Physiology of Ventilation

1. Physiology of Ventilation Principles of Ventilation

2. Educational Objectives • Define and differentiate between compliance, elastance, and resistance • List the normal values for the pressures associated with the act of breathing

3. Educational Objectives • Describe the distribution of ventilation within the lung, listing factors that affect how air is distributed • Describe the normal perfusion of the lung, listing the factors that affect blood flow • Define ventilation/perfusion ratio

4. Definitions • Ventilation – movement of air between the atmosphere and the alveoli • Respiration – movement of gas molecules across a membrane

5. Airflow Into And Out of The Lungs • Intrapulmonary Pressure (Palv) • Pressure at the alveolus; changes from positive to negative during ventilatory cycle (-5 to +5 cm H2O)

6. Airflow Into And Out of The Lungs • Intrapleural pressure (Ppl) • Always negative during normal breathing – (-5 to -10 cm H2O)

7. Airflow Into And Out of The Lungs

8. Normal Inspiration • Diaphragm contracts • Intrathoracic volume increases • Intrapleural pressure increases in negativity • Increase in volume causes decrease in intrapulmonary pressure

9. Normal Inspiration • Decrease in intrapulmonary pressure creates negative pressure gradient relative to the atmospheric pressure • Air flows into the lungs until pressures equalize

10. Normal Expiration • Diaphragm relaxes, moving upward • Intrathoracic volume decreases • Intrapleural pressure becomes less negative • Decrease in volume creates increase in intrapulmonary pressure

11. Normal Expiration • Increase in intrapulmonary pressure creates positive pressure gradient relative to the atmospheric pressure • Air flows out of the lungs until pressures equalize

12. Airflow Into And Out of The Lungs

13. Factors Affecting Lung Volume • Compliance • Elastance (Elasticity) • Resistance • Muscle strength and endurance

14. Compliance • The ratio of the change in volume to a given change in pressure • Normal value – 100 ml/cm H2O

15. Types of Compliance • Dynamic compliance – measured during normal breathing cycle Cdyn = Volume _ Peak Inspiratory Pressure

16. Types of Compliance • Static compliance – measured during breath-holding procedure Cplat = Volume _ Plateau Pressure

17. Elastance • The physical tendency of an object to return to its initial state after deformation • Inverse of compliance

18. Resistance • Opposition to a force; ratio of pressure change to flow change • Poiseuille’s Law – ΔP = 8nlV r4 • R = P1 – P2 Volume

19. Factors Affecting Muscle Strength and Endurance • Gender • Age • Training • Position

20. Factors Affecting Muscle Strength and Endurance • Underlying cardiac, pulmonary, and muscular disorders • Electrolyte imbalances • Acid-base disturbances

21. Factors Affecting Muscle Strength and Endurance • Endocrine abnormalities (e.g., thyroid disorders) • Prolonged use of steroids • Neuromuscular blocking drugs

22. Evaluation of Muscle Strength and Endurance • Measurement of transdiaphragmatic pressure • Maximum voluntary ventilation (MVV)

23. Distribution of Ventilation • Dead Space • Ventilation not involved in gas exchange

24. Dead Space • Anatomic dead space • Volume of ventilation in conducting airways • Alveolar dead space • Volume of ventilation in alveoli which are under perfused or not perfused

25. Dead Space • Physiologic dead space • Sum of anatomic and alveolar dead space

26. Normal Distribution of Ventilation (Upright Position) • Pleural pressure lower (more negative) at apex of lung • Greater transpulmonary pressure at apex

27. Normal Distribution of Ventilation (Upright Position) • Alveoli at apex more distended at FRC than those at base • Alveoli at base receive greater ventilation (are able to distend further) than the alveoli at apex

28. Factors Affecting Distribution of Ventilation • Increased regional resistance (inflammation) • Localized changes in compliance (blebs)

29. Distribution of Perfusion • Blood flow determined by difference between pulmonary vascular pressure and alveolar pressure • At apex, alveolar pressure greater than pulmonary vascular pressure – no blood flow (Zone 1) • At base, pulmonary vascular pressure greater than alveolar pressure – minimal ventilation (Zone 3)

30. Distribution of Perfusion • Blood flow determined by difference between pulmonary vascular pressure and alveolar pressure • Between Zones 1 And 3 (Zone 2), blood flow determined by the difference between pulmonary vascular pressure and alveolar pressure

31. Three Lung Zones

32. Distribution of Perfusion • Lowest resistance to blood flow is at FRC; resistance increases at either residual volume or total lung capacity

33. Ventilation/Perfusion Ratio • Ideally V/Q ratio is 1

34. Shunt • Perfusion Without Ventilation • V/Q Ratio is 0

35. Causes of Shunts • Atelectasis • Fluid in the alveolar space • Airway obstruction • Anatomic abnormalities

36. Modified Shunt Equation • Qs= (PAO2 – PaO2) x 0.003 _ QT (CaO2 – CvO2) + (PAO2) x 0.003

37. Ventilation/Perfusion

38. Oxygen Uptake and Diffusion Capacity • Time of transit of RBC through the pulmonary capillary • At rest – 0.75 seconds • During exercise – 0.25 seconds • Number of RBCs available

39. Oxygen Uptake and Diffusion Capacity • Biochemical characteristics of hemoglobin (e.g., sickle cell, carbon monoxide, presence of fetal hemoglobin) • Evaluation done by measuring single breath carbon monoxide diffusion