8.2 Vasodilators & Antianginal Agents

Vasodilator & Antianginal Pharmacology

Vasodilator and antianginal agents play a central role in managing Acute Coronary Syndromes (ACS), hypertensive emergencies, acute heart failure, and chronic stable angina. These medications decrease myocardial oxygen demand (MVO₂), augment myocardial oxygen supply, reduce cardiac filling pressures (preload), and lower systemic arterial resistance (afterload).

Organic Nitrates & Nitroglycerin Dynamics

• Mechanism of Action: Nitroglycerin (NTG) and organic nitrates undergo intracellular enzymatic biotransformation (primarily via mitochondrial aldehyde dehydrogenase) to yield nitric oxide (NO). Nitric oxide diffuses into vascular smooth muscle cells, activating soluble guanylyl cyclase (sGC) to convert guanosine triphosphate (GTP) into cyclic guanosine monophosphate (cGMP). Elevated cGMP activates Protein Kinase G (PKG), promoting intracellular calcium reuptake and dephosphorylation of myosin light chains, culminating in vascular smooth muscle relaxation.
• Dose-Dependent Hemodynamic Profile:
  • Low Infusion Rates (<100 mcg/min): Selectively dilate venous capacitance vessels. Venous pooling decreases venous return to the heart, lowering Left Ventricular End-Diastolic Volume (LVEDV), Left Ventricular End-Diastolic Pressure (LVEDP), and Pulmonary Capillary Wedge Pressure (PCWP). Decreasing LV volume lowers intramyocardial wall tension, substantially reducing MVO₂.
  • High Infusion Rates (>100 to 200 mcg/min): Dilation expands to systemic arterial resistance arterioles, producing significant reductions in SVR, MAP, and systolic blood pressure (afterload reduction).
  • Coronary Circulation Effects: Dilates epicardial coronary arteries, relieves coronary artery vasospasm, and enhances collateral blood flow to ischemic subendocardial myocardium.
• Nitrate Tolerance & Management: Continuous, uninterrupted exposure to organic nitrates over 24–48 hours induces nitrate tolerance (tachyphylaxis). Proposed mechanisms include intracellular sulfhydryl group depletion, neurohormonal activation (RAAS/sympathetic overdrive), and oxidative stress inactivating mitochondrial aldehyde dehydrogenase. To prevent tolerance, a nitrate-free interval of 10 to 12 hours daily is mandatory for transdermal patches and long-acting oral nitrates (e.g., Isosorbide mononitrate/dinitrate).
• Special AACN Nursing Mandates & Contraindications:
  • Polyvinyl Chloride (PVC) Absorption: Up to 80% of IV nitroglycerin binds to standard PVC plastic IV tubing and bags. Nitroglycerin continuous infusions MUST be prepared in glass bottles or polyolefin containers and administered using non-PVC polyethylene IV tubing.
  • Phosphodiesterase-5 (PDE-5) Inhibitor Absolute Contraindication: Concomitant administration of nitroglycerin with PDE-5 inhibitors (sildenafil, vardenafil within 24 hours; tadalafil within 48 hours) is strictly prohibited. PDE-5 inhibitors prevent cGMP breakdown; combining them with NO donors causes massive cGMP accumulation, precipitating profound, life-threatening, refractory hypotension and cardiac arrest.

Sodium Nitroprusside, Natriuretic Peptides & Specific Vasodilators

1. Sodium Nitroprusside (Nipride)

• Mechanism of Action: Ultra-short-acting, balanced direct donor of nitric oxide that dilates both arterial resistance arterioles and venous capacitance vessels equally without requiring enzymatic conversion.
• Clinical Indications: Hypertensive emergencies (aortic dissection, acute hypertensive encephalopathy), acute severe mitral regurgitation (rapid afterload reduction decreases regurgitant volume), acute aortic regurgitation, and refractory heart failure with severe vasoconstriction (SVR > 1600 dyn·s·cm⁻⁵).
• Pharmacokinetics & Titration: Onset within <1 minute; duration of action 1–10 minutes. Continuous IV infusion initiated at 0.25 to 0.5 mcg/kg/min and titrated every 5 minutes up to a maximum of 10.0 mcg/kg/min.
• Toxicity Profiles & Emergency Management:
  • Cyanide Toxicity: Nitroprusside contains 5 cyanide (CN⁻) ions per molecule. Free cyanide is buffered by hemoglobin and converted by hepatic rhodanase into thiocyanate using endogenous thiosulfate. High infusion rates (>2.0 mcg/kg/min for >24 hours) or hepatic dysfunction deplete thiosulfate, causing cyanide to bind mitochondrial cytochrome c oxidase. This halts aerobic respiration, driving severe anaerobic metabolism. Clinical signs include unexplained anion-gap metabolic acidosis, rising blood lactate, altered mental status, seizures, and elevated mixed venous oxygen saturation (SvO₂ > 80%, termed "venous hyperoxia" because tissues cannot extract oxygen). Treatment: Discontinue infusion; administer Hydroxocobalamin (Cyanokit, 5 g IV), which binds cyanide to form cyanocobalamin (Vitamin B12), or Sodium Thiosulfate (12.5 g IV).
  • Thiocyanate Toxicity: Thiocyanate (SCN⁻) is cleared renally with a half-life of 3–7 days. In patients with renal impairment (CrCl < 30 mL/min), thiocyanate accumulates, producing CNS toxicity: tinnitus, hyperreflexia, confusion, agitation, hallucinations, and seizures. Serum thiocyanate levels must be monitored (>10 mg/dL is toxic).
  • Light Sensitivity: Nitroprusside degrades rapidly upon light exposure; IV bags and tubing must be covered with opaque protective sleeves.

