Carbon Monoxide Poisoning: Diagnosis and Treatment

Question from Last Time: Hemolytic Anemia vs. CO Poisoning

• Scenario: Two patients with oxyhemoglobin at 60% of normal.

  • Patient A: Hemolytic anemia.

  • Patient B: Carbon monoxide poisoning.

• Question: Which patient will have worse tissue hypoxia?

• Answer: Patient B (CO poisoning).

• Explanation:

  • In hemolytic anemia, the 60% oxyhemoglobin is functional, though less than normal.

  • In CO poisoning, the left shift of the oxygen dissociation curve prevents oxygen release into tissues, rendering the 60% oxyhemoglobin useless.

Additional Effects of Carbon Monoxide

• CO inhibits cytochrome C oxidase (complex IV) in the mitochondria, acting as a cellular respiration poison.

• CO damages myoglobin, leading to traumatic rhabdomyolysis.

• CO poisoning is a silent killer because PaO2 remains normal, preventing the brain from triggering hyperventilation and wakefulness.

Causes of Carbon Monoxide Poisoning

• Incomplete combustion due to limited oxygen leads to CO production.

• CO is a chemical asphyxiant; carboxyhemoglobin is always abnormal.

• Common Causes:

  • Fires (most common cause of CO and cyanide poisoning).

  • Clogged vents.

  • Obstructed stoves and heaters (e.g., barbecues).

Mechanism of CO Poisoning

• CO interferes with cellular respiration by inhibiting cytochrome C oxidase (complex IV).

• Other inhibitors of complex IV: cyanide and hydrogen sulfide.

  • Cyanide leads to cyanide poisoning and cyanohemoglobin.

  • Hydrogen sulfide leads to sulfhemoglobin.

• Inhibition of complex IV depletes ATP, rendering mitochondria useless.

• CO competitively binds to hemoglobin, displacing oxygen and decreasing oxygen loading.

• CO prevents hemoglobin from releasing oxygen into tissues, causing a left shift in the oxygen dissociation curve.

Treatment of CO Poisoning

• Administering high concentrations of oxygen to compete with CO for binding to hemoglobin.

• 100% oxygen is given initially; if ineffective, a hyperbaric oxygen chamber is used.

Pulse Oximetry vs. Co-Oximetry

• CO poisoning causes carboxyhemoglobin, which decreases oxygen saturation (SaO2).

• Pulse Oximetry:

  • Detects oxyhemoglobin and deoxyhemoglobin.

  • Problem: Cannot accurately detect abnormal hemoglobins like methemoglobin and carboxyhemoglobin, potentially yielding falsely normal readings.

• Pulse Co-Oximetry:

  • Identifies abnormal hemoglobins like methemoglobin and carboxyhemoglobin.

  • Preferred method for CO poisoning diagnosis.

Physiological Consequences of CO Poisoning

• Carboxyhemoglobin decreases oxygen loading and unloading, leading to tissue hypoxia.

• Chronic cases can result in secondary polycythemia as the body attempts to compensate for hypoxia.

• CO disrupts mitochondrial function, preventing ATP formation and causing anaerobic glycolysis.

Impact on Organs and Metabolic Processes

• The brain and heart are most affected due to their high oxygen demands.

• Anaerobic glycolysis leads to lactic acidosis, resulting in high anion gap metabolic acidosis.

• CO-induced left shift prevents oxygen release to tissues, eliminating the oxygen concentration gradient.

• Increased oxygenated blood in veins causes cherry red skin.

Carbon Monoxide Binding

• CO binds to:

  • Hemoglobin (leading to a left shift of the oxygen dissociation curve).

  • Complex IV (decreasing ATP formation).

  • Myoglobin (decreasing oxygen in muscles, causing traumatic rhabdomyolysis).

Clinical Scenarios

• Entire family complaining of headaches after an indoor barbecue.

• Exposure to faulty exhaust systems in old vehicles without catalytic converters.

• Use of gasoline or kerosene heaters indoors.

• Leaving an engine running in a closed garage.

• Malfunctioning furnaces or HVAC systems.

• Building fires.

Clinical Presentation

• Symptoms: Nonspecific, often misdiagnosed.

  • Headache (most common).

  • Flu-like symptoms (malaise, fatigue, joint aches, chest pain, palpitations) without fever.

  • Eventual coma.

• Signs:

  • Neuropsychiatric symptoms (gradual, unlike stroke, caused by chronic smoking).

  • Lack of cyanosis due to increased venous PvO2.

  • Cherry red skin (rare but significant).

  • Cutaneous bullae (rare but significant).

  • Kidney hyperthermia.

  • Retinal hemorrhage.

  • Bright red retinal veins (due to increased PvO2).

  • Amnesia with confabulation.

Lab Findings

• Spectrophotometry: increased carboxyhemoglobin.

• Pulse co-oximetry: decreased SaO2 (pulse oximetry may show normal SaO2).

• Arterial blood gas: normal or increased PaO2 (in veins).

• High anion gap metabolic acidosis (low pH, low bicarbonate).

• CBC: increased white blood cell count.

• Blood chemistry: increased lactate level.

• Increased muscle CPK (creatine phosphokinase) due to myoglobin release.

• Increased BUN and creatinine due to myoglobin-induced acute kidney injury.

• Consider cyanide levels to rule out cyanide poisoning.

• EKG: nonspecific (sinus tachycardia is common).

Management of CO Poisoning

• Remove the patient from the source of exposure.

• Administer oxygen via non-rebreather mask in the ambulance.

• In the hospital, administer 100% oxygen; if ineffective, use a hyperbaric oxygen chamber.

• Avoid aggressive treatment of acidosis, as acidosis shifts the oxygen binding curve to the right, counteracting the left shift caused by CO poisoning.

Key Methodological Findings

• The brain and heart are most sensitive to CO poisoning due to high blood flow, poor tolerance to hypoxia, and high oxygen requirements.

• The amount of carboxyhemoglobin is more critical than the level of CO exposure.

Prognostic Factors

• Poor prognosis indicators: cardio-respiratory arrest, old age, exposure for more than 24 hours, acidosis, loss of consciousness.

• Survivors of intentional CO poisoning are at high risk for subsequent suicide attempts.

Side Effects of Oxygen Therapy

• Lungs: hyperoxic acute lung injury, atelectasis (collapse), bronchopulmonary dysplasia.

• Eyes: retinal damage and retinopathy of prematurity in young infants.