Receptors & Basics - The Receptor Rodeo
- Adrenergic Receptors: Key targets for inotropes & vasopressors.
- G-protein coupled receptors (GPCRs). Signal via 2nd messengers.
- 📌 Mnemonic (G-protein type): α1→Gq, α2→Gi, All βs→Gs. (Sequence: Q, I, S, S, S).
| Receptor | G-Protein (2nd Messenger) | Key Locations | Primary Effects |
|---|---|---|---|
| α1 | Gq (↑$IP_3$, $DAG$, $Ca^{2+}$) | Vascular smooth muscle, Iris, Liver | Vasoconstriction (↑BP), Mydriasis, ↑Glycogenolysis |
| α2 | Gi (↓$cAMP$) | Presynaptic nerve, Pancreas, Eye | ↓Norepinephrine release, ↓Insulin, ↓Aqueous humor |
| β1 | Gs (↑$cAMP$) | Heart (SA/AV, Myocardium), Kidney | ↑Heart Rate, ↑Contractility, ↑Conduction, ↑Renin release |
| β2 | Gs (↑$cAMP$) | Lungs, Skeletal muscle vessels, Uterus | Bronchodilation, Vasodilation, Uterine relaxation, ↑Glycogenolysis, ↑Gluconeogenesis |
| β3 | Gs (↑$cAMP$) | Adipose tissue | Lipolysis |
⭐ β1 receptors primarily increase heart rate and contractility, while β2 receptors cause bronchodilation and vasodilation.
Pure Inotropes - Pump Primers
| Feature | Dobutamine | Milrinone | Levosimendan |
|---|---|---|---|
| MoA | β1 > β2 > α1 agonist | PDE3 inhibitor; ↑cAMP | Ca²⁺ sensitizer (troponin C); KATP opener (vasodilation) |
| CO | ↑↑ | ↑↑ | ↑↑ |
| HR | ↑/↔ | ↑/↔ | ↑/↔ |
| SVR | ↓/↔ | ↓↓ | ↓↓ |
| PVR | ↓ | ↓ | ↓ |
| MAP | ↔/↓ | ↓ | ↓ |
| Key Indications | Cardiogenic shock, ADHF, Low CO states, Stress echo | ADHF (esp. with β-blockade), Post-cardiac surgery LV dysfunction | ADHF, Cardiogenic shock (esp. with β-blockers), Weaning from CPB |
| Adverse Effects | Tachyarrhythmias, Hypotension, Ectopy | Hypotension, Arrhythmias, Thrombocytopenia (rare) | Hypotension, Headache, Hypokalemia, Atrial fibrillation |
Vasopressors & Inopressors - Pressure Players
| Drug | Receptor Profile (α1,α2,β1,β2,D) | Hemodynamic Profile | Key Indications | Dopamine Doses (mcg/kg/min) | Major Adverse Effects |
|---|---|---|---|---|---|
| Norepinephrine | +++α1, ++α2, +β1 | ↑SVR, ↑MAP, +/- CO | Septic shock (1st line), cardiogenic shock | N/A | Arrhythmias, peripheral ischemia |
| Epinephrine | +++α1, +++α2, +++β1, ++β2 | ↑SVR, ↑MAP, ↑CO, ↑HR | Anaphylaxis, cardiac arrest, septic shock (2nd line) | N/A | Arrhythmias, anxiety, hyperglycemia |
| Dopamine | Dose-dep. (D, β, α) | Dose-dep. | Cardiogenic shock, bradycardia, septic shock | 1-3 (D), 3-10 (β), >10 (α) | Tachyarrhythmias, N/V |
| Phenylephrine | +++α1 | ↑SVR, ↑MAP, ↓CO (reflex brady) | Hypotension (anesthesia-induced) | N/A | Reflex bradycardia, ↓CO, tissue necrosis |
| Vasopressin | V1 (vascular), V2 (renal) | ↑SVR, ↑MAP (non-adrenergic) | Septic shock (adjunct), GI bleed | N/A | Myocardial ischemia, hyponatremia |
⭐ Norepinephrine is the first-line vasopressor in septic shock due to its potent α1-adrenergic effects with some β1 activity, leading to increased SVR and MAP with minimal direct chronotropic effects.
Clinical Application - Shock Selectors
- Goal: Restore tissue perfusion. Target MAP >65 mmHg.
- Selection: Based on shock type (see flowchart) & patient hemodynamics.
- Titrate to effect: Monitor BP, HR, UO, lactate.
- Fluids: Optimize volume status before/during vasopressor therapy.
⭐ In anaphylactic shock, Epinephrine is the drug of choice due to its α1 (vasoconstriction), β1 (inotropy/chronotropy), and β2 (bronchodilation, mast cell stabilization) effects.
High‑Yield Points - ⚡ Biggest Takeaways
- Dobutamine (β1 agonist) primarily ↑ contractility and CO; key for cardiogenic shock.
- Dopamine shows dose-dependent effects: D1 (renal vasodilation), β1 (↑inotropy), α1 (vasoconstriction).
- Adrenaline (α & β agonist) is crucial for anaphylaxis, cardiac arrest, causing ↑HR, ↑BP.
- Noradrenaline (potent α1 agonist, some β1) is first-line for septic shock, significantly ↑SVR.
- Phenylephrine (pure α1 agonist) rapidly ↑BP in spinal-induced hypotension via vasoconstriction.
- Milrinone (PDE3 inhibitor) acts as an inodilator, beneficial in acute decompensated heart failure.
- Vasopressin (V1 agonist) is used for catecholamine-refractory shock and vasoplegia.
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