Fluid flow physics and anaesthetic implication


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gas laws and fluid physics and its implication in anaesthesia

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Fluid flow physics and anaesthetic implication

  1. 1. Dr. Shraddha Naik
  2. 2.  Flow is defined as the quantity of a fluid i.e. a gas or liquid passing in unit time F=Q/t F=flow Q= quantity of liquid T=time
  3. 3. Flow is of three types: Laminar flow Transitional flow Turbulent flow
  4. 4. Laminar Flow A fluid flows in a steady manner No eddies or turbulence Present in smooth tubes Velocity is low Flow is greatest at centre ( 2x mean flow) To draw the fluid , a pressure difference must be present across the ends of tube.
  5. 5. Laminar flow Laminar flow Reynold’s number< 2000 low velocity Fluid particles move in straight lines Simple mathematical analysis possible
  6. 6. Laminar flow
  7. 7. Laminar flow
  8. 8. Determinants of laminar flow Pressure across tube Diameter of tube length of tube Viscosity of tube
  9. 9. All these factors are incorporated in an equationand known as the Hagen- Poiseuille equation
  10. 10. Viscosity Viscosity of fluid also affects the flow of fluid viscosity increase in following condition - policythemia -Increased fibrinogen level - hypothermia - cigarette smoking - Age Increased viscosity leads to increase risk of vascular occlusion .
  11. 11. Anaesthetic implication During fluid resuscitation, a short wide bore cannula e.g.14G is superior to a 20G cannula or a central line. Intubating patients with very small tube increases resistance to flow and thus pressure increases to deliver the same amount of flow through the tube.
  12. 12. Critical velocity This is the velocity for a given fluid for a given tube beyond which laminar flow gets converted into turbulent flow. When velocity of fluid exceeds this critical velocity , the character of flow changes from laminar to turbulent. This critical velocity applies only for a given fluid in a given tube.
  13. 13. Turbulent flow Reynoldss number > 4000 high velocity Particle paths completely irregular Average motion is in the direction of the flow Cannot be seen by the naked eye Changes/fluctuations are very difficult to detect. Must use laser. Mathematical analysis very difficult - so experimental measures are used Most common type of flow.
  14. 14. Onset of turbulent flow Turbulent flow occurs –2. Sharp increase of flow3. Increase in viscosity or density of the fluid4. Decrease in diameter of tube
  15. 15. Turbulent flow Laminar flow change to turbulent flow if constriction is reached Velocity of fluid increases Fluid is no longer in a smooth fashion Swirls in eddies Resistance is higher than for the same laminar flow . Flow is no longer directly proportional to pressure
  16. 16. Turbulent flow
  17. 17. Where turbulent flow is seen ? Turbulent flow is present where there is an orifice, a sharp bend and some irregulararity which may lead to local increase in velocity
  18. 18. Factors affecting flow and pressure duringturbulent flow Q α √P α 1/√ l α 1/ √ρ Q= flow P = Pressure l = length of tube ρ = density of fluid
  19. 19. All these factors are combined to an indexknown as Reynolds number  Reynold number = vρ∂ / ŋ v= linear velocity of fluid ρ = Density η =viscosity d = diameter of tube
  20. 20. Transitional flow Transitional flow Reynoldss number-2000-4000 medium velocity
  21. 21. Clinical Aspects Of Flow Laminar flow is present in bronchi, smaller air passage as they are narrower than trachea. Turbulent flow is present in corrugated rubber tubing . Sharp bend or angles increase turbulence In quiet breathing , the flow in resp tract is laminar, while speaking , coughing or taking deep breath turbulent flow tends to occur . A lining layer of mucus may affect the flow . In circulatory system, bruit and murmur can be heard due to turbulence of flow.
  22. 22. Variable orifice flowmeter In a variable orifice flow meter there is mixture of turbulent and laminar flow and for calibration purposes both viscosity and density is important. At low flows, gas flow depends on the viscosity of the gas. At higher flows, gas flow depends on the viscosity of the gas. Recallibration is required if flow meter is used for a different fluid than for what it was initially desigened.
  23. 23. How to measure the resistance A constant flow is passed through the apparatus Difference in pressure P1-P2 between the ends of tube is measured By dividing pressure difference by flow Provided the flow is laminar , resistance is independent of flow
  24. 24. Resistance Resistance= Pressure / Flow R= P/ Q
  25. 25. How to measure the resistance?
  26. 26. Bernoulli’s Principle Describes the relationship between the velocity and pressure exerted by a moving liquid. Applied to both liquids as well as gases. Venturi effect is based on the Bernoulli’s principle. Venturi effect is entrainment of fluid (gas or liquid ) due to the drop in pressure When a fluid flows through a constriction in the tube there is reduction in fluid pressure. The fluid velocity correspondingly increases in order to satisfy the law of conservation of energy.
  27. 27. Applications of Venturi effect Venturi masks used for oxygen therapy. Sander’s jet injector. Nebulisation chambers. Atomizers that disperse perfumes or spray paints. Water aspirators. Foam fire fighting nozzles and extinguishers. Modern vaporizers. Sand blasters to mix air and sand. Vehicle carburetors.
  28. 28. Venturi
  29. 29. Venturi
  30. 30. Venturi
  31. 31. Venturi oxygen mask