The primary method of myocardial protection during cardiopulmonary bypass (CPB) remains the administration of cardioplegia and the institution of hypothermia.
Cardioplegia solution composition varies between institutions but is generally composed of a combination of the following:
Potassium (10–40 mEq/L) for arresting the heart in diastole, which is an easily reversible state;
Blood and/or crystalloid as the carrier;
Bicarbonate and/or THAM solution for buffering the excessive acid metabolites on bypass;
Mannitol to help reduce edema;
Magnesium to reduce calcium overload; and
Citrate-phosphate-dextrose and amino acids such as glutamate and aspartate, for supplying any remaining metabolic demand of the heart which may only be present in the final “hot shot” which warms the heart and removes any remaining metabolites and cardioplegia.
This cardioplegia solution is administered in an either antegrade or retrograde fashion. With antegrade administration, cardioplegia is injected in the aortic root space between the aortic valve and the aortic cross clamp. As the pressure in the aortic root builds, cardioplegia is forced down the left main and right coronary arteries and is delivered to the myocardium. In some patients, such as those with high-grade coronary blockages or aortic insufficiency, retrograde cardioplegia is given by placing a cannula in the coronary sinus which can be palpated by the surgeon and placement confirmed with TEE.
Through this cannula, cardioplegia can be given in the reversed direction of normal blood flow, beginning with the venous system and then being delivered to the myocardium. This method avoids the need for a competent aortic valve and theoretically will deliver cardioplegia to those areas that are not perfused well with the antegrade approach due to high-grade coronary blockages, while avoiding LV distension and incomplete protection that would occur with aortic insufficiency during antegrade administration. The reason retrograde administration is not the only delivery method is that this route generally does not cover the left ventricle completely. Compared to antegrade administration, which is delivered through the arterial system and able to tolerate high flow pressures, the retrograde route must be monitored and the recommended high pressure limit is 40 mmHg in order to avoid hemorrhage from damage to the coronary sinus and venous systems which are difficult to repair. In most cases, a combination of antegrade and retrograde is used for the most complete coverage and protection of the myocardium.
Other modifications to this approach include the injection of cardioplegia directly into the coronary ostia after aortotomy using small hand-held cannulas and the injection of cardioplegia into the bypass grafts after the distal anastomoses are completed.
Dosing intervals are usually every 20 minutes or when cardiac activity is observed, whichever comes first. This is due to gradual washout and rewarming and the need to remove the metabolites that inhibit anaerobic metabolism.
The secondary method of myocardial protection is ...