2. Nesiritide (Natrecor)

• Mechanism of Action: Recombinant human B-type natriuretic peptide (rbNP). Binds to natriuretic peptide receptor-A (NPR-A) on vascular smooth muscle and vascular endothelium, elevating intracellular cGMP.
• Hemodynamic & Renal Actions: Produces balanced arterial and venous vasodilation, reducing PCWP, LVEDP, and SVR. Promotes natriuresis and diuresis by inhibiting renal tubular sodium reabsorption and suppressing the Renin-Angiotensin-Aldosterone System (RAAS) and endothelin-1. Does NOT increase myocardial contractility or MVO₂.
• Dosing & Monitoring: IV bolus of 2 mcg/kg over 1 minute followed by continuous infusion at 0.01 mcg/kg/min. Monitor closely for symptomatic hypotension and acute renal impairment.

3. Hydralazine & Isosorbide Dinitrate

• Hydralazine: Direct-acting arterial vasodilator that inhibits IP₃-mediated calcium release from vascular smooth muscle sarcoplasmic reticulum. Selectively lowers SVR and afterload with minimal effect on venous capacitance. Triggers reflex sympathetic activation (tachycardia, elevated renin), requiring concurrent beta-blocker therapy.
• Hydralazine + Isosorbide Dinitrate (BiDil): Combines arterial afterload reduction (hydralazine) with venous preload reduction (isosorbide dinitrate). Indicated as add-on therapy in self-identified African American patients with HFrEF (NYHA Class III-IV) or in patients intolerant to ACE inhibitors, ARBs, or ARNI due to renal failure or hyperkalemia.

4. Beta-Blockers, Calcium Channel Blockers & Ranolazine

• Beta-Adrenergic Blockers:
  • Cardioselective (beta₁): Metoprolol succinate/tartrate, Atenolol, Esmolol, Bisoprolol.
  • Non-Selective / Combined α₁/beta1,2: Carvedilol, Labetalol.
  • Indications: ACS, stable HFrEF (Carvedilol, Metoprolol Succinate, Bisoprolol improve survival), AFib rate control. Decreases HR, contractility, AV conduction, and MVO₂. Contraindicated in acute decompensated heart failure with hypoperfusion ("cold and wet"), severe bradycardia, or 2nd/3rd-degree AV block.
• Calcium Channel Blockers (CCBs):
  • Dihydropyridines (Amlodipine, Nicardipine, Clevidipine): Block L-type calcium channels in vascular smooth muscle → potent systemic arterial vasodilation with minimal nodal effects. Nicardipine IV and Clevidipine IV are first-line for most hypertensive emergencies and for acute stroke. They are not first-line in acute aortic syndromes: the 2022 ACC/AHA guideline requires an IV beta blocker first (Class 1) to lower dP/dt, with a vasodilator added only if blood pressure remains high afterward, because a pure arterial vasodilator given alone causes reflex tachycardia and raises aortic wall shear stress.
  • Non-Dihydropyridines (Diltiazem, Verapamil): Block L-type calcium channels in SA/AV nodes and myocardium → negative chronotropy, dromotropy, and inotropy. Primary indication: Rate control in atrial fibrillation. Absolutely contraindicated in HFrEF (LVEF < 40%) due to severe negative inotropic suppression.
• Ranolazine (Ranexa):
  • Mechanism of Action: Selective inhibitor of the late inward sodium current (INa, late) in ischemic cardiomyocytes. Prevents intracellular sodium overload, thereby stopping reverse-mode Na⁺/Ca²⁺ exchanger overload. Reduces diastolic wall tension and enhances microvascular subendocardial perfusion without altering heart rate or blood pressure.
  • Indication: Chronic refractory angina.
  • Safety Monitoring: Causes dose-dependent QTc prolongation. Baseline ECG, serum potassium, and magnesium monitoring required; contraindicated with strong CYP3A4 inhibitors.