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RSPT 1060
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  1. RSPT 1060 MODULE C – Applied Physics Lesson #1 - Mechanics

  2. OBJECTIVES • At the end of this module, the student should be able to… • define the terms and abbreviations used in the module. • draw & explain the equation of motion. • list the forces that oppose lung inflation & cause the work of breathing. • list the different types of compliance, their normal values & equations.

  3. OBJECTIVES • At the end of this module, the student should be able to… • differentiate between compliance & elastance. • list some pulmonary disorders that could change compliance. • explain the relationship between pressure, surface tension & radius. • explain how LaPlace’s Law relates to surface tension.

  4. OBJECTIVES • At the end of this module, the student should be able to… • explain the purpose of surfactant. • list the different types of resistance, their normal values and equations. • explain how Poiseuille’s Law relates to resistance. • list some pulmonary diseases that will alters resistance. • explain what the equal pressure point is.

  5. Ventilation vs. Respiration • Ventilation: The bulk movement of gas in and out of the lung. • Respiration: The exchange of gas (specifically oxygen and carbon dioxide) at the cellular level. • Internal Respiration: The exchange of gas between a peripheral capillary and a cell of the body. • External Respiration: the exchange of gas across the alveolar-capillary membrane.

  6. FRICTION STRETCH EQUATION OF MOTION

  7. Equation of Motion Mechanicalpositive pressure to inflate Resistance (gas flow) Muscle negative pressure to inflate Compliance (volume)

  8. Ventilator-Pump Thorax-Diaphragm Pump Contraction Pressure P MUS Mechanical Ventilator vs. Spontaneous Breathing

  9. Inflation pressure • Spontaneous Breathing • Contraction of muscles generates a negative pressure in lungs & gas is pulled into lungs. • Work done by patient. • Mechanical Ventilator • Positive pressure builds in the ventilator circuit & gas is pushed into the lungs. • Work done by machine.

  10. WORK OF BREATHING • FORCES OPPOSING INFLATION • Elastic (stretch) • Physical tendency of an object to resist stretching • Non-elastic (friction) • Occurs only when gas and the system is moving

  11. Work of Breathing Resistance Compliance Elastic Work (Stretch) Non-elastic Work (Friction) 35% 65% Tissue & Airway Lungs & Chest Wall (20%) (80%) Lungs Ribs Diaphragm Abd. organs Surfactant Tissue Lungs Pleura Skeletal Muscular Skin Airways Gas flow

  12. COMPLIANCE • ELASTIC OPPOSITION TO INFLATION • Elastic and collagen fibers found in lung parenchyma give the lungs elasticity. • Inflation - occurs as a result of forcibly stretching lung fibers during inspiration. (work) • Deflation or exhalation is normally passive. • The resting position of the lung is deflation.

  13. Compliance • Pressure-Volume loop High Compliance Normal Compliance Vol Low compliance Pressure

  14. Compliance • Pressure-Volume loop High volume (over-inflated) Vol Normal volume (filling) Low volume (opening) Pressure

  15. EXPERIMENT • Balloon • Initial inflation – easy or difficult? • Normal inflation – easy or difficult? • Over inflation – easy or difficult?

  16. Compliance • Compliance: Distensibility of the lung • Elastance: Property of resisting deformation or desire to return to original shape

  17. Comparison Example: Tennis ball vs. Balloon • Tennis ball • High elastance & low compliance • High resistance to change in shape • Low ability to stretch • Balloon • Low elastance & high compliance • Low resistance to change in shape • High ability to stretch

  18. Comparison Normal lung vs. Emphysema vs. Pneumonia • Normal lung • Normal elastance & Normal compliance • Returns to original shape easily • Easily filled • Emphysema • Low elastance & high compliance • Does not return to original shape easily (floppy) • Easily stretched until air-trapping occurs • Pneumonia • High elastance and low compliance • Readily returns to collapsed state • Very difficult to inflate

  19. Compliance Compliance ▲Volume (liters) = = ▲Pressurepl (cmH2O) • Total Compliance is composed of: • Lung Compliance • Chest Wall Compliance

  20. Lung Compliance Disease states that cause a change in lung compliance? • Decrease • Fibrosis • Adult Respiratory Distress Syndrome • Pulmonary Edema • Increase • Emphysema

  21. Lung Compliance • How will a patient with decreased lung compliance breathe? • Rapid • Shallow

  22. Experiment • Wrap belt tightly around chest. • Breathe slow & deep • Breath rapid & shallow • Which feels better?

  23. Chest Wall Compliance • “Stiffness” of chest wall • When thorax is intact, its resting level is FRC • With disruption to chest wall – lung collapse and chest wall expands (open pneumothorax) • Force of movement of chest wall is opposite that of lung • Chest Wall has tendency to expand (pull out) • Lung has tendency to collapse (pull in) • At rest – they balance (FRC)

  24. Chest Wall Compliance • Disease states that cause a change in chest wall compliance? • Decrease • Chest trauma • Chest burns • Kyphosis • Scoliosis • Chest wall deformity

