Pathophysiology
Clinical meaning
Low cardiac output syndrome (LCOS) is a clinical state of inadequate systemic oxygen delivery occurring in 25% of children within the first 6-18 hours after cardiac surgery. Cardiac output (CO) equals heart rate (HR) x stroke volume (SV), and stroke volume depends on preload, afterload, and contractility. In the postoperative pediatric cardiac patient, multiple factors reduce CO: myocardial stunning from cardiopulmonary bypass (inflammatory cascade, ischemia-reperfusion injury), residual structural lesions, arrhythmias, tamponade, increased afterload from hypothermia-induced vasoconstriction, and changes in preload from bleeding or third-spacing. Neonates and infants are particularly vulnerable because their myocardium has fewer contractile elements, limited ability to increase stroke volume (relatively non-compliant ventricles), and primary dependence on heart rate to augment cardiac output. Children compensate for low CO through tachycardia and peripheral vasoconstriction (maintaining blood pressure until late decompensation). Therefore, hypotension is a LATE and ominous sign in pediatric patients — tachycardia, poor perfusion, oliguria, metabolic acidosis, and rising lactate are earlier indicators. Inotropic support (milrinone, epinephrine, dopamine) targets improved contractility and optimized preload/afterload. Milrinone is particularly valuable because it provides inotropy AND reduces afterload (lusitropy), which...