Forensic Analysis of Injury and Death by Asphyxiation

TASA ID: 1785

If you were to dive into a pool and hold your breath, how long could you spend underwater?  Probably less than a minute unless you conditioned to lower your metabolic rate like a meditating Yogi.  However, with practice, many people can hold their breath for about two minutes.  What do breath-hold diving, suffocation, strangulation, and drowning have in common?  They all involve progressive asphyxia, concomitant low oxygen [hypoxia], high carbon dioxide [hypercapnia] and acidosis.  The latter is a result of the buildup of lactic acid, a by-product of anaerobic metabolism [without oxygen].  Most of us are familiar with the feeling of pain of lactic acidosis after a demanding workout.  Asphyxia can be limited to a regional tissue deprived of blood [e.g. ischemia] or manifest as blocked respiration in the body as a whole.  

There is a hierarchy within the body in terms of how long different tissues can withstand this deprivation.  In many cases, human extremities can be deprived of blood flow for more than 30 minutes without damage while the central nervous system, specifically those portions involved in consciousness, will not continue to function for more than a few seconds without oxygen. The disruption of cell metabolism in the tissues and the accumulation of toxic by-products result in patho-physiological consequences such as tissue necrosis, loss of consciousness and death.  Forensic interest may then become a question of causation and how long the asphyxia lasted before death occurred. The latter may be important in cases where family members witnessed the suffering of the decedent.

Lack of oxygen, either partial [hypoxia] or total [anoxia], can cause death. Normal room air is approximately 21% oxygen.  Impairment of cognitive and motor function can manifest at oxygen concentrations of 10-15%, loss of consciousness at less than 10%, while death usually occurs at less than 8%.  For example, although hypoxic endurance varies, a person can lose consciousness in 40 seconds and die within a few minutes at ambient oxygen levels as low as  4-6%.

Asphyxial deaths, whether accidental, suicidal or homicidal, are grouped by forensic scientists into categories based on mechanism.   The terminology used to describe cause of death may also vary based on the background of the investigator, causing some confusion to the client.  There are three generalized categories: Strangulation, Chemical Asphyxia and Suffocation.  Most reported murders by asphyxia involve strangulation.  An inhaled substance interfering with the body's ability to use oxygen [e.g. carbon monoxide, butane, and nitrous oxide] characterizes chemical asphyxia.  Carbon monoxide blocks the active binding site of hemoglobin [Hb], the protein that carries oxygen in red blood cells.  Another term, 'simple' asphyxia,  is sometimes used when oxygen is displaced by another gas. When water or another liquid fills the lungs causing asphyxia, this is called drowning.  There are several variations such as near-drowning, secondary drowning and immersion syndrome.  In some cases a coroner's report may list a significant medical or pathological condition such as Chronic Obstructive Pulmonary Disease [COPD] as cause of death rather than asphyxia during a drowning, further frustrating those who are unfamiliar with the terminology.  Autoerotic or 'sexual asphyxia' by self-strangulation, drowning, choking, and a variety of other means is increasingly reported, especially by the media.

Deaths due to suffocation can be subdivided by causation into those due to entrapment or environmental suffocation, smothering, choking, mechanical asphyxia, mechanical asphyxia combined with smothering, and suffocating gases.   Entrapment involves individuals trapped in air-tight enclosures [e.g. children trapped in abandoned refrigerators].    Environmental suffocation usually involves someone accidentally entering an area depleted of oxygen by a mechanism other than gaseous suffocation.   Another cause, inhaled fluid, displaces the oxygen during drowning.  Thus, drowning is a form of suffocation.  Fungal infestations in underground compartments can cause lethal oxygen reduction. Smothering by mechanical obstruction of the nose and mouth is rarely accidental.  However, examples are intoxication leading to loss of consciousness and subsequent face compression on pillows or bedding.   Baby cribs with gaps that might impair infant mobility after lying face down are another example.   Criminals sometimes inadvertently asphyxiate a victim with gags in the mouth or around the face. Choking asphyxia can be homicidal, accidental or even a natural result of inflammation of the mucosal tissues of the respiratory tract.  Steam inhalation can result in swelling to the point of obstruction in some reactive individuals.  Most reported choking deaths are accidental and involve obstruction of the pharynx and larynx by food.   Accidental inhalation of foreign material is another cause. 

