Mechanics Overview

1. Mechanics Overview Feroza Daroowalla, MD, MPH Pulmonary Systems Course 2009 Stony Brook University School of Medicine

2. Outline for Today Mechanics overview Pulmonary Function Testing COPD and Asthma basics Mechanics of the normal lung and chest wall Mechanics in COPD and asthma Skills in diagnosis of mechanics problems

3. Mechanics Overview - Objectives Explain how air moves in and out of the lungs Understand elastic properties of chest wall and lungs Surface tension Understand the role of the respiratory muscles Recognize the components of resistance to airflow

4. How to get air to move in and out of lungs? How do you create pressure differences so that air will flow in and out of lungs? Either push air into lungs or … Make a negative pressure in lungs so that air can rush in This is how humans breathe- when breathing spontaneously

5. Boyle’s Law For a given number of molecules of gas, volume x pressure remains constant As volume occupied by gas goes up, pressure in the gas goes down Gas flows from area of higher pressure to region of lower pressure Pressure in alveolus becomes negative during inspiration Achieved by increasing the volume in the lungs The volume of the gas is fixed ? the pressure drops Gas flows in

6. Boyle’s Law

7. How do we meet requirements Able to change volume (elastic): expandable chest wall (ribcage and diaphragm) expandable lung Walls do not collapse: Bones have rigidity Pleural space keeps lungs and chest wall connected and inflated Liquid film-keeps together but slide over each other Lungs and chest wall move together Power to change volume: respiratory muscles provide the work

8. Elastic structures- are deformable but return to resting unstressed volume May be thought of as springs with different recoil properties

9. Elastic properties Lungs and chest wall have different elastic properties Lungs alone- want to collapse Very small at the relaxed volume Held at a higher volume when in the chest wall Chest wall wants to increase in size Unstressed volume is greater than when combined with lung Unstressed volume of lung and chest wall together is called the Functional Residual Capacity (FRC) In between the volumes of either component alone

10. Functional Residual Capacity This the place where the system comes to rest at the end of a normal breath Opposing tendencies of the lungs to recoil inward and chest wall to recoil outward are evenly balanced These opposing forces create a negative pressure in the pleural space at FRC

11. Recoil force The lungs are pulled by muscles to a higher volume during inspiration Uses muscle energy At the top volume, Total Lung Capacity, both the lungs and even the chest wall want to recoil inwards This recoil force allows passive exhalation

12. Compliance Is a measure of the distensibility of a structure How much pressure need be applied to get a given change in volume Compliance= ?Volume/?Pressure = Slope of the pressure-volume curve of the structure

13. Steeper curve = more compliant lung Less steep curve = less compliant lung

14. The Real Situation So far we have been dealing with the lung as one giant alveolus Actually made up of millions of tiny air sacs with small diameters --lined with fluid but filled with air Great surface tension in alveoli could cause the lung to collapse Why doesn’t it?

15. Surface tension Alveoli are lined by layer of liquid Liquid molecules are more attracted to each other than to the gas molecule= high surface tension Could result in alveolar collapse Surface tension issues only come into play with a gas-liquid interface Goes away if breathing saline Surfactant reduces the surface tension in the alveolus Reduces forces between fluid molecules in alveolar lining

17. Surfactant Surface acting agent Reduces surface tension by reducing the forces between molecules of the fluid lining Surfactant decreases surface tension in a volume dependent manner As lung volume decreases, surface tension decreases

18. Air filled- No surfactant

19. The Alveolus

20. LaPlace Law Pressure generated in a spherical fluid lined structure Pressure is related to surface tension and radius of sphere How does a small alveolus keep from collapsing?

