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Work of Breathing

Work of Breathing. Components 1. Compliance work 65% (stretching lungs & chest wall) 2. Airways resistance work 30% 3. Moving tissues 5% Normally <1–3% Total Energy (E)  in exercise, still <3–5% total E output.  Volume. Compliance =.  Pressure. Lung resists stretching:.

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Work of Breathing

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  1. Work of Breathing Components 1. Compliance work 65% (stretching lungs & chest wall) 2. Airways resistance work 30% 3. Moving tissues 5% Normally <1–3% Total Energy (E)  in exercise, still <3–5% total E output

  2.  Volume Compliance =  Pressure

  3. Lung resists stretching: • Tissue expansion: Small componentnormally Fig. shows lung expansion involves an unfolding of elastin and collagen fibers in the alveolar walls. The actual lengths of the individual fibers change little.

  4. Major component: surface tension dynes / cm

  5. Lung surfactant • protein-phospholipid • from alveolar type 2 cells • Functions • 1.  surface tension of fluid in alveoli, so lungs able to expand with normal muscle activity. • 2. Stabilizes alveoli from collapse • surface tension is lowered more in small alveoli than in larger alveoli • P = 2T • r

  6. Small alveolus surfactant molecules more concentrated Large alveolus surfactant molecules more diluted Fig. shows surface tension alters alveoli stability. The smaller alveoli generate a greater pressure and cause air to flow into larger units. Surfactant promotes alveolar stability by lowering surface tension proportionately more in the small alveoli.

  7. Compliance work  1. where  surfactant ~ premature babies (smoking) 2. where lung tissue fibrotic (scar tissues) ~ coal miners ~ asbestosis 3. where chest wall expansion limited ~ scoliosis ~ tight bandages 4. at high lung volumes

  8.  compliance work Fig. shows static expiratory pressurevolume curves of lungs in normal subjects and subjects with pulmonary fibrosis.

  9. Airway Resistance pressure gradient Gas flow = resistance 1 with laminar flow resistance  4 radius • Airway resistance is increased by •  turbulent flow e.g. rapid breathing • airway narrowing

  10. Small peripheral airways • each has small diameter • but many of them • so total cross-sectional area is high • reactive smooth muscle in wall

  11. Fig. shows schematic representation of airway branching in the human lung with approximate dimensions.

  12. Causes of airway narrowing • in expiration vs inspiration • ~ expiratory forces tend to push airways shut • worse if little support for airways • ~ emphysema = lung tissue destruction floppy airways close during expiration air trapping • smooth muscle constriction e.g. asthma • inflammation, mucus, mucosal swelling • e.g. chronic bronchitis asthma

  13. Airway resistance is affected by: 1. Structure of lungs ~ resistance is higher in upper airways (trachea/bronchi etc) ~ lower in small airways (large total cross-sectional area, laminar flow) 2. Mechanical factors: Airway resistance is  in expiration vs inspiration expiratory forces tend to push airways shut worse if little support for airways ~ airway resistance is lower at high lung volumes – airways held open ~ airway resistance is lower during inspiration vs expiration

  14. 3. Smooth muscle tone in small airways ~ sympathetic input  relaxation ~ parasympathetic input  constriction ~ immune response e.g. asthma  allergen/cold  triggers immune response  inflammatory mediators released  smooth muscle constriction, mucosal swelling and mucus secretion 4. Local reflexes: Areas with  PCO2 cause bronchiolar smooth muscle contraction: important for ventilation-perfusion matching

  15. Mechanical factors cont: ~ airway resistance may be high during expiration due to dynamic airways collapse during expiration (forced expiration, “floppy” airways in emphysema) Emphysema = lung tissue destruction  floppy airways  close during expiration  air trapping

  16. Spirometry Forced expiratory volume in 1 sec F E V1 ~ measure of airways obstruction and/or dynamic airway compression ~ since F E V1 also affected by F V C (forced vital capacity) F E V1 / F V C ratio used N ratio  80%

  17. FEV1 = 3.2L FVC = 4L FEV1 = 1.5L FVC = 3.2L

  18. Peak flow

  19. Overall Work of Breathing 1.Compliance work is greatest at high lung volumes 2. Airways resistance work is greatest at rapid airflow rates ie highest respiratory rate / min minute ventilation VT = VT x resp rate L / min tidal vol

  20. Work of breathing (summary) • Compliance work. • ~ surfactant • measurement: static compliance curve spirometry • ~ airways resistance work • measurement: • alternative: peak flow meter • V  P F E V1 F V C

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