HYPOTHERMIC PRESERVATION: A PARTIAL SOLUTION
Static cold storage
Simple cooling markedly enhances ischaemic tolerance. All enzymic activity is temperature dependent—cooling diminishes metabolic activity, curtails oxygen demand, and slows degradation of energy stores. Hypothermia does not stop metabolism, but merely slows the metabolic clock and lessens the speed at which deterioration occurs. Cooling from (body temperature) to C (storage temperature) extends the tolerance of most organs to ischaemia from between 1 and 2 h to about 12 h. Unfortunately cold does not slow all biological functions uniformly but causes discordance in a variety of metabolic processes which occur in concert at 37°C. Transmembrane passive diffusion of ions is not appreciably affected by hypothermia, while active transport mechanisms, such as those governed by Na⫀, K⫀-ATPase and ATPase are inhibited below. Hypothermia alone cannot prevent cell swelling during storage.
A major requirement for a cold flushing solution is therefore that it includes an impermeant to provide osmotic force to oppose cellular oedema. Large anions such as lactobionate (molecular weight 358 Da), or non-electrolytes such as the saccharides raffinose (molecular weight 505 Da) or sucrose (molecular weight 342 Da), or chelates of citrate and magnesium (molecular weight approximately 1000 Da), can achieve this. Glucose (molecular weight 180 Da) can permeate the cell slowly and can stimulate an undesirable production of lactic acid and hydrogen ions by anaerobic glycolysis. Glucose in flushing solutions can be replaced by impermeant sucrose with benefit—especially for liver and pancreatic grafting, where cell membranes are even more permeable to glucose. An effective buffer (phosphate, citrate, or histidine) to counter intracellular acidosis is a second major requirement. The importance of these two mechanisms is indicated by the fact that a solution consisting solely of sucrose with an added phosphate buffer (PBS) is remarkably effective in kidney preservation—almost as effective as the very much more complex UW solution.
The electrolyte composition of flushing solutions can vary widely. The freely diffusible anion chloride is preferably replaced by an impermeant anion (lactobionate, gluconate, or chelated citrate). Flushing solutions often have high potassium and low sodium concentrations. Since the high potassium concentration is cardioplegic, these solutions cannot be used for early systemic intravenous use, and a systemic leak before the time of organ flushing and retrieval is highly dangerous. A high potassium concentration is also vasoconstrictive, slowing flushing rates and the rate of organ cooling. Solutions with the sodium/potassium ratio reversed, have been shown to be virtually as effective, provided suitable impermeants and buffers are present in the solution, and chloride concentration is kept low.
Magnesium has proved to be a useful additive in many solutions since it forms chelates with lactobionate and citrate which cannot pass through membranes. By contrast, calcium levels are kept low, or even excluded since cell damage is associated with calcium influx. Calcium (and magnesium) can precipitate in unstable solutions. Calcium is strongly chelated by lactobionate (and citrate), which may account partly for the usefulness of the latter substances in flushing solutions. This could be deleterious in heart preservation, contributing to harmful influx of calcium on reflow.
Preservation by simple hypothermic storage is limited by ultimate exhaustion of nutrients and accumulation of waste products. Preservation of kidneys can be extended to 3 days by simple storage using Collins, Citrate, PBS, and UW solutions. Preservation of liver and pancreas is more difficult, and was not consistently extended to 24 h until the advent of UW solution.
University of Wisconsin (UW) Solution
|
