Methemoglobinemia, Cyanide Poisoning, and Carbon Monoxide Poisoning Comparison

Methemoglobinemia, Cyanide Poisoning, and Carbon Monoxide Poisoning Comparison

Introduction

  • Comparison of methemoglobinemia, cyanide poisoning, and carbon monoxide (CO) poisoning.

  • Focus on vertical and horizontal analysis for effective studying.

  • Repetition is key to retaining information.

Etymology:

Methemoglobinemia

  • "Meta" means change.

  • "-emia" means blood.

  • "Hemoglobin" refers to hemoglobin in the blood.

Cyanide Poisoning

  • "Cyanide" comes from "cyan," which means blue.

  • Associated with Prussian blue color.

Definition

Methemoglobinemia

  • Normal Hemoglobin: Ferrous Fe2+Fe^{2+} (carries oxygen).

  • Methemoglobin: Ferric Fe3+Fe^{3+} (cannot bind oxygen).

  • Ferric iron is "hysteric" and cannot bind O2O_2.

Cyanide Poisoning

  • Exposure to cyanide.

Carbon Monoxide (CO) Poisoning

  • Exposure to CO from incomplete combustion.

  • Requires improper ventilation, as proper ventilation would yield non-toxic carbon dioxide (CO2CO_2).

Etiology

Methemoglobinemia

  • Acquired: Local anesthetics (e.g., lidocaine), sulfa drugs, nitrites, nitrates, sepsis, dehydration.

  • Congenital: Deficiency of NADPH-dependent cytochrome b5 reductase.

Cyanide Poisoning

  • Fires (most common cause).

  • Suicide attempts.

  • Ingestion of sodium nitroprusside.

  • Consumption of almonds and apricots in large quantities.

Carbon Monoxide Poisoning

  • Fires.

  • Suicide attempts.

  • Obstructed car mufflers.

  • Barbecues.

Pathophysiology

Methemoglobinemia

  • Methemoglobin (with Fe3+Fe^{3+}) cannot carry oxygen.

  • Decreased oxygen saturation leads to tissue hypoxia.

  • Increased erythropoietin (EPO) leads to secondary polycythemia (increased red blood cells).

  • Methemoglobin cannot release oxygen, causing a left shift of the oxygen dissociation curve.

  • Leads to lactic acidosis and high anion gap metabolic acidosis (HAGMA).

Cyanide Poisoning

  • Cyanide binds to ferric iron in complex IV (cytochrome c oxidase), inhibiting the electron transport chain.

  • No ATP production.

  • Cells use anaerobic glycolysis, leading to HAGMA due to increased lactic acid.

  • Increased oxygen in the vein (increased PvO2PvO_2) since oxygen is not utilized by the mitochondria.

Carbon Monoxide Poisoning

  • CO inhibits complex IV (cytochrome c oxidase).

  • Anaerobic metabolism and HAGMA.

  • Forms carboxyhemoglobin, reducing oxygen loading and unloading.

  • Affects hemoglobin and myoglobin, potentially leading to traumatic rhabdomyolysis.

  • Shifts the oxygen dissociation curve to the left.

Clinical Presentation

Methemoglobinemia

  • Chocolate-colored blood.

  • Dusky-colored skin.

  • Headache, weakness.

  • Chest pain.

Cyanide Poisoning

  • Bitter almond breath.

  • Dilated pupils.

  • Diaphoresis (sweating).

  • Headache.

  • Arrhythmia.

Carbon Monoxide Poisoning

  • Headaches among family members during winter.

  • Flu-like symptoms without fever.

  • Cherry-red skin.

  • Cutaneous bullae (skin blisters).

Diagnosis

Methemoglobinemia

  • Rule out G6PD deficiency.

  • Dark or chocolate-colored blood.

  • Filter paper test.

  • Enzyme assay: NADPH-dependent cytochrome b5 reductase.

Cyanide Poisoning

  • Arterial blood gas (ABG): High anion gap metabolic acidosis.

  • Venous blood gas (VBG): High PvO2PvO_2.

  • Increased plasma cyanide level.

Carbon Monoxide Poisoning

  • Spectrophotometry: Detects carboxyhemoglobin.

  • Pulse CO-oximetry: Detects decreased oxygen saturation.

  • Avoid standard pulse oximetry.

  • ABG and VBG: HAGMA and increased PvO2PvO_2.

Blood Gas Analysis

PaO2 (Partial Pressure of Oxygen in Arterial Blood)

  • O2 floating in arterial blood

  • Methemoglobinemia: Normal.

  • Cyanide Poisoning: Normal.

  • CO Poisoning: Normal.

SaO2 (Arterial Oxygen Saturation)

  • O2 on the hemoglobin

  • Methemoglobinemia: Decreased.

  • Cyanide Poisoning: Normal (Cyanide is a mitochondrial toxin).

  • CO Poisoning: Decreased.

Oxygen Dissociation Curve

  • O2 binding curve

  • Methemoglobinemia: Left shift.

  • Cyanide Poisoning: Normal.

  • CO Poisoning: Left shift.

Complex IV (Cytochrome C Oxidase, Cytochrome A/A3)

  • Methemoglobinemia: Normal.

  • Cyanide Poisoning: Inhibited.

  • CO Poisoning: Inhibited.

Treatment

Methemoglobinemia

  • Remove the cause.

  • Mild cases: Vitamin C and Vitamin B2.

  • Severe cases: IV methylene blue.

  • Hydrate, administer bicarbonate for acidosis, and provide oxygen.

Cyanide Poisoning

  • Remove clothes and give 100% oxygen.

  • Cyanide antidote triad:

    • Hydroxocobalamin.

    • Sodium nitrite.

    • Sodium thiosulfate.

Carbon Monoxide Poisoning

  • Remove from the environment and give 100% oxygen.

  • Severe cases: Hyperbaric oxygen chamber.