24. Hysteresis A quality shared by elastic materials and springs Lung acts differently during inflation and deflation An effect of surfactant As surface area gets bigger, surface tension gets higher As surface tension gets higher, it becomes more difficult to inflate

25. Air filled- No surfactant

26. Micelles move in and out of lining monolayer maintaining intermolecular distance and low surface tension The expiratory limb of the curve does NOT follow the inspiratory limb! Also note that as the tidal volume increases, the loop widens! What is happening? For any particular volume, the pressure on the expiratory curve is less than that on the inspiratory one. This is because the elastic recoil on expiration is always less than the distending transmural pressure gradient required to inflate the lung. This is a manifestation of loss of energy, and is a property that is common to all bodies that obey Hooke's law. We call the above property hysteresis. The expiratory limb of the curve does NOT follow the inspiratory limb! Also note that as the tidal volume increases, the loop widens! What is happening? For any particular volume, the pressure on the expiratory curve is less than that on the inspiratory one. This is because the elastic recoil on expiration is always less than the distending transmural pressure gradient required to inflate the lung. This is a manifestation of loss of energy, and is a property that is common to all bodies that obey Hooke's law. We call the above property hysteresis.

27. Alveoli are Interconnected and Support Each Other Surfactant promotes alveolar stability, prevents collapse of small alveoli Elastic fibers throughout the alveolar walls maintain patency of alveoli and airways

28. Surfactant Is like a detergent Reduces surface tension Reduces work of breathing Prevents alveolar collapse Small alveoli do not empty in to larger alveoli Prevents alveolar flooding

29. NEONATE RESPIRATORY DISTRESS SYNDROME Major cause of death in newborns Lungs develop late in gestation 85-90% complete at 34 weeks (term is 40 weeks) Premature birth results in infant with structurally intact but functionally immature lungs with low surfactant (breathing is labored due to high surface tension, pulmonary edema, and atelectasis)

30. Respiratory Distress in the Newborn Clinical Features Cyanosis Bluish discoloration of mucous membranes Increased % of deoxy-hemoglobin Grunting Audible sounds with breathing Reflexive vocal cord closure ? pressure to maintain FRC Poor feeding Lethargy

31. SURFACTANT DEFICIENCY IN ARDS Lungs are non-compliant Difficult to ventilate

32. Work of Breathing Done by the respiratory muscles During quiet breathing muscle work done during inhalation Exhalation is passive powered by elastic energy stored in the chest wall and lung Work in the respiratory system is of 2 kinds: Elastic work to overcome the recoil pressure of the lung and chest wall Elastic work is higher at higher lung volumes as recoil pressure increases Resistive work to overcome the resistance to airflow in the airway (and a small amount of “tissue resistance”) Resistive work is higher at lower lung volumes Resistive work is higher during exhalation

33. Respiratory Muscles Allow ventilation On inspiration create a more negative pressure in pleural space Air flows from atmosphere into alveoli Inspiratory muscles are most forceful at lower lung volumes Expiratory muscles generate maximum force at higher lung volumes

34. Flow resistance Air Flow is proportional to the pressure difference from one end of the tube to the other divided by the resistance to the airflow Resistance to airflow: Poiseuille’s Law R = 8nl/(pi)r4 n= viscosity l= length of tube r = radius increases with higher viscosity of gas is directly proportional to the length of the airway increases with 1/radius4 (if the radius halves resistance increases 16 fold) Major determinant of variability in resistance

35. Airway caliber (radius) Airway caliber is affected by: position in the lung (the more distal the smaller) bronchial smooth muscle contraction secretions lung volume pressure across the airway wall

37. Total cross-sectional resistance Even though peripheral airways are smaller there are many more of them so that total resistance is lowest in the periphery and highest centrally.

38. Measuring Airflow and Volumes PFT Spirometry Lung Volumes

39. Mechanics Summary Respiratory muscles change the volume of respiratory system in accordance with the compliance of system Air flows along a pressure gradient towards or away from alveoli depending on the phase of the respiratory cycle Airflow depends on airways resistance as well as the pressure driving the gas The work of breathing depends on the airways resistance and the energy required to change the volume of the respiratory system