Carboxyhemoglobin (COHb), Carbon monoxide (CO) poisoning

Sample

1. Collect the venous blood immediately when CO poisoning is suspected.
   1. Because CO is rapidly cleared from the hemoglobin when the patient exposed to fresh air.
2. Whole blood in heparin is stable >4 months infilled well-caped bottle.

Precaution

1. Don’t use oxalate for the blood sample.
2. Collect the blood sample before oxygen therapy is started.
3. O₂ saturation and oximetry are inaccurate in the CO-poisoning because it measures all forms of O₂ – saturated hemoglobin and CO-Hb.

Purpose of the test (Indications)

• This test is done to detect carbon monoxide poisoning.
• This is done in patients who are exposed to smoke inhalation, exhaust fumes, and fires.

Pathophysiology

1. CO is a colorless, odorless, and tasteless gas, undetectable unless it is mixed with the visible or odorous pollutant.
   1. It is produced by the incomplete combustion of such fuels like gasoline.
   2. CO is a chemical agent, which causes injury which produces hypoxic injury due to O₂ deprivation.
2. Carbon monoxide combines with hemoglobin with 200 times more affinity as a comparison to oxygen.
   1. CO + Hb = CO-Hb
   2. CO-Hb produces a cherry – red or violet color of the blood and skin.

3. So there are few sites available for O2 to combine with Hb.
4. Less O2 is available for the tissue respiration leads to Hypoxemia because CO-Hb is not capable of transporting O₂.

5. CO poisoning is directly toxic to the intracellular oxidative mechanism and produces more production of nitric oxide (NO).
6. When CO poisoning takes place, ask for Hb analysis for CO-Hb.
7. CO is readily cleared by breathing normal air.
8. Sources of CO are:
   1. Tobacco smoke.
   2. Automobile exhaust fumes.
   3. Natural gas heaters where there is less ventilation.
   4. Defective gas stoves.
   5. Petroleum and natural gas fuel fumes.
9. Workers who are prone to get CO-poisoning:
   1. In defective furnaces.
   2. A worker such as coal mining.
   3. The firefighter.
   4. Smokers of cigarettes, cigars, or pipes.

Signs and symptoms of Carbon monoxide poisoning:

1. This CO poisoning takes place from the house fire, gas heaters, stoves, and engine exhaust.
2. There are nausea and weakness.
3. There is a headache, dizziness, and vomiting.
4. Tinnitus is ringing in the ears.
5. It upsets the stomach.
6. The patient may have chest pain.
7. The patient will be confused.
8. Drunk or sleeping people may die due to CO poisoning.
9. Clinical effects  and source of CO-Hb:
   1. CO-Hb poisoning is due to smoking, exhaust fumes, and fires.
   2. Hemolytic disease.
   3. Hemorrhage in the intestine.
   4. Newborn where the fetal Hb breakdown that yields endogenous CO.
   5. There is a direct relationship between the CO  and symptoms of heart disease, angina, and myocardial infarction.

CO-saturated Hb %	Signs and Symptoms
10	Slight dyspnoea
20	A headache
30	Irritable, impaired judgment, Loss of memory
40	Confusion, decreased vision, weakness
50	Fainting, ataxia, and collapse
60	Coma
> 60    (>70%)	death

Measurement 

1. For CO estimation, can use venous or arterial blood.
2. CO is measured by CO-oximeter.
3. A blood gas analyzer measures oxyhemoglobin by calculation.

Normal

Source 1

• CO-Hb = <0.5% of total hemoglobin.
• Newborn = upto 5 % saturation of total Hb.
• Nonsmoker = <3 % saturation of total Hb.
  • Smoker Light = 2 to 5 % saturation of total Hb.
  • Smoker heavy = 5 to 10 % saturation of total Hb.
• Critical value = >20 % saturation of total Hb.
• Lethal value   =  >50 % saturation of Hb.

Source 2

Clinical condition	% saturation of Hb
Nonsmokers	0.5 to 1.5
Smokers
1 to 2 packs /day	4 to 5
>2 packs/day  (heavy smoker)	8 to 9
Toxic level	>20
Lethal level	>50

Source 3

Saturation of hemoglobin

• Nonsmoker = <3%
• Smoker = ≤ 12%
• Newborn = ≥ 12%

Source 2

Clinical values and its effects:

COHb %	Clinical signs and symptoms
20 to 30%	Dizziness, headache,  and disturbance in judgment
30 to 40%	Tachycardia, hyperpnea, hypotension, and confusion
50  to 60%	Coma
>60%	Death

Source 4

• Nonsmoker = <2% of total Hb.
• Light smoker = 4 to 5% of total Hb.
• Heavy smoker = 6 to 8% of total Hb.
• Newborn = 10 to 12% of the total Hb.

Increased level of CO:

• This is 50 to 60% seen in CO poisoning.

Treatment:

• CO- toxicity needs a high concentration of O₂ to replace the CO-Hb.
• In the case of severe CO, toxicity may be treated with hyperbaric oxygen.

 

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