Question

A previously healthy factory worker was found unresponsive in his work place. He was afebrile, anicteric, tachypnoeic, drowsy and "blue" all over with clear lung fields and hyperdynamic cardiovascular findings. His ABG with 100% oxygen after intubation was:
pH = 7.30
pO2 = 580 mm Hg
pCO2 = 30 mm Hg
SaO2 = 50%
What is the most likely diagnosis?

• A. Methemoglobinemia
• B. Adult Respiratory Distress syndrome
• C. Carbon monoxide poisoning
• D. Organophosphorus poisoning

Hint:

A high pO2 but low SaO2 gap in a factory worker points to carbon monoxide binding haemoglobin.

Answer 1

The Correct Option is C

Step 1: List the clues.
A factory worker is found unresponsive, afebrile, anicteric, breathing fast, drowsy, and described as "blue" all over. His lungs sound clear and his heart is working in an overactive, hyperdynamic way. This points to a toxic exposure rather than an infection or primary lung disease.

Step 2: Read the ABG carefully.
The pO2 on the blood gas machine is very high at 580 mm Hg because the patient is on 100% oxygen, and a standard blood gas analyzer calculates this from the dissolved oxygen in plasma. But the SaO2 is only 50%. This is a mismatch: dissolved oxygen (pO2) is high, but the amount of oxygen actually being carried and released by haemoglobin (SaO2) is low.

Step 3: Explain why this mismatch happens.
A standard ABG machine calculates oxygen saturation from the pO2 using a formula that assumes normal haemoglobin. If a poison is stuck to haemoglobin and stopping it from carrying real oxygen, the calculated SaO2 can look falsely normal or high, while a directly measured (co-oximetry) SaO2 shows the true low value. Carbon monoxide binds to haemoglobin about 200 times more strongly than oxygen, forming carboxyhaemoglobin, which cannot carry oxygen. This gives exactly this pattern: high pO2, because the patient is breathing 100% oxygen and gas exchange in the clear lungs is fine, but low true SaO2, because haemoglobin is blocked by carbon monoxide.

Step 4: Match the clinical picture.
Factory workers are exposed to carbon monoxide from combustion engines, furnaces or faulty ventilation. Classic features include a bluish or cherry red discoloration, drowsiness progressing to coma, clear lungs on exam (unlike pulmonary oedema), and a hyperdynamic circulation as the heart tries to compensate for poor oxygen delivery to tissues. All of this fits carbon monoxide poisoning.

Step 5: Rule out the other options.
Methemoglobinemia also causes a low SaO2 with a normal pO2, but it typically causes chocolate brown blood and cyanosis that does not improve with oxygen, and is linked to certain drugs or chemicals, not factory combustion fumes. Adult Respiratory Distress Syndrome would show poor oxygenation with an abnormal chest exam and a low pO2 despite 100% oxygen, not clear lungs and a high pO2. Organophosphorus poisoning causes excess secretions, pinpoint pupils, bradycardia and muscle fasciculations, not this ABG pattern with clear lungs.

Step 6: Final Answer.
The most likely diagnosis is
\[ \boxed{\text{Carbon monoxide poisoning}} \]