Vertical caving terminology and methods > SRT basic terms
Loss of blood flow returning from the legs resulting from being held upright or sitting for too long without moving (typically 20 minutes or more, but can be as quick as 6 minutes, with symptoms starting as quickly as 2 minutes for some people). This, in turn, can cause loss of blood supply to the brain, resulting in unconsciousness. This could be while sitting in a sit harnesses or full body harness, hanging in a chest harness, standing in a rope walking harness, or just being propped upright without a harness. The effect takes place surprisingly quickly, and is most serious for an incapacitated caver hanging on a rope. Cavers who are not incapacited will notice the uncomfortable feeling in their legs before it becomes a problem, and shift their weight around, or stand up in a footloop to relieve the discomfort, and that motion in the muscles allows the blood to start flowing normally again.
The effect happens quickest when held in a standing position, and is not caused by the harness restricting blood flow. The effect takes place a little slower when held in a seated position. The effect is slowest when held lying fully forwards from an attachment point on the back of a full body harness (which includes a sit harness and chest harness), but while these are used in rope access work, they are totally impractical for caving. Chest harnesses when used alone without a sit harness produce their own breathing issues and blood flow issues in addition to the potential for suspension syncope and general discomfort. The effect does not happen at all when held in a completely horizontal position, such as when lying on a bed, or when held in a position with the legs raised above the head. (Statements claiming the opposite are sometimes repeated by cavers from areas where step systems are popular, and used as a way to promote their preferred prusiking systems. It is important to read the actual research rather than the nonsense threads by a misinformed internet warrior. It is also worth noting that almost all cavers will be in a seated position when using a descender, and there is not normally a chest harness to keep them more upright while doing so.)
After hanging that way for 3-4 hours, the effects become more serious, as muscles start to break down (rhabdomyolysis) due to lack of oxygen, and lack of blood flow causes toxins from that process to build up in those muscles. Once blood flow resumes, those toxins can cause shock syndrome. Removing the affected person from the rope, using an improvised rescue or dedicated rescue, and lying them down as soon as possible, reduces the amount of toxins and increases the chances of survival without complications. Propping someone up for a lengthy period used to be commonly advised as a way to reduce the sudden impact of the toxins, but this approach has been shown to have no overall positive effect in the situations cavers will experience, and prolongs the time that the blood is not flowing properly, increasing the severity of rhabdomyolysis. However, you should note that the author of this text is not a medical researcher, and you should take the advice here in the same way as anything else you read on the non-peer-reviewed internet; potentially inaccurate and full of mistakes. Read the actual research instead. The British Caving Association have published a summary of the research, and instructions for how best to deal with suspension syncope.
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