Why does suspension trauma happen?

Question

This question once more brought up the topic of suspension trauma.

I tried to read up on the topic, but found that the Wikipedia page was not as informative as I would have liked. Especially I'm still unclear as to what exactly it is that leads to suspension trauma:

• Is it being unconscious while standing/hanging/... upright?
• Or is it the actual suspending (generally in some sort of harness) that somehow interferes with the blood flow in a way that will eventually harm you?

I guess I was wondering about this because the exact reason will also change how one should go about situation in which suspension trauma could become an issue. Were #1 above correct then one wouldn't have to be terribly concerned as long as the person was still conscious. In case #2 on the other hand we're basically racing a clock from the second somebody is suspended somewhere... However for #2 I guess the issue could be mitigated by special gear which prevents the circulation/suspension issues all-together?

Answer 1

Most climbing harnesses support the majority of your weight, if freely suspended, around the tops of the thighs. While the waist belt may be designed to take the brunt of loads in a fall once you are suspended the weight will tend to settle onto the leg loops, depending of course on the position that you end up in. But either way the harness will end up putting pressure on your lower body.

Even under normal circumstances blood has to flow quite a long way to circulate through your legs which contain large muscles and are lower than the heart. Normal circulation relies on some movement of the legs to keep blood moving effectively. Simply standing or sitting still for long periods can cause problems. It is not that uncommon for soldiers to faint on parades when standing still for long periods.

Clearly being suspended by leg loops greatly exacerbates this especially as the femoral artery, one of the largest blood vessels in the body is quite close to the surface around the inner thigh.

Being unconscious makes the situation worse as the victim is unable to move or transfer their weight eg to an improvised foot loop but even when conscious the fundamental problem still exists.

Also the waist belt can put pressure on the abdomen constricting the diaphragm and making breathing more difficult.

The net result is that the heart has to work much harder to circulate blood as it is fighting both against gravity and the restriction around the thighs plus any other shock and trauma which occurred during the initial fall. To put it another way blood tends to pool in the lower body. Which has a knock-on effect for blood flow and thus oxygen supply to the brain.

There is also a potential issue that when blood flow is restored to a limb which has been restricted accumulated toxins can overwhelm the body's ability to deal with them potentially causing organ damage. This can also happen with crush injuries.

Answer 2

The main problem is the disruption of the normal blood circulation. The blood pressure generated by the heart alone is not able to persistently pump back the blood from the feet, it needs the venes and the "muscle pump" generated by movement of the legs, especially of the feet against the ground.

The loss of consciousness is a symptom, but after a person loses consciousness, the problem is expedited because she doesn't move at all.