Article: Positional asphyxia
Positional asphyxia, is also known as postural asphyxia, is a form of asphyxia which occurs when someone's position prevents them from breathing adequately. A small but significant number of people die suddenly and without apparent reason during restraint by police, prison (corrections) officers and health care staff.
Positional asphyxia may be a factor in some of these deaths. Obviously, there are parallels with restraint for pleasure and there have been deaths, usually as a result of unmonitored and especially long-term bondage or self-bondage.
• People may die from positional asphyxia by simply getting themselves into a breathing-restricted position they cannot get out of, either through carelessness or as a consequence of another accident.
Research has suggested that restraining a person in a face down position is likely to cause greater restriction of breathing than restraining a person face up. Many law enforcement and health personnel are now taught to avoid restraining people face down or to do so only for a very short period of time. Risk factors which may increase the chance of death include obesity, prior cardiac or respiratory problems, and the use of illicit drugs such as cocaine.
Almost all subjects who have died during restraint have engaged in extreme levels of physical resistance against the restraint for a prolonged period of time. Other issues in the way the subject is restrained can also increase the risk of death, for example kneeling or otherwise placing weight on the subject and particularly any type of restraint hold around the subject's neck. Research measuring the effect of restraint positions on lung function suggests that restraint which involves bending the restrained person or placing body weight on them, has more effect on their breathing than face down positioning alone
There is a degree of controversy amongst researchers regarding the extent to which restraint positions restrict breathing. Some researchers report that when they conducted laboratory studies of the effects of restraint on breathing and oxygen levels, the effect was limited. Other researchers point out that deaths in real life situations occur after prolonged, violent resistance which has not been studied in laboratory simulations.
The conclusion being: "Based on these findings, factors other than body positioning appear to be more important determinants for sudden, unexpected deaths in individuals in the hogtie custody restraint position. Illicit drug use (including sympathomimetic, hallucinogenic, and psychomotor stimulant drugs), physiologic stress, hyperactivity, hyperthermia, catechol hyperstimulation, and trauma from struggle may be more important factors in the deaths of these individuals. Although restraints in general increase the psychological and physiologic stress on the individual, no evidence suggests that body position alone causes hypoventilation, respiratory compromise, or positional asphyxia in the hogtie custody restraint position."
NB: It does refer only to 'sudden unexpected deaths' so it doesn't mean that long-term unattended hog-ties are safe!