Compliance, airway resitance, work of breathing

1. Compliance, airway resitance, work of breathing

2. Design of the ventilatory apparatus Function : to move the air in and out of lungs Made up two expansible chambers one inside the other Lungs Pleural space Chest wall

3. Mechanics of thorax and lungs (alone and combined) Development of pleural pressure (negative) Lungs and thoracic cage are both elastic structures Display a constant relationship between distending pressure and change in volume

4. Compliance Change in volume per unit change in pressure Measure of distensibility

5. Apply negative/positive pressure around the thorax and record the change in the volumes Determination of the combined compliance of thorax and lungs C

6. Inhale an amount of air, relax the muscles and measure the pressure developed due to recoil AP PP IP V is same for both lungs and chest wall P for lungs is difference between alveolar pressure and pleural pressure P for thoracic is difference between atmospheric pressure and pleural pressure

7. Pressure-Volume curve of the Lungs Ideally should be a straight line

8. Compliance of lungs --------------0.2L/cm water • What will happen if…….. • There is only one lung • In children • Specific compliance…………..compliance/FRC

9. Compliance of thoracic cage 0.2L/cm water in adults Compliance of total respiratory system………..0.1L/cm of water Formula : 1/CT=1/CL+1/CW

10. Meaning of the Rahn Diagram

11. T P1 r1 T P2 r2 Surface Tension • Force acting across an imaginary line 1 cm. long in a liquid surface • Laplace’s Law: For each surface of a bubble, pressure is equal to twice the tension divided by the radius Result: Small Bubble Collapses

12. Role of surfactant T/R = t/r P1 = P2 Small alveoli will not empty into large alveoli

13. Airflow through tubes Resistance during flow • Difference of pressure exists between the ends • Pressure difference depends on the rate and pattern of flow • 3 patterns of flow • Laminar flow • Turbulent flow • Transitional flow

14. Laminar flow P2 P1 (for most of respiratory system) Turbulent flow

15. What decides whether flow through a tube will be laminar or turbulent? Reynold‘s number Re = Vdρ/η If reynold;s number > 2000, flow will be turbulent Laminar airflow in tubes<2mm

16. Pressure flow characteristics By poisseuill, for laminar flow Critical importance of radius For turbulent flow P= KV2 For airways P= K1V +K2V2

17. Airway resistance Parallel branching Large diameter

18. Airway resistance • Airway resistance is defined as the ratio of the pressure drop between the mouth and alveoli to the volume flow rate • Chief site of airway resistance: medium sized bronchi • Smaller airways contribute very little to total resistance due to their numerous parallel branching resulting in a large cross-sectional area • Average normal airway resistance in healthy adults ……1.6cm water/L/s R = P/V

19. Factors determining airway resistance Radius of the tube Nature of flow : turbulence and velocity of flow Factors influencing airway resistance State of contraction of bronchial musculature Lung volume Breathing….expiration much more difficult in asthmatic attack Dust and smoke

20. Work of breathing W.D = F X D W.D = P X V Plot the change in interpleural pressure against change in volume

21. Work done during inspiration WTR 7% WE 65% WAR 28% V WVR 35% P Work done to overcome elastic recoil of lungs and chest wall Work done to overcome viscous resistance

22. Work done during expiration Dissipated as heat V P

23. Energy cost of breathing The magnitude of elastic component of the work depends on degree of expansion of the lungs Patients with restrictive lung diseases……………. shallow breaths, increase frequency Magnitude of viscous component of work of breathing depends on the velocity of air flow Obstructive lung disease patients……….slow and deep breaths

24. Thankyou