Reperfusion injury

A reperfusion injury occurs when tissue such as muscle is deprived of blood and oxygen for an extended period of time (more than several minutes) then has blood circulation restored. Instead of starting a healing process, this situation instead results in inflammation and the return of oxygen causes damage to the tissues. This is largely due to the fact that the widespread injury has now opened up passages between cellular tissues that are otherwise segregated in a healthy body. The metabolism of the cells now changes and produces even more reactions that cause tissue damage. The returning white blood cells attract further response by the immune system to the injured area. Ironically, the influx of white blood cells can actually result in further loss of blood flow to the area, resulting in more damage. 
Reperfusion injuries are not limited to traumatic events. They are also common in cases of cardiac arrest or stroke. 
When reperfusion is a concern (particularly during procedures such as a liver transplant, where it is a common complication), physicians should be ready to control continued blood flow to the affected area. In most cases, surgical clamps are used for the purpose. The use of reduced temperatures also slows the process, with much the same effect as using ice on a strain or sprain. Ciclosporin has also been found to be useful to counteract the reaction. 
Reperfusion injury at Wikipedia