What are the mechanisms of action and clinical applications of adrenaline?

Epinephrine is commonly available in injectable form. Its mechanisms of action generally include activating α receptors to constrict blood vessels, stimulating β1 receptors to enhance myocardial contraction, activating β2 receptors to relax smooth muscles, promoting glycogen breakdown to elevate blood glucose levels, and modulating central nervous system excitability. Clinically, epinephrine is commonly used in resuscitating cardiac arrest, treating anaphylactic shock, relieving acute bronchial asthma attacks, local hemostasis, and prolonging the duration of local anesthetics. A detailed explanation is as follows:

1. Mechanism of Action

1. Vasoconstriction via α Receptor Activation: Epinephrine binds to α receptors on vascular smooth muscles, causing vasoconstriction, particularly in small arteries and veins, thereby increasing blood pressure to ensure adequate blood perfusion to vital organs and also reducing local blood flow.

2. Enhanced Myocardial Contraction via β1 Receptor Activation: By acting on β1 receptors in the heart, epinephrine enhances myocardial contractility, increases heart rate, boosts cardiac output, and improves cardiac pumping function, thereby enhancing circulation when cardiac function is compromised.

3. Smooth Muscle Relaxation via β2 Receptor Activation: Epinephrine binds to β2 receptors in areas such as the bronchi, effectively relaxing smooth muscles, alleviating airway spasms, dilating bronchi, improving ventilation, and reducing symptoms of dyspnea.

4. Promotion of Glycogen Breakdown and Elevation of Blood Glucose: Epinephrine acts on the liver and muscle tissues to promote glycogen breakdown into glucose, rapidly increasing blood glucose levels and providing sufficient energy for the body under stress conditions.

5. Modulation of Central Nervous System Excitability: Epinephrine stimulates the central nervous system, enhancing alertness and response capability, enabling the body to rapidly enter a state of stress response and improving its ability to cope with emergencies.

2. Clinical Applications

1. Resuscitation from Cardiac Arrest: During cardiac arrest, epinephrine enhances myocardial contractility, restores spontaneous heart rhythm, increases coronary artery perfusion, and is a key medication during cardiopulmonary resuscitation (CPR). It exerts rapid effects when administered intravenously.

2. Treatment of Anaphylactic Shock: In anaphylactic shock, epinephrine constricts blood vessels to raise blood pressure, alleviates laryngeal edema, dilates bronchi to improve ventilation, and simultaneously inhibits the release of allergic mediators. It is the first-line drug for treating anaphylactic shock.

3. Relief of Acute Bronchial Asthma Attacks: Epinephrine rapidly relaxes bronchial smooth muscles, reduces airway narrowing, alleviates dyspnea and wheezing caused by asthma, and quickly improves ventilation function.

4. Local Hemostasis: Diluted epinephrine can be applied to local bleeding wounds to induce vasoconstriction and reduce blood flow to the bleeding site, achieving hemostatic effects. It is commonly used for hemostasis on mucous membranes such as the nasal cavity and gums.

5. Prolongation of Local Anesthetic Action: Adding a small amount of epinephrine to local anesthetics causes vasoconstriction, slows the absorption rate of the anesthetic, prolongs its local effect, enhances the anesthetic effect, and reduces the required drug dosage.

Epinephrine is an emergency medication and must be strictly administered according to medical instructions by trained healthcare professionals. It should not be used arbitrarily outside of emergencies. Vital signs such as blood pressure and heart rate should be closely monitored after administration. If adverse reactions such as palpitations or sudden blood pressure elevation occur, appropriate interventions should be promptly taken.