A lot of research on this topic comes from the Law Enforcement/Corrections community. It's one of the major risks when subduing someone. So ther will be references to Officers used in this topic. I do strongly suggest that you research this topic more thoroughly and ensure that you are well versed in this topic before using any bondage technique on any person including yourself. Please be aware of the risks you take when using bondage or restraint. Grometsplaza is in no way responsible for any actions that any person/s take when reading the material contained on this site. Comments are welcomed

Positional asphyxia, also known as postural asphyxia, is a form of asphyxia which occurs when someone's position prevents them from breathing adequately. 
 

• Positional asphyxia is a potential danger of many physical restraint techniques, and of some sexual bondage techniques. 
• People may die from positional asphyxia by simply getting themselves into a breathing-restricted position they cannot get out of, either though carelessness or as a consequence of another accident. 
• Small children under two are particularly at risk from positional asphyxia. 

Restraint Asphyxia 
The (US) National Law Enforcement Technology Centre defines restraint asphyxia, positional asphyxia: "as death as a result of body position that interferes with breathing".

According to experts, it arises from use of neck-holds which restrict breathing or when a person is laid on their stomach during restraint and/or transportation. This position compromises a person’s ability to breathe. 

Handcuffing a person behind their back also restricts a person’s ability to breathe. Any weight applied to the back in this position (such as pressure by a law enforcement officer, including an attempt to keep a person still) increases breathing difficulty further. 

A "natural reaction" to oxygen deficiency is increased physical struggle. In the face of such a struggle law enforcement officials are likely to apply additional pressure/compression to subdue the restrained person. Yet further compromising the restrained person’s ability to breathe. 

Factors which may increase dangers of restraint asphyxia include: obesity; enlarged heart; alcohol and drug use or other things that impede the ability to breathe including, for example, the presence of chemical agents. 

Guidelines to minimize the risk of positional asphyxia include restraining a person other than laying them on their stomach and monitoring the restrained person’s breathing and health.

Prone restraints are dangerous because: - 

1. Prone reduces ventilatory volume & ability to breathe 
2. Prone increases recovery time after exertion 
3. Prone is only dangerous in combination with other, pre existing risk factors 
      (i.e. health impairments etc)
4. Prone is a potentially fatal position, without the presence of other risk factors 

Various mechanisms have been suggested which act to increase risk in prone restraint. These include:- 
 

• Positional Asphyxia:- in which the free action of the diaphragm and intercostal muscles is compromised, thereby causing hypoxia, disturbed heart rhythm and death. This factor will be significantly increased in prone restraints, or any procedure in which the movement of the chest is impeded or in which pressure or weight is applied to the back or chest. 
• Obesity – so the adipose tissue is pushed up into the abdominal cavity during prone restraints
• Excited or Agitated delirium:- A state of mental and physiological arousal, agitation, hyperexia and epiphora and hostility. Symptoms include sweating, bizarre behaviour and speech combined with extreme strength and endurance. This condition is often, but not inevitably, associated with substance or stimulant abuse.
• Acidosis:- the extreme exertion encountered in restraint situations produces lactic acid. In situations of prolonged struggle the individual may be unable to adequately clear lactic acid production leading to acidosis due to severe exertion, with consequent cardio-vascular collapse. 
• Psychological response:- Individuals may find a restraint position so aversive and horrifying leading to struggle beyond the point of exhaustion, with consequent physiological consequences. This is similar to the “capture myopathy” observed in trapped wild animals who die due to exhaustion when netted. This may be particularly significant for learning disabled individuals, people on the Autistic spectrum for whom hyper stimulation is frightening and/or young people who have suffered previous sexual abuse, for whom prone restraints replicate the experience. 

1. A patent airway
2. the lungs where gas exchanges occur
3. a ventilatory apparatus or bellows that moves gases between the external environment and the lungs ( Chan 1998) 

Fatality occurs where one of these is compromised. 

Positional Asphyxia
 

When a hog-tied subject dies, forensic pathologists generally attribute the death in whole or in part to positional asphyxia result-ing from respiratory compromise. Positional asphyxia occurs when  the position of the body interferes with the person's ability to breathe. Breathing involves interaction of the chest wall, the diaphragm, and the muscles of the rib cage and abdomen. Interference with proper breathing produces an oxygen deficiency (known as hypoxia) in the blood, which disturbs the body's chemistry and creates the conditions for a fatal rhythm disturbance in the heart.

Contributing Factors
 

An examination of reported positional asphyxia-related deaths revealed that several other readily observable factors can increase a subject's susceptibility to sudden death when officers place the subject in the hog-tied and prone position. These include obesity and psychosis, which covers both organic (i.e., paranoia, schizophrenia, etc.) and drug- or alcohol-induced psychotic behavior.

A large, bulbous abdomen (a beer belly) presents significant risks because it forces the contents of the abdomen upward within the abdominal cavity when the body is in a prone position. This puts pressure on the diaphragm, a critical muscle responsible for respiration, and restricts its movement. If the diaphragm cannot move properly, the person cannot breathe.

The second significant contributing factor is psychosis, often induced by drugs or alcohol. Drugs, such as methamphetamine, cocaine,3 and LSD, create a state of delirium frequently accompanied by violent muscular activity. The mind-altering character of these drugs, combined with the added outbursts of violent and vigorous muscular activity, can cause individuals to lose their breath more quickly than expected. When the hog-tied position prevents recovery from such an oxygen deficit, cardiac rhythm disturbances can occur, resulting in death.4

Other pre-existing physical conditions also can contribute to sudden in-custody death. Any condition that impairs breathing under normal conditions will put a subject at a higher risk of respiratory failure when a situation escalates to the point that hog-tied restraint must be employed. Heart disease, asthma, emphysema, bronchitis, and chronic pulmonary disease fall into this category. Clearly, officers subduing a violent subject will not be able to pause in the midst of the action to inquire about the subject's medical history. It might be prudent, however, for correctional officials to be aware of inmates' medical records and be alert for potential problems when using hog-tied restraint.

Prevention

Vigilance in monitoring the subject's condition. The process of hypoxia is insidious, and subjects might not exhibit any clear symptoms before they simply stop breathing. Generally, it takes several minutes for significant hypoxia to occur, but it can happen more quickly if the subject has been violently active and is already out of breath. If the subject experiences extreme difficulty breathing or stops breathing altogether, officers must take steps to resuscitate the subject and obtain medical care immediately.

Links

http://www.perils-of-pleasure.com/BDSM/Hogtie.html
http://www.fetishlink.co.uk/article-print-147.html

Medical Realities of Breath Control Play by Jay Wiseman