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VISCOSITY, HEMATOCRIT AND OXYGEN TRANSPORT Rajesh Kumarswami, MD (2003-2004) Hematocrit (def.) – The percentage of blood that is cells Normal ranges (adult): 40- 45% Male: 42-45% Female: 38-46% Viscosity and flow are interrelated, that is, flow is inversely related to viscosity In a homogenous liquid viscosity is constant In a two phase medium (blood) viscosity depends on several factors: 1) Increased HCT- increased viscosity - Produces increased friction between successive layers of blood - More internal friction causes increased viscosity If water = 1, whole blood (with hematocrit 42-45%) – 3.6 times greater - (3-4 times more pressure required to force whole blood through a tube than water) - At high altitude blood viscosity may be 10 times water Viscosity of plasma 1.3 times water due to plasma proteins (not significant for hemodynamic studies) 2) Fahreus – Lidquist (sigma effect - blood less viscous in small vessels than large) Very prominent in capillaries (viscosity is one-half what it is in large vessels) Red blood cell deformity (i.e., sickle cell anemia) is associated with increased viscosity Leftward shift of hemoglobin-oxygen dissociation curve with cold and increased viscosity reduces oxygen delivery Summary – All of these effects are offsetting (some increase viscosity whereas others decrease viscosity). It is unclear as to how hematocrit affects viscosity in small vessels. Current principles assume that overall viscosity in small vessels equivalent to that in large vessels. Editor: Dean F. Connors, MD, PhD Author: Rajech Kumarswami, MD (2003-2004) VISCOSITY, HEMATOCRIT AND OXYGEN TRANSPORT For the Boards… PHYSIOLOGY Effects on oxygen transport: Blood oxygen content: CaO2= (Hb x 1.39 x SaO2) + (PaO 2 x .003) Where: Hb = Hemoglobin SaO2 = Oxygen saturation of blood PaO2 = Partial Pressure of oxygen in dissolved blood Increasing hematocrit increases arterial oxygen content and oxygen transport to cells But it increases viscosity and may increase the work associated with moving blood. Increasing hematocrit to about 50% increases oxygen transport. Beyond that oxygen transport is compromised. With altitude blood volume increases and optimum hematocrit also increases . This increases oxygen transport. Reference: Pharmacology and Physiology in Anesthetic Practice, 3rd edition. Robert K. Stoelting (ed). Lippincott, Williams & Wilkins, 1999, pp. 737-739. Rajesh Kumarswami, MD 2003-2004 2
