Shock is not a blood pressure diagnosis. Shock is inadequate tissue perfusion. The blood pressure may be low, normal, or temporarily supported by compensatory physiology. The key bedside task is identifying what is limiting oxygen delivery.
A useful approach is to separate shock into major physiologic patterns. Distributive shock is primarily a vascular tone problem. Sepsis, anaphylaxis, and some toxicologic states cause vasodilation and maldistributed blood flow. Cardiogenic shock is primarily a pump problem. The heart cannot generate enough forward flow to meet metabolic demand.
Obstructive shock is a flow limitation problem. Tamponade, massive PE, tension pneumothorax, and severe dynamic hyperinflation can prevent adequate cardiac output despite potentially normal intrinsic myocardial function. Hypovolemic shock is a volume problem. Hemorrhage, dehydration, burns, and severe third spacing reduce venous return and preload. Many critically ill patients have mixed shock.