When blood stops reaching tissue, restoring it can cause fresh harm
Ischemia means a part of the body is not getting enough blood. Oxygen runs short, nutrients stop arriving and waste piles up. Strangely, when the flow comes back, the returning blood can do further damage, sometimes worse than the original shortage. Doctors call that twist reperfusion injury.
The word comes from Greek roots meaning to hold back and blood. Blockage can be partial or total, and it usually stems from trouble in the vessels: a clot forming on a fatty plaque, a clot travelling from elsewhere, an aneurysm, or an injury that crushes or tears an artery. Less obvious triggers include low blood pressure, sickle cell disease, the g-forces of aerobatic and military flying, frostbite, and outside pressure on a vessel from a tumour.
What it does depends on where it strikes. In the heart, a shortage of blood to the muscle may cause chest pain known as angina, and ischemic heart disease is the leading cause of death for men and women in most Western countries. In the brain, a clot can cause an acute stroke, while brief episodes lasting minutes, called transient ischemic attacks, are followed by a serious stroke within a year in about a third of patients. Long-term shortage in the brain can lead to vascular dementia, and the gut and kidneys can be affected too.
Medical students learn the signs of a suddenly starved limb as the six Ps: pain, pallor, pulselessness, pins and needles, paralysis and a limb that takes on the surrounding temperature. Paralysis comes late and means nerves are dying, and because nerves are so sensitive to low oxygen, the damage can persist even after flow is restored. Untreated, tissue can die and turn gangrenous within hours.
At the cellular level, the harm unfolds as an ischemic cascade: waste accumulates, membranes fail, mitochondria are damaged and digestive enzymes leak out. When blood returns it floods cells with oxygen that fuels destructive free radicals and with calcium that can trigger dangerous heart rhythms. Cooling tissue slows its metabolism, one reason lowering body temperature can blunt both the shortage and the rebound.
Source: Ischemia
