Mechanism of Action and Immediate Physiological Effects
Adrenaline (epinephrine) is a potent sympathomimetic catecholamine, meaning it mimics the body's natural 'fight-or-flight' response. When administered intravenously, its effects are nearly instantaneous because it is delivered directly into the bloodstream. Adrenaline exerts its effects by acting on both alpha ($\alpha$) and beta ($\beta$) adrenergic receptors throughout the body, leading to a cascade of physiological changes.
Receptor-Specific Actions
- Alpha-1 Receptors: Stimulation of these receptors, especially at higher concentrations, leads to significant vasoconstriction in the peripheral blood vessels. This increases systemic vascular resistance, which helps to raise blood pressure.
- Beta-1 Receptors: Adrenaline's affinity for these receptors, primarily located in the heart, causes an increase in heart rate (chronotropic effect) and the force of contraction (inotropic effect). This results in a higher cardiac output and can raise systolic blood pressure.
- Beta-2 Receptors: Activation of these receptors promotes smooth muscle relaxation, most notably in the airways, causing bronchodilation. This effect is crucial for treating respiratory distress in anaphylaxis. These receptors also cause vasodilation in skeletal muscle vascular beds, though the overall peripheral effect is often dominated by alpha-receptor-mediated vasoconstriction.
Systemic Responses
As a result of these receptor interactions, intravenous adrenaline triggers a systemic response that includes:
- Cardiovascular: A rapid and dramatic increase in heart rate, palpitations, and blood pressure. High doses can induce dangerous cardiac arrhythmias, such as ventricular fibrillation, and increase the risk of myocardial infarction.
- Respiratory: Relaxation of the bronchial smooth muscles, relieving airway constriction and improving breathing.
- Central Nervous System: Feelings of anxiety, nervousness, apprehension, dizziness, and headache are common due to central stimulation.
- Metabolic: Increases in blood glucose levels (hyperglycemia) and lactic acid (lactic acidosis).
- Renal: Decreased renal perfusion due to vasoconstriction, which can lead to reduced urine output.
Clinical Applications and Necessary Precautions
Given its potency and rapid onset, intravenous adrenaline is reserved for critical, life-threatening situations within a controlled medical environment. It is not the standard first-line treatment for all allergic reactions, as intramuscular administration is often safer and sufficient.
Primary Uses of IV Adrenaline
- Cardiac Arrest: During cardiopulmonary resuscitation (CPR), intravenous or intraosseous (IO) adrenaline is administered to increase aortic diastolic blood pressure and improve coronary and cerebral perfusion.
- Severe Anaphylaxis: For patients who do not respond to intramuscular adrenaline or are experiencing profound shock, a titrated IV infusion may be required.
- Septic Shock: Adrenaline is used as a vasopressor to increase blood pressure in hospitalized patients with septic shock that has caused dangerously low blood pressure.
Risks of Inadvertent Administration
Inadvertent or excessively rapid IV administration of adrenaline can have severe consequences, including fatal cardiac arrhythmias, uncontrolled hypertension, and even cerebral hemorrhage. Therefore, IV adrenaline is always administered with extreme caution by trained medical personnel, often as a carefully monitored continuous infusion rather than a rapid push, except during cardiac arrest. If extravasation occurs (the drug leaks from the vein into surrounding tissue), it can lead to tissue necrosis due to intense vasoconstriction.
Comparison of IV and IM Adrenaline Administration
| Feature | Intravenous (IV) Administration | Intramuscular (IM) Administration |
|---|---|---|
| Onset of Action | Immediate (seconds) | Slower (minutes) |
| Dose Control | Titratable via continuous infusion | Fixed dose; cannot be adjusted post-injection |
| Safety Profile | Higher risk of severe adverse effects (e.g., arrhythmias, severe hypertension) if not carefully controlled | Generally safer, with a lower incidence of severe cardiac events |
| Clinical Use | Reserved for critical care (cardiac arrest, septic shock, unresponsive anaphylaxis) | First-line treatment for most anaphylaxis cases |
| Administration | Requires venous access, careful dilution, and infusion pump; managed by trained professionals in a controlled setting | Can be administered quickly and easily, often by auto-injector outside of a hospital setting |
Conclusion
Intravenous adrenaline delivers an immediate and powerful systemic effect by acting on adrenergic receptors to rapidly increase heart rate, blood pressure, and bronchodilation. While essential in critical care scenarios like cardiac arrest and severe septic or anaphylactic shock, this potent route of administration carries significant risks, including severe cardiac events, if not carefully managed. It stands in contrast to the generally safer intramuscular route, which is preferred for initial anaphylaxis treatment. Consequently, IV adrenaline must be handled with the utmost precision by trained medical professionals, making what happens when adrenaline is given IV a matter of life-saving science and meticulous clinical practice.
For more detailed pharmacological information on epinephrine, see the comprehensive resource on the National Institutes of Health (NIH) bookshelf.
