Cardiovascular Physiology, Shock, and Hemodynamic Support
Physiology
The circulation delivers oxygen: DO2 = cardiac output x CaO2 x 10. Cardiac output equals heart rate times stroke volume, and stroke volume depends on preload, afterload, and contractility (Frank-Starling). In children, tachycardia is the earliest and most sensitive compensatory response; hypotension is a late sign and often means decompensated shock.
Classify shock by physiology: hypovolemic, distributive (septic, anaphylactic, neurogenic), cardiogenic, obstructive (tamponade, tension pneumothorax, ductal-dependent lesions), and endocrine. Cold shock shows delayed capillary refill, mottling, and narrow pulse pressure; warm shock shows flash refill, bounding pulses, and wide pulse pressure.
Management
Give 10-20 mL/kg isotonic crystalloid aliquots with reassessment after each bolus; in settings with limited intensive care, malnutrition, severe anemia, or malaria, use smaller aliquots because of the risk of fluid overload. Start epinephrine for fluid-refractory septic shock and norepinephrine for warm shock. Echocardiography helps distinguish vasodilated from myocardial dysfunction. Monitor lactate clearance, urine output, mentation, and ScvO2.
Traps
A normal blood pressure does not exclude compensated shock. Hepatomegaly, rales, or a gallop after boluses signals fluid overload and a need to stop. Bradycardia in a neonate is usually a respiratory problem. Do not attribute persistent shock to hypovolemia after adequate filling.
Worked rule
A child with Hb 8 g/dL, SaO2 100%, and cardiac output 3 L/min has CaO2 of about 10.7 mL/dL and DO2 of roughly 321 mL/min. Raising cardiac output to 5 L/min increases delivery to about 535 mL/min, showing why inotropy matters when transfusion is not immediately available.