THE AUSTRALIAN NATIONAL UNIVERSITY

1. THE AUSTRALIAN NATIONAL UNIVERSITY Mechanical Properties of Lung and Chest WallChristian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - ANUChristian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/Mechanics.pptx

3. Aims At the end of this lecture students should be able to • explain different types of air flow conditions; • identify determinants of airway resistance (RAW); • illustrate the concepts of static and dynamic compliance and how these are measured; • demonstrate why a small lung volume is harder to inflate than a larger; and • point out how surfactants increase compliance.

4. Contents • Airway resistance • Air flow conditions • Locations and determinants of RAW • Transmural pressure • System compliance and its elements • Static & dynamic compliances • Alveolar surface tension • Laplace’ law and alveolar pressure • Surfactants and compliance

5. Determinants of RAW

6. Air Exchange Berne et al., 2004 • Conducting airways: blood supply via bronchial artery. • Bronchioles: no skeleton; exposed to transmural pressure. • Respiratory unit = physiological unit, where O2 and CO2are exchanged; blood supply via pulmonary artery.

7. Consequences for Air Flow Borom & Boulpaep, 2003 • Flow over vocal cords is biggest and decays later to small values in alveolar airways. • Functional consequence: ~ turbulent flow over vocal cords; but ~ laminar flow in alveolar airways.

8. Flow Conditions in Airways • Since airways are bifurcated, turbulence arises at bifurcation points. • Flow in airways is transitional (in between laminar and turbulent). • Ohm’s law is used to determine RAW(airway and tissue deformation): • Contribution to RAW: Boron & Boulpaep, 2003

9. Determinants of RAW • Under laminar flow conditions,with ηviscosity, l length and r radius. • Normally, viscosity is constant (air); altered with pressure (altitude, diving) & gas mixtures. • Elements of RAW (around TV) • Rvisc ~ 40% (dynamic parameter; flow dependent). • Laminar and turbulent conditions (80%) • Tissue resistance (“friction” between elastic fibres; 20%) • Inertia (very little) • Relast~ 60% (static parameter; volume dependent).

10. RAW and Lung Volume Modified from Boron & Boulpaep, 2003 • Lung volume affects airway diameter, particularly airways without skeleton: during E, tension release (alveolar size ↓) and positive pressure on bronchioli → r ↓; during I,vice versa. • It is easier to breath in than out (air trapping…). • COPD: r↓ → RAW↑. To maintain ventilation, flow↑.

11. Transmural Pressures • Affects bronchioles • During forced I, positive transmural pressure keeps small airways open. • During forced E, when Ppl> 0, transmural pressure can become ≤0; i.e. airways collapse. • Can be seen in flow-volume loop: airway closure. Modified from Hlastala & Berger 2001

12. Modulation of RAW Berne et al., 2004 • Smooth muscle tone • Parasympathetic: bronchial constriction and mucus production ↑. • Sympathetic:β2-action (smooth muscle relaxation, secretion ↓). • With ↑ → local airway dilation; ↓→ local airway constriction.

13. Compliance of Breathing System Static compliance: no flow, volume fixed Dynamic compliance: both flow, volume change CT = total compliance (breathing system) CL = lung compliance CCW = thorax (chest wall) compliance

14. How to Measure Compliances • Shown with plethysmograph. • Required for Cdyn. • Not necessary for Cstatic (no flow…). • Cstaticwith valve and spirometer only. • Measured during expiration (see later). • PAand ΔVLmeasured simultaneously after halting flow (= Poral): at each volume, PA measured. Modified from Boron & Boulpaep, 2003

15. Static Lung Compliance (No Flow) • Total system compliance (CT) can be measured after breathing out (“relaxation curve”); linear within range of TV. • Both lung (CL; fibrosis – too small; emphysema – too large) and chest-wall compliance (CCW; skoliosis) are needed clinically. • How CT is related to CCWand CL: • Requires that Pplbe measured with each volume. • Within TV, CL ~ CCW ~ 2 CT, ~ 0.1 L/cm H2O. Modified from Hlastala & Berger 2001

