Metoprolol
Lopressor · Toprol-XL
Cardioselective β1-adrenergic antagonist
Selective β1 blockade → ↓ HR, contractility, AV conduction, and myocardial O₂ demand. Cardioselectivity is dose-dependent (lost at high dose).
Indications
- •Rate control (AF/SVT)
- •Perioperative ischemia / rate control
- •Heart failure (Toprol-XL)
- •Post-MI
- •Hypertension
Dosing
| Context | Adult | Pediatric |
|---|---|---|
| IV (acute rate/ischemia) | 2.5–5 mg IV q5 min up to 15 mg | — |
| Oral | 25–100 mg PO BID (tartrate) or daily (succinate ER) | — |
Pharmacokinetics
IV onset ~5 min. Half-life ~3–7 h. Hepatic CYP2D6.
Hemodynamic effects
↓ HR + BP; more sustained than esmolol (choose esmolol for brief/titratable control).
Respiratory effects
Relatively lung-safe at low dose (β1-selective) but can still trigger bronchospasm at higher doses.
Side effects
- !Bradycardia, AV block, hypotension
- !Bronchospasm (high dose)
- !Masks hypoglycemia symptoms
- !Withdrawal rebound tachycardia/HTN
Contraindications
- ×Decompensated HF
- ×High-grade AV block/bradycardia
- ×Cocaine-induced ischemia (unopposed α)
- ×Severe reactive airway (relative)
Clinical pearls
- ★Longer-acting than esmolol — use esmolol when you want on/off titratability, metoprolol for sustained control.
- ★Do NOT stop chronically-taken β-blockers perioperatively (rebound ischemia/HTN — continue through surgery).
Other drugs in Cardiac / BP
- Epinephrine
α1 (vasoconstriction), α2, β1 (inotropy + chronotropy), β2 (bronchodilation, vasodilation in skeletal muscle). Dose-dependent receptor preference: low-dose β-predominant, high-dose α-predominant.
- Norepinephrine
Strong α1 → vasoconstriction. Mild β1 → modest inotropy. Minimal β2.
- Phenylephrine
Pure α1 agonist → vasoconstriction. No β activity.
- Dexmedetomidine
α2 agonist (locus coeruleus) → sedation + analgesia + anxiolysis without significant respiratory depression.
- Vasopressin
Endogenous nonapeptide hormone. V1 receptor agonist on vascular smooth muscle (Gq → IP3 → Ca²⁺ → vasoconstriction). V2 on renal collecting ducts (Gs → cAMP → aquaporin insertion → water reabsorption). At pressor doses (0.01–0.04 U/min), V1 effects dominate.
- Esmolol
Selective β1-adrenergic receptor antagonist. Decreases HR, contractility, conduction velocity, and AV node refractoriness. Selectivity for β1 over β2 reduces (does not eliminate) bronchospasm risk vs non-selective beta-blockers.
- Magnesium Sulfate
Multiple mechanisms: (1) NMDA receptor antagonism (anticonvulsant, analgesic); (2) Voltage-gated calcium channel blockade in vascular + uterine smooth muscle (vasodilation, tocolysis); (3) Decreased ACh release at neuromuscular junction (NMB potentiation); (4) Membrane stabilization (antiarrhythmic, especially torsades).
Suggested reading
- •Stoelting & Hines, Pharmacology & Physiology in Anesthetic Practice, 6e
- •Miller's Anesthesia, 9e
- •FDA package insert
