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Typically, St. Thomas Hospital cardioplegia solution is used to arrest the heart within seconds of the solution being introduced into the coronary blood stream. The cardioplegia consists of a potassium concentration four times the normal concentration found in the blood (17 mmoles/L). Consequently, the myocardial cells in the conduction pathway are depolarized by the influx of potassium ions. The cells remain at a neutral polarity due to the potassium imbalance and repolarization will not occur until the extracellular potassium is removed. Not only does cardioplegia prevent normal conduction from the SA node to the rest of the myocardium, but it also prevents the occurrence of ectopic beats, which may originate in any cell of the treated myocardium.

During the explantation procedure no oxygen is delivered to the heart via the coronary system (this state is known as global ischemia). Thus, any cardiac activity that occurs while the heart is ischemic will result in depletion of vital energy needed for the cell to survive. Ischemia has also been speculated to cause an increase in oxygen free radical formation and altered capillary membrane permeability. By continuing the flow of cardioplegia during the entire explantation procedure and inhibiting any cardiac activity, the amount of ischemic damage to the myocardium is sought to be minimized.

The effects of various methods to improve ischemic tolerance are currently being investigated using the Visible Heart® preparation; e.g. By changing the osmolarity, pH, and/or composition of the cardioplegia solution, the in vitro performance of the heart can be improved and the life of the preparation been prolonged. Additionally, preconditioning of the heart with various therapeutic agents is another pharmacological approach currently being pursued in our lab to improve post-ischemic recovery of the heart. (see references)

Crystalloid versus Colloid Perfusates

One of the most important attributes of the modified Krebs-Henseleit buffer used in this preparation is its transparency. This crystalloid solution is essentially a balanced salt solution with glucose added as an energy substrate; it may or may not contain proteins. Potential disadvantages of a crystalloid perfusate are its decreased ability to carry oxygen over extended lengths of tubing and its low viscosity. Furthermore, the addition or removal of small amounts of salt can cause large changes in the osmolarity, an important determinate of water weight gain by the heart tissue (edema). A solution containing colloids such as cells, proteins, and synthetic macromolecules minimizes changes in osmolarity and increases the viscosity of the solution to more physiological levels (i.e. that of blood). However, these colloid additives may compromise the transparency of the solution and if they cross the endothelial membrane of the blood vessels, edema could be accelerated. Our lab is actively investigating such modification of our perfusates.
