Blood Pressure Fundamentals
Blood pressure (BP) is determined by the product of cardiac output (CO) and total peripheral resistance (TPR). The relationship can be expressed as:
BP = CO × TPR
Cardiac output itself is the product of stroke volume (SV) and heart rate (HR):
CO = SV × HR
Understanding these components helps explain why various drug classes target different aspects of the cardiovascular system.
Regulatory Systems
The primary hormonal system that controls blood pressure among the options listed is the Renin‑Angiotensin‑Aldosterone System (RAAS). RAAS adjusts both blood volume and vascular tone through angiotensin II and aldosterone, making it a central target for antihypertensive therapy.
Drug Classes that Lower Blood Pressure
- Diuretics: Reduce blood volume by inhibiting sodium reabsorption in the kidney, increasing water and salt excretion.
- Alpha blockers: Block vascular α‑adrenergic receptors, causing vasodilation.
- Beta blockers: Block β‑1 cardiac receptors, decreasing heart rate and contractility.
- ACE inhibitors: Inhibit the conversion of angiotensin I to angiotensin II by blocking the angiotensin‑converting enzyme.
Antiplatelet Therapy
Platelet aggregation can be prevented by targeting distinct pathways:
| Drug | Mechanism | Duration of Effect |
|---|---|---|
| Aspirin | Irreversibly inhibits COX‑1, blocking thromboxane A2 synthesis | 8‑10 days (lifespan of platelet) |
| Clopidogrel | Irreversibly antagonizes the ADP P2Y12 receptor | 8‑10 days (lifespan of platelet) |
Aspirin’s target enzyme is COX‑1, the cyclooxygenase isoform most important for platelet function. By permanently disabling this enzyme, aspirin provides a lasting antiplatelet effect.
Statins and Cholesterol Management
Statins are widely used to lower plasma cholesterol. Their primary pharmacologic action is the inhibition of HMG‑CoA reductase, the rate‑limiting enzyme in cholesterol biosynthesis. This inhibition reduces synthesis of LDL and VLDL particles, leading to lower circulating cholesterol levels.
It is a common misconception that statins increase cholesterol synthesis; the correct statement is that they decrease it.
Atherosclerosis and Foam Cell Formation
The development of atherosclerotic plaques begins when low‑density lipoprotein (LDL) penetrates the endothelial layer of arteries. Once inside the intima, LDL is taken up by macrophages, which become engorged with lipid droplets and transform into foam cells. This process is a key early event in plaque formation.
Vasodilators: Nitroglycerin (GTN)
Glyceryl trinitrate (GTN) is metabolized to nitric oxide (NO), which activates guanylate cyclase in vascular smooth muscle, raising cyclic GMP (cGMP) levels and causing vasodilation. The typical half‑life of sublingual GTN is about 6 minutes, reflecting its rapid absorption and clearance.
Heart Failure and Loop Diuretics
Furosemide, a loop diuretic, acts on the Na⁺/Cl⁻ cotransporter in the thick ascending limb of the loop of Henle. By inhibiting this transporter, furosemide increases urine output, reduces plasma volume, and alleviates pulmonary congestion in heart‑failure patients.
Adrenergic Agents
Adrenaline (epinephrine) does not selectively stimulate only beta‑adrenergic receptors; at clinical concentrations (e.g., 1:10,000), it activates both alpha and beta receptors, producing combined vasoconstrictive and inotropic effects.
Anti‑arrhythmic Drugs
Amiodarone belongs to class III anti‑arrhythmics and primarily blocks potassium currents during repolarization, prolonging the refractory period and stabilizing cardiac rhythm.
Myocardial Infarction (MI) Types
ST‑segment elevation myocardial infarction (STEMI) involves a transmural, full‑thickness infarct, whereas non‑ST‑segment elevation myocardial infarction (NSTEMI) results in a subendocardial, partial‑thickness injury.
Complications of STEMI
A serious complication of STEMI is cardiac tamponade, where fluid accumulates in the pericardial sac, impeding diastolic filling and reducing cardiac output.
Oxygen Therapy in Acute MI
Routine supplemental oxygen is not universally beneficial for all acute myocardial infarction (AMI) patients. Evidence indicates that providing oxygen to patients with adequate peripheral oxygen saturation (SpO₂) may be unnecessary or even harmful.
Summary Table of Key Concepts
| Concept | Key Point |
|---|---|
| Blood Pressure Equation | BP = CO × TPR; CO = SV × HR |
| Primary BP Regulator | RAAS adjusts volume and tone |
| Diuretics | Inhibit renal Na⁺ reabsorption → ↓ plasma volume |
| ACE Inhibitors | Block conversion of Ang I → Ang II |
| Aspirin | Irreversible COX‑1 inhibition → ↓ thromboxane A2 |
| Clopidogrel | Irreversible P2Y12 blockade → ↓ ADP‑mediated aggregation |
| Statins | Inhibit HMG‑CoA reductase → ↓ cholesterol synthesis |
| Foam Cell Formation | Macrophage uptake of LDL → lipid‑laden foam cells |
| GTN | NO → ↑ cGMP → vasodilation; half‑life ≈ 6 min |
| Furosemide | Loop of Henle Na⁺/Cl⁻ blockade → diuresis |
| Adrenaline | Stimulates both α and β receptors |
| Amiodarone | Class III anti‑arrhythmic; K⁺ channel block |
| STEMI vs NSTEMI | Transmural vs subendocardial injury |
| Cardiac Tamponade | Pericardial fluid compresses heart |
| Oxygen in AMI | Not always indicated; assess SpO₂ |
