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PULMONARY GAS EXCHANGE IN HEALTH AND DISEASE

PULMONARY GAS EXCHANGE IN HEALTH AND DISEASE. PETER D. WAGNER, M.D. UCSD, La Jolla, California. Antalya, Turkey, April 2006. GAS EXCHANGE IN HEALTH AND DISEASE. NORMAL. ASTHMA. COPD. INTERSTITIAL FIBROSIS. PULMONARY THROMBOEMBOLISM.

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PULMONARY GAS EXCHANGE IN HEALTH AND DISEASE

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  1. PULMONARY GAS EXCHANGE IN HEALTH AND DISEASE PETER D. WAGNER, M.D. UCSD, La Jolla, California Antalya, Turkey, April 2006

  2. GAS EXCHANGE IN HEALTH AND DISEASE NORMAL ASTHMA COPD INTERSTITIAL FIBROSIS PULMONARY THROMBOEMBOLISM

  3. MEASURING VENTILATION / PERFUSION MISMATCH MIGET PRINCIPLES

  4. INERT GAS ELIMINATION FROM THE BLOOD VENTILATION INERT GAS IN VENOUS BLOOD BLOOD FLOW SOME MOLECULES ELIMINATED BY VENTILATION SOME RETAINED IN BLOOD

  5. RETENTION DEPENDS ON : PARTITION COEFFICIENT (l)

  6. RETENTION DEPENDS ON : PARTITION COEFFICIENT (l) VENTILATION/PERFUSION RATIO (VA/Q) RETENTION IS HIGH WHEN VA/Q IS LOW RETENTION IS LOW WHEN VA/Q IS HIGH RETENTION = l / (l + VA/Q)

  7. RETENTION = l / (l + VA/Q)

  8. RETENTION = l / (l + VA/Q) HOMOGENEOUS LUNG, VA = QT = 6.0 L/min VA/Q RATIO = 1.0 Ether Acetone Enflurane RETENTION ( ) Cyclopropane Ethane SF6 PARTITION COEFFICIENT (l)

  9. RETENTION = MIXED ARTERIAL/VENOUS RATIO EXCRETION = MIXED EXPIRED/VENOUS RATIO IN A HOMOGENEOUS LUNG WITH ANATOMIC DEADSPACE (VD/DT), EXCRETION = (1 – VD/VT) x RETENTION - FOR ALL INERT GASES

  10. HOMOGENEOUS LUNG, VA/Q RATIO = 1.0 AND 30% ANATOMIC DEADSPACE EXCRETION = 70% OF RETENTION RETENTION ( ); EXCRETION ( ) PARTITION COEFFICIENT (l)

  11. HOMOGENEOUS LUNG VA ( ) AND Q ( ) R ( ) AND E ( ) PARTITION COEFFICIENT VA/Q RATIO

  12. NORMAL LUNG R VA ( ) AND Q ( ) R ( ) AND E ( ) E No shunt or Low VA/Q alveoli No high VA/Q alveoli PARTITION COEFFICIENT VA/Q RATIO

  13. LUNG WITH 50% SHUNT (VA/Q = 0) VA ( ) AND Q ( ) R ( ) AND E ( ) PARTITION COEFFICIENT VA/Q RATIO

  14. LUNG WITH 50% LOW VA/Q AREAS R ( ) AND E ( ) VA ( ) AND Q ( ) PARTITION COEFFICIENT VA/Q RATIO

  15. LUNG WITH HIGH VA/Q AREAS R ( ) AND E ( ) VA ( ) AND Q ( ) PARTITION COEFFICIENT VA/Q RATIO

  16. NORMAL SUBJECT NORMAL VA/Q VENTILATION ( ), BLOOD FLOW ( ) NO SHUNT NO LOW VA/Q NO HIGH VA/Q VENTILATION / PERFUSION RATIO

  17. VENTILATION / PERFUSION MISMATCH IN CHRONIC LUNG DISEASE ASTHMA

  18. SEVERE, CHRONIC ASTHMA, PaO2 = 53; FEV1 = 35% Predicted NORMAL VA/Q LOW VA/Q VENTILATION ( ), BLOOD FLOW ( ) NO HIGH VA/Q NO SHUNT VENTILATION / PERFUSION RATIO

  19. ASYMPTOMATIC ASTHMA, PaO2 = 79; FEV1 = normal NORMAL VA/Q VENTILATION ( ), BLOOD FLOW ( ) LOW VA/Q NO HIGH VA/Q NO SHUNT VENTILATION / PERFUSION RATIO

