Mechanism of Action and Cardioselectivity
The main distinction of this drug class lies in their ability to selectively target $\beta_1$-adrenergic receptors, which are located predominantly in the heart. Effects on $\beta_2$-receptors (located in the bronchi and blood vessels) are minimal.
The cardioselectivity index is used to evaluate this selectivity. It represents the ratio of the potency of $\beta_1$ to $\beta_2$ receptor blockade. For example, metoprolol and betaxolol have an index of 25/1 (moderate selectivity), while nebivolol reaches 290/1 (high selectivity). For comparison, the non-selective agent propranolol has a ratio of 1/1.8.
Key Representatives
The most commonly used cardioselective beta-blockers in clinical practice include:
- Metoprolol
- Bisoprolol
- Nebivolol
- Atenolol
- Betaxolol
- Talinolol
- Esmolol
Advantages and Clinical Application
Due to their low affinity for $\beta_2$-receptors, these drugs offer several advantages over non-selective analogues:
- Less increase in bronchial tone.
- Weaker effect on peripheral vascular tone.
- Less interference with carbohydrate metabolism (less increase in insulin resistance and less impact on glucose-lowering medications).
Main indications include hypertension, stable angina, myocardial infarction, and tachyarrhythmias. In coronary artery disease, it is clinically significant that these drugs decrease cardiac output while virtually not increasing afterload or total peripheral vascular resistance.
Therapy in Chronic Heart Failure (CHF)
In CHF, the body attempts to compensate for reduced pumping function by activating the sympathoadrenal and renin-angiotensin systems. Excess norepinephrine and angiotensin II lead to increased heart rate, myocardial overload, and ultimately cardiomyocyte apoptosis, fibrosis, and arrhythmias.
Cardioselective beta-blockers counteract the toxic effects of norepinephrine. They slow the heart rate, prolong diastole (improving myocardial perfusion), and suppress renin secretion. Therapy is always initiated at the lowest doses and gradually increased (titration). These drugs are combined with ACE inhibitors, and a sustained positive effect develops over 2–3 months.