equipment - Anaesthesia Equipment P RESSURE Def'n: Newton...

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PRESSURE Def'n: Newton (N): the force that will accelerate a mass of 1 kg at 1.0 m.s -2 Gravity (g): 9.81 m.s -2 Pascal (Pa): 1 Pa = 1 N acting over an area of 1 m 2 therefore, the force of gravity on 1 kg will be 9.81 N so, 1 Newton is equivalent to 1/9.81 kg = 102 gram weight 102 g acting over a square metre is small and cumbersome kPa atmospheric pressure at sea level = 101.325 kPa = 760 mmHg Def'n: 1 kPa = 10.2 cmH 2 O = 7.5 mmHg mercury is 13.6 x density of water 1 bar = 100 kPa = 750 mmHg 1 mbar ~ 1.02 cmH 2 O *guage on an "Oxylog" FLUID FLOW Laminar Flow Hagen-Poiseuille Equation but as R = δ P/Q, so where eta, h = the viscosity of the fluid in Pascal seconds there are no eddies or turbulence flow is greatest at the centre, being ~ twice the mean flow near the wall 0 flow is directly proportional to the driving pressure Q . = π r 4 . δ P 8 η l R = 8 η l π r 4 Anaesthesia Equipment 1
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Turbulent Flow the velocity profile across the lumen is lost flow becomes directly proportional to the square root of the driving pressure therefore, as pressure flow is not linear, resistance is not constant, and the flow at which the resistance is measured must be specified other factors in turbulent flow may be summarised, where, k = a constant r = rho, the density of the fluid in kg.m -3 thus, radius has less of an effect on turbulent flow the likelihood of the onset of turbulent flow is predicted by, Reynold's number (Re) = ρ vd η where, d = the diameter of the tube v = the velocity of flow ρ = rho , the density of the fluid in kg.m -3 η = eta , the viscosity of the fluid in Pascal seconds empirical studies show that for cylindrical tubes, if Re > 2000 turbulent flow becomes more likely for a given set of conditions there is a critical velocity at which Re = 2000 Clinical Aspects thus the transition from laminar to turbulent flow depends on the mixture of gases present in the patient's airway the gases are humidified, contain CO 2 and are warmed the net effect is an increase in the critical velocity, due to a reduction in density due to warming of the gases for a typical anaesthetic mixture, critical flow (l/min) ~ airway diameter (mm) as breathing is cyclical, with peak flows > 50 l/min, turbulent flow usually predominates during peak flow, while laminar flow is present during other times in the respiratory cycle due to the great reduction in velocity in the bronchi and smaller airways, flow through them tends to be laminar in general, during quiet breathing flow tends to be laminar, while during speaking, coughing, or deep breathing flow becomes turbulent in the larger airways Q . = k . r 2 . δ P ρ l Anaesthesia Equipment 2
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Tension Laplace's Law P = T.h.(1/r 1 + 1/r 2 ) thus, for straight tubes, P = T.h./r and, for spheres, where, T = the tangential force in N/m, acting along a length of wall h = the thickness of the wall (usually small) thus, as the diameter of a vessel becomes smaller, the collapsing force becomes greater this can lead to vessel closure at low pressures, the critical closing pressure also seen in alveoli, leading to instability with small alveoli tending to fill larger ones
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This note was uploaded on 10/02/2011 for the course CHEM 131 taught by Professor Hanna during the Spring '10 term at Henry Ford College.

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equipment - Anaesthesia Equipment P RESSURE Def'n: Newton...

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