Mechanism of antianginal action
The antianginal effect of the drugs is based on the blockade of cardiac $\beta_1$-adrenergic receptors. This leads to two key outcomes:
- Negative inotropic effect — reduction in the force of cardiac contractions;
- Negative chronotropic effect — decrease in heart rate.
As a result, the overall workload of the heart decreases, and myocardial oxygen demand drops significantly. With long-term administration (more than a week), blood pressure also decreases, which further unloads the myocardium.
Classification and features of drugs
Depending on their effect on receptors, drugs are divided into two main groups:
- Cardioselective ($\beta_1$): atenolol, metoprolol, bisoprolol, talinolol. The described cardiac mechanism is fully manifested in them.
- Non-selective ($\beta_1, \beta_2$): propranolol. It is characterized by a unique ability to redistribute coronary blood flow in favor of ischemic areas by increasing the tone of healthy vessels.
Nebivolol deserves special attention, possessing super-selective $\beta_1$-receptor blockade and the ability to stimulate endothelial nitric oxide production for additional vasodilation.
Side effects and safety
The use of these agents carries the risk of certain adverse reactions:
- Cardiac effects: suppression of AV conduction, excessive bradycardia, a drop in contractile force, and arterial hypotension.
- Specific risks of non-selective agents: bronchospasm and peripheral vascular spasm with a sensation of cold extremities.
Abrupt discontinuation of treatment is unacceptable, as it provokes withdrawal syndrome — a sudden increase in the severity and frequency of angina attacks.
Combination therapy with nitrates
Co-administration with nitrates is considered a classic and highly rational example of drug synergy:
- Beta-blockers completely eliminate the reflex tachycardia caused by nitrates.
- Nitrates, in turn, neutralize the potential vasospastic effect of beta-blockers and reduce left ventricular end-diastolic pressure and volume.
Briefly: answers to topical questions
Which drugs belong to cardioselective beta-blockers?
Cardioselective beta-blockers (selective $\beta_1$-receptor blockers) include the following drugs:
- Atenolol (atenolol) is a selective $\beta_1$-blocker, a hydrophilic drug.
- Metoprolol (metoprolol) — a lipophilic selective drug.
- Bisoprolol (bisoprolol) — an amphiphilic drug with high selectivity.
- Nebivolol (nebivolol) — a drug with the highest cardioselectivity and vasodilating properties.
- Betaxolol (betaxolol) — a lipophilic selective drug.
- Talinolol (talinolol) — a representative of cardioselective agents.
What are the absolute contraindications to beta-blockers?
Absolute contraindications to beta-blockers include the following conditions:
- Cardiogenic shock (cardiogenic shock) — severe cardiovascular impairment.
- Severe chronic obstructive pulmonary disease (chronic obstructive pulmonary disease) in the acute stage.
- Atrioventricular block (atrioventricular block) grade II–III in patients without a functioning artificial pacemaker.
- Presence of a functioning artificial pacemaker (artificial cardiac pacemaker).
- Allergy (allergy) or intolerance to drug components.
How do beta-blockers affect lipid and carbohydrate metabolism?
Beta-blockers exert a specific effect on metabolic processes:
- Carbohydrate metabolism: inhibition of hepatic glycogenolysis prolongs hypoglycemia, while blockade of pancreatic $\beta_2$-receptors decreases insulin secretion. Non-selective drugs mask the symptoms of hypoglycemia and increase tissue insulin resistance in type 2 diabetes.
- Lipid metabolism: metabolic effects include a decrease in triglyceride concentration, a reduction in total cholesterol, and a decrease in high-density lipoprotein (HDL) levels.
Which drugs belong to non-selective beta-blockers?
Non-selective beta-blockers ($\beta_1$- and $\beta_2$-receptor blockers) include the following drugs:
- Propranolol (propranolol) — the reference standard drug of the group.
- Nadolol (nadolol) — a long-acting hydrophilic drug.
- Timolol (timolol) — a representative of non-selective agents.
- Pindolol (pindolol) — a drug with intrinsic sympathomimetic activity.
- Oxprenolol (oxprenolol) — a drug from the group of non-selective blockers.
Beta-blockers are a "brake" for the engine: fewer beats per minute (chronotropy), weaker contraction (inotropy), and lower myocardial oxygen demand.
Class and exam questions
Why are beta-blockers contraindicated in vasospastic angina?
In vasospastic (Prinzmetal) angina, their use is prohibited due to the high risk of increasing coronary vascular tone and subsequent worsening of ischemia.
How do non-selective drugs redistribute coronary blood flow?
By blocking $\beta_2$-receptors, they increase the tone of unaffected coronary vessels. Because the lumen of vessels in the plaque area is rigid, blood rushes under high resistance into the ischemic area in need of it.
What is the essence of nebivolol's combined action?
Nebivolol combines super-selective $\beta_1$-receptor blockade with additional stimulation of endothelial nitric oxide production, which ensures reliable vasodilation.
What else to study on this topic
- Detailed pharmacokinetic differences between bisoprolol and metoprolol
- Selectivity indices of various beta-blockers ($\beta_1 / \beta_2$)
- Pathophysiology of withdrawal syndrome upon abrupt cessation
- Peculiarities of drug use in concomitant chronic heart failure
- Comparison of antiarrhythmic activity of class II agents
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