Relative Dose Potencies of Beta Blockers
The relative potency of beta blockers varies significantly, with propranolol serving as the reference standard against which other beta blockers are compared. Atenolol is approximately 1:2 as potent as propranolol (meaning 100 mg atenolol ≈ 50 mg propranolol), while metoprolol is approximately 1:1 with propranolol on a milligram basis.
Comparative Potency Table
| Beta Blocker | Selectivity | Partial Agonist Activity | Usual Dose Range | Relative Potency to Propranolol |
|---|---|---|---|---|
| Propranolol | None | No | 20-80 mg BID | 1 (reference) |
| Metoprolol | Beta-1 | No | 50-200 mg BID | 1:1 |
| Atenolol | Beta-1 | No | 50-200 mg daily | 1:2 |
| Nadolol | None | No | 40-80 mg daily | 2-3:1 |
| Timolol | None | No | 10 mg BID | 4:1 |
| Acebutolol | Beta-1 | Yes | 200-600 mg BID | 1:4 |
| Betaxolol | Beta-1 | No | 10-20 mg daily | 5:1 |
| Bisoprolol | Beta-1 | No | 10 mg daily | 4:1 |
| Esmolol (IV) | Beta-1 | No | 50-300 mcg/kg/min | 1:10 (short-acting) |
| Labetalol | None* | Yes | 200-600 mg BID | 1:3-5 (for beta blockade) |
| Pindolol | None | Yes | 2.5-7.5 mg TID | 4:1 |
| Carvedilol | None* | Yes | 6.25-25 mg BID | 2-3:1 (for beta blockade) |
*Labetalol and carvedilol also have alpha-blocking properties
Clinical Implications of Beta Blocker Potency
Beta-1 selectivity: Drugs like metoprolol, atenolol, and bisoprolol preferentially block cardiac beta-1 receptors at lower doses, potentially causing fewer bronchospastic effects in patients with reactive airway disease 1.
Conversion between agents: When converting between beta blockers, the relative potency must be considered. For example, 200 mg of labetalol is approximately equivalent to 75 mg of metoprolol tartrate 2.
Pharmacokinetic differences: The elimination half-life varies significantly between agents (propranolol: 3-5 hours; atenolol: 6-7 hours; bisoprolol: 9-12 hours), affecting dosing frequency 3, 4, 5.
Lipophilicity: Propranolol is highly lipophilic and crosses the blood-brain barrier readily, potentially causing more central nervous system effects than hydrophilic agents like atenolol 1.
Important Considerations When Selecting Beta Blockers
Receptor selectivity: Beta-1 selective agents (metoprolol, atenolol, bisoprolol) may be preferred in patients with asthma or COPD, though selectivity is lost at higher doses 5.
Intrinsic sympathomimetic activity (ISA): Agents with ISA (pindolol, acebutolol) may cause less resting bradycardia and less reduction in cardiac output 6.
Alpha-blocking properties: Labetalol and carvedilol have additional alpha-blocking effects that cause greater vasodilation, which may be beneficial in hypertension but can cause more orthostatic hypotension 7.
Duration of action: Longer-acting agents (nadolol, atenolol, bisoprolol) allow for once-daily dosing, potentially improving adherence 8.
Common Pitfalls in Beta Blocker Dosing
Assuming equivalent potency: Milligram-for-milligram equivalence does not exist across all beta blockers. For example, 100 mg of atenolol is not equivalent to 100 mg of metoprolol 9.
Ignoring pharmacokinetics: Poor CYP2D6 metabolizers may have several-fold higher plasma concentrations of metoprolol, potentially requiring dose adjustment 3.
Overlooking combined properties: When converting from a non-selective agent with alpha-blocking properties (like carvedilol) to a selective beta blocker (like metoprolol), blood pressure effects may differ even at "equivalent" beta-blocking doses 7.
Disregarding clinical evidence: Selection of a specific agent should be guided by trial data rather than assumed class effects, as not all beta blockers have demonstrated the same benefits in specific conditions 10.
By understanding these relative potencies and pharmacological differences, clinicians can make more informed decisions when selecting and dosing beta blockers for various cardiovascular conditions.