5 inhibitors), as synergistic cGMP accumulation precipitates fatal refractory hypotension. Ranolazine inhibits the late inward sodium current (I_Na,L) in ischemic myocardium to reduce diastolic Ca²⁺ overload and wall tension without altering heart rate or blood pressure.

Intravenous vasodilators used in hypertensive emergencies are selected according to target-organ pathology and metabolic clearance. Sodium nitroprusside is an ultra-short-acting nitric oxide donor that dilates both arterial resistance vessels and venous capacitance beds equally (↓ preload and ↓ afterload); however, each nitroprusside molecule contains five cyanide (CN⁻) radicals that are liberated upon erythrocyte reaction and converted by hepatic rhodanese (requiring thiosulfate) into renally excreted thiocyanate. Prolonged infusions (>24–48 hours) or renal/hepatic dysfunction cause cyanide toxicity—presenting with almond breath, altered mental status, tachyphylaxis, high-anion-gap lactic acidosis, and elevated venous O₂ saturation—treated with hydroxocobalamin and sodium thiosulfate. Fenoldopam is a selective peripheral D₁ dopamine receptor agonist (G_s) that dilates coronary, splanchnic, and renal afferent/efferent arterioles while promoting natriuresis, making it the ideal parenteral agent in hypertensive emergencies complicated by acute kidney injury (contraindicated in glaucoma as it raises IOP). Nicardipine and clevidipine (hydrolyzed by plasma esterases) provide pure arteriolar DHP calcium channel blockade in acute ischemic or hemorrhagic stroke.

Three non-Vaughan-Williams antiarrhythmic agents serve specialized roles in acute and chronic rhythm management. Adenosine (half-life <10 seconds, administered as a rapid 6 mg IV push followed by an immediate normal saline flush) binds cardiac A₁ purinergic receptors (G_i) to activate inwardly rectifying K⁺ channels (I_KADO, causing deep hyperpolarization) and inhibit L-type Ca²⁺ currents—producing transient complete AV nodal block that terminates paroxysmal supraventricular tachycardia (AVNRT/orthodromic AVRT). Adenosine causes transient facial flushing, chest tightness, and bronchospasm; its effects are competitively blocked by methylxanthines (theophylline, caffeine) and potentiated by dipyridamole (which blocks cellular adenosine reuptake) and in denervated transplanted hearts. Ivabradine selectively blocks hyperpolarization-activated cyclic nucleotide-gated (HCN) channels responsible for the pacemaker "funny" current (I_f, phase 4 Na⁺/K⁺ influx) in the SA node, slowing heart rate without negative inotropy in HFrEF patients who remain in sinus rhythm with HR ≥70 bpm despite maximally tolerated beta-blockade (characteristic side effect: luminous visual phosphenes from retinal I_h channel blockade). Intravenous magnesium sulfate is the immediate first-line therapy for polymorphic VT with long QT (torsades de pointes) regardless of baseline serum magnesium concentration.

A hypertensive emergency is defined by severe blood pressure elevation (typically >180/120 mmHg) accompanied by acute target-organ damage (hypertensive encephalopathy, intracranial hemorrhage, acute ischemic stroke, aortic dissection, acute coronary syndrome, flash pulmonary edema, acute kidney injury, or eclampsia). In the absence of target-organ injury (hypertensive urgency / severe asymptomatic hypertension), aggressive IV blood pressure lowering is harmful and precipitates cerebral or myocardial watershed ischemia; treatment consists of gradual oral antihypertensive re-initiation over 24–48 hours.

In true hypertensive emergencies, the general rule is to reduce Mean Arterial Pressure (MAP) by no more than 20–25% within the first hour to prevent hypoperfusion below the rightward-shifted cerebral autoregulatory curve. Two critical exceptions override this rule: (1) Acute Aortic Dissection, where systolic BP must be lowered rapidly to <120 mmHg and heart rate <60 bpm within 20 minutes using an IV beta-blocker FIRST (esmolol or labetalol) before adding a vasodilator (nicardipine or nitroprusside) to prevent reflex tachycardia and increased aortic shear stress (dP/dt); and (2) Preeclampsia with severe features / Eclampsia, requiring immediate BP lowering below 160/110 mmHg alongside IV magnesium sulfate for seizure prophylaxis.

