65 - Reperfusion Injury

KYJ 65 - Reperfusion Injury.
The hottest of topics over recent years has included this paradox that ischaemic tissue is actually damaged as much by the restoration of oxygen than the deprivation of oxygen.

Reperfusion injury as the name implies is cellular damage that occurs to tissue when it's oxygen supply has been replenished.  We see it in cardiac tissue, brain tissue, In fact, all tissue.

Recently the world has seen a reversal of the routine administration of oxygen to stroke patients , and those with cardiac chest pain and MI.  Many nurses and doctors have struggled with this.  At the outset, it seems counterintuitive to withhold oxygen to tissue that starves for it; but read on.

Review that all cells have a membrane made from a dual layer of fat (lipid). Technically this lipid bilayer is a water repelling wall that has tiny holes (pores or channels) that allows gases and chemicals to enter and exit cells.

The damage of those membranes makes cells lose control of these pores, allowing water to enter the cell and burst it.  The death of the cell is called necrosis, and if it is water that killed it, it's called hydrolysis.

Now the damage to these cell membranes can be caused by many things. Toxins, mechanical damage or chemicals that dissolve the lipid bilayer are all examples.

Now let's look at oxygen . O2 is two atoms of oxygen.  When oxygen enters cells, some of it splits into two singlet oxygen atoms that are negatively charged. These are called highly reactive oxygen species or most commonly "Oxygen free radicals".

It can form damaged O2- which is called Superoxide.

It can bond to hydrogen to form OH- (hydroxyl)

If O joins H2O is becomes hydrogen Peroxide (H2O2).

If it joins a nitrogen atom it becomes Nitric Oxide (NO).

It can even bind to another O2 causing the molecule called Ozone  (O3).

All of these are oxygen free radicals and they react chemically with lipid cell membranes (lipid peroxidation) and this punches holes in the membrane of recently ischaemic cells.

The more oxygen available to tissue, the more free radicals (oxidants) are produced, which means the more cells get damaged.

In a nut shell this is called Reperfusion injury.

So you can see why oxygen delivery is now considered a double inch sword.  While cells that had been starved of it, needed it; when it is restored, it actually causes further necrosis.

We can minimise this reperfusion by restoring blood flow, but not giving too much oxygen.

How much is too much??  We just don't know.  But we do know that the oxygen we had been using is damaging.

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