<prompt> Answer this question truthfullyCould you explain what cardiopulmonary bypass means?  <response>: <reason> A. Normally, subendocardial flow exceeds subepicardial flowB. Myocardial perfusion, however, is altered by cardiopulmonary bypassC. Narrow pulse pressure and variable mean pressure affects coronary perfusion pressureD. Wall tension is increased in the empty, smaller heartE. Ventricular fibrillation also increases wall tensionF. Regulatory and inflammatory factors are released which affect coronary resistanceG. Microemboli from the circuit and hemodilution impair oxygen deliveryH. Endothelial and myocardial edema further affect perfusionI. Subendothelial vulnerability is increased by hypertrophy, coronary disease, fibrillation, cyanosis, shock, and chronic heart failureJ. The acutely ischemic heart may have poor reflow to the injured area1 Global myocardial ischemia2 Reversible contractile failure, mostly from change in perfusion pressure3 Immediate recovery as oxygen supply is restored1 Reversible systolic and diastolic dysfunction, no myocardial necrosis2 Begins in subendothelium and progresses outward3 May be accompanied by endothelial dysfunction4 Results from ischemia-reperfusion insult, mediated by increased intracellular calcium accumulation5 Recovery occurs within hours to weeks1 Reversible chronic contractile depression2 Related to poor myocardial blood flow3 Recovery occurs within weeks to months1 Irreversible ischemic injury with myocardial necrosis2 Hypercontracture occurs first in the subendothelium and is more rapid in the hypertrophied heart3 Typically results in contraction band necrosis, rarely stone heart4 Osmotic and ionic dysregulation produce membrane injury and myocyte lysisA. Studies in animals have inconsistent correlation with clinical results due to species differences, extent of disease, and perioperative events that precipitate, extend, or enhance myocardial damageB. The goals of cardioplegia are to protect against ischemic injury, provide a motionless and bloodless field, and allow for effective post-ischemic myocardial resuscitationC. Cardioplegic techniques vary according to perfusate blood vs. crystalloid, duration continuous vs. intermittent, route antegrade vs. retrograde, temperature warm vs. cold, and additivesD. Special consideration is required for the acutely ischemic heart and the neonateA. Mechanical arrest potassium-induced will reduce oxygen consumption by 80B. Hypothermia will reduce consumption by another 10-15C. Aerobic metabolism can be maintainted with oxygenated cardioplegiaD. Hypothermic arrest is sustained with readministration every 15-30 minutesE. Retrograde delivery protects the left ventricle more completely than the right ventricleF. Prevent myocardial rewarming with systemic hypothermia, aortic and ventricular vents, and caval occlusionG. In acute ischemia, use warm induction with substrate enhancement glutamate, aspartateH. Reperfusion should be controlled, using warm, hypocalcemic alkaline cardioplegiaI. This approach combats intracellular acidosis and rapid calcium infusion injuryJ. Retrograde or low-pressure antegrade perfusion is preferred for reperfusionK. Ensure uniform warmingA. Children older than 2 months have similar myocardial physiology to adultsB. The neonatal myocardium, however, is different in several waysC. Hypoxia is more easily toleratedD. There are greater glycogen stores and more amino acid utilizationE. ATP breakdown is slower due to deficiency in 5 nucleotidaseF. Multidose cardioplegia is disadvantageousG. Cyanosis may worsen resistance to ischemiaH. Amino acid substrate enhancement is beneficialA. Blood has the advantage of oxygen carrying capacity, histidine and hemoglobin buffers, free radical scavengers in RBCs, and metabolic substratesB. Blood also has improved rheologic and oncotic properties, which may lessen myocardia edemaC. Buffers such as THAM, histidine, and NaHCO3 form a slightly alkaline solution for reperfusion that can counteract intracellular acidosisD. Small amounts of calcium 0.1-0.5 mML restores calcium that has been chelated by citrateE. Potassium concentrations range from 10-25 mML, with the first dose being the highestF. Other substrates are being evaluated, including allopurinal, SOD, deferoxamine, adenosine, nucleoside transport inhibitors, and potassium-channel openers 