1. Gururaj

2. Briefly describe factors affecting Lung Compliance

3. Resistance to Breathing  Elastic resistance ~ 65% ( of lung and chest wall)  Non-elastic resistance ~ 35% (frictional resistance to gas flow, inertia associated with movement of gas and tissues)

4. Elastic Resistance  Elastic Recoil of the Lungs elastic lung tissue recoils from the chest wall & results in a sub-atmospheric intrapleural pressure at FRC, the mean intrapleural pressure ~ 4-5 cmH 2 0 sub-atmospheric

5. Compliance  a measure of the elasticity, or distensibility, of pulmonary or thoracic tissues for an elastic body, this is given by the relation between the distending force and length for the lung, this is given by the relationship of pressure and volume may be measured under static conditions, ie. zero air flow, or under dynamic conditions  units of compliance,  V/  P = litres/cmH 2 0

6. Static Lung Compliance  the relationship between volume change of lung and the transpulmonary pressure change, i.e., airway - intrapleural pressure change, under known static conditions (zero airflow)  normal value for a 70 kg adult ~ 200 ml/cmH 2 0  the value decreases as lung volume increases due to the limitations of the non-elastic components of the lung/chest wall system

7. Static Lung Compliance : 2  static P/V curves for the lung  sigmoid curve varying degrees of hysteresis volume at any given pressure being greater during deflation

8. Reasons for Hysteresis  Changes in Surfactant activity: Surface tension greater in inspiration  Stress relaxation: Inherent property of elastic tissues ( crinkled structure of collagen in the lung)  Redistribution of gas: fast and slow alveoli

9. Static Lung Compliance : 3

10. Static Lung Compliance : 4  compliance is directly related to lung volume   transpulmonary pressure 1.0 cmH 2 0 will inflate, two lungs by 0.2 l one lung by 0.1 l  the lung of a neonate absolute compliance~ 0.006 l/cmH 2 0 specific compliance~ 0.067 l/cmH 2 0/l.V L the later being identical to that of an adult lung

11. Specific Compliance  a true measure of the distensibility of lung tissue  defined as, C S = Lung Compliance = (  V  P  V Lung Volume = Lung Compliance FRC

12. Static Compliance: Factors 1  lung volume the bigger the lungs the greater the compliance  posture due to changes in lung volumes ? Related to measurement of intrapleural pressure in supine position does not affect specific compliance  pulmonary blood volume pulmonary venous congestion from any cause will decrease compliance

13. Static Compliance: Factors 2  age many studies have failed to demonstrate any change in compliance when allowing for changes in lung volumes this is consistent with the notion that most of the elastic recoil is due to surface forces  restriction of chest expansion causes only temporary changes in compliance  recent ventilatory history  pulmonary disease

14. Compliance : Disease  emphysema static C L is increased, as is FRC however, the distribution of inspired gas may be grossly abnormal, therefore dynamic C L is frequently reduced  asthma P/V curve is displaced upwards without a change in C L the elastic recoil is reduced at normal transmural pressure, thus the FRC is increased  most other types of pulmonary disease decrease the C L, both static & dynamic

15. Dynamic Compliance : PV Loop  points of flow reversal (zero airflow)

16. Dynamic Compliance : 2

17. Dynamic Compliance : 3  measurements made using these points reflect dynamic compliance  in normal lungs at low and moderate frequencies, dynamic and static lung compliance are approximately equal  however, dynamic C L is less than static C L at higher frequencies in normal lungs at normal frequencies in abnormal lungs

18. Dynamic Compliance : 4  pressure equilibrium between applied pressure and alveolar pressure is not obtained lung appears artefactually stiffer  the time to fill an alveolus depends on the product of airway resistance and the compliance of the alveolus = the exponential time constant  the higher the airway resistance, or regional lung compliance, the longer to fill a given alveolus

19. C Dynamic : Factors  decreased dynamic lung compliance is seen especially with increased airways resistance asthma, chronic bronchitis and emphysema principally due to the prolonged time constants  emphysema increases specific lung compliance but, due to its effect on the time constant, produces the phenomenon of frequency dependent compliance

20. Time Constant  numerically, the time required for an exponential process to reach 63% of its final change  alternatively, the time which would be taken to complete volume change, if the initial rate of volume change (  V/  t), were maintained  for the lung:  tau  C L × R A

21. Surface Forces and Lung Recoil  elastic lung recoil is dependent on, surface tension : (dynes/cm, SI units = N/m) ○  produces > 50% of normal lung recoil tissue elastic fibres  the recoil pressure of a saline filled lung is lower determined only by the elastic recoil of pulmonary tissue

22. Laplace’s Law P = 2T/r Which one has higher transmural pressure?? R1: 0.1 T1: 20 R2: 0.05 T2: 20

23. Surface Tension  surface active agents, surfactants, exert smaller attracting forces for other molecules  when concentrated at the surface they dilute the molecules of a liquid and lower surface tension  ordinary detergents lower surface tension, however tension does not alter with changes in surface area with pulmonary surfactant, as the surface area decreases, so surface tension also decreases

24. Pulmonary Surfactant  synthesised in type II alveolar cells, granular pneumocytes  elimination half life :t ½ ~ 14 hrs  dipalmitoyl phosphatidyl choline (DPPC), a phospholipid, is the main component hydrophilic and hydrophobic ends, therefore forms a lipid monolayer

25. Surfactant : Actions  reduces T s in alveoli reduces lung recoil and work of breathing  stabilises alveoli of variable size as surface tension is proportional to surface area prevents small alveoli tending to "fill" larger ones  promotes alveolar “dryness” a high T s tending to draw fluid into alveoli as well as promoting collapse

26. Surfactant  RDS of new-born  hyperoxia- O 2 toxicity of lung  Smoking  gross over distension of alveoli  ARDS

27. Elastic Recoil: Thoracic  resting volume for thoracic cage ~ FRC + 600-700 ml  thoracic cage compliance is calculated from total compliance of the thoracic cage + the lungs, and from pulmonary compliance when measured simultaneously, where, 1/C TOT = 1/C L + 1/C CW

28. Elastic Recoil: Thoracic 2  FRC = equilibrium point for both systems not quite true, as FRC is 400-500 ml above the equilibrium point due to the tonic activity of the diaphragm  thoracic cage compliance is decreased in, kyphoscoliosis, ankylosing spondylitis scleroderma muscle spasticity abdominal distension, obesity

29. Non-Elastic Resistance  this is composed of, airway flow resistance~ 80% pulmonary tissue resistance, or viscous resistance~ 20%  increases markedly with rapid respiration, or narrowing of the airways proportional to the rate of airflow  P for a given airflow depends upon whether the flow is laminar, or turbulent

30. Laminar Flow  Hagen-Poiseuille Equation n Pressure gradient = Flow X Resistance therefore, by rearrangement,

31. Turbulent Flow  the likelihood of flow becoming turbulent is predicted by the Reynold's Number n V: velocity, d: diameter viscosity (  -eta) is relatively less important viscosity (  -eta) is relatively less important ä viscosities of respirable gasses do not vary greatly, cf. densities may vary considerably density (  -rho) decreases flow proportionately density (  -rho) decreases flow proportionately

32. Reynold’s Number  Re < 2000, laminar flow becomes more likely Re > 4000 : predominantly turbulent  Flow is square front:  Fresh gas has to fill the volume of the tube  Better at purging the contents of the tube  Gas representative at all points theoretically, the required driving pressure becomes inversely proportional to the fifth power of the tube radius:Fanning equation