Principles of Cardioplegic Solutions
Energy conservation during myocardial ischemia can be achieved by inducing chemical arrest through two primary mechanisms:
Preventing the conduction of the myocardial action potential by inhibiting the fast sodium current can be achieved through one or more of the following methods:
a. Extracellular hyperkalemia
b. Sodium channel blockers (e.g., lidocaine)
c. KATP channel openers (e.g., adenosine)
Preventing myocyte contraction by inhibiting calcium activation of myofilaments can be accomplished through one or more of the following approaches:
a. Reducing extracellular calcium to zero
b. Using L-type calcium channel blockers (e.g., magnesium)
c. Directly inhibiting myofilaments with agents such as 2,3-butanedione monoxime (BDM)
Cold Cardioplegia
There are two types of solutions used: asanguinous solutions and those mixed with blood at a 2:1 or 4:1 blood-to-solution ratio. These solutions can be categorized as extracellular or intracellular based on their potassium concentrations. It is believed that the composition of these solutions, particularly the potassium levels, should be adjusted according to factors such as the solution temperature, infusion timing (initial, maintenance, and terminal phases), and the myocardium's energy state. Generally, potassium concentrations are reduced during maintenance, and energy substrates are added for hearts that are energy-depleted.
Delivery methods can be either intermitten (multidose) or continuous; continuous delivery requires a lower potassium concentration compared to intermittent delivery. Hyperkalemic cold sanguinous cardioplegia is often preferred for its advantages, though asanguinous cardioplegia may be equally effective. The Buckberg formulation, which includes a cold, oxygenated, hyperkalemic blood-crystalloid mixture with reduced free calcium, added glucose, and buffering capacity, is considered preferable to simple hyperkalemic blood. However, simple hyperkalemic blood is less expensive, as it involves only transferring blood perfusate from the CPB oxygenator to a separate reservoir-heat exchanger-pump system and adding sufficient potassium chloride to achieve cardioplegia (potassium concentration of about 22 mmol · L−1).
