Lung Volumes and Gas Distribution

1. Lung Volumes and Gas Distribution RET 2414 Pulmonary Function Testing Module 3.0

2. Lung Volumes / Gas Distribution • Objectives • Describe the measurement of lung volume using direct and indirect spirometry • Explain two advantages of measuring lung volumes using the body plethysmograph

3. Lung Volumes / Gas Distribution • Objectives • Calculate residual volume and total lung capacity from FRC and the subdivisions of VC • Identify restriction from measuring lung volumes

4. Lung Volumes / Gas Distribution • Direct Spirometry • Used to measure all volumes and capacities EXCEPT for RV, FRC and TLC

5. Lung Volumes / Gas Distribution • Indirect Spirometry • Required for the determination of RV, FRC and TLC • Most often, indirect spirometry is performed to measure FRC volume • FRC is the most reproducible lung volume and it provides a consistent baseline for measurement

6. Lung Volumes / Gas Distribution • Indirect Spirometry • Two basic approaches • Gas dilution • Body plethysmography • Measurements are in Liter or Milliliters • Reported at BTPS

7. Lung Volumes / Gas Distribution • Gas dilution techniques • All operate on a principle SIMILAR to Boyle’s Law (P1 V1 = P2 V2), which states, In isothermic conditions, the volume of a gas varies inversely with its pressure Fractional concentration of a known gas is used instead of its partial pressure C1 V1 = C2 V2

8. Lung Volumes / Gas Distribution • Gas dilution techniques • By having a known (or measured) gas concentration at the start and end of the study and a single known volume, the unknown volume can be determined. For example: V1 = C2 V2 C1

9. Lung Volumes / Gas Distribution • Gas dilution techniques • Can only measure lung volumes in communication with conducting airways !!!

10. Lung Volumes / Gas Distribution • Gas dilution techniques • Obstruction or bullous disease can have trapped, noncommunicating air within the lungs • FRC may be measured as being less than its actual volume

11. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • The natural volume of nitrogen in the subject’s lungs at FRC is washed out and diluted with 100% oxygen • Test must be carefully initiated from the FRC baseline level

12. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • All exhaled gas is collected in a Tissot (large volume) spirometer for measurement of its volume • Analyzer in the breathing circuit monitors nitrogen concentrations

13. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout

14. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout

15. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • Approximately 3-7 minutes of breathing 100% O2 to wash out N2 from the lungs • If oxygen-induced hypoventilation is a documented problem (as in COPD), a different method of FRC determination is needed

16. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • Test is successfully completed when the N2 levels decrease to become less than 1.5% for at least 3 successive breaths(subjects without obstructive disorders) • Premature discontinuation may occur due to: • System leak • Patient unable to continue • Tissot spirometer is full

17. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • The FRC has a N2 concentration of approximately 0.75, based on the atmospheric nitrogen minus CO2 and water vapor at BTPS: (CAlvN2) = 0.75

18. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • The final collected volume of exhaled gas in the Tissot spirometer (VExh) • Has a measurable concentration of N2 (CExhN2)

19. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • FRC determination is based on the following equation: VFRC = (CExhN2)(VExh) CAlvN2

20. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • In the actual FRC determination by this method, the calculation is more complex Do not get scared ! You will not be asked to do the calculation!

21. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • The small final concentration of alveolar N2 remaining in the lung needs to be subtracted from the original CalvN2 • Deep breath of O2 at the end of the test and slowly exhaled. The end-expiratory CN2 is used as the CFN2 (This volume should not be exhaled into the spirometer)

22. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • The second correction is the volume of nitrogen released from the body tissues during the washout procedure (body tissue N2 factor or BTN2) • Rages from 30 – 50 ml/minute of the washout procedure (TTest)

23. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • Final Calculation VFRC = (CExhN2 X (VExh +VD) ) - BTN2 Factor X TTest CAlvN2 – CFN2 • Must be BTPS converted • Test can be repeated after 15 minutes (longer if COPD)

24. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • Modern computer-operated pneumotachometer systems do not require collection of total VExh or measurement of the CExhN2 • Breath-by-breath CExhN2 and VExh measurements are made

25. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout

26. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout

27. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout • Leak

28. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout Criteria for Acceptability • The washout tracing/display should indicate a continually falling concentration of alveolar N2 • The test should be continued until the N2 concentration falls to <1.5% for 3 consecutive breaths

29. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout Criteria for Acceptability • Washout times should be appropriate for the subject tested. Healthy subjects should washout N2 completely in 3-4 minutes • The washout time should be reported. Failure to wash out N2 within 7 minutes should be noted