All ACE inhibitors, ARBs, and direct renin inhibitors (aliskiren) are strictly contraindicated throughout pregnancy due to fetal renal dysgenesis (Potter sequence: oligohydramnios, pulmonary hypoplasia, limb contractures, calvarial hypoplasia). Safe antihypertensive agents in pregnancy can be remembered via the classic board mnemonic "Hypertensive Moms Love Nifedipine": Hydralazine (direct arteriolar vasodilator via cGMP; causes drug-induced lupus in slow acetylators and reflex tachycardia), Alpha-Methyldopa (prodrug converted to alpha-methylnorepinephrine, a central alpha₂ agonist; causes positive direct Coombs test/hemolytic anemia and sedation), Labetalol (combined alpha₁/beta₁/beta₂ antagonist; first-line IV and oral agent), and extended-release Nifedipine (dihydropyridine L-type Ca²⁺ blocker).

HMG-CoA Reductase Inhibitors (Statins: atorvastatin, rosuvastatin [high-intensity, ≥50% LDL reduction]; simvastatin, pravastatin, lovastatin) competitively inhibit the rate-limiting enzyme of hepatic cholesterol synthesis (conversion of HMG-CoA to mevalonate). Depletion of intrahepatic cholesterol activates SREBP-2, upregulating surface hepatic LDL receptors to clear circulating LDL-C and conferring plaque-stabilizing, anti-inflammatory pleiotropic mortality benefits. Adverse effects include hepatotoxicity (check baseline LFTs), statin-associated myopathy/rhabdomyolysis (due to depletion of coenzyme Q10 / ubiquinone and farnesyl/geranylgeranyl intermediates; worsened by CYP3A4 inhibitors or gemfibrozil), and a modest increase in incident Type 2 diabetes.

Ezetimibe selectively blocks the Niemann-Pick C1-Like 1 (NPC1L1) sterol transporter at the jejunal enterocyte brush border, preventing dietary and biliary cholesterol absorption and lowering LDL-C by an additional 18–25% when added to a statin.

PCSK9 Inhibitors (evolocumab, alirocumab; inclisiran [siRNA against PCSK9 mRNA]) are subcutaneous monoclonal antibodies (or small interfering RNA) that prevent Proprotein Convertase Subtilisin/Kexin Type 9 from binding to the hepatic LDL receptor. Normally, PCSK9 targets the LDLR-LDL complex for lysosomal degradation; inhibiting PCSK9 allows continuous recycling of LDL receptors to the hepatocyte surface, lowering LDL-C by 50–70% in familial hypercholesterolemia or statin-intolerant ASCVD patients.

Fibrates (fenofibrate, gemfibrozil) act as ligands for the nuclear transcription factor PPAR-α (Peroxisome Proliferator-Activated Receptor-alpha), which upregulates lipoprotein lipase (LPL) and ApoA-I/ApoA-II synthesis while downregulating ApoC-III (an endogenous inhibitor of LPL). This accelerates VLDL and chylomicron triglyceride hydrolysis, lowering serum triglycerides by 35–50% (first-line to prevent acute pancreatitis when TG >500–1,000 mg/dL). Because fibrates inhibit cholesterol 7α-hydroxylase, they supersaturate bile with cholesterol and predispose to cholesterol gallstones (cholelithiasis).

Bile Acid-Binding Resins (cholestyramine, colestipol, colesevelam) are non-absorbable positively charged polymers that sequester bile acids in the intestinal lumen, forcing the liver to consume intracellular cholesterol via 7α-hydroxylase to synthesize new bile acids. While safe in pregnancy and useful for cholestatic pruritus or bile acid diarrhea, they cause bloating, impair absorption of fat-soluble vitamins (A, D, E, K) and medications (warfarin, digoxin, levothyroxine), and paradoxically increase VLDL/triglycerides (contraindicated if TG >300–400 mg/dL).

Niacin (Vitamin B₃) inhibits hormone-sensitive lipase in adipose tissue and hepatic VLDL synthesis, producing the largest increase in HDL-C (+15–35%) alongside lowered LDL, TG, and Lp(a). Its clinical utility is limited by lack of incremental mortality benefit over statins and signature toxicities: prostaglandin D₂/E₂-mediated cutaneous flushing and pruritus (prevented by pre-treatment with aspirin 325 mg or NSAIDs, NOT antihistamines), hyperuricemia/acute gout, and insulin-resistant hyperglycemia. Finally, icosapent ethyl (highly purified EPA omega-3 fatty acid ester) reduces hepatic VLDL-TG secretion and significantly lowers major cardiovascular events in statin-treated patients with TG 135–499 mg/dL.