1. CS 2015 Mechanical Properties of Lung and Chest Wall Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/Mechanics.pptx Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/Mechanics.pptx THE AUSTRALIAN NATIONAL UNIVERSITY

2. CS 2015

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 (R AW ); 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. CS 2015 Contents Airway resistance –Air flow conditions –Locations and determinants of R AW –Transmural pressure System compliance and its elements –Static & dynamic compliances –Alveolar surface tension –Laplace’ law and alveolar pressure –Surfactants and compliance

5. CS 2015 Determinants of R AW Air flow conditions Locations and determinants of R AW Transmural pressure Modulation of R AW

6. CS 2015 Air Exchange Conducting airways: blood supply via bronchial artery. Bronchioles: no skeleton; exposed to transmural pressure. Respiratory unit = physiological unit, where O 2 and CO 2 are exchanged; blood supply via pulmonary artery. Berne et al., 2004

7. CS 2015 Consequences for Air Flow 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. Borom & Boulpaep, 2003

8. CS 2015 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 R AW (airway and tissue deformation): Contribution to R AW : Boron & Boulpaep, 2003

9. CS 2015 Determinants of R AW Under laminar flow conditions, with η viscosity, l length and r radius. Normally, viscosity is constant (air); altered with pressure (altitude, diving) & gas mixtures. Elements of R AW (around TV) –R visc ~ 40% (dynamic parameter; flow dependent). Laminar and turbulent conditions (80%) Tissue resistance (“friction” between elastic fibres; 20%) Inertia (very little) –R elast ~ 60% (static parameter; volume dependent).

10. CS 2015 R AW and Lung Volume 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↓ → R AW ↑. To maintain ventilation, flow↑. Modified from Boron & Boulpaep, 2003

11. CS 2015 Transmural Pressures Affects bronchioles During forced I, positive transmural pressure keeps small airways open. During forced E, when P pl > 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. CS 2015 Modulation of R AW Smooth muscle tone –Parasympathetic: bronchial constriction and mucus production ↑. –Sympathetic: β 2 -action (smooth muscle relaxation, secretion ↓). With ↑ → local airway dilation; ↓→ local airway constriction. Berne et al., 2004

13. CS 2015 Compliance of Breathing System Static & dynamic compliances Alveolar surface tension -Laplace’ law and alveolar pressure -Surfactants and compliance

14. CS 2015 Compliance of Breathing System Static compliance: no flow, volume fixed Dynamic compliance: both flow, volume change C T = total compliance (breathing system) C L = lung compliance C CW = thorax (chest wall) compliance

15. CS 2015 How to Measure Compliances Shown with body plethysmograph. –Required for C dyn. –Not necessary for C static (no flow…). C static with valve and spirometer only. –Measured during expiration (see later). –P A and ΔV L measured simultaneously after halting flow (= P oral ): at each volume, P A measured. Modified from Boron & Boulpaep, 2003

16. CS 2015 Static Lung Compliance (No Flow) Total system compliance (C T ) can be measured after breathing out (“relaxation curve”); linear within range of TV. Both lung (C L ; fibrosis – too small; emphysema – too large) and chest-wall compliance (C CW ; skoliosis) are needed clinically. C T is related to C CW and C L via Requires that P pl be measured with each volume change. Within TV, C L ~ C CW ~ 2 C T, ~ 0.1 L/cm H 2 O. Modified from Hlastala & Berger 2001

17. CS 2015 Static C L and Pathology Static C L important in pathophysiology. Emphysema (“overblown” lung) has large compliance at FRC: loss of recoil (elastance; 1/C L ). Conversely, fibrosis reduces C L and FRC: too much recoil … Modified from Boron & Boulpaep, 2003

18. CS 2015 Dynamic Compliance Example for TV Hysteresis (CCW move) C dyn 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 C stat ≈ average C dyn (which is typically a bit smaller). Effort sets width of hysteresis. Modified from Despopoulos & Silbernagl 2003

19. CS 2015 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 R AW ). RDS has low static and dynamic compliance and TV at high pressures. Modified from Koller, 1979

20. CS 2015 Alveolar Surface Tension Laplace’ law Surfactants

21. CS 2015 Surface Tension and Compliance C L ↑ when lung filled with saline - but finite. Surface tension is largest factor determining C L : –Laplace’ law. How to minimise surface tension? –Detergents (soap) –Surfactants … Modified from Boron & Boulpaep, 2003

22. CS 2015 What Every Child Knows… What is the hardest part to blowing up a balloon? –Initial volume change… –Becomes easier as you inflate… –Ultimately so easy, it can be blown apart…

23. CS 2015 Laplace’ Law P recoil 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

24. CS 2015 Surfactants and Surface Tension Surfactant (surface-active agent) Reduces surface H 2 O and hence surface tension: it is an attractive force of surface molecules that tends to minimise surface area. Combination of dipalmitoylphosphatidyl- choline and apoproteins (SP-A/B/C/D). Secreted by alveolar type II cells Can easily be destroyed with O 2. 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

25. CS 2015 Surface Expression Surfactants form micelles. Dynamic system: –During I, as alveolar surface increases and [surfactant] decreases, surfactant from micelles is recruited to surface. –During E, alveolar surface de- creases, [surfactant] is higher and micelles re-form. Role: –Reduction in surface tension: keeps alveoli “open”. –Keeping alveoli dry. Modified from Hlastala & Berger 2001

26. CS 2015 Ventilation and Surfactants Rapidly expanding alv. → [surfactant]↓ → C A ↓ → ventilation↓. Slowly expanding alv. → [surfactant]↑ → C A ↑ → ventilation↑. Homeostatic principle to open alveoli to ~ similar volume. Modified from Boron & Boulpaep, 2003

27. CS 2015 Take-Home Messages Flow in bronchi is transitional, in alveoli laminar. R AW is volume dependent; is neurally modulated. C L is ~2 x C T ; 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.

28. CS 2015 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 H 2 O and the alveolar pressure from 5 to 0 cm H 2 O. What is the best estimate of her static lung compliance? A.0.5 L / cm H 2 O B.5.0 cm H 2 O / L C.0.1 L / cm H 2 O D.2.0 cm H 2 O / L E.0.2 L / cm H 2 O

29. CS 2015 That’s it folks…

30. CS 2015 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 H 2 O and the alveolar pressure from 5 to 0 cm H 2 O. What is the best estimate of her static lung compliance? A.0.5 L / cm H 2 O B.5.0 cm H 2 O / L C.0.1 L / cm H 2 O D.2.0 cm H 2 O / L E.0.2 L / cm H 2 O