Vascular Resistance Principles

A blood pressure reading is the endpoint of two moving parts: cardiac output and vascular resistance. When mean arterial pressure falls, ask whether forward flow has fallen, vascular tone has fallen, or both have failed. A patient with profound hypotension, warm extremities and preserved or increased output suggests low systemic vascular resistance; a patient with hypotension, cool mottled skin, oliguria and low cardiac index suggests low output with compensatory vasoconstriction. The same blood pressure can therefore require opposite treatment. This model prevents a common error: treating the number before identifying the haemodynamic problem. Increasing resistance can restore pressure in vasodilatory shock, but can worsen afterload when the ventricle is failing. Conversely, reducing resistance may improve forward flow in a selected afterload-dependent state, but can cause dangerous hypotension if circulating volume or contractility is inadequate. The sections that follow connect the physiology to measurement, treatment selection and surveillance of tissue perfusion.