Low cardiac output syndrome (LCOS) is a flow problem: the heart does not deliver enough blood, and therefore enough oxygen, to meet the body's needs. It may be brief myocardial stunning after cardiopulmonary bypass, persistent postoperative pump dysfunction, or severe cardiogenic shock requiring mechanical circulatory support. The term describes a clinical state and severity spectrum, not one universally defined disease. Cardiac output can fall because the heart rate is too slow or ineffective, stroke volume is reduced by inadequate preload, excessive afterload or poor contractility, or several of these mechanisms at once. A normal blood pressure does not rule out LCOS. Vasoconstriction may temporarily preserve pressure while the kidneys, skin, brain and gut are already receiving inadequate flow. Conversely, hypotension caused only by vasodilation is not automatically cardiogenic shock; inflammation after bypass can create a mixed cardiogenic–vasodilatory picture. The experienced nurse looks for a pattern rather than waiting for one dramatic number: a new tachyarrhythmia, cool or mottled skin, delayed capillary refill, falling urine output, altered mentation, increasing lactate or worsening acidosis. In a child after cardiac surgery, this pattern...