Cardioplegia

Cardioplegia solution is used to arrest and protect the heart. The solution used at Harbor consists of 30 or 60 mEq/liter K+ D5-1/4 normal saline containing solution buffered with Tham and CPD. The solution is diluted in a 1:4 mixture with blood drained from the patient. Thus, the final concentration of potassium in the low and high potassium containing solutions is in the order of 10 and 20 mEq/liter respectively. The high potassium containing cardioplegia is used for the initial arrest. This is then switched to low potassium cardioplegia to minimize the amount of total potassium load that the patient receives to prevent hyperkalemia and arrhythmias while attempting to wean from cardiopulmonary bypass. In general, at Harbor cardiac arrest and myocardial protection is achieved with both antegrade and retrograde cold blood cardioplegia. There are several factors to consider regarding cardioplegia: 1) cold versus warm; 2) antegrade versus retrograde; 3) blood versus crystalloid. In general, we prefer cold blood antegrade cardiople-gia at 4°C. There are many advocates of continuous warm blood cardioplegia. It prevents damage to enzyme systems resulting from cold temperatures, and thus the sodium potassium ATPase is kept intact, helping to limit the amount of cellular swelling and third space edema within the myocardium. Also it prevents damage to the coagulation enzymes, thus reducing bleeding once cardiopulmonary bypass is terminated. Also it appears that weaning from cardiopulmonary bypass is easier with warm cardioplegia, and the patients more often come off in normal sinus rhythm. It should be noted that 90% of oxygen utilization by the myocardium is eliminated simply by arresting the heart, which warm cardioplegia does quite efficiently; only another 10% of myocardial protection is added by cooling the myocardium. It should be noted, however, that advocates of continuous warm blood cardioplegia still maintain that in any condition where there is a question of myocardial protection, standard cold cardioplegia should be immediately instituted, since this is a time-honored method of protecting the heart. At Harbor we tend to use cold blood cardioplegia since it can be given intermittently rather than continuously and affords a small additional decrease in myocardial oxygen consumption. One additional problem with warm continuous cardiople-gia is that it obscures the surgical field due to blood. Another is safety. If a problem arises with the pump while warm, the margin of safety is greatly diminished.

Blood cardioplegia is felt to be superior to crystalloid cardioplegia because blood delivers some amount of oxygen to the myocardium, even though the myocardial oxygen utilization is at a low rate. Nonetheless, the oxygen utilization of the myocardium does exist and blood facilitates its delivery.

Retrograde cardioplegia is felt to be superior to the antegrade method. In general, we start off with an antegrade dose to arrest the heart and then switch to retrograde soon thereafter. The retrograde system is particularly useful for redo coronary surgery and eliminates the possibility of atheromatous embolization of debris from old grafts ("trash heart"). It also provides more uniform cooling of the myocardium as well as better distribution of cardioplegia solution beyond blockages in the coronary arteries. Finally, retrograde cardioplegia administration has the advantage that the cardioplegia may be given continuously and the heart may be manipulated and moved around without concern for aortic regurgitation when administered through the aortic root.

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