1. Pharmacologic Class

• Endogenous purine nucleoside
• Classified clinically as an antiarrhythmic (Class V) and diagnostic coronary vasodilator

2. Mechanism of Action

Adenosine acts via adenosine (A₁, A₂A, A₂B, A₃) receptors, all G-protein–coupled receptors.

Cardiac Effects (A₁ receptor – primary clinical relevance)

• Located predominantly in the AV node
• Activates Gi proteins → ↓ adenylate cyclase → ↓ cAMP
• Results in:
  • ↓ Ca²⁺ influx
  • ↑ K⁺ efflux
  • Hyperpolarization of AV nodal tissue
• Clinical effect: Transient AV nodal conduction block

➡ This terminates reentry circuits involving the AV node

Coronary Vasodilation (A₂A receptors)

• Activates Gs proteins → ↑ cAMP
• Causes potent coronary vasodilation
• Basis for pharmacologic cardiac stress testing

3. Indications

FDA-Approved

1. Acute termination of paroxysmal supraventricular tachycardia (PSVT)
   • Specifically AVNRT and AVRT
   • Ineffective in atrial fibrillation/flutter except for diagnostic slowing
2. Pharmacologic stress testing
   • Alternative to exercise stress tests

4. Pharmacokinetics

• Onset: Seconds
• Half-life: <10 seconds (typically 1–6 sec)
• Metabolism:
  • Rapid uptake by erythrocytes and endothelial cells
  • Metabolized to inosine → hypoxanthine → uric acid
• Elimination: Intracellular metabolism (not renal or hepatic dependent)

➡ Explains brief efficacy and short-lived adverse effects

5. Dosing (PSVT)

• Initial: 6 mg IV rapid bolus
• Second dose: 12 mg IV if no response (may repeat once)
• Must be followed immediately by 20 mL saline flush
• Administer via proximal IV site

6. Adverse Effects (Common but Transient)

Usually resolve within seconds due to short half-life.

⚠ Patients should be warned prior to administration

7. Contraindications & Precautions

Absolute

• 2nd- or 3rd-degree AV block (without pacemaker)
• Sick sinus syndrome
• Active bronchospastic disease (e.g., severe asthma)

Relative

• Heart transplant patients (↑ sensitivity)
• Concomitant AV nodal–blocking agents

8. Drug Interactions

➡ Caffeine intake can reduce efficacy (clinically relevant in stress testing)

9. Clinical Pearls

• Not effective for terminating AF or atrial flutter—only slows AV conduction
• Diagnostic utility: unmasks atrial activity
• Expect brief asystole — reassure staff and patient
• Adenosine sensitivity is increased post–heart transplant

10. Counseling & Monitoring (Pharmacist Role)

• Prepare patient for brief but intense symptoms
• Verify rhythm and contraindications before administration
• Ensure resuscitation equipment is available
• Monitor ECG continuously during administration