You may see your horse lying down routinely and think lying down is normal for horses, and you would be right assuming the horse only does so for a short period of time. One study found that normal horses lie down during deep sleep for an average of 23 minutes in one study, with the longest period being only one hour. However, horses that are trapped in an emergency such as a barn collapse, or have an illness preventing them from rising, can suffer decreased circulation and muscle inflammation from recumbency within 2 hours. Dr. Tom Gimenez from Clemson indicates in the Equine Veterinarian that after the horse rises and muscle compression is released, a reperfusion injury (damage to tissue when blood supply is restored after some time without sufficient circulation) occurs, and that can cause injured tissue to die. Recumbency also effects the lungs, so the volume of air that is taken in is reduced, which can lead to collapsed lungs and lung congestion. Reperfusion injury can also lead to damage of the lung. The circulation is also decreased with prolonged recumbency, which will lead to increased potassium concentrations that can adversely affect the heart. The damage to muscle cells from prolonged recumbency will cause a release of toxins that can cause severe kidney damage.
So whether the horse is trapped by a collapsed barn or down on the ground due to an injury or disease, they must be picked up as soon as possible, preferably within 4 hours, to prevent further injury to organs. Handling a recumbent horse is a major problem. Most referral centers have slings and equipment to get these horses up and help prevent further damage. If you have a horse that is down and will not rise, allowing the horse to remain recumbent for over 7 hours is really not an option if the horse is to survive.