CARBON MONOXIDE
Carbon monoxide (CO) is a poisonous gas. However it
is not an irritant and is odorless. Hence it is insidious and concentrations
can build up with the victim being unaware of any danger. Carbon monoxide is no
longer present in domestic gas in the UK but is still a major cause of
poisoning, both accidental and intentional and results in the deaths of several
hundred people in the UK each year. Sources of CO include domestic fires, ovens
and boilers, coal gas, furnace gas, cigarette smoke, burning plastic and car
exhausts, although catalytic converters have reduced the CO output from car
petrol engines. Thus traffic policemen, firemen, those trapped in fires and
some factory workers are all potentially at a greater risk. In the UK, the
major cause of poisoning results from exposure to inefficient oxidation in
engines and poorly maintained gas fires, ovens and boilers, especially where
ventilation is inadequate.
The mechanism of CO poisoning is well understood at
the biochemical level. The gas is absorbed rapidly through the lungs and binds
to the iron atom of hemoglobin at same site as dioxygen, but about 240 times
more strongly. This prevents the efficient distribution of oxygen to the
tissues. The product of CO binding is carboxyhemoglobin. Carbon monoxide is
potentially extremely poisonous at low concentrations. Given that air contains
21% O2, approximately 0.1% CO will
saturate 50% of the hemoglobin. Concentrations of 60% carboxyhemoglobin in the
blood are usually fatal, even if maintained for only a few minutes. A
concentration of 20% carboxyhemoglobin may not present obvious symptoms but the
ability to perform tasks can be impaired. At 20–30%, the victim may have a
headache, with raised pulse, dulling of the senses and feelings of weariness.
Concentrations of 30–40% accentuate these symptoms and decrease the blood
pressure so that exertions may lead to faintness. At 40–60% and above, the
victim becomes unconscious and will suffer convulsions. Other clinical features
include pink skin, nausea, vomiting, loss of hearing, hyperpyrexia,
hyperventilation, a decrease in light sensitivity, renal failure and acidosis.
The main target organs of CO poisoning are the heart
and brain. These organs extensively utilize aerobic metabolic pathways and so
their abilities to sustain an oxygen debt are relatively poor. Death is due to
brain tissue hypoxia although cardiac arrythmias and heart and respiratory
failures may also occur. The duration of exposure is also a factor since
hypoxic cell death is not instantaneous. Also, some individuals, for example
those with anemia , are more sensitive to CO poisoning than healthy people.
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