ventilation and perfusion

1. Ventilation and Perfusion John W. Kreit, M.D. Division of Pulmonary and Critical Care Medicine University of Pittsburgh School of Medicine

2. Ventilation The repetitive movement of gas into and out of the lungs Ventilation delivers the oxygen and removes the carbon dioxide that is exchanged across the alveolar-capillary interface.

3. Ventilation and Gas Exchange

4. Lung Volumes and Capacities

5. Lung Volumes and Capacities Tidal Volume (VT) The volume of gas inhaled or exhaled during a breath Residual Volume (RV) The volume of gas remaining in the lungs at the end of a maximal expiration

6. Lung Volumes and Capacities Expiratory Reserve Volume (ERV) The volume of gas that can be forcefully exhaled after a normal tidal expiration Inspiratory Reserve Volume (IRV) The maximum volume of gas that can be inhaled after a normal tidal inspiration

7. Lung Volumes and Capacities Functional Residual Capacity (FRC) The volume of gas remaining in the lungs after a normal, tidal expiration FRC = RV + ERV Total Lung Capacity (TLC) The volume of gas in the lungs at the end of a maximal inspiration

8. Lung Volumes and Capacities Vital Capacity (VC) The maximum volume of gas that can be exhaled beginning at the end of a maximal inspiration VC = TLC � RV Inspiratory Capacity (IC) The volume of gas entering the lungs during a maximal inspiration that begins at the end of a tidal expiration IC = IRV + VT

9. Lung Volumes and Capacities

11. Partial Pressure of Respiratory Gases In a gas mixture, Pgas = Ptotal x Fgas At sea level, total gas pressure (PB) = 760 mmHg In dry air at sea level: PO2 = 760 x 0.21 = 160 mmHg PN2 = 760 x 0.79 = 600 mmHg PCO2 = 760 x 0.0004 = 0.3 mmHg

12. Partial Pressure of Respiratory Gases In inspired, humidified air at sea level: Pgas = (PB � PH2O) x FIgas PH2O = 47 mmHg PO2 = (760 � 47) x 0.21 = 150 mmHg PN2 = (760 � 47) x 0.79 = 563 mmHg PCO2 = (760 � 47) x 0.0004 = 0.3 mmHg

13. Partial Pressure of Respiratory Gases In alveolar gas at sea level: PAO2 is calculated using the alveolar air equation: PAO2 = (PB � PH2O) FIO2 � PACO2 / R PACO2 = PaCO2 = 40 mmHg R = VCO2/VO2 = 0.8 PAO2 = (760 � 47) x 0.21 � (40 / 0.8) = 100 mmHg

14. Partial Pressure of Respiratory Gases Air Airways Alveoli Arterial Mixed venous PO2 160 150 100 95 40 PCO2 0 0 40 40 46 PH2O 0 47 47 47 47 PN2 600 563 573 573 573

15. The Alveolar-arterial Oxygen Gradient PAO2 is estimated using the alveolar gas equation. PaO2 is measured. A difference always exists between PAO2 and PaO2. Alveolar-arterial oxygen gradient (PA-aO2) Normally 8 � 12 mmHg Increased by ventilation � perfusion imbalance, shunt, and diffusion impairment

16. Dead Space Volume and Alveolar Volume Dead Space Volume (VD) The volume of gas that enters the physiologic dead space Anatomic dead space Alveolar dead space Alveolar Volume (VA) The volume of gas entering the lungs that participates in gas exchange. VA = VT - VD

17. Minute Ventilation and Alveolar Ventilation Minute Ventilation (VE) The total amount of gas entering or leaving the lungs each minute. VE = VT x RR Dead Space Ventilation (VD) The amount of gas entering or leaving the physiologic dead space each minute. VD = VD x RR

18. Minute Ventilation and Alveolar Ventilation Alveolar Ventilation (VA) The volume of gas entering or leaving the lungs each minute that participates in gas exchange. VA = VA x RR VA = VE - VD

19. Alveolar Ventilation and PCO2 PACO2 and PaCO2 are directly related to the rate at which CO2 enters the alveoli determined by the rate at which CO2 is produced by the tissues (VCO2) PACO2 and PaCO2 are inversely related to the rate at which CO2 is removed from the alveoli determined by alveolar ventilation (VA)

20. Alveolar Ventilation and PCO2 PACO2 ? VCO2 / VA PACO2 = K x VCO2 / VA PACO2 = PaCO2 = K x VCO2 / VA PaCO2 = K x VCO2 / (VE - VD)

21. Alveolar Ventilation and PAO2 Alveolar ventilation influences PAO2 only through its effect on PACO2. PAO2 = (PB � PH2O) x FIO2 � PACO2 / R When PACO2 rises, PAO2 falls. When PACO2 decreases, PAO2 increases.

22. Distribution of Alveolar Ventilation Pleural pressure increases from the apex to the base of the lungs. This leads to a progressive Decrease in trans-pulmonary pressure Decrease in end-expiratory alveolar volume Increase in alveolar compliance

23. Distribution of Alveolar Ventilation These factors lead to a progressive increase in end-inspiratory alveolar volume. Ventilation increases from the non-dependent to the dependent regions of the lungs.

25. The Pulmonary Circulation Two circulations Pulmonary Bronchial When compared with systemic arteries, the pulmonary arteries have thinner walls larger lumens very little smooth muscle

26. The Pulmonary Circulation These characteristics cause the pulmonary arteries to be much more compressible and distensible than systemic arteries. pulmonary vascular resistance (PVR) to normally be much less than systemic vascular resistance.

27. Determinants of PVR Active factors Neural factors Sympathetic Parasympathetic Humoral factors Catecholamines Prostaglandins Alveolar PO2 Passive factors Lung Volume Cardiac Output Gravity

28. Determinants of PVR Lung volume Alveolar vessels Resistance varies directly with lung volume. Extra-alveolar vessels Exposed to pleural pressure Resistance decreases during spontaneous inspiration and increases during forced exhalation below FRC.

30. Lung Volume and PVR

31. Lung Volume and PVR

32. Determinants of PVR Cardiac Output An increase in cardiac output is accompanied by a fall in PVR and little change in arterial pressure. The decrease in PVR is due to two processes: Recruitment Distention

33. Cardiac Output and PVR

34. Determinants of PVR Gravity Intravascular pressure is higher in dependent than in non-dependent lung regions. The higher the intravascular pressure, the greater the vascular distention and the lower the PVR.

36. Distribution of Perfusion Since vascular pressure and resistance are influenced by gravity, Blood flow increases in the more dependent and decreases in the less dependent regions of the lungs.

37. Zones of the Lung Since intravascular pressure varies and alveolar pressure is uniform, the relationship between the pressure in the arteries (Pa), veins (Pv), and alveoli (PA) varies throughout the lung Zones of the lung Zone 1: PA > Pa > Pv Zone 2: Pa > PA > Pv Zone 3: Pa > Pv > PA

38. Zones of the Lung

39. Fluid Flow Acrossthe Pulmonary Capillaries The net movement of fluid across the pulmonary capillariesis determined by: Pressure within the capillaries (Pc) and interstitium (Pi) Osmotic pressure of the plasma (?p) and interstitium (?i) Permeability of the capillaries to fluid (Kf) and solute (?)

40. Fluid Flow Acrossthe Pulmonary Capillaries Qf = Kf [(Pc � Pi) � ?(?p � ?i)]

41. Fluid Flow Acrossthe Pulmonary Capillaries Pulmonary edema can result from Increased pulmonary capillary pressure Increased capillary permeability Decreased plasma oncotic pressure Impaired lymphatic drainage