Asphyxia

Asphyxia can literally be translated from the Greek as meaning 'absence of pulse', but is usually the term given to deaths due to 'anoxia' or 'hypoxia'.

The term 'asphyxia' is thought by some forensic pathologists to be a vague and confusing term. In its broadest sense it refers to a state in which the body becomes deprived of oxygen while in excess of carbon dioxide (ie. hypoxia and hypercapnoea). This results in a loss of consciousness and/or death. However, prior to any death the body usually reaches a low oxygen-high carbon dioxide state, and so an 'asphyxial' death is therefore one in which the oxygen deprived state has been achieved unnaturally.

Categorising asphyxial deaths

	Neck Compression
	Chest Compression
	Postural/ Positional Asphyxia
	Airway Obstruction
	Exhaustion or Displacement of Environmental Oxygen

Neck Compression

Mechanisms of Death

mechanical constriction/ squeezing of the soft tissues of the neck - the most common mechanism is that of compression of the jugular veins, with or without that of the carotid arteries, leads to reduced oxygen reaching the brain, loss of consciousness, and if sustained for a sufficient interval (minutes) death. The time interval of compression to loss of consciousness is approx. 10 secs if both carotid arteries are compressed and a minute if only the jugulars are compressed. The time interval from loss of consciousness to death is said to be in the region of minutes.

airway obstruction - this is a contributory factor in some hangings, where the hyoid bone and tongue are pushed upwards and backwards against the laryngo-pharynx. This type of obstruction produces 'air hunger', which is a frightening sensation and which is not a feature of vascular compression in the neck.

cardiac arrhythmia - this is a controversial postulated mechanism whereby pressure over the carotid artery at the carotid sinus provokes a reflex slowing of the heart (bradycardia), which may provoke a fatal arrythmia (particularly in the elderly or those with underlying cardiac disease). This mechanism is unlikely to be responsible where there are petechiae or congestion which would suggest that the heart had been beating for a more lengthy period than this mechanism would support.

Classic signs of asphyxia ...

	congestion of the face - due to venous congestion (venous return to the heart is prevented)
	facial oedema - increased venous pressure causes tissue fluid transudation (remember those Starling forces !)
	cyanosis - excess de-oxygenated haemoglobin in the venous blood
	petechial haemorrhages in the skin and eyes (particularly the eyelids, conjunctiva, sclera, face, lips and behind the ears) - due to raised venous pressure

Petechial Haemorrhages

Unfortunately the presence of petechial haemorrhages does not automatically point to asphyxia as a cause of death. They are fairly non-specific in that they can be produced whenever there is a marked or sudden increase in vascular congestion of the head that causes rupture of capillaries.

The areas of the head that are most characteristically involved are those that have little surrounding soft tissue support, such as the conjunctiva, eyelids, lining of the mouth/ larynx etc. Petechiae can therefore be produced not only during vascular compression of the neck, but also where valsalva manouvres operate, such as during labour, straining at stool, coughing (eg in asthma), sneezing, vomiting etc.

Other examples include chest compression, where the right heart is compressed, but the left heart is still capable of pumping (and acute right heart failure due to disease). In these circumstances, the level of congestion would be just above the heart on the chest wall.

Where bodies are found lying prone with the head at a lower level than the rest of the body, there may be coarse petechiae present in the areas of intense congestion.

Traumatic Asphyxiation

This is the term given to the condition most often seen after mass disasters, such as the Hillsborough football stadium disaster, or where people have been crushed by collapsing trenches, or by the weight of grain etc in silos.

The thorax is transfixed, preventing respiratory movements. There are classic signs of congestion, cyanosis and petechiae, but there may be no other signs of injury on the body. The florid signs of congestion usually finish at the level of the clavicles.

Postural asphyxia is a related condition, recently coming to the fore due to interest in deaths in police custardy etc, and may involve splinting of the diaphragm during restraint, coupled with the additional requirements for oxygen during a struggle. Research into this aspect is ongoing.

Obstruction of the airway

When oxygen is not able to reach the lungs because of external occlusion of the mouth and/ or nose, or the airway at the level of the larynx is obstructed (eg by a bolus of food), the cause of the asphyxial death is 'obstruction of the airways'. There are no specific autopsy findings that would support the main types of airway obstruction deaths, and circumstantial evidence, physical evidence (eg plastic bags used by the deceased) and the scene of death would be relied on to support the diagnosis.

	smothering - the covering of the mouth or nose (or external occlusion) eg by a plastic bag or in overlay deaths (may see abrasions etc in a homicidal smothering if the victim could put up a struggle)
	gagging - the tongue is pushed backwards and upwards, and the gag becomes saturated with saliva and mucus causing further obstruction.
	foreign body obstruction (those at risk being children/ infants, the intoxicated and those with neurological difficulties with swallowing etc)
	swelling of the airway lining (anaphylactic hypersensitivity reactions, or thermal/ heat injury.

Exhaustion or Displacement of Environmental Oxygen (Suffocation)

This may occur in tight or confined spaces, where toxic fumes are released from bedding etc in cots, or in drowning (the inhaled water displaces the oxygen).

This is 'pure' asphyxia and results in a fairly rapid, painless loss of consciousness, followed by death if not discovered. There are no diagnostic autopsy findings.

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Wikipedia Links

	Asphyxia (http://en.wikipedia.org/wiki/Asphyxia)
	Hanging (http://en.wikipedia.org/wiki/Hanging)
	Strangulation (http://en.wikipedia.org/wiki/Strangulation)
	Autoerotic asphyxia (http://en.wikipedia.org/wiki/Erotic_asphyxia)

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