  20. PATHOPHYSIOLOGICAL INFERENCES 1. Even mild, asymptomatic asthma creates significant VA/Q mismatch - in spite of a normal FEV1 2. VA/Q mismatch with normal FEV1 suggests small airway obstruction 3. Despite this significant VA/Q mismatch, PaO2 remains high 4. PaO2 remains high because cardiac output is elevated 5. VA/Q pattern is similar in severe asthma - Bimodal distribution with normal and low VA/Q modes • Shunt is rarely present despite VA/Q regions as low as 0.01 - • suggesting a role for collateral ventilation in preventing shunt • Bimodality suggests small airways either badly obstructed or • little obstructed

  21. VENTILATION / PERFUSION MISMATCH IN CHRONIC LUNG DISEASE ASTHMA COPD

  22. NORMAL LUNG EMPHYSEMATOUS LUNG

  23. COPD: PREDOMINANT EMPHYSEMA NORMAL VA/Q VENTILATION ( ), BLOOD FLOW ( ) HIGH VA/Q NO SHUNT LOW VA/Q VENTILATION / PERFUSION RATIO

  24. MECHANISMS OF AIRWAY OBSTRUCTION AIRWAY WALL THICKENING, BRONCHO- CONSTRICTION MUCUS PLUGGING

  25. COPD: PREDOMINANT CHRONIC BRONCHITIS NORMAL VA/Q VENTILATION ( ), BLOOD FLOW ( ) LOW VA/Q NO HIGH VA/Q NO SHUNT VENTILATION / PERFUSION RATIO

  26. COPD: CHRONIC BRONCHITIS AND EMPHYSEMA NORMAL VA/Q VENTILATION ( ), BLOOD FLOW ( ) LOW VA/Q HIGH VA/Q NO SHUNT VENTILATION / PERFUSION RATIO

  27. PATHOPHYSIOLOGICAL INFERENCES 1. COPD causes extensive VA/Q mismatch 2. Three characteristic patterns are seen 3. Patients with a high VA/Q mode are clinically emphysematous 4. Patients with a low VA/Q mode are often clinically bronchitic 5. However, such patients commonly also have emphysema • The high VA/Q mode probably represents ventilation in poorly • perfused destroyed emphsyematous regions • The low VA/Q mode probably represents peripheral airway obstruction • due to mucus/remodeling/bronchoconstriction 8. Despite a low DLCO, hypoxemia is due only to VA/Q mismatch

  28. VENTILATION / PERFUSION MISMATCH IN CHRONIC LUNG DISEASE ASTHMA COPD INTERSTITIAL FIBROSIS

  29. INTERSTITIAL FIBROSIS NORMAL VA/Q VENTILATION ( ), BLOOD FLOW ( ) SHUNT LOW VA/Q NO HIGH VA/Q VENTILATION / PERFUSION RATIO

  30. PATHOPHYSIOLOGICAL INFERENCES 1. Interstitial Fibrosis causes extensive VA/Q mismatch 2. A characteristic pattern is seen: Shunt + very low VA/Q areas 3. Hypoxemia is variable, due at rest wholly to VA/Q mismatch 4. - Unless DLCO < 50% Predicted, when diffusion limitation occurs 5. Diffusion limitation during exercise causes hypoxemia • Limited cardiac output response to exercise worsens hypoxemia • Shunt + low VA/Q areas probably represent perfusion of capillaries • buried in thick fibrotic alveolar walls – extreme diffusion limitation

  31. VENTILATION / PERFUSION MISMATCH IN CHRONIC LUNG DISEASE ASTHMA COPD INTERSTITIAL FIBROSIS PULMONARY THROMBOEMBOLISM

  32. PULMONARY EMBOLISM NORMAL VA/Q VENTILATION ( ), BLOOD FLOW ( ) HIGH VA/Q NO SHUNT NO LOW VA/Q VENTILATION / PERFUSION RATIO

  33. PULMONARY EMBOLISM NORMAL VA/Q VENTILATION ( ), BLOOD FLOW ( ) HIGH VA/Q SHUNT NO LOW VA/Q VENTILATION / PERFUSION RATIO

  34. PATHOPHYSIOLOGICAL INFERENCES 1. Pulmonary thromboembolism causes extensive VA/Q mismatch 2. A characteristic pattern is seen – very high VA/Q areas 3. High VA/Q areas are due to low blood flow in embolized vessels 4. Hypoxemia is variable; hypercapnia would occur without hyperventilation 5. Hypoxemia is explained by VA/Q mismatch 6. Diffusion limitation is not seen • Shunt occurs in some patients. It probably represents one of: • a) perfusion through a patent foramen ovale • b) scattered microatelectasis (? Due to loss of surfactant activity)

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