METHODS OF MYOCARDIAL MANAGEMENT DURING CARDIAC SURGERY, part 2
Moderately Hypothermic Intermittent Global Myocardial Ischemia
Intermittent cardiac ischemia with moderate cardiac hypothermia requires performing cardiopulmonary bypass (CPB) with a perfusate temperature between 25°C and 30°C. During this method,
the surgeon operates on or in the heart for 10 to 15 minutes while the ascending aorta is clamped (stopping coronary perfusion) or individual coronary perfusion is paused. After these intervals, the aortic clamp is released or perfusion is resumed for 3 to 5 minutes, ideally allowing the heart to beat rather than fibrillate. This technique, prevalent in the 1960s and early 1970s, is still used by some surgeons.Clinical outcomes with this method can be positive, as shown by McGoon and Bonchek. McGoon's study of patients with valved extracardiac conduits found no correlation between mortality and cumulative aortic clamp time. However, 35% of patients experienced low cardiac output postoperatively, with a mortality rate of 52% in this group, indicating potential myocardial damage. Reduto and colleagues reported no significant difference in early left ventricular performance between this method and cold cardioplegic myocardial protection.
While this technique provides reasonable conditions for coronary artery bypass grafting (CABG), it is less optimal for internal heart procedures. The heart often continues to beat during ischemic periods, complicating precise repairs. Each resumption of coronary perfusion carries a risk of coronary and systemic air embolization, despite precautions. Additionally, the heart receives a substantial amount of blood during perfusion, which can stress intracardiac suction systems, increase blood damage, and disrupt the operation. Furthermore, uncontrolled coronary perfusion may lead to reperfusion injury.
Profoundly Hypothermic Global Myocardial Ischemia
The heart can be deeply cooled using a very cold saline solution in the pericardium, by cooling the perfusate, or both, before clamping the aorta. The cardiac surgery is then performed during a single period of aortic clamping. Typically, myocardial temperature with these methods is around 22°C, which most surgeons believe allows for 45 to 60 minutes of safe global myocardial ischemia.
This approach offers improved operating conditions compared to earlier methods and has achieved favorable results. However, a randomized study on aortic valve replacement indicated that this technique causes as much myocardial necrosis as continuous individual coronary perfusion.
For infant cardiac surgery, profoundly hypothermic cardiac ischemia without cardioplegia is often preferred, particularly when hypothermic circulatory arrest is used. This preference is partly because no perfusate circulates through the heart during arrest, avoiding the potential issues of rewarming or inadequate reperfusion associated with ongoing cardiopulmonary bypass.