16. Static CL and Pathology • Static CL important in pathophysiology. • Emphysema (“overblown” lung) haslarge compliance at FRC: loss of recoil (elastance; 1/CL). • Conversely, fibrosis reduces CL and FRC: too much recoil … Modified from Boron & Boulpaep, 2003

17. Dynamic Compliance • Example for TV • Hysteresis (CCW move) • Cdyn at end of E > than at beginning of I. • For both I and E, smaller at beginning than at end. • Elastic recoil > at end of I which helps at start of E • Cstat ≈ average Cdyn(typically a bit smaller). • Effort sets width of hysteresis. Modified from Despopoulos & Silbernagl 2003

18. Compliances in Disease • Emphysema with a high static compliance and a wide dynamic hysteresis (work! - recoil lost). • Asthma increases compliance; TV at FRC↑; large expiratory work (increased RAW). • RDS has low static and dynamic compliance and TV at high pressures. Modified from Koller, 1979

19. Alveolar Surface Tension Laplace’ law Surfactants

20. Surface Tension and Compliance • CL↑ when lung filled with saline - but finite. • Surface tension is largest factor determining CL: • Laplace’ law. • How to minimise surface tension? • Detergents (soap) • Surfactants … Modified from Boron & Boulpaep, 2003

21. What Every Child Knows… • What is the hardest part to blow a balloon up? • Initial volume change… • Becomes easier as you inflate… • Becomes so easy, it can be blown apart…

22. Laplace’ Law • Precoil in B is 2 x that in A. • If A and B are coupled in series, what happens? • B blows A up. • To counter this, alveoli are • interdependent: physically interconnected with each other; and • lined with surfactant. Modified from Boron & Boulpaep, 2003

23. Surfactants and Surface Tension • Surfactant (surface-active agent) • Reduces surface H2O and hence surface tension: it is an attractive force of surface molecules that tends to minimise surface area. • Combination of dipalmitoyl-phosphatidylcholine and apoproteins (SP-A/B/C/D). • Secreted by alveolar type II cells • Can easily be destroyed with O2. • Produced shortly before birth; problem in premature babies (respiratory distress syndrome). • Steroid priming for 2-3 d can initiate surfactant expression. Modified from Boron & Boulpaep, 2003

24. Surface Expression • Surfactant can form micelles. • Dynamic system: • During I, as alveolar surface increases and [surfactant] decreases, surfactant from micelles is recruited to surface. • During E, alveolar surface decreases, [surfactant] is higher and surfactant then re-forms micelles. • Role: • Reduction in surface tension: keeps alveoli “open”. • Keeping alveoli dry. Modified from Hlastala & Berger 2001

25. Ventilation and Surfactants • Rapidly expanding alv. → [surfactant]↓ → CA↓ → ventilation↓. • Slowly expanding alv. → [surfactant]↑ → CA↑ → ventilation↑. • Homeostatic principle toopen alveoli to ~ similarvolume. Modified from Boron & Boulpaep, 2003

26. Take-Home Messages • Flow in bronchi is transitional, in alveoli laminar. • RAW is volume dependent; is neurally modulated. • CL is ~2 x CT; is linear in range of TV. • A small alveolus requires a larger pressure to increase its volume than a large one; • Hysteresis in V-P loop is result of surface tension and Laplace’ law; and • Surfactants reduce surface tension and ease alveolar ventilation.

27. MCQ Anna May, a 43 year-old female, has an extensive lung function analysis. As she exhales under static conditions from FRC + 1 L to FRC, her oesophageal pressure changes from -10 to -5 cm H2O and the alveolar pressure from 5 to 0 cm H2O. What is the best estimate of her static lung compliance? • 0.5 L / cm H2O • 5.0 cm H2O / L • 0.1 L / cm H2O • 2.0 cm H2O / L • 0.2 L / cm H2O

28. That’s it folks…

29. MCQ Anna May, a 43 year-old female, has an extensive lung function analysis. As she exhales under static conditions from FRC + 1 L to FRC, her oesophageal pressure changes from -10 to -5 cm H2O and the alveolar pressure from 5 to 0 cm H2O. What is the best estimate of her static lung compliance? • 0.5 L / cm H2O • 5.0 cm H2O / L • 0.1 L / cm H2O • 2.0 cm H2O / L • 0.2 L / cm H2O