  25. Total Compliance • Normal Lung compliance = 0.2 L/cmH2O • Normal Chest wall compliance = 0.2 L/cmH2O • Normal Total compliance =0.1 L/cmH2O • Why is total less than the individual compliances? • Lungs at rest = collapse • Chest wall at rest = expansion • They are working in opposite directions

  26. Calculation of Total Compliance • Clinically we measure Dynamic and Static Lung Compliance With air movement Tidal Volume {Vt} (liters) CDYN = Peak Pressure – PEEP (cmH2O) No air movement Tidal Volume {Vt} (liters) CSTAT = Plateau Pressure – PEEP (cmH2O) NOTE: PEEP stands for Positive End Expiratory Pressure. It is the BASELINE or starting pressure.

  27. Calculations • Normal compliance = 0.1 Liter/cmH2O Volume = 0.5 liters = 0.1 L/cmH2O Pressure 5 cmH2O

  28. Calculations • Low compliance = 0.05 Liter/cmH2O Volume = 0.5 liters = 0.05 L/cmH2O Pressure 10 cmH2O Stiffer lung needs more inflation pressure.

  29. Calculations • High compliance = 0.17 Liter/cmH2O Volume = 0.5 liters = 0. 17 L/cmH2O Pressure 3 cmH2O Floppier lung needs less inflation pressure.

  30. SURFACE TENSION • The alveoli are like bubbles lined with fluid and filled with air. • Surface tension is the attractive force exerted by like molecules at the liquid’s surface. • Surface tension forces cause the bubble to collapse.

  31. The Force of Surface Tension in a drop of liquid. Cohesive force (arrows) attracts molecules inside the drop to one another. Cohesion can pull the outermost molecules inward only, creating a centrally directed force that tends to contract the liquid into a sphere.

  32. LaPlace’s Law Two bubbles of different sizes with the same surface tension. Bubble A, with the smaller radius, has the greater inward or deflating pressure and is more prone to collapse than the larger bubble B. Because the two bubbles are connected, bubble A would tend to deflate and empty into bubble B. Conversely, because of bubble A's greater surface tension, it would be harder to inflate than bubble B.

  33. Calculation of Surface Tension • LaPlace’s formula: 4ST Pressure in a bubble = r P P ST = surface tension If surface tension increases, the pressure to inflate the bubble increases. R = radius If the radius decreases, the pressure to open the alveoli increases.

  34. Surfactant • Alveoli are lined with a surface-tension lowering agent (surfactant) produced by alveolar type II cells. • Surfactant has a low attractive force exerted by its molecules. • Surfactant helps stabilize the alveoli so they do not collapse completely on each exhalation. • Destruction of surfactant will significantly decrease compliance and increase the work of breathing.

  35. Surface Tension • Disorders altering or destroying surfactant: • Prematurity • Adult respiratory Distress Syndrome • Oxygen toxicity

  36. RESISTANCE • INELASTIC OPPOSITION TO INFLATION • Occurs only when the system is in motion and air is moving. (friction) • Tissue Viscous Resistance • Airway Resistance

  37. Tissue Resistance • Tissue Viscous Resistance (20%) • Things that increase tissue resistance: • Obesity • Fibrosis • Abdominal distention

  38. Airway Resistance • Airway Resistance (80%) • Mainly in upper airway • Only 20% in small airways (less than 2 mm) • Things that increase airway resistance: • High gas flow • Turbulent gas flow • Narrow airway • Long airway • Viscous gases

  39. Poiseuille’s law Pressure = ŋ 8 l V ╥r4 ŋ = gas viscosity l = tube length V = gas flow r = tube radius Pressure increases with increased tube length and gas viscosity. Pressure increases with decreased radius

  40. Poiseuille’s law • Reducing the radius of a tube by ½ requires an increase in pressure 16 fold to maintain the same speed of gas flow through the tube. Pressure = 1 cmH2O Pressure = 16 cmH2O

  41. Poiseuille’s law • Egan: • Rule of Thumb – page 215 • Mini Clini – page 215

  42. Airway Size • If a smaller radius causes increased resistance then why is resistance less in the smaller airways? • More cross section • Slower flow • Laminar flow • Large airways have less cross section & higher more turbulent flows thus more resistance.

  43. Decreased resistance Increased resistance

  44. Calculation of Airway Resistance RAW Pressure (cmH2O) = X 60 Flow (Liters/min.)

  45. Calculations • Normal Resistance = 0.5 – 2.5 cmH2O/L/sec Pressure = 10cmH2O = 2 cmH2O/L/sec Flow 5 L/sec

  46. Calculations • Increased Resistance = 4 cmH2O/L/sec Pressure = 20 cmH2O = 4 cmH2O/L/sec Flow 5 L/sec Narrower airways require more pressure.

  47. Resistance • Diseases that cause an increase in airway resistance • Asthma • Emphysema • Excessive sputum production • Tumors • Things that decrease airway resistance • Bronchodilators • Anti-inflammatory agents