Adenosine 

1. Overview

Adenosine is an endogenous purine nucleoside 

It is a Class V antiarrhythmic drug (miscellaneous) per Vaughan-Williams classification.

2. Mechanism of Action 

Adenosine acts via A1 receptors (Gi protein-coupled):

  • ↓ cAMP → ↓ Ca²⁺ influx
  • ↑ K⁺ efflux → hyperpolarization

 Result:

  • Marked AV node suppression
  • ↓ conduction velocity
  • ↑ refractory period

 3. Electrophysiological Effects

Site

Effect

SA node

Mild suppression

AV node

Profound block (main action)

Atria

Minimal effect

Ventricles

No direct effect

 Important:

  • Works ONLY in AV node–dependent arrhythmias

 4. Pharmacokinetics 

Parameter

Value

Half-life

< 10 seconds (ultrashort)

Onset

Immediate (seconds)

Duration

10–30 sec

Route

Rapid IV bolus only

Must be given via:

  • Large bore IV
  • Followed by 20 mL saline flush
  • Preferably proximal line (antecubital / central line)

 5. Indications in ICU

 A. Therapeutic

  1. Paroxysmal SVT (PSVT)
    • Especially:
      • AVNRT
      • AVRT (orthodromic)

 First-line drug per:

  • ACLS guidelines
  • ESC tachycardia guidelines

B. Diagnostic

  1. Wide complex tachycardia (regular)
    • Helps differentiate:
      • SVT with aberrancy vs VT
  1. Unmask atrial activity
    • Atrial flutter
    • Atrial tachycardia

 Not Effective In

  • Atrial fibrillation
  • Atrial flutter (terminates rarely, but unmasks waves)
  • Ventricular tachycardia

 6. Dosing 

Adult Dose (ACLS Standard)

  1. Initial:
    • 6 mg rapid IV bolus
  1. If no response (1–2 min):
    • 12 mg rapid IV bolus
  1. Repeat once:
    • 12 mg

Special Situations

Condition

Dose Adjustment

Central line

Lower dose (3 mg)

Heart transplant

Very sensitive → 3 mg initial

Dipyridamole use

↓ dose

Theophylline/caffeine

↑ dose needed

7. ECG Effects

Before → After Adenosine

  • Sudden pause (asystole-like)
  • Then:
    • Conversion to sinus rhythm (SVT)
    • OR reveal underlying rhythm

9. Adverse Effects

 Common 

  • Flushing
  • Chest pain
  • Dyspnea
  • Sense of impending doom
  • Headache

These last < 30 sec (due to short half-life)

 Serious

Effect

Mechanism

Bronchospasm

A2B receptor activation

Prolonged asystole

Excess AV suppression

Atrial fibrillation

Triggered arrhythmia

Hypotension

Vasodilation

10. Contraindications

Absolute

  • 2nd/3rd degree AV block (without pacemaker)
  • Sick sinus syndrome
  • Severe asthma / bronchospasm

Relative

  • COPD
  • Hypotension
  • Heart transplant

11. Drug Interactions 

Drug

Effect

Dipyridamole

↑ Adenosine effect

Theophylline

↓ effect (antagonist)

Caffeine

↓ effect

 Mechanism:

  • Adenosine receptor modulation

 12. Special Clinical Scenarios

1. Wide Complex Tachycardia

  • Safe only if regular and monomorphic
  • Avoid in:
    • Irregular Wide Complex Tachy  → may worsen (e.g., AF with WPW)

 2. WPW Syndrome

  • Orthodromic AVRT → effective
  • AF with WPW →  dangerous (may cause VF)

3. Asthma Patients

  • Avoid → risk of severe bronchospasm

 4. Heart Transplant Patients

  • Denervated heart → hypersensitive
  • Use lower dose