Carbon Monoxide Poisoning
A colourless, odourless gas resulting from incomplete combustion, carbon monoxide (CO) can be found in vehicle exhaust fumes (more so in those without catalytic converters), house fires, poorly maintained oil or gas heaters and the fumes of certain solvents. The fact that there are no obvious signs to the presence of the gas has led to it being termed the "silent killer".
Gas boilers, as used in central heating systems, should be inspected regularly by qualified service personnel to ensure correct operation. The pilot light flame should appear blue - the presence of a yellow flame, or soot marking, indicates an increased risk of carbon monoxide poisoning in the home. Absence of yellow, however, does not mean that the system is safe.
An approved CO detector should be fitted to alert occupants to the presence of the gas. These units are inexpensive and should be considered as being as important for users of gas or oil heating systems as a smoke alarm.
Normally, oxygen binds with the haemoglobin (Hb) in red blood cells to form oxyhaemoglobin (O2Hb) and dissolves only in small amounts in the liquid component of blood (plasma). Carbon monoxide is particularly dangerous because it has an affinity for haemoglobin 240 times greater than oxygen - forming carboxyhaemoglobin (COHb). This means that a small amount of inhaled gas can be fatal. It also interferes with cellular function in the body by attaching itself to myoglobin and cytochrome enzymes.
Carbon monoxide (CO) should not be confused with carbon dioxide (CO2), which is released naturally by cells as a waste product of metabolism. CO2 can also become toxic if concentrations are high enough, although CO is by far the most dangerous.
Signs and Symptoms
Symptoms of CO poisoning include:
- Rapid pulse.
- Loss of consciousness.
- Neurological effects can become evident some weeks following the exposure, even after apparent recovery.
It should be noted that the 'cherry red' appearance of finger tips and lips attributed to carbon monoxide poisoning is of little use in initial diagnosis. "Cherry Red means Dead".
- As with any first aid intervention the safety of the rescuer takes priority - keep safe.
- Remove casualty to fresh air - administer 100% oxygen if available.
- Call for an ambulance as soon as possible.
- Check breathing and perform CPR if required - cyanosis may be absent due to effect of carbon monoxide on the blood.
- If unconscious (but breathing) place into the recovery position.
- Check pulse, breathing and consciousness level regularly until ambulance arrives.
Diagnosis of CO poisoning should be based mainly on the patient's history - which may reveal an increase in symptoms when indoors at home or work (indicating a possible heating system fault), being present in a house fire or in attempted suicide. Blood results can help to confirm the severity of poisoning but emphasis must be placed on the patient's history if the results are inconclusive.
Non-invasive pulse oximeters are commonly used to measure oxygen saturation (SpO2) - the amount of oxygen being carried by red blood cells. They work by emitting light at two separate frequencies (660 and 940nm) and measuring the difference in absorption of the light as it passes through part of the body (usually a finger or ear). COHb absorbs the same frequency of light as O2Hb and therefore will result in a falsely high reading. Other factors such as nail varnish, intravenous dyes, external light sources and cold peripheries also cause inaccurate results.
Patients diagnosed with CO poisoning may be referred by medical staff in the Accident and Emergency department to the on call hyperbaric consultant. Where patients refuse treatment, consideration should be given to the possibility of hypoxia induced confusion or underlying psychiatric conditions. Even after symptoms have subsided, patients might still benefit from hyperbaric oxygen therapy.
If suitable for HBO therapy, the patient will be admitted to the unit as an emergency and treated using a modified U.S. Navy Table 5 dive profile (right).
Green areas on the graph represent the patient breathing pure oxygen delivered via a sealed hood - or a respiratory ventilator in the case of intubated patients. Blue areas represent five minute intervals when the patient breathes the chamber's air atmosphere to reduce the risk of oxygen toxicity.
Because there are many more oxygen molecules present in a given volume when under pressure, hyperbaric oxygen dissolves in the blood in far greater amounts enabling it to be transported to the cells. Hyperbaric oxygen (HBO) also helps rid haemoglobin of the tenacious CO molecules, freeing it up for normal use once more. The actual amount of oxygen molecules at a pressure of 3ata (20msw) in a fixed volume is equal to 3 times the amount at the surface.
In cases of carbon monoxide poisoning, it normally takes over four hours for the amount of CO in the body to fall by one half, during which time the tissues are hypoxic due to replacement of O2Hb with COHb. Hyperbaric oxygen at 3ata reduces this to 20 minutes, during which time the extra oxygen dissolved in the blood alleviates hypoxia.
The normal amount of CO bound to haemoglobin (COHb) in people living in cities is less than 1.5% - or up to 10% in smokers.
Carbon monoxide is also produced naturally in the body by the breakdown of red blood cells (haemolysis) and is also a neurotransmitter produced in the brain. Certain conditions that increase haemolysis, e.g. sickle cell anaemia, pregnancy, malaria or large bruises (haematomas), can raise COHb levels.
Further information about Carbon Monoxide can be found here.
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