--- module: 008-02 language: en chapter: 8 title: "Haemodynamics, Vascular Control, and Blood Pressure" module_title: "Vascular impedance, microcirculatory failure, and pressure phenotypes" source_sha256: e7602b9000f21d8fcfcaa55b336fad1fd9b6f01dc6104ecbff115cd746af6b67 --- # Vascular impedance and microcirculatory failure ## Impedance beyond steady resistance ### A pulsatile circulation, so resistance is incomplete ### Resistance links mean pressure to mean flow ### Impedance adds compliance, inertia, reflection, frequency ### Same mean resistance, different systolic load ### Arteriolar constriction raises mean load, redistributes flow ### Large-artery stiffness raises pulsatile load ### Drug classes differ in volume, rate and wave effects ## Wave reflection and pulse pressure ### Compliant arteries store ejected volume in systole ### Diastolic recoil maintains forward flow ### Lost compliance raises pulse-wave velocity ### Reflections return in late systole, not diastole ### Central systolic pressure and ventricular work rise ### Diastolic support of coronary perfusion may fall ### Wide pulse pressure: stiff arteries or large stroke volume ### Narrow pulse pressure: small stroke volume, or artefact ### Peripheral and central pulse pressure differ ## Perfusion pressure and autoregulation ### Inflow pressure minus downstream or surrounding pressure ### Cerebral: arterial against intracranial pressure ### Renal: pressures across glomerulus and capsule ### Coronary: aortic diastolic against ventricular pressure ### Arteriolar tone holds flow across a pressure range ### Chronic hypertension shifts the operating range ### Abrupt lowering can starve brain or kidney ### Severe hypertension exceeds the upper limit ### Carbon dioxide is a potent cerebral dilator ### Renal myogenic response and tubuloglomerular feedback ### Afferent and efferent tone set glomerular pressure ## Glycocalyx and fluid movement ### A thin, active endothelial surface layer ### Permeability, mechanosensing, coagulation, leukocyte traffic ### Subsurface protein sets the effective oncotic gradient ### Inflammation, hyperglycaemia, reperfusion and shear damage it ### Net filtration along much of the capillary ### Sustained venous-end reabsorption is the exception ### Lymphatics carry long-term balance ## Interstitial compliance and oedema ### Compliance sets how fast tissue pressure rises ### Low-compliance spaces: compartment and intracranial swelling ### Loose tissue holds volume before pressure climbs ### Danger depends on site as well as amount ### Oedema can coexist with intravascular depletion ### Protein-rich leak lowers effective circulating volume ### Low albumin acts with sodium, pressure and lymph capacity ## Venous function and stressed volume ### Unstressed volume fills without much pressure ### Stressed volume stretches vessels and drives return ### Venoconstriction raises mean systemic filling pressure ### Return falls as atrial pressure approaches filling pressure ### Very negative atrial pressure collapses thoracic veins ### Right-heart failure congests without excess volume ### Venous pressure impairs renal and hepatic drainage ### Incompetent valves give ambulatory venous hypertension ### Compression only after assessing arterial supply ### Spontaneous inspiration supports venous return ### Positive pressure cuts the return gradient ### Positive pressure may still unload the left ventricle ## Microcirculatory and distributive failure ### Restored pressure does not guarantee perfusion ### Heterogeneous flow: swelling, adhesion, microthrombi ### Shunting and stiff red cells limit extraction ### Distributive shock separates pressure from flow ### Output may be high while distribution fails ### Later depression, leak and lost reserve cut output ### Vasopressors restore pressure, not the syndrome ### Lactate rises by several routes, not oxygen alone ### A severity and trajectory marker, read in context ## Pressure phenotypes and measurement ### Clinic, home, day and night sample different states ### White-coat: high in clinic, lower outside ### Masked hypertension leaves risk undetected ### Absent nocturnal fall flags apnoea or kidney disease ### Orthostatic testing needs timing and symptom correlation ### Cuffs infer pressure from arterial oscillations ### Arrhythmia, tremor, stiffness and poor fit reduce accuracy ### Repeat extreme readings while assessing organ injury ### Ask what generates the pressure ### Trends under posture, exercise and treatment show reserve ### Mean pressure can rise while cool skin and oliguria persist ### Volume responsiveness is not an indication to give fluid