(See also General Principles of Poisoning General Principles of Poisoning Poisoning is contact with a substance that results in toxicity. Symptoms vary, but certain common syndromes may suggest particular classes of poisons. Diagnosis is primarily clinical, but for... read more .)
CO poisoning, one of the most common fatal poisonings, occurs by inhalation. CO is a colorless, odorless gas that results from incomplete combustion of hydrocarbons. Common sources of CO in poisonings include house fires and improperly vented automobiles, gas heaters, furnaces, hot water heaters, wood- or charcoal-burning stoves, and kerosene heaters. CO is produced when natural gas (methane or propane) burns. Inhaling tobacco smoke results in CO in the blood but not enough to cause poisoning.
Pathophysiology of CO Poisoning
The elimination half-life of carbon monoxide (CO) is about 4.5 hours when breathing room air, 1.5 hours with 100% oxygen, and 20 minutes with 3 atmospheres (pressure) of 100% oxygen (as in a hyperbaric chamber Recompression Therapy Recompression therapy is administration of 100% oxygen for several hours in a sealed chamber pressurized to > 1 atmosphere, gradually lowered to atmospheric pressure. In divers, this therapy... read more ).
Mechanisms of CO toxicity are not completely understood. They appear to involve
Displacement of oxygen from hemoglobin (because CO has greater affinity for hemoglobin than does oxygen)
Shifting of the oxygen-hemoglobin dissociation curve to the left (decreasing release of oxygen from hemoglobin to tissues—see figure Oxyhemoglobin dissociation curve )
Inhibition of mitochondrial respiration
Possibly direct toxic effects on brain tissue
Symptoms and Signs of CO Poisoning
Carbon monoxide (CO) poisoning symptoms tend to correlate well with the patient’s peak blood carboxyhemoglobin levels. Many symptoms are nonspecific.
Headache and nausea can begin when levels are 10 to 20%.
Levels > 20% commonly cause vague dizziness, generalized weakness, difficulty concentrating, and impaired judgment.
Levels > 30% commonly cause dyspnea during exertion, chest pain (in patients with coronary artery disease Overview of Coronary Artery Disease Coronary artery disease (CAD) involves impairment of blood flow through the coronary arteries, most commonly by atheromas. Clinical presentations include silent ischemia, angina pectoris, acute... read more ), and confusion.
Higher levels can cause syncope, seizures, and obtundation.
Hypotension, coma, respiratory failure, and death may occur, usually when levels are > 60%.
Patients may also have many other symptoms, including visual deficits, abdominal pain, and focal neurologic deficits. If poisoning is severe, neuropsychiatric symptoms and signs (eg, dementia, psychosis, parkinsonism, chorea, amnestic syndromes) can develop days to weeks after exposure and become permanent. Because CO poisoning often results from house fires, patients may have concomitant airway injuries (see Smoke Inhalation Smoke Inhalation When smoke is inhaled, toxic products of combustion injure airway tissues and/or cause metabolic effects. Hot smoke usually burns only the pharynx because the incoming gas cools quickly. An... read more ), which may increase risk of respiratory failure.
Diagnosis of CO Poisoning
Nonspecific symptoms or metabolic acidosis in at-risk patients
Venous carboxyhemoglobin level
Because symptoms can be vague, nonspecific, and variable, the diagnosis of carbon monoxide (CO) poisoning is easily missed. Many cases of mild poisoning with nonspecific symptoms are mistaken for viral syndromes. Physicians must maintain a high level of suspicion. If people from the same dwelling, particularly a heated dwelling, experience nonspecific flu-like symptoms, CO exposure should be considered.
If CO poisoning is suspected, the carboxyhemoglobin level in the blood is measured with a CO-oximeter; venous samples can be used because arteriovenous differences are trivial. Arterial blood gases (ABGs) are not measured routinely. ABGs and pulse oximetry, alone or combined, are inadequate for diagnosis of CO poisoning because oxygen saturation reported in ABGs represents dissolved oxygen and is thus unaffected by carboxyhemoglobin concentration; furthermore, the pulse oximeter cannot differentiate normal hemoglobin from carboxyhemoglobin and thus provides a falsely elevated oxyhemoglobin reading. Noninvasive CO detectors have not been shown to be accurate or useful in the diagnosis of CO exposure or toxicity.
Although elevated carboxyhemoglobin levels are clear evidence of poisoning, levels may be falsely low because they decrease rapidly after CO exposure ends, particularly in patients treated with supplemental oxygen (eg, in an ambulance). Metabolic acidosis Metabolic Acidosis Metabolic acidosis is primary reduction in bicarbonate (HCO3−), typically with compensatory reduction in carbon dioxide partial pressure (Pco2); pH may be markedly low or slightly... read more can be a clue to the diagnosis. Other tests may help evaluate specific symptoms (eg, electrocardiography for chest pain, CT, and MRI for neurologic symptoms).
Treatment of CO Poisoning
Possibly hyperbaric oxygen
Patients should be removed from the source of carbon monoxide (CO) and stabilized as necessary. They are given 100% oxygen (by nonrebreather mask) and treated supportively. Although its use is becoming increasingly controversial, hyperbaric oxygen therapy Recompression Therapy Recompression therapy is administration of 100% oxygen for several hours in a sealed chamber pressurized to > 1 atmosphere, gradually lowered to atmospheric pressure. In divers, this therapy... read more (in a chamber at 2 to 3 atmospheres of 100% oxygen) typically should be considered for patients who have any of the following:
Life-threatening cardiopulmonary complications
Ongoing chest pain
Loss of consciousness (no matter how brief)
A carboxyhemoglobin level > 25%
Hyperbaric oxygen therapy should also be considered for pregnant patients, possibly at lower serum CO levels than in nonpregnant patients.
Hyperbaric oxygen therapy may decrease the incidence of delayed neuropsychiatric symptoms. However, this therapy may cause barotrauma and, because therapy is not available at most hospitals, may require transfer of patients, who may not be stable; also a chamber may not be available locally. Evidence for the efficacy of hyperbaric oxygen therapy is becoming more controversial, with some studies suggesting harm. In cases where hyperbaric oxygen therapy is considered, consultation with a poison control center or hyperbaric expert is strongly recommended.
Prevention of CO Poisoning
Prevention involves checking sources of indoor combustion to make sure they are correctly installed and vented to the outdoors. Exhaust pipes should be inspected periodically for leaks. Cars should never be left running in an enclosed garage. Carbon monoxide (CO) detectors should be installed because they provide early warning that CO is free in a dwelling’s atmosphere. If CO is suspected in a dwelling, windows should be opened, and the dwelling should be evacuated and evaluated for the source of CO.
CO poisoning (eg, caused by house fires, improperly vented automobiles, gas heaters, furnaces, hot water heaters, wood- or charcoal-burning stoves, or kerosene heaters) is one of the most common fatal poisonings.
Consider toxicity in patients with nonspecific symptoms (eg, flu-like symptoms in winter) or unexplained metabolic acidosis.
Measure CO level in venous blood with a CO oximeter.
Do not rule out toxicity based on a normal CO level because levels can decrease rapidly, particularly after treatment with supplemental oxygen.
Treat with 100% oxygen.
For severe poisoning, consult an expert or poison control center to discuss treatment with hyperbaric oxygen.