Overview
Clinical Meaning
Vascular resistance is the opposition to blood flow within the circulatory system, determined by vessel radius, vessel length, and blood viscosity as described by Poiseuille's l...
Vascular resistance is the opposition to blood flow within the circulatory system, determined by vessel radius, vessel length, and blood viscosity as described by Poiseuille's law: Resistance = (8 × viscosity × length) / (π × radius⁴). The radius is the most powerful determinant because resistance is inversely proportional to the fourth power of the radius -- halving vessel radius increases resistance 16-fold. Systemic vascular resistance (SVR) represents total peripheral resistance to left ventricular ejection and is calculated as SVR = (MAP - CVP) / CO × 80 dynes·sec/cm⁵ (normal: 800-1200 dynes·sec/cm⁵). Pulmonary vascular resistance (PVR) represents resistance to right ventricular ejection: PVR = (mPAP - PCWP) / CO × 80 dynes·sec/cm⁵ (normal: 100-250 dynes·sec/cm⁵). Mean arterial pressure (MAP) is determined by the equation MAP = CO × SVR, meaning blood pressure depends on both cardiac output and vascular resistance. In clinical practice, SVR is modulated by the sympathetic nervous system (alpha-1 adrenergic vasoconstriction), the renin-angiotensin-aldosterone system (angiotensin II vasoconstriction), endothelial-derived factors (nitric oxide vasodilation, endothelin vasoconstriction), and circulating hormones (vasopressin, catecholamines). In shock states, SVR patterns are diagnostic: distributive...