When external pressure prevents breathing by compressing the lungs and diaphragm, the term mechanical asphyxia is used. Traumatic asphyxia, positional asphyxia and "riot-crush" deaths are subtypes of mechanical asphyxia.  In traumatic asphyxia, a large mass or heavy weight presses on the victim's chest or upper body preventing breathing.  Survival is surprisingly common even if there is a short loss of consciousness. More severe cases have included people pinned under a vehicle after a motorway accident or a vehicle falling or rolling onto someone attempting repairs.  Traumatic asphyxia during police restraint when officers attempt to subdue someone by sitting on his chest often involves meth-amphetamine abuse and taser application, further complicating the analysis of death.  Positional asphyxia is often an accidental result of someone trapped in a position that prevents breathing.  Self-imposed suspended or strapped positions with the head lowered for autoerotic benefits are examples of accidental or suicidal positional asphyxia.   Many cases involve alcohol or drug intoxication.    When someone is prevented from breathing or crushed by the bodies of others during sports games and rock concerts, the asphyxial death is termed "riot-crush" for obvious reasons.

A combination of traumatic asphyxia and smothering can be accidental or homicidal.  The former scenario might involve rolling over onto an infant placed in bed with parents or an older sibling.  Burials in a collapsed mineshaft or cave are other examples.  During the early 19th century, "resurrectionists" Burke and Hare excavated graveyard bodies to sell to medical schools.  They decided preying upon live alcoholics would make their job easier.  Burke sat on the victim's chest, used one hand to cover the victim's nose and mouth and the other to close the victim's jaws, resulting in traumatic asphyxia, thereby providing a body without digging.  This is an example of homicidal traumatic asphyxia in combination with smothering, now called "Burking."  There have been reports of police custody deaths attributed to this mechanism. 

During deaths by suffocating gases, ambient oxygen is displaced by another gas.   Examples are carbon dioxide and methane, which are found in mines, sewers and natural gas used for cooking. An interesting case that has received media attention recently involves the mass mortality of people living near volcanoes[].  Lake Nyos and Lake Kivu formed in craters made by dormant volcanoes in Africa.  Fissures and springs beneath the craters feed the lakes and deliver high concentrations of carbon dioxide gas that has poisoned entire villages.  The locals call these ground hugging plumes of odorless, volcanic carbon dioxide  "mazuku," which means "evil wind".  Children are often victims because they breathe air nearest the earth [CO2 is heavier than air]. 

Tolerance to ischemia and asphyxia vary with not only age and special adaptation but also with past medical history and present state of health.  For example, those who have a history of cardiovascular or pulmonary disease may be more susceptible [e.g. heart attack, asthma]. Medications can also affect the body's ability to defend itself against asphyxial threat. 

Postmortem examinations, review of medical records, accident reports and photos taken at the scene are used to analyze and classify asphyxial deaths.  There are non-specific physical signs used to attribute death to asphyxia.  These include visceral congestion via dilation of the venous blood vessels and blood stasis, petechiae, cyanosis and fluidity of the blood.  Petechiae are tiny hemorrhages. Blood vessels, usually small veins, are broken by high intravascular pressure.  They can occur in various parts of the body, such as over the surface of the heart and organs, in the eye, the skin and the scalp.  If a large area is affected, they may be termed ecchymoses and appear as bruising.   Hemoglobin [Hb] in red blood cells turns from red to blue when it loses oxygen.  This loss of oxygen is the reason veins are described as blue since they carry blood that has lost oxygen to the body's cells back to the lungs where it can be reoxygenated. As asphyxia progresses and more oxygen is depleted, a dark discoloration of the skin and tissues called cyanosis develops.  Cyanotic tissue is described as blue, black or purplish in color.  After death, changes in blood chemistry and the breakdown of clotting factors such as fibrin lower the viscosity of the blood; this is sometimes called 'fluidity'. The study of flow is called rheology, thus; those who specialize in the study of blood flow behavior are called rheologists or, more specifically, hemorheologists.  

As stated earlier, these physical signs are non-specific to asphyxia, meaning they can be present after death from other causes.  Furthermore, a case may be complicated by pathology or injuries additional to asphyxia. This information is used by forensic scientists in combination with data describing the place and manner of death to perform analyses and form opinions. Reconstruction of asphyxial death often involves a combination of experts who may be clinicians, biomechanical experts and automotive experts who perform accident reconstruction, and chemical or biological scientists.   Communicating the results of these studies and the terminology to attorneys, families and a jury can be a significant contribution to a successful case.  

This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances.  Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.

This article may not be duplicated, altered, distributed, saved, incorporated into another document or website, or otherwise modified without the permission of TASA.

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