Why Allopurinol is Given in Cyanide Poisoning

Step 1: How Cyanide Causes Injury

• Arrests oxidative phosphorylation and ATP production
• Forces cells into anaerobic metabolism, causing severe lactic acidosis
• Produces profound cellular hypoxia despite normal or elevated blood oxygen levels
• Causes rapid cell death, especially in CNS and cardiac tissue

Step 2: The Reperfusion Injury Component

Hypoxanthine + O₂  --[Xanthine Oxidase]-->  Xanthine + Superoxide (O₂•-)
Xanthine + O₂      --[Xanthine Oxidase]-->  Uric Acid + Superoxide (O₂•-)

Step 3: How Allopurinol Helps

1. Prevents superoxide burst - the initial ROS surge on reperfusion is dramatically reduced
2. Reduces lipid peroxidation - protecting cell membranes, especially in the brain and heart
3. Limits neutrophil activation - preventing the inflammatory cascade that amplifies tissue injury
4. Preserves mitochondrial membrane potential - by reducing ROS-mediated mitochondrial permeability transition pore (mPTP) opening
5. Protects ATP synthesis - downstream mitochondrial function is preserved

Summary Table

Important Note

1. Hydroxocobalamin (Cyanokit) - binds CN- to form non-toxic cyanocobalamin (vitamin B12) - preferred, especially with concurrent CO poisoning
2. Sodium nitrite + sodium thiosulfate (cyanide antidote kit) - methemoglobin induction followed by thiocyanate formation
3. 100% O₂, supportive care

Why Allopurinol is Given in Carbon Monoxide Poisoning

Step 1: How CO Causes Toxicity - Beyond Hemoglobin Binding

Step 2: The NO → Xanthine Oxidase → Free Radical Cascade

CO exposure
    ↓
↑ Nitric Oxide (NO) synthesis
    ↓
NO enhances neutrophil adhesion to endothelium
(via β-integrin adherence molecules)
    ↓
Neutrophil activation → activates Xanthine Oxidase (XO)
    ↓
XO catalyzes:  Hypoxanthine + O₂  →  Xanthine + O₂•⁻ (superoxide)
               Xanthine + O₂      →  Uric acid + O₂•⁻ (superoxide)
    ↓
Free radicals oxidize brain lipids (lipid peroxidation)
    ↓
DELAYED NEUROLOGICAL SEQUELAE (DNS)

• Mental deterioration and memory impairment
• Mutism
• Gait disturbances
• Urinary and fecal incontinence
• Personality changes, parkinsonism

Step 3: How Allopurinol Intervenes

1. Blocks superoxide generation - prevents the free radical burst driven by NO-activated neutrophils
2. Reduces brain lipid oxidation - protecting neuronal and myelin membranes
3. Prevents DNS - the primary goal of its use in CO poisoning
4. Also has a secondary antioxidant effect (allopurinol and its metabolite oxypurinol can directly scavenge hydroxyl radicals)

Comparison: Allopurinol in CN vs CO Poisoning

Bottom Line

Ventricular Arrhythmias - Overview

1. Definition and Classification

2. Mechanisms

A. Reentry (Most Common)

1. Two pathways with different conduction velocities and refractory periods
2. Unidirectional block in one pathway
3. Slow enough conduction in the other to allow recovery of the blocked pathway

"Reentry is a fundamentally different mechanism of arrhythmia-genesis than automaticity or triggered activity. Circus movement reentry occurs when an activation wavefront propagates around an anatomical or functional core, and reexcites the site of origin." - Fuster and Hurst's The Heart, 15th Ed.

B. Abnormal Automaticity

• Accelerated idioventricular rhythm (AIVR) - Purkinje fibers
• Early post-MI period (enhanced automaticity in ischemic cells)
• Digitalis toxicity

C. Triggered Activity

3. Types of Ventricular Arrhythmias

A. Premature Ventricular Complexes (PVCs)

• Single ectopic ventricular beats occurring earlier than expected
• ECG: Wide, bizarre QRS (>120 ms), no preceding P wave, compensatory pause
• Two in a row = couplet; 3 or more in a row = VT by definition
• PVCs alternating with normal beats = bigeminy; every 3rd beat = trigeminy
• Usually benign in structurally normal hearts, but frequent PVCs (>15-20% of beats) can cause PVC-induced cardiomyopathy

B. Ventricular Tachycardia (VT)

• Most common in ischemic heart disease (scar-related reentry)
• Also: RVOT tachycardia (idiopathic, LBBB + inferior axis), fascicular tachycardia

• Associated with acute ischemia, electrolyte disorders, channelopathies
• Degenerates easily to VF

C. Torsades de Pointes (TdP)

• Name means "twisting of the points" - QRS axis rotates around the baseline
• Triggered by EADs on a long QT
• Causes: Hypokalemia, hypomagnesemia, bradycardia, drugs (class Ia/III antiarrhythmics, antipsychotics, antimicrobials), congenital long QT syndrome
• Often self-terminating but can degenerate to VF
• Treatment: IV magnesium, correct electrolytes, increase heart rate (pacing/isoprenaline), withdraw offending drug

D. Ventricular Fibrillation (VF)

• Completely chaotic, disorganized electrical activity with no coordinated ventricular contraction
• No effective cardiac output - equivalent to cardiac arrest
• ECG: irregular chaotic baseline with no recognizable QRS complexes
• Requires immediate defibrillation
• Most common cause of sudden cardiac death

4. ECG Diagnosis: VT vs SVT with Aberrancy

"All wide complex tachycardias should be assumed to be VT unless there is conclusive evidence of a supraventricular tachycardia." - Symptom to Diagnosis: An Evidence-Based Guide, 4th Ed.

ECG Features Strongly Favoring VT:

5. Common Substrates / Causes

6. Key Diagnostic Pointers (Summary)

• Sustained VT = lasts ≥30 seconds or requires termination due to hemodynamic compromise
• Non-sustained VT = ≥3 beats but <30 seconds, self-terminates
• Bundle branch reentry VT - special form using the bundle branches as the reentrant circuit; seen in dilated cardiomyopathy; QRS looks like typical bundle branch block
• Bidirectional VT (alternating QRS axis beat-to-beat) = classic for digoxin toxicity and CPVT