Suspension trauma (Syn. "orthostatic shock while suspended"), also known as harness hang syndrome (HHS), or orthostatic intolerance, is an effect which occurs when the human body is held upright without any movement for a period of time. If the person is strapped into a harness or tied to an upright object they will eventually suffer the central ischaemic response (commonly known as fainting). If one faints but remains vertical, one risks death due to one's brain not receiving the oxygen it requires. Since there is no evidence that these effects are specifically due to trauma, or caused by the harness itself, climbing medicine authorities have argued against the terminology of suspension trauma or harness hang syndrome and instead termed this simply "suspension syndrome". 
People at risk of suspension trauma include people using industrial harnesses (fall arrest systems, abseiling systems, confined space systems), people using harnesses for sporting purposes (caving, climbing, parachuting, etc.), stunt performers, circus performers, and so on. Suspension shock can also occur in medical environments, for similar reasons.
The most common cause is accidents in which the person remains motionless suspended in a harness for longer periods of time. Motionlessness may have several causes including fatigue, hypoglycemia, hypothermia or traumatic brain injury.
Onset of symptoms may be after just a few minutes, but usually occurs after at least 20 minutes of free hanging. Typical symptoms are pallor, sweating, shortness of breath, blurred vision, dizziness, nausea, hypotension and numbness of the legs. Eventually it leads to fainting, which may result in death due to oxygen deprivation of the brain.
If someone is stranded in a harness, but is not unconscious or injured, and has something to kick against or stand on (such as a rock ledge or caving leg-loops) it is helpful for them to use their leg muscles by pushing against it every so often, to keep the blood pumping back to the torso. If the person is stranded in mid-air or is exhausted, then keeping the legs moving can be both beneficial and rather dangerous. On the one hand, exercising the leg muscles will keep the blood returning to the torso, but on the other hand, as the movements become weaker the leg muscles will continue to demand blood yet they will become much less effective at returning it to the body, and the moment the victim ceases moving their legs, the blood will immediately start to pool. "Pedaling an imaginary bicycle" should only be used as a last-ditch effort to prolong consciousness, because as soon as the "pedaling" stops, fainting will shortly follow. If it is impossible to rescue someone immediately, then it is necessary to raise their legs to a sitting position, which can be done with a loop of rigging tape behind the knees or specialized equipment from a rescue kit.
When workers are suspended in their safety harnesses for long periods, they may suffer from blood pooling in the lower body. This can lead to suspension trauma. Once a worker is back on the ground after a fall has been arrested on a fall protection system, a worker should be placed in the “W” position. The “W” position is where a worker sits upright on the ground with their back/chest straight and their legs bent so that their knees are in line with the bottom of their chin. For added stability, make sure that the worker’s feet stay flat on the ground. In this position, a KED board can still be used if there are any potential spinal injuries and a worker needs stabilization before transport.
Once the worker is in this position, they will need to stay in that position for at least 30 minutes. Try to leave the worker in this position until their symptoms begin to subside. The time in the “W” position will allow the pooled blood from the legs to be slowly re-introduced back into the body. By slowing the rate at which the pooled blood reaches different organs, you are giving the body more of an opportunity to filter the pooled blood and maintain internal homeostasis. http://www.rigidlifelines.com/blog/entry/suspension-traumasymptoms-and-treatment
Prevention of suspension trauma is preferable to dealing with its consequences. Specific recommendations for individuals doing technical ropework are to avoid exhausting themselves so much that they end up without the energy to keep moving, and making sure everyone in a group is trained in single rope rescue techniques, especially the "single rope pickoff", a rather difficult technical maneuver that must be practiced frequently for smooth performance.
- Reflow syndrome, which occurs when toxins that accumulated in pooled blood suddenly return to the body when the person lies down following suspension trauma
- Compartment syndrome, a dangerous condition that sometimes occurs with suspension trauma
- Seddon P.: Harness suspension: review and evaluation of existing information. In: Health and Safety Executive - CONTRACT RESEARCH REPORT 451/2002, page 3, hier online
- Hawkins SC, Simon RB, Beissinger JP, Simon D. Vertical Aid: Essential Wilderness Medicine for Climbers, Trekkers, and Mountaineers. New York: The Countryman Press, 2017.
- Suspension Trauma Article on the Prevention and Treatment of Suspension Trauma
- Harness suspension: review and evaluation of existing information
- Harness Hang Syndrome: Fact and Fiction
- Will Your Safety Harness Kill You?, from Occupational Health & Safety magazine, Vol. 27, No. 3, pages 86–90, March 2003