Learning Objectives
Master the differentiation between Cyanide and Carbon Monoxide (CO) poisoning. Understand their unique mechanisms of inhibiting aerobic metabolism, their classic clinical presentations (e.g., “cherry-red” skin), and the distinct pharmacological interventions required for each.
1. Exposure and Mechanisms
Both toxins disrupt the Electron Transport Chain (ETC), but they attack different components of oxygen delivery and utilization.
| Feature | Cyanide | Carbon Monoxide (CO) |
|---|---|---|
| Source | House fires, Nitroprusside use, Apricot seeds (Amygdalin), Suicide attempts. | House fires, Motor exhaust, Space heaters, and tobacco smoke. |
| Primary Target | Complex IV (Cytochrome c oxidase); binds Fe3+ in cytochrome a3. | Hemoglobin (binds Fe2+ competitively) AND Complex IV. |
| Effect | Inhibits oxidative phosphorylation; tissues cannot use O2. | Decreases O2 carrying capacity and prevents O2 unloading. |
2. Clinical Presentation and Diagnostics
While both can cause “cherry-red” skin, the underlying reason for this color—and the laboratory findings—differs significantly.
| Finding | Cyanide | Carbon Monoxide (CO) |
|---|---|---|
| Skin/Breath | Pink/flushed (bright red venous blood); Bitter almond odor. | Cherry-red skin; Bullous skin lesions (in severe cases). |
| MRI Brain | Rarely affects the globus pallidus. | Bilateral globus pallidus lesions. |
| Labs | Severe Lactic Acidosis (High Anion Gap). Normal SaO2 initially. | Carboxyhemoglobin on co-oximetry. Pulse ox is misleadingly normal. |
| ODC Shift | Curve is normal (O2 is there, just unused). | Left Shift (Increased affinity, decreased unloading). |
3. Antidotes and Management
Treatment strategies differ based on the need to either displace the toxin or chemically neutralize it.
| Poisoning | Standard Treatment | Mechanism |
|---|---|---|
| Cyanide | Hydroxocobalamin | Binds CN- to form Cyanocobalamin (Vitamin B12) for renal excretion. |
| Nitrites (Amyl nitrite) | Induces MetHb (Fe3+), which sequesters CN- away from mitochondria. | |
| Sodium Thiosulfate | Enhances the conversion of CN- to Thiocyanate by rhodanese. | |
| Carbon Monoxide | 100% O2 or Hyperbaric O2 | Competitively displaces CO from Hb and reduces half-life. |
Activity:
High-Yield Clinical Pearls:
- Fire Victims: Suspect both poisons. Use Hydroxocobalamin first. Avoid nitrites in fire victims because nitrites induce MetHb, which would further decrease O2 delivery in someone who already has CO poisoning.
- The Pulse Ox Trap: Pulse oximeters cannot tell the difference between Oxyhemoglobin and Carboxyhemoglobin. A CO patient may look “100% saturated” while their tissues are starving.
- Venous O2: In Cyanide poisoning, the O2 content of venous blood is high (bright red) because the tissues are unable to extract it from the arterial blood.