30. Lung Volumes / Gas Distribution • Open-Circuit Nitrogen Washout Criteria for Acceptability • Multiple measurements should agree within 10% • Average FRC from acceptable trials should be used to calculate lung volumes • At least 15 minutes of room-air breathing should elapse between repeated trials, >1 hour for patients with severe obstructive or bullous disease

31. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution • FRC is calculated indirectly by diluting the gas in the lungs at the end-expiration level with a known concentration of helium (an inert gas)

32. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution FRC

33. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution • Procedure • Spirometer is filled with a known volume of air with added oxygen of 25 – 30% • A volume of He is added so that a concentration of approximately 10% is achieved • System volume (spirometer, tubing) and He concentration are measured

34. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution C1 V1 = C2 V2 (C1 initial He concentration)(V1 system volume)

35. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution • Procedure • The patient breathes through a free-breathing valve that allows either connection to both room air or the rebreathing system • The patient is switched into the rebreathing system at end-expiration level (FRC) • The patient rebreathes the gas in the spirometer, until the He concentration falls to a stable level

36. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution O2 Added CO2 Absorbed H2O Absorbed

37. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution He Concentration System Volume

38. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution

39. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution • Procedure • Once the He reaches equilibrium between the spirometer and the patient, the final concentration of He is recorded • The FRC can then be calculated FRC

40. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution C1 V1 = C2 V2 (CIHe)(SV) = (CFHe) (FRC) FRC = (%HeInitial - %HeFinal) x System volume %HeFinal

41. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution Volume Corrections • A volume of 100 ml is sometimes subtracted from the FRC to correct loss of He to the blood • The dead space volume of the breathing valve and filter should be subtracted from the FRC

42. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution Criteria for Acceptability • Spirometer tracing should indicate no leaks (detected by a sudden decrease in He), which would cause an overestimation of FRC • Test is successfully completed when He readings change by less than 0.02% in 30 seconds or until 10 minutes has elapsed

43. Lung Volumes / Gas Distribution • Closed-Circuit Helium Dilution Criteria for Acceptability • Multiple measurements of FRC should agree within 10% • The average of acceptable multiple measurements should be reported

44. Lung Volumes / Gas Distribution • Body Plethysmography (BP) • Measurement of FRC by body plethysmograph is based on an application of Boyle’s law P1V1 = P2V2 or V1 = P2V2 P1

45. Lung Volumes / Gas Distribution • Body Plethysmography (BP) • Unlike gas dilution tests, BP includes both air in communication with open airways as well as air trapped within noncommunicating thoracic compartments • In patients with air trapping, plethysmography lung volumes are usually larger those measured with gas dilution methods • Volume measured is referred to as thoracic gas volume (TGV or VTG) • ATS is recommending term be dropped and changed to “plethysmographic lung volume” (VL, pleth), and “FRC by body plethysmography” or TGV at FRC (FRCpleth)

46. Lung Volumes / Gas Distribution • Body Plethysmography (BP) • Procedure • Patient is required to support cheeks with both hands and pant with an open glottis at a rate of 0.5 - 1 Hz (30 – 60 breaths/min) • BP shutter is suddenly closed at end-expiration prior to inspiration • Panting is continued for several breaths against closed shutter (no air flow)

47. Lung Volumes / Gas Distribution • Body Plethysmography (BP) • Procedure • The thoracic-pulmonary volume changes during panting produce air volume changes within the BP cabinet • Decreases in cabinet volume are an equal inverse response to thoracic volume increase (As thoracic volumes increase with panting inspiration, BP cabinet volume decreases and visa versa)

48. Lung Volumes / Gas Distribution • Body Plethysmography (BP) • Criteria of Acceptability • Panting maneuver shows a closed loop without drift • Tracing does not go off the screen • Panting is 0.5 – 1 Hz • Tangents should be within 10% • At least 3 FRCpleth values should agree within 5% and the mean reported

49. Lung Volumes / Gas Distribution • Body Plethysmography (BP) • Airway Resistance (Raw) and Specific Airway Conductance (SGaw) can be measured simultaneously during open-shutter panting (1.5-2.5 Hz) • Most plethysmographs have built-in pneumotachometers and allow VC maneuvers to be performed during the same testing session

50. Lung Volumes / Gas Distribution • Single-Breath Nitrogen Washout • Measures Distribution of Ventilation • Closing Volume • Closing Capacity