Asphyxia means death due to lack of oxygen to the brain. The following are the different ways a person can asphyxiate:
1. Compression of the neck (hanging and strangulation)
2. Blockage of the airway (suffocation, gagging)
3. Compression of the chest, neck, or face (postural or positional asphyxia)
4. Chemical and lack of available oxygen in the atmosphere
In hanging (usually suicide), the neck can be compressed by rope, wire, or articles of clothing. Pressure on the neck will usually occlude the vasculature, but not necessarily the airway (larynx or trachea). Very little pressure is needed to occlude the blood vessels. It is a misconception that the airway must be occluded to asphyxiate. Ruptured blood vessels in the tissues after prolonged hanging, especially in the lower extremities, are called Tardieu spots.
The neck can also be compressed manually by strangulation or throttling. An assailant must compress either the airway or the blood vessels to render a victim unconscious. The time it takes to render an individual unconscious is quite variable (seconds to over a minute). Once a victim becomes unconscious, pressure must be continued in order to cause death.
Signs of trauma to the neck are generally evident in manual strangulation and hanging. There may be contusions or abrasions but rarely lacerations. An object used to compress the neck often leaves an abraded, imprinted mark. If the ligature is thin like a rope, the depressed mark on the neck is usually apparent and the pattern can be matched to the particular ligature. If the ligature is wide, like a towel or shirt, there will be no specific pattern of the ligature. There may be superficial fingernail cuts from either the victim or assailant; however, they are usually from the victim.
Pinpoint hemorrhages, or petechiae, are commonly present in the eyes after manual compression of the neck. Petechiae may be on either the bulb of the eye, or on the lids, or both. Petechiae may also be found on the face, especially the forehead, and around the eyes. They are caused by the buildup of vascular pressure which causes capillaries to rupture. They are not often found in suicidal hanging. Petechiae are not specific for asphyxiation and may occur in sudden natural death.
A unique subgroup of asphyxial deaths are autoerotic deaths which occur during purposeful attempts to reduce blood flow to the brain by neck compression during masturbation. Any object which compresses the neck can be used. Most of the time a towel or some soft object is placed between the ligature and the neck to prevent visible scrapes or bruises. The diagnosis is readily made at the scene because the decedent is usually naked with pornographic material nearby. Often there is evidence of repeated behavior at the scene, such as worn grooves in the rafters where ropes or pulleys have been placed. The manner of death is accidental.
Blockage of the airway (suffocation, aspiration, gagging)
If the airway is blocked, then oxygen cannot get into the lungs, and asphyxiation results. A pillow or hand, for instance, can be placed over the mouth, prevent a person from breathing, and cause suffocation. An unchewed peanut or small parts of toys can become lodged in an infant's or child's airway. Individuals without teeth or with a history of stroke or other debilitating disease may have trouble chewing and aspirate food into the airway. Those under the influence of alcohol are also more likely to aspirate. There are usually no signs of trauma in these deaths.
Compression of the chest, neck, or face
(postural or positional asphyxia)
Postural asphyxiation occurs when a person cannot breathe because of an inability to move one's chest, or the airway is compressed against the neck or face. This type of circumstance is commonly seen during motor vehicle accidents when the vehicle overturns on a victim or a driver may become trapped between the steering wheel and seat. There may be surprisingly few injuries except for other signs of blunt trauma and petechiae of the eyes and face.
Chemical and lack of available oxygen in the atmosphere
If the atmosphere's oxygen is replaced by another chemical or gas, or if a person's red blood cells are unable to deliver oxygen to bodily tissues, a person will asphyxiate. Depletion of atmospheric oxygen usually occurs in a relatively closed environment. Examples include gas which can accumulate and displace oxygen in improperly vented mine shafts, sewers, or chemical storage tanks. It is common to encounter multiple deaths in such cases because rescuers can also be overcome by fumes and lack of oxygen.
Examples of chemical asphyxia by interfering with oxygen delivery to the tissues include carbon monoxide and cyanide. When a car is left running in a closed garage, carbon monoxide from burning gasoline competes with oxygen on the red blood cells. Carbon monoxide can incapacitate a person very quickly. The most common cause of death in fires is carbon monoxide poisoning. Cyanide causes livor mortis to be red as in carbon monoxide poisoning. The cyanide gas may smell like bitter almonds. Both deaths can occur quickly, especially cyanide poisoning.
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