Adrenaline (epinephrine) is a potent vasoconstrictor commonly used as an adjuvant to local anaesthetics . Its primary role is to prolong the duration of anaesthesia , reduce systemic absorption , and minimize toxicity by confining the local anaesthetic to the site of injection.
Mechanism of Action
- Adrenaline acts on α-adrenergic receptors , causing vasoconstriction of blood vessels at the injection site.
- This reduces local blood flow , leading to:
- Slower absorption of the anaesthetic
- Prolonged action at the site of injection
- Decreased risk of systemic toxicity
- Reduced intraoperative bleeding (relative ischemia)
Clinical Effects of Adrenaline with Local Anaesthetics
Prolonged local anaesthetic effect
Decreased rate of systemic absorption
Localized drug action with reduced systemic toxicity
Improved surgical field visibility due to reduced bleeding
Clinical Use
Adrenaline is commonly added to local anaesthetics in:
- Nerve blocks
- Infiltration anaesthesia
- Dental procedures
Not recommended in:
- Spinal (intrathecal) or epidural anaesthesia with bupivacaine
- Anaesthesia in end-arterial areas (e.g., fingers, toes, ears, nose, penis )
Risk of ischemia and tissue necrosis
Pharmacokinetics
- Reduced perfusion at the injection site leads to:
- Lower concentration gradient between endoneural space and capillaries
- Reduced diffusion of anaesthetic into systemic circulation
Alternative Vasoconstrictors
- Norepinephrine – occasionally used but less common
- Felypressin – a vasopressin analogue , used when epinephrine is contraindicated
- No arrhythmogenic action
- Less reactive hyperemia
- Higher risk of coronary artery vasoconstriction
Adverse Effects of Catecholamine Vasoconstrictors (e.g., Adrenaline)
- Reactive hyperemia after washout
- Cardiostimulation if systemically absorbed
- Tachycardia
- Hypertension
- Arrhythmias
Dosage and Concentration
Adrenaline concentrations in local anaesthetic solutions:
| Ratio | Concentration (µg/mL) | Example Max Volume |
|---|---|---|
| 1:80,000 | 12.5 µg/mL | 20 mL (250 µg) |
| 1:100,000 | 10 µg/mL | 25 mL (250 µg) |
| 1:200,000 | 5 µg/mL | 50 mL (250 µg) |
Expression : Adrenaline concentration is written as the ratio of grams per mL.
E.g., 1:100,000 = 0.01 mg/mL or 10 µg/mL.
Maximum Safe Dose
- 250 micrograms of adrenaline is considered a general maximum safe dose in local anaesthesia.
- This corresponds to:
- 20 mL of 1:80,000 , or
- 50 mL of 1:200,000
High yield Points
- Always avoid adrenaline in end-artery regions .
- Monitor cardiovascular status , especially in cardiac patients.
- Use lowest effective concentration to minimize systemic effects.
- Be aware of drug interactions (e.g., tricyclic antidepressants, MAO inhibitors).
