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 are David Blaine, an Ama Pearl Diver, or 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 [i.e. without oxygen]. Most of us are familiar with the 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 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 results 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] causes 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%. 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 often grouped by forensic scientists into three generalized categories: Suffocation, Strangulation and Chemical Asphyxia. Chemical asphyxia is characterized by an inhaled substance interfering with the body’s ability to use oxygen [e.g. carbon monoxide, which blocks oxygen from binding to hemoglobin in red blood cells]. The term ‘simple’ asphyxia also describes a case of oxygen displacement by another gas. Most reported murders by asphyxia involve strangulation. When water or another liquid fills the lungs causing asphyxia this is called drowning. There are several possible variations such as near-drowning, secondary drowning and immersion syndrome. Autoerotic or ‘sexual asphyxia’ by self-strangulation, drowning, choking, and a variety of other means is increasingly reported, especially by the media. Owing to cause and pathologic presentation, types of suffocation are distinguished as entrapment or environmental suffocation, smothering, choking, mechanical asphyxia, mechanical asphyxia combined with smothering, and lastly, suffocating gases. When external pressure prevents breathing, the term mechanical asphyxia is used. Vehicle accidents and police custody incidents are some of the cases we have seen in our firm. Traumatic asphyxia, positional asphyxia, and riot-crush are subtypes of mechanical asphyxia while ‘Burking’ is a combination of mechanical asphyxia and smothering. Further descriptions can be found at www.fiskebrown.com in the articles section. A recent PBS documentary investigated the suffocation deaths of people living near volcanoes due to excess carbon dioxide. These examples can be found at www.pbs.org/wgbh/nova/volcanocity.
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. 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 other 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 is the oxygen carrier in red blood cells. It turns from red to blue when it loses oxygen causing the blue or black discoloration of the skin and other tissues called cyanosis. After death, changes in blood chemistry and the breakdown of clotting proteins such as fibrin lower the blood's viscosity; this is sometimes called 'fluidity'.
As stated earlier, these signs are non-specific and can be present after death without asphyxia. Furthermore, a case may be complicated by pathology or injuries additional to asphyxia. This information is used in combination with data on place and manner of death by forensic scientists to perform analyses and form opinions. Investigation into asphyxial death often involves a combination of experts who may be clinicians, biomechanical and automotive experts who perform accident reconstructions, and chemical or biological scientists.
If your case has you in a bind, don't hold your breath, call a forensic consultant!
This article written by Lori Wickham, Ph.D.was published in the San Diego Daily Transcript on July 20, 2006 as part of the Forensic Consultants Association Newsletter. Dr. Wickham is a member of John Fiske Brown Associates, www.fiskebrown.com., San Diego’s most experienced forensic science and engineering group.
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