Carboxyhemoglobin (COHb), Carbon monoxide (CO) poisoning
- Collect the venous blood immediately when CO poisoning is suspected.
- Because CO is rapidly cleared from the hemoglobin when the patient exposed to fresh air.
- Whole blood in heparin is stable >4 months infilled well-caped bottle.
- Don’t use oxalate for the blood sample.
- Collect the blood sample before oxygen therapy is started.
- 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.
- CO is a colorless, odorless, and tasteless gas, undetectable unless it is mixed with the visible or odorous pollutant.
- It is produced by the incomplete combustion of such fuels like gasoline.
- CO is a chemical agent, which causes injury which produces hypoxic injury due to O2 deprivation.
- Carbon monoxide combines with hemoglobin with 200 times more affinity as a comparison to oxygen.
- CO + Hb = CO-Hb
- CO-Hb produces a cherry – red or violet color of the blood and skin.
- So there are few sites available for O2 to combine with Hb.
- Less O2 is available for the tissue respiration leads to Hypoxemia because CO-Hb is not capable of transporting O2.
- CO poisoning is directly toxic to the intracellular oxidative mechanism and produces more production of nitric oxide (NO).
- When CO poisoning takes place, ask for Hb analysis for CO-Hb.
- CO is readily cleared by breathing normal air.
- Sources of CO are:
- Tobacco smoke.
- Automobile exhaust fumes.
- Natural gas heaters where there is less ventilation.
- Defective gas stoves.
- Petroleum and natural gas fuel fumes.
- Workers who are prone to get CO-poisoning:
- In defective furnaces.
- A worker such as coal mining.
- The firefighter.
- Smokers of cigarettes, cigars, or pipes.
Signs and symptoms of Carbon monoxide poisoning:
- This CO poisoning takes place from the house fire, gas heaters, stoves, and engine exhaust.
- There are nausea and weakness.
- There is a headache, dizziness, and vomiting.
- Tinnitus is ringing in the ears.
- It upsets the stomach.
- The patient may have chest pain.
- The patient will be confused.
- Drunk or sleeping people may die due to CO poisoning.
- Clinical effects and source of CO-Hb:
- CO-Hb poisoning is due to smoking, exhaust fumes, and fires.
- Hemolytic disease.
- Hemorrhage in the intestine.
- Newborn where the fetal Hb breakdown that yields endogenous CO.
- There is a direct relationship between the CO and symptoms of heart disease, angina, and myocardial infarction.
|CO-saturated Hb %||Signs and Symptoms|
|30||Irritable, impaired judgment, Loss of memory|
|40||Confusion, decreased vision, weakness|
|50||Fainting, ataxia, and collapse|
|> 60 (>70%)||death|
- For CO estimation, can use venous or arterial blood.
- CO is measured by CO-oximeter.
- A blood gas analyzer measures oxyhemoglobin by calculation.
- 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.
|Clinical condition||% saturation of Hb|
|Nonsmokers||0.5 to 1.5|
|1 to 2 packs /day||4 to 5|
|>2 packs/day (heavy smoker)||8 to 9|
Saturation of hemoglobin
- Nonsmoker = <3%
- Smoker = ≤ 12%
- Newborn = ≥ 12%
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|
- 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.
- 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.